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Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing (2014)

Chapter: 2 Active Electro-Optical Sensing Approaches

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Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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2

Active Electro-Optical Sensing Approaches

In this chapter the committee discusses mature and fielded active electro-optical (EO) sensing technologies. Emerging or future EO technologies are discussed in Chapter 3.

RANGE MEASUREMENT TECHNIQUES

One of the most widely exploited features of active EO sensing is the ability to measure range very accurately. While some lidars and ladars (such as 2-D imagers) do not make use of accurate range measurement capability, most of them do. Range can be measured using active EO sensors in many fashions. The earliest, and in many ways simplest, method is to emit a short laser pulse and measure the time when it returns. This method is simplest, but it has limitations. This is the method of range measurement used with direct detection ladars.

With a single pulse, there is a trade-off between the precision of a range measurement and the precision of a velocity measurement, since a high-resolution range measurement requires a short pulse, and a high-resolution Doppler velocity measurement requires precise measurement of frequency, which takes time. Range measurement precision for a simple pulsed ladar is based on a convolution of the detector response and the return pulse shape. To have both precise measurement of range and velocity at the same time, a series of laser pulses can be used. The range precision comes from the rise time and width of a single pulse. The velocity precision comes from the pulse trains, assuming phase coherence across the pulses in the measurement.

High range precision does not necessarily require a short pulse. Any waveform that has wide bandwidth can provide high range resolution. In addition to short pulses and an array of short pulses, range can be measured with great precision by using a linear frequency modulated (LFM) chirp or a pseudo random coded waveform with sufficient bandwidth. The range resolution will be approximately the inverse of the bandwidth, multiplied by the speed of light, and then divided by a factor of 2 because the light has to go both ways. A good rule of thumb is that 1 nsec provides one-half foot—or 0.15 m—of range resolution, due to the two-way path.

To measure frequency modulation requires heterodyne detection. One major benefit of LFM as a method of range measure is it enables chirping of the local oscillator (LO) along with the emitted signal. Chirping the LO time delay compared to the linear chirp of the master oscillator is sometimes called stretch processing. The time delay of the linear chirp limits the difference in frequency between the LO and the return signal without limiting the ladar bandwidth associated with range resolution. For example, Bridger Photonics recently published results1 with 3.6 THz LFM, resulting in 50 µm range resolution, but there are no detectors capable of bandwidths even close to that level. Of course, if one only needs range resolution consistent with available detector bandwidth then one does not need this advantage. Even if one has available detectors with sufficient bandwidth, use of stretch processing may save money by allowing use of cheaper, lower bandwidth detectors.

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1 R.R. Reibel et al., 2009, “Ultrabroadband optical chirp linearization for precision length metrology applications,” Optics Letters, 34: 3692.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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Digital holography, or spatial heterodyne (see Chapter 3), can measure range by using more than one wavelength. As the two or more wavelengths walk off from each other in phase, range can be determined. Usually the two wavelengths are separated by large bandwidths, resulting in very fine range resolution, but also very fine unambiguous range.

LASER RANGE FINDERS

Laser range finders are very simple forms of laser radar. A range finder determines the range to a target object based on the round-trip time-of-flight of a laser pulse to and from the object. Most laser range finders are direct-detection ladars, meaning the return pulse hits the detector and triggers a range measurement. A combination of the return pulse shape and the detector response determines how accurately one can measure the round trip time.

An early application of a laser range finder was the determination of the precise distance from the Earth to the Moon.2 “The first laser ranging retroreflector was positioned on the Moon in 1969 by the Apollo 11 astronauts (see Figure 1-10).”3 By beaming a 60-Joule ruby laser pulse at the reflector from Earth, scientists were able to determine the round-trip travel time, which gave the distance between the two bodies at any time to a precision of about 3 cm.4

While the U.S. military pioneered laser range finders,5 they are now available for construction, surveying, golf, hunting, or many other applications where one wants to know the range to an object. Laser range finders, also called laser rangers, are widely available commercially and cost from $100 up to tens of thousands of dollars. Usually, commercial range finders are direct detection pulsed ladars.6

The main limitation on range finders is the atmosphere. If the laser beam cannot be transmitted to the target, or received from the target, then one cannot use a laser ranger. For long-range measurements, slight variations in the index of refraction of the atmosphere can slow the speed of light, introducing slight inaccuracies in the range measurement.

ONE-DIMENSIONAL RANGE PROFILE IMAGING LADAR

A pulsed laser and a single detector can provide a range profile from an object with a range resolution that depends on the transmitter pulse rise time, pulse width, or in the case of coherent ladars, the width of the frequency chirp (e.g., how short the laser pulses are). Of course one must match the transmitter pulse with how fast the detector can resolve the pulse, and a wide frequency chirp must be matched by a detector with sufficient bandwidth. The range profile does not depend on the size of the receiving optical apertures, except for signal-to-noise considerations. This can be very useful in environments that are not very cluttered and for objects that are far away, where the transverse dimensions of the object are not resolved by the optical system and the only spatial information about the object comes from the range profile.

A high-range-resolution profile is most useful when the orientation of the object is known; for example, one generally assumes that an airplane is oriented so the nose is pointed in the direction that the plane is traveling. Figure 2-1 shows how an airplane’s scattering signal will vary with range, depending on what scatterers are in a given range bin. Starting from the time when the detector is first triggered by

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2 P.F. McManamon, M. Huffaker, and G. Kamerman, 2010, “A history of laser radar in the United States,” Proceedings of SPIE, 7684: 76840T.

3 Lunar Planetary Institute, 1994, “Apollo Laser Ranging Experiments Yield Results,” LPI Bulletin 72. http://eclipse.gsfc.nasa.gov/SEhelp/ApolloLaser.html.

4 Ibid.

5 J.T. Correll, 2010, “The emergence of smart bombs,” Air Force Magazine, March: 60.

6 See, for example, patent publication number US5359404 A.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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FIGURE 2-1 Range profile of an airplane viewed in one dimension (1-D) from the front, although the image shown is from the side. SOURCE: Courtesy of Matthew Dierking, Air Force Research Laboratory, Dayton, Ohio.

the large scattering signal from the nose, the dimensions of the rest of the airplane can be calculated by the relative time delay.

Tomographic Imaging Using Multiple 1-D Profiles from Various Angles

Tomographic methods, which reconstruct an image from a set of its projections, are well known in radio astronomy and medical imaging. The goal of reflective tomography is to estimate an object’s surface features using a set of reflective projections (as opposed to transmissive projections). These projections are measured in angular increments around the object to obtain 3-D information.7 Each angular resolution cell in the projection represents the energy reflected off the corresponding illuminated surface of the object at that angle. For any laser radar, the received signal represents information about the surface of the object illuminated by the radar from a given line of sight. A series of signals along the range resolution coordinate produces a reflective projection of the object that can be used to reconstruct the object.8Figure 2-1 shows one such 1-D image.

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7 F.K. Knight, S.R. Kulkarni, R.M. Marino, and J.K. Parker, 1989,” Tomographic techniques applied to laser radar reflective measurements,” Lincoln Laboratory Journal 2 (2): 143.

8 X. Jin and R. Levine, 2009, "Bidirectional reflectance distribution function effects in ladar-based reflection tomography," Appl. Opt. 48: 4191-4200.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
×

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FIGURE 2-2 Two-dimensional projection reconstruction of an F-4 fighter for (a) measurements from 0 to 180 degrees and (b) measurements within 20 degrees around broadside. SOURCE: M.P. Dierking, F. Heitkamp, and L. Barnes, 1998, “High temporal resolution laser radar tomography for long range target identification,” OSA Signal Synthesis & Reconstruction Conference, June.

In this technique, a pulse whose physical length is short compared to the range extent of the object is first reflected off an object. The resulting time-dependent return signal is collected by single detector in a 1-D optical system (single-element detector), which provides signal as a function of range.9 This 1-D signal relates to a 1-D slice of the spatial 3-D profile of the object.10 When the object rotates, different slices of the 3-D profile can be obtained.11 If a sufficient number of 1-D slices are collected, a 2-D image can be reconstructed. A scanning ladar can be employed as long as the 2-D images are registered in angle.12

Figure 2-2 shows a 2-D image, obtained by Air Force researchers in 1998, using range profiles determined at multiple angles. Various algorithm approaches can be used to combine multiple range profiles to make a more complete image from multiple 1-D profile images at different angles to the target. Wider-angle reconstruction yielded better detail, as expected.

Range profiles from various angles can be obtained when the object moves with respect to the 1-D ladar. The object may be stationary, as a target on the ground, or moving, as an airplane, or rotating, as a satellite. In each case suitable mathematics has been developed to provide images. The more complex the object, the more interest there is in obtaining range profile information over multiple angles, but even with a single range profile 1-D imaging can be useful for recognizing some objects, as discussed in the previous section.

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9 J.B. Lasche, C.L. Matson, S.D. Ford, W.L. Thweatt, K.B. Rowland, V.N. Benham, 2009, “Reflective tomography for imaging satellites: Experimental results,” Proc. SPIE 3815: 178, Digital Image Recovery and Synthesis IV.

10 Ibid.

11 J. B. Lasche, C. L. Matson, S. D. Ford, W. L. Thweatt, K. B. Rowland; V. N. Benham, 2009, “Reflective tomography for imaging satellites: experimental results,” Proc. SPIE 3815: 178, Digital Image Recovery and Synthesis IV.

12 R.M. Marino, T. Stephens, R.E. Hatch, J.L. McLaughlin, J.G. Mooney, M.E. O’Brian, G.S. Rowe, J.S. Adams, L. Skelly, R.C. Knowlton, S.E. Forman, and W.R. Davis, 2003, “A compact 3-D imaging laser radar system using Geiger-mode APD array,” Proceedings SPIE, 5086: 1.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
×

When a coherent detection system is employed and data are collected from multiple views, an object’s Doppler spectrum can be used to help determine the angle from which a given 1-D image has been collected.13 With Doppler techniques, the rotation speed of an object can be precisely measured.

The ability of range-resolved reflective tomography to image satellites was demonstrated in 2001.14 The researchers used a CO2 laser mode-locked to produce nanosecond pulses, and the data were collected using a coherent laser radar system. “Key components of the image reconstruction process included the calculation of tomographic angles and the alignment of the intensity projections to an appropriate center of rotation.”15

Researchers from the Electronic Engineering Institute of the State Key Laboratories of Pulsed Power Laser Technology and Electronic Restriction in Anhui Province, Hefei, China recently reported their verification by computer simulation of ladar reflective tomography imaging of space objects.16 This paper concluded that atmospheric turbulence has little effect on the coherent detection signal at 10.6 µm, because the atmospheric coherence length is on the order of meters. They found that chirped pulse signals are advantageous for reflective tomography imaging of space targets and that the “optimal pulse repetition is decided by the target altitude, target size, and the cutoff frequency in the spectrum domain.”17 Background work on reflective tomography has been ongoing at Key Laboratory of Space Laser Communication and Testing Technology at the Shanghai Institute of Optics and Fine Mechanics in China.18

Most early work on 1-D ladar, and tomographic imaging with 1-D ladar used low-duty-cycle pulsed ladar sensors. More advanced range measurements techniques can be used, such as linear frequency-modulated (FM) chirp or pseudo-random coded range measurements. This provided another method of achieving both 1-D imaging and tomographic imaging.19

TWO-DIMENSIONAL ACTIVE/GATED IMAGING

Two dimensional (2-D) gated imaging is one of the more straightforward and mature methods of active EO imaging. It is a useful imaging technique for seeing through haze and smoke, for seeing objects in shadow, for imaging at higher angular resolution in darkness and at longer range (approximately 10-20 km) than can be achieved with passive MWIR systems, and for seeing objects more clearly against a cluttered background. “Range-gated active imaging is widely used in night vision, underwater imaging, three-dimensional scene imaging, and other applications because of its ability to suppress backscatter from fog and other obscurants, as well as its high signal-to-noise ratio, high angular resolution, long detection range, and direct visualization.”20

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13 C. Matson and D. Mosley, 2001, “Reflective tomography reconstruction of satellite features? Field Results,” Appl. Opt. 40(14): 2290-2296.

14 C. Matson and D. Mosley, 2001, “Reflective tomography reconstruction of satellite features? Field Results,” Appl. Opt. 40(14): 2290-2296.

15 C. Matson and D. Mosley, 2001, “Reflective tomography reconstruction of satellite features? Field Results,” Appl. Opt. 40(14): 2290-2296.

16 F. Qu, Y. H., and D. Wang, 2011, “Lidar reflective tomography imaging for space object,” Proc. of SPIE 8200: 820015.

17 Ibid

18 X. Jin, J. Sun, Y. Yan, Y. Zhou, and L. Liu, 2012, “Imaging resolution analysis in limited-view laser radar reflective tomography, Optics Communications, 285 (10-11): 2575.

19 J. Murry, J. Triscari, G. Fetzer, R. Epstein, J. Plath, W. Ryder, and N.V. Lieu, 2010, “Tomographic Lidar,” OSA Conference Paper, Applications of Lasers for Sensing and Free Space Communications, San Diego, CA, February.

20 X. Wang, Y. Zhou, S. Fan, Y. Liu, H. Liu, 2009, “Echo broadening effect in the range-gated active imaging technique.” Proc. SPIE 7382:738211, International Symposium on Photoelectronic Detection and Imaging 2009: Laser Sensing and Imaging.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
×

2-D gated imagers are typically used in conjunction with laser illuminators and often share the same pulsed laser. They use the pulsed laser to illuminate a scene, then collect the returning reflected light at a predetermined delay time based on the range of the target. These sensors are gated both to eliminate the reflection from obscurations and to reduce the noise coming into the detector (Figure 2-3). The range information is typically first acquired with the co-located laser illuminator. The gated imager uses this information to set the start of the integration window of a high-speed focal plane receiver to coincide with light returning from the range of interest in the scene. This provides a range slice image each time the scene is illuminated. The scene can be interrogated at a series of ranges by varying the delay in turning on the receiver. An example is shown in Figure 2-4.

Collecting imagery using a 2-D gated camera has three primary advantages over passive imaging:

1. For scenes with clutter (such as particles in the air or haze) or obstructions near the camera or clutter in the background behind the area of interest, 2-D gated imaging effectively eliminates the visually confusing effects of clutter by only imaging the slice of the scene of interest. In these cases the object of interest is either not visible or very indistinct when viewed with standard visible or IR cameras.

2. 2-D gated imaging is sometimes referred to as “bring your own flashlight.” It effectively brings out features in shadowed or recessed areas, such as inside doorways or other openings or under overhanging structures. By shining the laser into those areas, it is easy to see what is not otherwise visible. It also works when thermal imaging has limited signal, such as thermal crossover.

3. An active EO eye-safe laser 2-D gated camera operating at 1.55 µm will have approximately 3 times better angular resolution than an MWIR sensor. Higher F-number optics allow the user to make use of the improved diffraction limit resulting from operating at shorter wavelengths to see finer details, perform better target detection and recognition, and to see details that have contrast at the laser wavelength but not in the thermal region.

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FIGURE 2-3 A gated active EO imaging system can eliminate clutter and obscurants that might otherwise degrade the image. Although laser returns from image clutter, smoke, mist and foreground obstructions can be range-gated, effects of atmospheric turbulence remain. SOURCE: I. Baker, S. Duncan, and J. Copley, 2004, “A Low Noise, Laser-Gated Imaging System for Long Range Target Identification,” Proc. of SPIE, Vol. 5406. Courtesy of Selex ES Ltd.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
×

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FIGURE 2-4 Illustration of how the outline of a man can be extracted from the deep gate by knowledge of the individual pixel ranges. SOURCE: I. Baker et al., 2008, “Advanced infrared detectors for multimode active and passive imaging applications,” Infrared Technology and Applications XXXIV, edited by Bjørn F. Andresen, Gabor F. Fulop, Paul R. Norton, Proc. of SPIE 6940: 69402L. Courtesy of Selex ES Ltd.

2-D gated imagers have processing requirements equivalent to passive imaging systems, which are significantly less than those required for 3-D imaging. An inexpensive 2-D active imager that can form both passive and active images has recently been reported.21 However, moderate spatial resolution 3-D images can be constructed using gated imaging in the case where a large number of images are sequentially collected and integrated.22

2-D gated imagers are currently fielded in the United States and in Europe. Performance characteristics of two types are given in Table 4-4. In addition to U.S. Air Force work in the late 1990s on the ERASER Program,23 much of the early development work was done in the United Kingdom at BAE Systems and in Denmark.24 Commercial units are relatively compact and are used on almost all types of platforms, from handheld and tripod-mounted viewers to ground vehicles and ships, and in aircraft. Current providers of component hardware and systems include Intevac (see Figure 2-5), which is working on an export-approved unit, Sensors Unlimited, SELEX (UK), DRS Technologies, and Northrop Grumman. China’s interest is shown in publications referred to above, which are mostly theoretical and do not appear yet to be state of the art.

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21 R.H. Vollmerhausen, 2013, “Solid state active/passive night vision imager using continuous-wave laser diodes and silicon focal plane arrays,” Optical Engineering, 52: 043201, April.

22 J. Busck and H. Heiselberg, 2004, “Gated viewing and high-accuracy three-dimensional laser radar,” Applied Optics 43 (24): 4705.

23 P.F. McManamon, M. Huffaker, and G. Kamerman, 2010, op. cit.

24 McManamon et al., op. cit., and Busck and Heiselberg, op. cit.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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FIGURE 2-5 Intevac Laser Illuminated Viewing and Ranging (LIVAR) camera. SOURCE: Courtesy of Intevac, Inc.

The fundamental limit to this technology is atmospheric transmission, which limits the range of the sensor. Increasing the laser pulse energy and detector sensitivity and/or reducing the illuminated area can extend the range of the sensor. Atmospheric turbulence is not usually a significant concern as long as the transmission is adequate to close the link budget. The snapshot time duration is short enough that the atmosphere is essentially frozen for each image, and the atmospheric distortion is usually not the limitation in angle/angle resolution. In general the short-pulse, high-energy lasers used for illuminators operate, by their nature, on a number of longitudinal (and often spatial) modes, and thus may be broadband enough in wavelength to eliminate speckle in a single pulse. Techniques such as broadband seeding of the high-energy source by a low-power semiconductor laser may be used to further spoil the laser coherence. If necessary, any remaining speckle can be averaged out using multiple 2-D images.

While not a physical fundamental limit, gated imaging requires high per-pulse energy to illuminate the scene (tens to hundreds of mJ) at any useful range. Therefore, the operating wavelength must be in the eye-safer region of the spectrum where optical radiation cannot focus on the retina (see Box 1-4), requiring solid state lasers with high per-pulse energy and detectors in the 1.55-2 µm range, which are more challenging to manufacture than 1 µm or visible lasers. Single fiber lasers cannot achieve this illuminator performance. Intevac cameras have limited life due to plasma etching of internal components, which limits them to low-repetition-rate systems.

Over the next decade, the committee expects to see the proliferation of 2-D gated imagers, particularly for marine and airborne systems. Their advantages are relative simplicity, technical maturity, good utility, and the ability to utilize existing laser rangefinder transmitters that are already in targeting systems for added functionality. Improvements in detector sensitivity and laser efficiency will extend the useful range of 2-D active imaging systems.

THREE-DIMENSIONAL DIRECT-DETECTION ACTIVE IMAGING

Ladar has been used extensively to create accurate and precise high-resolution angle/angle/rangeresolution digital elevation models of tactically significant geographic areas. These can provide true 3-D point clouds25 with a high density of information. The digital terrain elevation data (DTED) point spacing depends practically on the position of the sensor: 1-10 m angle/angle resolution for airborne laser

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25 A point cloud is a set of data points in some coordinate system, often intended to represent the external surface of an object.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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scanning (ALS), and millimeter to centimeter angle/angle resolution for terrestrial laser scanning (TLS). Helicopter-based ALS can give a higher angle/angle resolution than aircraft-based ALS and allows orienting the scanner in all directions. A recent book26 provides a comprehensive compilation of the principles and fundamentals of ladar technologies as well as the state-of-the-art of the performance aspects of the sensors (components, calibration, waveform analysis, quality control of ladar data, filtering and feature extraction techniques). In most references the term lidar has been used for 3-D mapping, but since the convention in this study is to use the word ladar for active EO imaging of surface reflecting targets, the committee will use ladar for 3-D mapping.

Scanning 3-D Ladar

A scanned laser radar is one technology for collecting 3-D images. Figure 2-6 is a block diagram of a typical bistatic,27 direct-detection scanned ladar. A laser generates an optical pulse that is shaped and expanded to reduce its divergence. It is then directed toward the scene to be interrogated by a scanner. The backscattered light is directed into a collector (e.g., a telescope), where it is focused onto a photosensitive element. The resulting electronic signal is filtered to remove noise and analyzed to determine the time of arrival of the reflected optical signal. The distance to the scene at the point where the transmitted light was reflected is determined by the round-trip time of the radiation. The scanner repositions the interrogation point and the process is repeated.

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FIGURE 2-6 Block diagram of a typical bistatic, scanned 3-D imaging laser radar.

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26 J. Shan and C.K. Toth, eds., 2008, Topographic laser ranging and scanning: principles and processing, CRC Press.

27 Bistatic means that separate apertures are used to transmit and receive the optical radiation.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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FIGURE 2-7 One scanning 3-D ladar scan pattern.

If the scanner follows a raster or line pattern, as shown in Figure 2-6, then the data will resemble a television image. Here, however, instead of the data consisting of two angles and intensity, it is composed of two angles and range. It is a representation of the geometric shape of the scene portrayed in spherical coordinates originating at the sensor. By trigonometric coordinate transformations, these data can be converted into another format such as Cartesian coordinates. If the location and orientation of the sensor are known, then the Cartesian representation closely resembles a high-resolution digital topographic map.

Such “flying spot” scanned systems are currently the rule for commercial airborne topographic mapping as well as ground-based (tripod) systems. These commonly use a line scan in one dimension, typically across the aircraft flight path. The physical motion of the aircraft itself accomplishes scanning along the flight path (Figure 2-7). A swath is then mapped. Subsequently, overlapping swaths are stitched together to map wider areas, as shown in the figure. Frequently, the ground spatial resolution or ground sampling distance in flying spot systems is limited by the transmitter spot size at the target, and not by the receiver optics. The instantaneous field of view (IFOV) of the receiver optics is often designed to be larger than the transmitter spot size in order to simplify transmitter to receiver alignment. This is in contrast to the focal plane systems to be described below. A typical commercial airborne topographic mapping system is shown in Figure 2-8.

This class of instrument typically uses only a single photosensitive element to receive the reflected radiation. The acquisition cost of sensors that employ this “one pulse-one pixel” methodology is lower than sensors with multiple detector elements but the methodology limits the data collection rate, which in turn limits the area coverage rate at the desired ground sampling distance. The commercial collection strategy requires multiple passes and additional flight time to cover a given collection area. This is acceptable for commercial topographic mapping systems but may not be acceptable for military

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
×

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FIGURE 2-8 RIEGL LMS-Q780 Airborne Laser Scanner.

SOURCE: Courtesy of Riegl USA.

applications. Area coverage rate is a key attribute of military sensors as multiple passes are frequently not permissible. Multiple passes are not practical in a hostile airspace or where this collection strategy might alert opposing forces.

Commercial flying spot scanners, or airborne line scanners, usually operate at lower altitudes than do military systems, which must operate at higher altitudes in order to avoid hostile fire. Operating at lower altitudes permits the use of lower sensitivity detectors, smaller aperture sizes, and lower power lasers. Overall, this reduces the acquisition cost of the laser radar and limits eye-safety concerns. This lower cost is highly desirable in a commercial environment but these commercial sensors cannot be applied to many military applications as they require altitudes lower than the effective altitude of many common air defense threats.

Commercial, ground-based survey laser radar systems generally use a second, orthogonal scan element instead of gross sensor motion to generate a raster. The area coverage rate of these ground-based laser radar systems is also limited by the one-pulse-one-pixel collection strategy. It is notable that even though flying spot scanned 3-D imaging laser radars were first demonstrated in the United States, U.S. industries have not taken advantage of this early technological advantage. The majority of commercial airborne mapping and all ground-based surveying laser radars are currently produced outside the United States. Based on total operating units, the commercial market is currently dominated by Optech (Canada), Riegl (Austria), and Leica Geosystems (overall headquarters in Switzerland).28

3-D Flash Imaging

As illustrated in Figure 2-9, flash imaging is achieved by flood-illuminating a target scene or a portion of a target scene. The receiver collects the backscattered light and directs an image of the target

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28 M.S. Renslow, ed., 2012, Manual of Topographic Lidar, American Society for Photogrammetry and Remote Sensing, 34.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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FIGURE 2-9 Concept for 3-D laser radar that creates a 3-D image from a single laser pulse. IMU, inertial measurement unit. SOURCE: Lincoln Laboratory MIT, 2011, Tech Notes: Airborne Ladar Imaging Research Testbed, www.ll.mit.edu/publications/technotes/TechNote_ALIRT.pdf.. Reprinted with permission of MIT Lincoln Laboratory, Lexington Massachusetts.

scene onto a 2-D array. This array pattern can then also be scanned for further scene coverage. Using the location and pointing information from the sensor platform, the range for each pixel can be converted into a location in absolute space.

Imaging over a larger area presents the challenge of achieving both detector sensitivity and speed. For flash imagers, the energy returning to the receiver is divided among multiple detectors. Thus, imaging of targets containing many pixels in angle/angle space requires high peak illumination power and/or very sensitive detectors compared to single-pixel scanners. If multiple range returns are contained in a single detector angular subtense (DAS), then it can divide up the returned signal even more, further increasing the required energy per pulse. One method to increase sensitivity of a direct detection receiver is to immediately amplify the return signal, so that this signal level is above noise sources introduced downstream from the initial detection. Avalanche photodiodes (APDs), discussed further in Chapter 4, are often used to increase sensitivity. Linear mode APD detectors have a gain that provides a linear relationship between the number of photons received and the amplitude of the signal output from the detector. Geiger-mode APDs have a huge gain regardless of the number of photons input. Fiber preamplification before detection is also possible, but to date has not been used with arrays of ladar detectors, though it is used with single detectors to increase sensitivity.

Intensity-encoded imaging addresses a different issue. Instead of having high-bandwidth readout circuits, intensity-encoded imagery uses polarization rotation to measure range, which means that simple framing cameras can be used to do 3-D flash imaging. Each of these technologies is described below.

Figure 2-10 shows an early 3-D flash imager. As described above, scanning limits the speed of image acquisition and adds to the size, weight, and power (SWaP) requirements as well as the cost of the system.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
×

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FIGURE 2-10 An early flash imager, ASC’s 3-D FLC ladar. SOURCE: TigerEye 3D Flash LIDAR Camera (45° fixed lens); 2006 for robotics. Used by permission Advanced Scientific Concepts, Inc. (left). The TigerCub 3D camera captures 16,384 pixels with each laser pulse up to 30 frames per second. Used by permission Advanced Scientific Concepts, Inc. (right).

ASC’s 3-D ladar, shown in Figure 2-10, “illuminates the scene, records time-of-flight laser pulse data onto a detector array, and generates precise “point cloud” data (video streams) on a per frame basis.”29 “Each pixel is “triggered” independently, allowing capture of 16,384 range data points to generate the 3-D point cloud image.”30

High-speed and stable optics are needed to steer the beam since any pointing errors can cause gaps or overlap in the image or cause the image to jump from frame to frame. Optical stability needs to be a fraction of the DAS. Also, since it takes a finite time to scan a complete frame, any relative motion between the target and the sensor platform will cause the image to be distorted.31 Therefore, for applications requiring coverage of larger areas, or real-time imaging of moving targets, it is desirable to capture the 3-D image with a large array of detectors over a very short period of time (<30 Hz). Ideally, these collections could be carried out with a single laser pulse.32 Systems that utilize arrays of detectors to collect in this manner are referred to as flash imaging systems. 3-D flash imaging can be ideal for these applications and has been implemented in several operational airborne ladar systems (e.g., ALIRT, HALOE). 3-D flash imaging can be divided up into 3-D imagers that collect all the information on a single pulse and 3-D imagers that must take multiple snapshots of the same area many times. The multiple-snapshot flash imagers can suffer from some of the same motion limitations suffered by 3-D scanning ladars, since multiple pulses over time are required to form an image. They do however gain the benefit of not requiring high-resolution tracking to a fraction of a pixel.

APD-Based Imaging

Arrays of Geiger- and linear-mode APDs are often used to amplify the return signal to bring it above other noise sources in imaging applications. The fundamental differences between these systems revolve around how the gain is achieved by the two types of detectors. A comprehensive description of

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29 V. Molebny, G. Kamerman, O. Steinvall, 2010, “Laser radar: from early history to new trends,” Proc. SPIE 7835:783502, Electro-Optical Remote Sensing, Photonic Technologies, and Applications IV.

30 Ibid.

31 R. Stettner, H. Bailey, and R. Richmond, 2004, “Eye-Safe Laser Radar 3-D Imaging,” RTO-MP-SCI-145.

32 B. Aull et al., 2002, “Geiger-mode avalanche photodiodes for three-dimensional imaging,” Lincoln Laboratory Journal 13 (2): 335.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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image

FIGURE 2-11 ALIRT imagery of the Grand Canyon obtained during a single flight. Colors represent heights over a 1,000+ meter range. The inset displays a wide-angle visible-camera image of a similar view. SOURCE: Lincoln Laboratory MIT, 2011, Tech Notes: Airborne Ladar Imaging Research Testbed, www.ll.mit.edu/publications/technotes/TechNote_ALIRT.pdf. Reprinted with permission of MIT Lincoln Laboratory, Lexington Massachusetts.

linear- and Geiger-mode APD (GM-APD) arrays, including discussion of materials and fabrication techniques, can be found in Chapter 4. An example of an APD-based 3-D flash image is shown in Figure 2-11.

The ultimate performance of APD-based flash ladar systems depends on the interplay of a number of parameters, including the number and sensitivity of the pixels in the APD array, the power and pulse repetition frequency (PRF) of the laser, and the operating range/altitude of the platform carrying the system. This interplay is illustrated in Figure 2-12, which shows the area collection rate as a function of the aircraft altitude above the ground (or range) for a fixed ground sample distance (GSD) of 30 cm. While this example uses a GM-APD based system to illustrate the trades, the same fundamental trade-offs (but not the exact numbers) hold for linear-mode systems.

The horizontal lines in Figure 2-12 show the performance for three different APD formats (from bottom to top, 32 × 128, 64 × 256, and four 64 × 256 arrays). As the size of the array increases, so does the instantaneous area collection rate. The dashed diagonal lines show the performance for different laser powers and aperture sizes. As expected, as the power and aperture increase the area coverage increases as well, while the performance at a fixed GSD degrades as the altitude increases. The power lines in this chart each assume a 30 cm GSD. A final limitation is imposed by the scan width limitations (most flash imaging ladars still scan to increase their area rate) and the speed of the platforms carrying the system.

Consider a baseline laser design with a 2 W, 20 kHz PRF laser and a single 32 × 128 GM-APD behind a 10 cm aperture. As shown by the red dot in Figure 2-13, the optimal performance point for this system is at approximately 12,000 ft. Increasing the laser power to 30 W alone will allow operation at higher altitudes but will not increase the overall area coverage (green dot). At this point the APD size is limited. If the APD size is increased to 64 × 256, the area rate can be increased, but the aircraft must operate at a lower altitude to achieve this coverage (blue dot). In this case the limit is the laser power. For

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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image

FIGURE 2-12 Example laser radar performance trade-offs for fixed GSD. Geiger mode is assumed here for illustrative purposes. SOURCE: MIT Lincoln Laboratory.

some scenarios, operation at lower altitudes is acceptable, but high altitude operation is desirable where survivability is a concern. Finally, increasing the size of the APD to four 64 × 256 arrays in parallel and increasing the laser power and aperture allows operation at the optimal point in the scan-limited regime at higher altitudes (black dot).

Geiger-Mode Imaging

Geiger-mode imaging refers to imaging based on a detector technology that utilizes a massive amplification of signal no matter how many photons are received. This makes GM-APDs very sensitive photon-counting detector arrays. The term photon-counting is often used interchangeably with detector sensitivity capable of single-photoelectron detection. In fact photon counting is exactly that—the ability to count the exact number of received photons. This means that GM-APDs have the ability to detect the arrival of a single photon. Active sensors that can detect very weak signals of interest can have relatively superior performance in mission areas that are SWaP-constrained and/or require extended ranges or rapid decisions, because an increase in detector sensitivity directly reduces required laser power. There are several methods of implementing photon-counting detector arrays, but Geiger-mode APD array-based systems are currently the most mature and widely used.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
×

As discussed in Chapter 4, Geiger-mode imaging takes advantage of the fact that for low incident photon rates per laser pulse (~0.2 photons per pulse), the measured photon rates for a GM-APD ladar system approach that of a “perfect” detector (i.e., one that detects every incident photon at every pulse). Hence, Geiger-mode ladar systems operate most efficiently at a low per-pulse probability of detection where they collect multiple pulses to obtain high probabilities of detection of a target surface. The output of the detector is then an ensemble of independent measurements that are histogrammed to give a single range measurement. An example of imagery obtained with ALIRT, a fielded GM-APD system, is shown in Figure 2-11.

The assembly of a device with a high probability of detection using multiple pulses creates a sensitivity to motion not unlike the sensitivity scanning sensors have; however, the timescale for the measurements is much shorter than for scanning systems. Still, any scene motion must be compensated for prior to adding detections. GM-APD arrays can operate with 10-200 kHz frame rates, which enables the ensemble detections to be made very quickly (e.g., 1-10 msec for 10-100 measurements at 10 kHz). The fast frame rates also allow the use of lasers that run at rather high PRF with low energy per pulse. “This low energy requirement allows the use of 1.06 µm radiation without eye hazard because of low single-pulse intensities.”33 This is a significant feature since it enables high-performance active sensing with SWaP-constrained platforms.34 High frame rates also increase the laser duty cycle, which is favorable from a point of view of laser cost, weight, and efficiency and enables fiber laser solutions. However, operation at high pulse repetition rates reduces the unambiguous range35 for the system.

Initial Geiger-mode flash ladar systems were pioneered by MIT Lincoln Laboratory (MIT/LL), which has made Geiger-mode APD cameras with up to 64 × 256 detectors.36 MIT/LL developed the ALIRT laser radar system (shown in Figure 2-13), which was deployed in 2010 to Afghanistan and provides wide area mapping and point targets. The DARPA High Altitude Lidar Operational Experiment (HALOE) also uses Geiger-mode APD arrays to provide high angle/angle resolution (20 cm), wide area, off-nadir 3-D mapping and target information. This system was also deployed in the Afghanistan theater.37,38 These two systems have been responsible for wide area 3-D maps of Afghanistan, providing digital elevation data of the country with unprecedented resolution. More recently, MIT/LL has been developing the MACHETE ladar system for USSOUTHCOM. This ladar, which was based on the ALIRT system, represents the state of the art for GM-APD-based flash ladar systems with 16 times more pixels and a much more capable laser developed by Raytheon Space and Airborne Systems.

Two U.S. companies have commercialized Geiger-mode APD arrays. Both Princeton Lightwave and Boeing Spectrolab have 32 × 32 array based cameras available at 1.06 µm and at 1.55 µm (see, for example, Figure 2-14), and both are developing 32 × 128 format cameras.39 Initial Geiger-mode APD cameras operated at a PRF of about 20 kHz, but advances in the readout integrated circuits (ROICs) by

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33 P. McManamon, 2012, “Review of ladar: A historic, yet emerging, sensor technology with rich phenomenology,” Optical Engineering 51(6): 060901.

34 P.F. McManamon et al, op. cit.

35 The unambiguous range is the range at which the returned signal can be known to be unquestionably at that range.

36 P. McManamon, 2012, “Review of ladar: A historic, yet emerging, sensor technology with rich phenomenology,” Optical Engineering 51(6): 060901.

37 DARPA 2011 Congressional testimony, available at http://www.darpa.mil/WorkArea/DownloadAsset.aspx?id=2929.

38 B. Thompson, 2011, “AFRL plays pivotal role in response to urgent operational need in Afghanistan,” Inside WPAFB, http://www.wpafb.af.mil/news/story.asp?id=123281012.

39 P. McManamon, 2012, “Review of ladar: A historic, yet emerging, sensor technology with rich phenomenology,” Optical Engineering 51(6): 060901.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
×

image

FIGURE 2-13 The ALIRT 3-D laser radar system and associated hardware. SOURCE: MIT Lincoln Laboratory.

image

FIGURE 2-14 Princeton Lightwave 32 × 32 GM-APD-based flash ladar camera. SOURCE: Courtesy of Princeton Lightwave, Inc.

Princeton Lightwave allow operations up to 186 KHz for the 32 × 32 array.40 The 32 × 128 array will have a lower maximum frame rate, probably about 105 kHz.41 The commercial availability of such cameras may lower the barrier to entry for others seeking this type of capability.

Linear-Mode Active Imaging

Another method for flash imaging uses the same overall system architecture shown in Figure 2-13, but uses linear-mode APD arrays instead of Geiger-mode APDs. Linear-mode APDs are designed with the goal of operating below the breakdown voltage with high gain and low noise. The operation of

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40 P.F. McManamon et al., op. cit.

41 Personal communication from Mark Itzler, Princeton Lightwave.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
×

image

FIGURE 2-15 ASC’s 128 × 128 pixel DragonEye 3-D flash ladar camera shown alone and as installed on the NASA Endeavour space shuttle. SOURCE: Left: DragonEye 3D Flash LIDAR Space Camera (45°FOV) 2011 for International Space Station rendezvous and berthing. Used by permission Advanced Scientific Concepts, Inc. Right: NASA.

these APDs produces an average photocurrent that is linearly proportional to the incident optical flux (hence the name linear mode).42 A detailed description of linear-mode APD arrays can be found in Chapter 4.

Unlike Geiger-mode systems, which must geo-register and aggregate multiple-high-frame rate returns, linear-mode systems have the advantage of collecting range and intensity information in a single frame, assuming the ROICs are made to support this (making them true “flash imaging” systems). However, these systems usually operate at lower frame rates (~30 Hz), requiring higher laser energy per pulse. Historically, linear-mode APDs also have not been as sensitive as GM-APDs, requiring additional total laser energy per sample. Both factors currently combine to limit the ability to use lower SWaP, efficient, fiber lasers. More sensitive linear-mode APDs operating at higher frame rates may be able to use fiber lasers for sources. As single-photon-counting linear-mode APDs that operate at higher frame rates become widely available, the SWaP for future linear-mode APD 3-D ladars will be reduced.

Linear-mode 3-D flash imaging systems and cameras have become commercially available. ASC pioneered this approach, especially for commercial applications, and offer several different camera models for different applications. All of the available cameras utilize 128 × 128 pixel arrays that frame at 1 to 20 Hz or at 30 Hz in burst mode.43 The DragonEye system (shown in Figure 2-15) was developed for space applications and was flown on the Endeavour Space Shuttle in 2009.

Recently, there has been significant progress in developing high sensitivity linear-mode APD arrays by companies such as Raytheon and DRS Technologies.44 High gain in the APD reduces the effect of any noise introduced after the amplification stage, as is discussed in more detail in Chapter 4. This increased sensitivity enables photon detection and counting in single-to-few pulses, thus ultimately reducing the discussion of APD-based flash ladars to photon-counting and non-photon-counting systems rather than to Geiger-mode and linear-mode APDs.

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42 B. Aull et al., op. cit.

43 P. McManamon, 2012, “Review of ladar: A historic, yet emerging, sensor technology with rich phenomenology,” Optical Engineering 51(6): 060901.

44 Ibid.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
×

Comparison of Linear- and Geiger-Mode Systems

In this section the relative qualitative advantages and disadvantages of Geiger-mode and linear-mode 3-D flash imaging systems are discussed (a comparison at the detector level is deferred to Chapter 4).

Long-Range Applications

The single-photon sensitivity, low readout noise, and accurate timing enabled by the Geiger-mode ladar design mean that high-precision and high-accuracy spatial information (range or time-of-flight) can be measured with minimal return optical signal, making it well-suited to long range and/or high altitude applications. These situations also tend to have limited angular motion, making the required motion compensation less difficult.

Currently fielded linear-mode flash ladar systems require hundreds of photons to create images. The requirement for more photons drives the need for higher laser energy per pulse at relatively low repetition rates and makes it harder to use fiber lasers. Current systems are not designed for precision wide-area mapping at long ranges. The ongoing development of highly sensitive, linear-mode APD arrays will reduce the need for these high-energy-per-pulse lasers and provide high-quality, long-range sensing capability.

High Background Noise Environments

In high background noise environments (e.g., reflections from sunlight, etc.), the photons from the background will be captured along with the target returns. Since linear-mode systems capture the entire return in a signal pulse, the laser power and detection threshold can be turned up so that only the signal return is increased and the background noise remains constant on a per pulse basis.

As described above, Geiger mode systems operate best with low per-pulse probability of detections, so the practice of “turning up the volume” to overcome the background noise is not an effective solution. Instead, Geiger-mode systems spread the energy across multiple (tens to hundreds of) pulses and histogram the returns to create a detection. In a high-background noise environment, the noise will be present in every pulse and therefore may be binned instead of the signal. This challenge may be overcome in many situations by implementing range gating, since smaller range gates offer less time and fewer opportunities for background photons to be collected. If there is a priori knowledge of where to set the range gates (which is often true for ground target imaging in the open), the range gate can be set to be very small (e.g., nanoseconds to microseconds45). This is generally sufficient to block out most background events, especially in the open. In practice, fielded Geiger-mode APD systems have not found blocking losses to be a significant issue in actual operation. High background noise may still present challenges for the Geiger-mode systems in foliage penetrating or underwater imaging regimes, but again, this has not been found to be an issue in operations. In fact, as described in the “Imaging Through Obscurants” section below, Geiger-mode systems work well in foliage penetrating environments. Moving Targets

The binning of multiple pulses for Geiger-mode systems drives the need for some platform motion compensation before the returns are summed. If the target itself is moving, there may be blurring in the 3-D image as the target moves during the image collection time. However, due to the high frame rates of the Geiger-mode systems, the speed at which the vehicle must move to induce significant blurring is generally high and will usually only be an issue for fast-moving targets in regimes requiring long collection times (e.g., in very dense foliage).

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45 M. Entwistle, M.A. Itzler, et. al., “Geiger-mode APD Camera System for Single Photon 3-D Ladar Imaging,” Proc. SPIE 8375, Advanced Photon Counting Techniques VI, 83750D (May 1, 2012).

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
×

Since linear-mode systems collect all of the return information on a single pulse, it is possible to create a snapshot of a moving target with minimal blurring even for fast movers. This snapshot imaging capability also reduces the processing required to form images. Due to motion compensation issues, forming an image with a GM-APD sensor is more complex than forming the same image with an LMAPD sensor.

Resolution

As with the 3-D scanning systems described in the preceding section, the range resolution of a flash ladar system is driven by the pulse width of the laser and cross range resolution by the detector angular subtense of the detectors and possibly the diffraction limit of the receive optics.

In Geiger-mode systems, the timing readout circuitry is realized digitally. It is read out noiselessly and hence can handle larger bandwidths (~1 GHz) without being subject to increased noise. This enables operation with pulse widths of ~1 ns to obtain 10-20 cm range resolution.

Because linear-mode systems use analog electronics for the readout circuitry, larger bandwidths introduce more noise to the system. Hence, these systems tend to operate in the hundreds of MHz regime, resulting in lower range resolution (although the range precision can be quite high with thresholding). Voxtel has reported the development of 128 × 128 multi-gain stage InGaAs APD arrays with gigahertz bandwidth,46 but these have not yet been put into a ladar system. There is ongoing work to develop linear-mode systems with gigahertz bandwidths.47

Gray Scale Intensity Images

It is often desirable to form an intensity or gray scale image of an area or target in addition to the typical range/height images. Overlaying this type of information on a 3-D image provides a more camera-like picture that may be more intuitive to interpret. No matter what type of sensor is used, measuring a gray scale dynamic range requires enough samples to form that level of gray scale.

Linear-mode systems can measure gray scale on a single pulse, since the output is proportional to the reflected light.48 However, in order to measure gray scale, enough photons must be returned in order to see the dynamic range associated with a given gray scale. Therefore, the required energy per frame for a photon-counting LM-APD sensor will increase as the desired gray scale dynamic range increases.

With Geiger-mode imaging there is no ability to measure the signal intensity per pixel (or gray scale image) on each pulse, because any received photon causes the same massive signal response.49 However, in the regime where the probability of detection per pulse is kept low, a higher reflectance area will have a higher probability of return, causing more events to trigger in those areas.50 Accounting (histogramming) for the total number of counts (signal trigger events) from each pixel across multiple pulses results in an effective gray scale but reduces the effective frame rate of the system. It also increases the energy required for a frame.51 Geiger-mode flash imaging becomes less efficient for gray scale or range profiles because multiple (typically ~1,000) looks are needed to bin enough pulses to create these products. However, the high frame rates used in these systems still make this an efficient way to gather these data, and for typical operating parameters the number of samples (and hence “frame” energy)

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46 See for example http://voxtel-inc.com/products/single-element-detectors-and-pixelated-detector-arrays/.

47 J. Asbrocket et al., 2008,” Ultra-high sensitivity APD based 3-D LADAR sensors: linear-mode photon counting LADAR camera for the Ultra-Sensitive Detector program,” Proc. of SPIE 6940: 2-3.

48 P. McManamon, 2012, “Review of ladar: a historic, yet emerging, sensor technology with rich phenomenology,” Optical Engineering 51(6): 060901

49 Ibid.

50 Ibid.

51 Ibid.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
×

required to form the gray scale image is well below what is required to measure the fundamental limit of dynamic range.

Linear-mode cameras can measure multiple range returns on a single pulse but require a more complex ROIC in order to collect the multiple range bins. Once multiple range returns are collected, images can be range gated to show only the returns beyond an obscurant (e.g., foliage, camouflage, or dust). Figure 2-16 shows gated imagery through a sand cloud, from an ASC 3-D imager.52

Geiger-mode systems can also be operated in a way to enable penetration of obscurants. However, the emitted laser energy must be set so that the system can detect the returns from bright scatterers (e.g., treetops or other obscurants) without saturating while still detecting weaker signals beyond the obscurant with the right probability of detection. If one has pixels with mixed range returns and collects enough samples to detect both the weak and strong target returns with some probability, “one can play essentially the same trick as used with gray scale in to map the returns as a function of range…If the probability of triggering is low for any event, one will get events triggering at longer ranges” through the holes in the trees.53 By peeling away the canopy layer in processing, the features of the underlying layer can be seen, as shown in Figure 2-17. In this case, the high-PRF operation of the Geiger-mode systems are an advantage because there are increased opportunities “per frame” to see through different holes in the canopy, thus increasing the probability of detecting the target as well as increasing the aspect diversity on the target, making the target easier to pull out of the background clutter. Unlike linear-mode sensors, which require a more complicated ROIC to measure multiple range returns, the ROIC complexity for a GM-APD does not change.

image

FIGURE 2-16 Flash ladar image showing penetration through dust. Visible imagery is completely obscured, while ladar imagery shows the potential hazards. SOURCE: Helicopter Brownout Landing. Used by permission Advanced Scientific Concepts, Inc.

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52 NAVAIR Public Release 11-033, “3-D Flash LADAR helicopter landing sensor for brownout and reduced visual cue.”

53 P. McManamon, 2012, “Review of ladar: A historic, yet emerging, sensor technology with rich phenomenology,” Optical Engineering 51(6): 060901.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
×

image

FIGURE 2-17 Puerto Rico foliage penetration (FOPEN) data collected by ALIRT over El Yunque National Forest, Puerto Rico. Left: EO image of the scene; Center: Ladar image with canopy (color indicates height above mean sea level); Right: Ladar image with canopy removed (color indicates height above ground). SOURCE: Dale Fried, 2012, “Photon counting laser radar,” presentation at 2012 SPIE Defense Security and Sensing Conference (April 25). Approved for public release, unlimited distribution. Post-Detection Processing. MIT Lincoln Laboratory.

The high frame rates used by Geiger-mode APDs produce a large amount of data that must be read out and processed in order to produce 3-D images. This data processing increases the SWaP of the ladar systems and limits the current utility of Geiger-mode flash ladar systems for real-time operations. Instead, the current airborne systems store the data and process it later.

The processing requirements are much less demanding for linear-mode sensors because more processing is done on the ROIC, and there are fewer pulses per image to process. The image is formed directly out of the ROIC for most linear-mode sensors. However, as previously described, the more complicated ROIC impacts the performance in other areas.

Many of the limitations of the APD-based ladar systems arise from the operational limits of the APD array itself. For Geiger-mode systems, “there is a dead time after each triggered event. During the dead time, the detector will not detect any received photons.”54 Crosstalk and dark currents can also affect the performance of the ladar system. Fielded linear-mode arrays have a noise floor that is significantly higher than a single photon. Therefore, the energy required to image a given area at a given range is significantly higher for the linear-mode receiver than for a Geiger-mode receiver. Also, the bandwidth for linear-mode receivers currently limits the range resolution achievable with these systems. These attributes, and the potential advancements to overcome these limitations, are discussed in the various references previously provided and in Chapter 4.55

The area that can be imaged in a single “flash” will depend on the detector and the laser. While larger format and higher frame rate detector arrays will continue to develop to support coverage of wider areas, the development cycle for lasers that can provide the same energy per pulse over a larger area may be the limiting factor. SWaP may also limit the desirability of going to large flash illumination areas for longer ranges.

Another limitation of Geiger-mode laser radar is the processing needed for the ladar data. Because the data are being collected at such a high frame rate and the detector arrays are large, the data processing needs can be quite high. For example, a 64 × 256 array being read out at 20 kHz will produce on the order of 300 MB per second of data. This drives the need for large on-board data storage and/or data processing that can increase the SWaP of the sensor, and it also limits the ability to obtain real-time data from these types of systems. As the size of both linear and Geiger-mode arrays increases and linear-mode arrays are able to capture more data per pulse at higher pulse rates, the need for this kind of processing will continue to grow.

As described above and in Chapter 4, improvements in the ladar components will improve the APD-based ladar system performance but must be coupled in a smart way to optimize that performance.

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54 P. McManamon, 2012, “Review of ladar: A historic, yet emerging, sensor technology with rich phenomenology,” Optical Engineering 51(6): 060901.

55 P.F. McManamon et al., op. cit.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
×

Improvements in Geiger-mode APD array technology (larger arrays with better quantum efficiency, faster reset, less crosstalk, higher timing precision, faster or asynchronous ROICs, and so on) and laser technology improvements—for example high-power, short-pulse, medium-PRF (20-200 kHz) systems with high wall-plug efficiencies—are required for longer range capability. Reducing the processing and data readout times would enable production of real-time images. The development of on-chip processing (histogramming) and/or compression would reduce the amount of data that need to be read out. Conversely, the development of wideband off-chip communications paths (20-200 GB/s) would allow more data to be read out.

Continued progress in the development of high-sensitivity, photon-counting, linear-mode APDs will improve the linear-mode system performance. As described previously, there is significant ongoing work in this direction. Improvements in the APDs to reduce the readout noise will enable larger bandwidth measurements and hence, better range resolution. Like Geiger-mode systems, performance (area coverage, cross-range angle/angle resolution) will also improve with the development of larger arrays with smaller pixel pitch.

With the exception of MWIR HgCdTe-based APDs, most of the detectors discussed so far, linear- or Geiger-mode, operate in the SWIR and near infrared (NIR) spectral bands. HgCdTe APDs in MWIR and LWIR bands have been demonstrated in recent years that exhibit sensitivities comparable to the SWIR band. Lasers suitable for active sensing, such as quantum cascade lasers (QCLs), are becoming increasingly available. However, active sensing in the MWIR and LWIR bands under standard terrestrial conditions is hampered by the high thermal background flux. Thus the laser flux required is significantly greater to operate in these bands. Furthermore, optical resolution that can be achieved in LWIR and MWIR is also reduced. The advantage of operating in the MWIR and LWIR bands is that laser illumination in 2-D and 3-D modes in a narrow wavelength region can be selectively tuned for gas and or chemical sensing. These bands also enable reduced turbulence effects and atmospheric scattering compared to the SWIR band.

As both types of APD-based ladar systems also utilize scanners to increase their area rates, the development of large, lightweight, agile pointing apertures would also improve the system performance.

Geiger- and linear-mode ladar technology development is an active area of R&D, and advances such as those described above would be published in literature and would probably be presented at conferences, although some advances may be held as proprietary.

Conclusion 2-1: The distinction between linear- and Geiger-mode systems is likely to blur as highly sensitive linear-mode arrays become more mature. Instead of focusing on one type or the other, it is more important to focus on the photon counting performance, since this drives the best achievable sensitivity and therefore the system size, weight, and power.

This technology is well suited for high-resolution 3-D imaging applications from the ground, air, or space. Like all direct-detect 3-D ladar systems, it is well suited for application requiring “geometrical” information, such as 3-D mapping, line-of-sight maps, and geometric change detection. It is also the best technology to obtain high-resolution images through obscurants and under foliage over relatively wide areas. It is anticipated that a global, 3-D database derived from ladar measurements will support civil, commercial, and military needs as the foundation layer for organizing geospatial information of all types.

The United States is the leader in the development of 3-D flash ladar systems, but there is a great deal of foreign development work in APD arrays. CEA-LETI/Sofradir (France/Israel) have demonstrated HgCdTe (MCT) arrays for flash ladar in “near photon counting” mode. Selex Galileo (U.K.) has done flash ladar with MCT APDs.56 The Milan group (Milano Politechnico) has also done work in the development of single photon sensitive APD arrays. First Sensor A.G. (Germany) sells small APD arrays

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56 See http://proceedings.spiedigitallibrary.org/proceeding.aspx?articleid=1342598.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
×

for visible and near-infrared ladar,57 but it only makes the arrays and not the electronics required for time-of-flight measurements.

Hamamatsu (Japan) also makes linear-mode and Geiger-mode APD arrays58 (silicon for visible/near-IR) but neither is directly useful for flash ladar. In principle, the company could produce imaging arrays. The leading GM-APD group in China is at East China Normal University,59 though their array work is still early in development.

With the development and commercialization of ladar cameras and arrays, it is likely that flash ladar systems will become more widely proliferated and less expensive. It is anticipated that commercial applications will drive this proliferation.

Intensity-Encoded 3-D Flash Imaging

As mentioned previously, one of the challenges of flash imaging is having a large enough focal plane array to detect an area-based object with a single pulse.60 Like Geiger-mode systems, performance (area coverage, cross-range angle/angle resolution) will also improve with the development of larger arrays with smaller pixel pitch. While APD-based flash imagers use arrays with high bandwidth readout circuits to measure high-resolution 3-D images of an area of interest, intensity-encoded flash imaging leverages commercially available large framing CCD arrays to obtain 3-D images. This technique was developed to take advantage of existing laser illuminators and CCD arrays at a time when APD technology was in its infancy. In the early 1990s, multiple patents were awarded for the Laser Imaging and Ranging System (LIMARS),61,62 which replaces the high-speed camera with a Pockels cell and two low-frame-rate cameras for flash 3-D imaging.63 In the LIMARS receiver, temporal (range) resolution is provided by a high-speed Pockels cell, so that high-bandwidth cameras are not required.64Figure 2-18 shows a diagram of the LIMARS receiver.

This concept uses a high-power, short-pulse laser to illuminate a scene of interest. The reflected light enters the receiver and is passed through a polarizer, where a single polarization of return light is isolated (alternatively, twice as many cameras could be used to detect both polarizations). The light next passes into a Pockels cell, which is key in converting the time of flight of the pulse to intensity information.65 A ramp is placed on the Pockels cell, where a phase shift between the components of the optical field is proportional to the applied voltage, as shown in the bottom of Figure 2-20. The output of the Pockels cell is in general elliptically polarized, and is passed into a polarizing beam splitter which splits the polarized energy between two CCD cameras.66 In any given detector, representing a certain solid angle, the ratio of power in one camera to power in the other camera provides range information,

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57 See http://www.first-sensor.com/en/news/newspress/2010-11-09-matrix-apd-detector-arrays-lidar, and http://www.first-sensor.com/en/products/optical-sensors/rd.

58 See http://www.hamamatsu.com/us/en/index.html.

59 See, for example, Min Ren et al., 2011, “Laser ranging at 1550 nm with 1-GHz sine-wave gated InGaAs/InP APD single-photon detector,” Optics Express, 19 (14): 13497.

60 P. McManamon, 2012, “Review of ladar: A historic, yet emerging, sensor technology with rich phenomenology,” Optical Engineering 51(6): 060901.

61 L. Tamborino and J. Taboda, 1992, “Laser imaging and ranging system, one camera,” U.S. Patent No. 5,162,861.

62 J. Taboda and L. Tamborino, 1992, “Laser imaging and ranging system using two cameras,” U.S. Patent No. 5,157,451.

63 P. McManamon, op. cit.

64 P. McManamon, 2012, “Review of ladar: A historic, yet emerging, sensor technology with rich phenomenology,” Optical Engineering 51(6): 060901.

65 R. Goldstein, 1968, “Pockels cell primer,” Laser Focus, Feb.: 21.

66 M.B. Mark, 1992, “Laser Imaging and Ranging System (LIMARS) Range Accuracy Analyses,” WL-TR-92-1053

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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since targets at different ranges will have returns to the Pockels cell at different times in the waveform. If the cameras are aligned so that the same pixels on each camera image the same part of the target, spatially and range-resolved measurements are obtained.

The range information within each detector is driven by the slope of the waveform on the Pockels cell. A steeper slope provides more accurate range information but also repeats more quickly67 and results in a smaller ambiguous range. This problem is not unique to polarization-based imaging, and a number of standard techniques (e.g., chirping the length of the ramps) can be applied to expand the unambiguous range. Any number of available CCD cameras can work for this application, including silicon-based TV cameras, NIR cameras, and military hardened SWIR cameras. All of these cameras have formats that are larger than those of the high-bandwidth cameras described in the preceding section. This technique has been used in several programs. Figure 2-19 shows two images from the DARPA SPI 3-D effort—a program aimed at developing airborne active EO sensors capable of positive target identification at standoff ranges—which uses this approach.68Figure 2-20 shows a very high resolution short-range image from a small company, Tetravue, again using this technique.69

Comparison of Intensity-Encoded and APD-Based 3-D Imaging

The main advantage of intensity-encoded flash imaging is the ability to create high-rangeresolution 3-D images using standard framing cameras. These framing cameras often have a larger format than the APD arrays used in the systems above and can cover a wider area with a single pulse. In addition, the readout of these arrays can be very fast, enabling real-time imaging and even the formation of 3-D videos.

image

FIGURE 2-18 Diagram of the LIMARS intensity-encoded 3-D flash ladar concept. The bottom portion of the diagram shows a typical Pockels cell voltage. SOURCE: P. McManamon, 2012, “Review of ladar: a historic, yet emerging, sensor technology with rich phenomenology,” Optical Engineering 51(6): 060901.

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67 P. McManamon, 2012, “Review of ladar: A historic, yet emerging, sensor technology with rich phenomenology,” Optical Engineering 51(6): 060901.

68 T. Tether, 2004, testimony before the House Armed Services Committee Subcommittee On Terrorism, Unconventional Threats And Capabilities, http://www.globalsecurity.org/military/library/congress/2004_hr/04-03-25tether.htm.

69 See http://www.tetravue.com.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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image

FIGURE 2-19 SPI 3-D images. SOURCE: P. McManamon, op. cit.

image

FIGURE 2-20 Polarization-based 3-D ladar image. SOURCE: High resolution 3D image, TetraVue, Inc. 2011.

The use of the Pockels cell to rotate polarization does introduce some challenges. As described above, there is a trade-off between ambiguous range and range resolution. Pockels cells traditionally require high voltage and have a narrow field of view.70 Like linear-mode sensors, these systems operate at low frame rates (~30 Hz). However, these systems have lower sensitivity than the APD arrays and require higher SNR and, hence, high-power lasers that operate at low PRF. Therefore, polarization-based imaging sensors may be disadvantageous in SWaP-constrained applications. The high SNR requirement will also limit the suitability of this technique for long-range applications. Using detectors with low detector noise levels or increasing the aperture size would improve the ability to carry out long-range measurements. Another option is to effectively concentrate the received energy onto fewer pixels by reducing the beam divergence to only illuminate the target while reducing the focal length to have the target occupy a reduced number of pixels. However, this will reduce the coverage area. At this writing the committee is aware of only two companies pursuing the 3-D ladar polarization imaging approach—one for the military market, and one for the short-range commercial market.

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70 K. Ayer and W. Martin, “Laser Imaging and Ranging System (LIMARS): A proof of concept experiment,” Proc. SPIE 1633, 54-69, (1992).

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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Conclusion 2-2: Polarization imaging offers the potential for high-resolution 3-D imaging without the need for large avalanche photodiode arrays and is well suited for close-range imaging. However, this technique is less likely to be used for long-range applications, particularly those that require low size, weight, and power.

ACTIVE POLARIMETRY

Light is a transverse electromagnetic wave (i.e., the oscillations are perpendicular to the direction of propagation), similar in some respects to the waves that propagate down a taunt string when it is shaken. Consequently, the wave may oscillate horizontally, vertically, in a right- or left-handed spiral or in some combination of the four. Mathematically, the four components of the polarization are represented by a set of order quadruples known as the Stokes vector.71

The polarization of the light reflected from a surface contains information not contained in the intensity or even the spectral reflectivity. For example, measurements of polarization of ambient reflected or emitted light have shown the potential to differentiate man-made from naturally occurring materials, even when the two materials are perfectly spectrally matched.

One of the more common approaches to polarization measurement is to pass the received light through one or more polarization analyzers (e.g., a Polaroid filter) and measure the intensity of the filtered light that emerges. Multiple measurements with filters rotated with respect to each other can identify the degree of polarization (DOP) of the light received. The DOP72 is strongly affected by the roughness of the surface. Since the roughness may be different on different surfaces having the same reflectivity, the DOP provides an additional degree of contrast when analyzing imagery. Surface roughness seems to be one of the parameters that influences the ratio of light reflected at different polarization states. People often wear polarized sunglasses to reduce glare from smooth surfaces. Passive DOP measurements have been limited by the uncertainty in the initial polarization state of the ambient illumination and the angle of incidence of the light. In addition, if more than one material is present within the field of view (FOV) of a single pixel (e.g., when leaves or camouflage are in front of a vehicle), the intensity and the DOP measured will be the result of the summation of all of the surfaces within the FOV and each surface will contribute in the proportion to which it is visible to the receiver. Since these proportions are not known, the “mixed pixel” effect adds to the uncertainty.

Active EO polarimetric measurements can eliminate this uncertainty by controlling direction of illumination and the initial state of its polarization. In addition, the bandwidth of the laser modulation allows high range resolution imaging. The signal generated by foliage or camouflage can be separated in range from the signal generated by whatever lies behind or in front of the object of interest. This minimizes the mixed pixel effect unless the two surfaces are separated by less than the range resolution of the ladar. Active polarimetry has not been used operationally, but is being investigated as part of emerging ladar sensors.

Conclusion 2-3: Active polarimetric imaging can provide much more powerful characterization of surface roughness than passive polarimetric imaging, and this can aid target detection and automated target recognition.

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71 Any nondegenerate representation of the polarization states can be used to represent the Stokes vector. Commonly, the four elements used to represent the Stokes vector are the horizontal linear polarization, vertical linear polarization, 45° linear polarization, and right hand circular polarization.

72 The DOP measurements usually are based on the ratio of only a subset of the elements of the Stokes vector. DOP includes degree of linear polarization (DOLP), degree of circular polarization (DOCP) and ellipticity, which is the ratio of DOLP to DOCP.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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UNDERWATER SENSING

An important application for underwater active EO sensing is detection of underwater mines. This technology represents some of the opportunities and challenges for active electro-active sensing. Four wavelengths are useful simultaneously for mine detection: 1,064 nm, 532 nm, one wavelength band in the red region (~650-670 nm), and one wavelength band in the near infrared (~750-810 nm). For simplicity the latter two will be referred to as ~650 nm and ~810 nm. These four wavelengths enable the application of multi-modal detection strategies to address the complex issues involved in mine detection, including target contrast with the background and false detections. Table 2-1 summarizes the laser wavelengths and their applications.

The 532-nm wavelength provides the primary imaging capability for both in- and above-water detection. The choice of 532 nm leverages prior Navy experience in mine detection, relatively good seawater transmission in the green spectral region, and the well-proven, frequency-doubled laser wavelength and high pulse energy that can be produced using a Nd:YAG laser source. The fundamental laser wavelength output from the Nd:YAG laser is 1,064 nm. Efficient frequency doubling to produce 532-nm laser radiation leaves approximately 40 to 50 percent of the 1,064-nm energy available. This can be used both for (1) interrogation of changes to the reflectivity of the ocean surface (due to waves and surface roughness) for signal normalization and (2) frequency conversion to produce two additional wavelengths that will aid in target discrimination. The rapidly absorbed 1,064-nm light provides only near-surface backscatter signals.

The final two wavelengths, ~650-670 nm and ~750-810 nm, are primarily used for target discrimination using both reflectance and fluorescence spectroscopy. A metric that is commonly used in remote sensing for determining the presence of vegetation (a common cause of false detections) is the Spectral Vegetation Index (SVI), which takes advantage of the “NIR vegetation rise” in reflectance. There is a distinct difference in reflectance between red and NIR light associated with the presence of chlorophyll. Chlorophyll absorbs strongly in the red (<700 nm) and reflects strongly in the near infrared

TABLE 2-1 Four Laser System Output Wavelengths and Their Application for Mine Detection

Laser Wavelength Properties / Application
1,064 nm

•   Very shallow water penetration depth (cm)

•   Measurement of ocean surface effects (waves/ripples) on reflectivity

•   Normalization/interpretation of signals at other wavelengths

532 nm

•   Deep water penetration depth (tens of meters)

•   In-water imaging and target identification

•   Above-water imaging

~650 nm

•   Moderate water penetration depth (m)

•   Excitation wavelength for chlorophyll fluorescence-based detection of vegetation

•   Low reflection wavelength for spectral vegetation index (SVI)

•   Application in very shallow water (VSW), surf zone (SZ), and beach detection

~810 nm

•   Poor water penetration depth, but located at a minimum in the water NIR absorption, allowing some water penetration

•   High reflection wavelength for SVI-based detection of vegetation

•   Application in VSW, SZ, and beach detection

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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image

FIGURE 2-21 (a) Chlorophyll a and b absorption spectra; (b) Water transmission throughout the visible and infrared spectral regions. Note the sharp rise starting just above 700 nm and the dip near 810 nm.

SOURCE: (a) By Chlorophyll_ab_spectra2.PNG: Aushulz derivative work: M0tty [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], via Wikimedia Commons. See http://upload.wikimedia.org/wikipedia/commons/6/68/Chlorophyll_ab_spectra2.png.(b) Q-Peak, Inc.

(>700 nm). Most other common materials, such as sand, bare soil, exposed rock, or concrete, generally show a steady rise in reflectance (with no dramatic jumps) as wavelength increases from the visible to the NIR. Water, in contrast, has much higher reflectance in the red than in the NIR.73,74

In satellite remote-sensing applications, the measurement takes advantage of ambient sunlight, but for laser-based applications it can be done through selection of appropriate wavelengths to conduct the measurement of interest. A laser wavelength pair at ~650 and ~810 nm (red and NIR) can be used to measure the SVI based on either the spectral reflectance (SR) or the normalized difference vegetation index (NDVI). As shown in Figure 2-21a, absorption by both chlorophyll a and b are elevated at 650 nm, meaning the reflectance would be low, while at 810 nm, reflectance will be high. The ~650 nm wavelength will also excite chlorophyll fluorescence that can be detected in the vicinity of both 700 and 735 nm, the two chlorophyll emission peaks. This dual approach—vegetation detection by both fluorescence and SVI—has advantages for the application proposed here, for which the system should work in very shallow water (VSW), surf zone (SZ), and beach environments. Vegetation detection by SVI in water of any significant depth is degraded due to strong NIR absorption by water (Figure 2-21b). While 650 nm is not an ideal wavelength for penetration of the water column, it can penetrate far enough to reach shallow targets and excite fluorescence. The deeper red fluorescence emission then only has to travel a one-way path exiting the water, as opposed to reflectance signals, which require a two-way transit.

The selection of a laser wavelength near 810 nm as the NIR wavelength for SVI detection works as the high-reflectance band for vegetation, and it is also the NIR wavelength with the greatest penetration in the water column, enabling limited in-water use of SVI to complement fluorescence detection of

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73 See http://www.deepocean.net/deepocean/index.php?science07.php. Also see http://extension.usu.edu/nasa/ntm/on-target/near-infrared-tutorial.

74 J. Weiqi, C. Fengmei, W. Xia, L. Guangrong, H. Youwei, Q. Huaichuan, and S. Fei, 2008, “Range-gated underwater laser imaging system based on intensified gate imaging technology,” Proc. of SPIE 6621: 66210L.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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chlorophyll-containing targets. Absorption by water goes up strongly in the infrared, but there is a local minimum at 810 nm, with steep rises on either side of this wavelength. The 810-nm wavelength is also well separated from the chlorophyll fluorescence wavelengths, enabling the use of broader detection bandpass filters if required.

Active Illumination and Detection from Above the Water Surface

Considerable effort has been spent investigating the possibility of sensing underwater from above the water, with many applications being relevant to ecological assessment. When incident from above the surface of the water, a fraction of incoming light is reflected away; the amount depends on the state of the water itself. If it is calm and smooth, theory determines that the fraction of light reflected depends on the polarization of the incident light. When vertically polarized light is incident at 57° to the surface, no light will be reflected. At angles more vertical than this, the reflectivity increases to a maximum of 5 percent. Angles more shallow are not very practical, as the reflectivity increases dramatically.

Incident illumination can easily be aimed at an underwater target when the water is calm. If the surface of the water is turbulent, however, with many waves, light will refract into the water at different time-varying angles, effectively spreading out the beam. With enough froth, most of the illumination may be reflected rather than refracted into the water. Light returning from under the water to a sensor above the water can also be modulated by the changing waves on the surface. This can reduce resolution, unless motion of the surface can be compensated for.

Light may be scattered or absorbed by solid particles. Most of the visible light spectrum is absorbed within 10 meters of the water’s surface, and almost none penetrates below 150 meters of water depth, even when the water is very clear. Shallow water near a shore is typically more turbid (cloudy) due to particles and will show a decrease in light transmission. Large numbers of particles are brought in by river systems, by biological production of microorganisms, and by waves, tides, and other water movement that picks up debris on the ocean floor.

As the light enters the water and begins to scatter, the beam increases in size, as shown in Figure 2-22. It has been suggested that the nonlinear self-filamentation of intense laser light might keep the beam from spreading out, but it is unclear how fundamental losses in water would affect such behavior.75 Experimental demonstration with filaments indicated that they remained at the same size in pure water for a distance of 4.5 m from the iris.

Range-Gating to Mitigate Backscatter

The impact of scattering is not only the reduction in signal reaching the target, but also the possibility of backscatter blinding the detector. Figure 2-23 shows an example of a backreflected signal as a function of time, measured by naval engineers in China.76 It is clear that gating the detector to measure signal only in the expected range will substantially increase the SNR. When a laser pulse is emitted, a clock is started to trigger the detector array at the proper distance, as discussed in the section on 2-D active/gated imaging, above.

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75 K. Wang, B.D. Strycker, D.V. Voronine, P.K. Jha, M.O. Scully, R.E. Meyers, P. Hemmer, and A.V. Sokolov, 2012, “Remote sub-diffraction imaging with femtosecond laser filaments,” Optics Letters 37 (8): 1343.

76 G. Wei-Long, H. Hong-Weei, Z. Siao-Hui, and X. Xiang, 1999, “A new kind of underwater photoelectric imaging system,” Proc. of SPIE 7382: 73824T.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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image

FIGURE 2-22 On the same scale, a light beam at 20 m below the surface of the ocean (left) and at 200 m depth (right). The assumed scattering attenuation coefficient was 0.4 m-1. SOURCE: Y. Xiao-li, Y. Hong, X. Li-ming, and W. Fu, 2009, “Analysis of characteristics of blue-green laser propagation through ocean water,” Proc. of SPIE 7382: 738212. © SPIE. Reprinted with permission.

image

FIGURE 2-23 Reflected image temporal profile in the time domain, for clear water with attenuation coefficient α = 0.26 m-1 and absorption coefficient 0.04 m-1. The front, middle, and tail of the actual signal are identified as 1, 2, and 3. SOURCE: G. Wei-Long, H. Hong-Weei, Z. Siao-Hui, and X. Xiang, 1999, “A new kind of underwater photoelectric imaging system,” Proc. of SPIE 7382: 73824T. © SPIE. Reprinted with permission.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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Practical distances are still limited to where water is very transparent. Jaffe77 reports typical values for minimum attenuation of deep-ocean water, coastal water, and bay water as 0.05 m-1,0.2 m-1 and 0.33 m-1, respectively. An acceptable attenuation for a completely reflected frequency-doubled, Q-switched Nd:YAG laser with 80 mJ per pulse energy, assuming suitably gated measurements, could be the detection of one photon per pulse. Because the input corresponds to 2.1 × 1017 photons per pulse, an acceptable round-trip attenuation would be exp(-40) = 4.2 × 10-18. This corresponds to a one-way distance of L = 20/α. Using Jaffe’s suggestions for loss coefficients, visibility in deep ocean water should be 400 m; in coastal water the depth would be 100 m, and in bay water would be 60 m for a single-element detector. For imaging arrays, the distances would be reduced by the logarithm of the number of elements in the array.

In recent years, many research groups around the world including in Sweden, Denmark and Israel, have engaged in this technology. A special symposium of SPIE, held in 2009 to explore laser sensing and imaging, examined many of these technologies.78

A variety of methods have been tried to decrease the problems with scattering of the illumination signal. Rejecting backscatter by properly designing polarizing filters has been extensively investigated, but none of the methods seems to have increased the effectiveness of long-range underwater sensing. It appears that researchers in the United States are no longer pursuing advanced polarization techniques, at least in the open literature.

Coastal Sea Floor Imaging

Coastal waters provide another important application of active EO sensing, with aerial systems common since 1995. Recent systems under development are designed to be operated by helicopters or unmanned aerial vehicles (UAVs). Active optical range-imaging sensors collect three-dimensional coordinate data from object surfaces and can be useful along the sea coast, in littoral waters up to 50 meters deep, along the surf-line, and onto the beach. Performance of ocean-sensing ladar sensors depends on laser propagation effects in a scattering medium and the manner in which a particular system collects the ladar data. A few examples are discussed below.

The Scanning Hydrographic Operational Airborne Lidar Survey (SHOALS) system, developed for the U.S. Army Corps of Engineers, was fully operational by 1995. Employing scanning ladar to remotely collect accurate, high-density measurements of both bathymetry and topography in coastal regions, SHOALS had by 1999 completed 230 projects totaling 5000 km.79 The SHOALS technology is marketed today by Optech.80 It uses green laser pulses (second harmonic of the Nd:YAG laser) and measures time-of-flight of the reflected light. For bathymetry, the system sends out both the fundamental as well as the harmonic. Since the fundamental is absorbed by water within in a short distance, its sole signal is reflection is from the surface of the water. This means ocean depth is given by the time difference between these two different signals. SHOALS can thus measure both land heights and ocean littoral regions.

Coastal zone mapping and imaging lidar (CZMIL) is a multisensor airborne mapping system, also designed by Optech for the U.S. Army Corps of Engineers. The Optech CZML summary specification sheet cites the system produces simultaneous “high-resolution 3-D data and imagery of the beach and shallow water seafloor, including coastal topography andperforms particularly well in shallow, turbid waters… Its bathymetric ladar is integrated with a hyperspectral

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77 J.S. Jaffe, 1990, “Computer modeling and the design of optimal underwater imaging systems,” IEEE J. Ocean Eng. 15: 101.

78 International Symposium on Photoelectronic Detection and Imaging 2009: Laser Sensing and Imaging, edited by F. Amzajerdian, C.-Q. Gao, and T.-Y. Xie, Proc. of SPIE 7382.

79 J.L. Irish and W.J. Lillycrop, 1999, “Scanning laser mapping of the coastal zone: the SHOALS system,” ISPRS Journal of Photogrammetry & Remote Sensing 54: 123.

80 See http://www.optech.ca/pdf/Brochures/shoals_shoals.pdf.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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imaging system and digital metric camera.”81 OptechHydroFusion, an end-to-end software suite, handles all three sensors—from mission planning through to fused ladar and imagery data sets. For optimal object detection, particularly around surf, with its breaking waves and foam, a circular scan pattern provides two looks at any target. Tuell et al. describes the

CZMIL scanner is based on a rotating Fresnel prism having a 20-cm clear aperture. The segmented detector holds the possibility of producing sub-meter, 3-D seafloor images in very shallow water… This bathymetric ladar is integrated with a commercial imaging spectrometer and digital metric camera. The data processing system employs new algorithms and software designed to automatically produce environmental image products by combining data from the three sensors within a data fusion paradigm.82

Northrop Grumman’s active 3-D ocean sensor system is the Airborne Laser Mine Detection System (ALMDS), shown in Figure 1-11, whose initial roll-out was in 2007. The pulsed laser radar in this system, made by Arete Associates, scans a wide swath of ocean and is designed to locate and identify mines accurately in a single pass over a target area. “It transmits a fan-shaped beam of laser light to establish its swath width, and then relies on the forward motion of the helicopter to sweep the light over the water in a push broom manner.”83 “Four cameras are arranged to cover the same swath illuminated by the laser fan beam.”84 Time-resolution is obtained by streak-tube imaging light (STIL) detection. The streak tube uses a photocathode to detect the 1-D spatial information that comes from the slice of ground illuminated by each fan-shaped laser pulse and converts it to a line swath of electrons whose lateral density is proportional to the lateral variations in light intensity. The electron swath within the tube is accelerated and deflected by high voltage applied to plates. As the electron swath is rapidly swept vertically, so that when the electrons hit a phosphor screen, each portion forms a streak that provides time resolution along the third dimension. Thus, for any portion of the initial illumination slice, upward deviation along the streak represents depth into the ocean. Arete Associates have also developed a multiple-slit STIL system that uses several slits to provide additional capabilities, used for flash mode, 3-D polarimetry, and multispectral 3-D fluorescence imaging.85

Arete was also involved in an experimental program of the Office of Naval Research (ONR) called Anti-Invasion Mine Signature Measurement and Assessment for Remote Targeting (AIMSMART), which investigated 3-D flash ladar and 3-D polarimetric data on sea and land mines scattered throughout the surf and beach zones. Moran et al. describes the Rapid Overt Airborne Reconnaissance (ROAR) system:

Under the Office of Naval Research’s Organic Mine Countermeasures Future Naval Capabilities (OMCM FNC) program, Lite Cycles, Inc. is developing ROAR, a highly compact airborne active sensor system for mine and obstacle detection in very shallow water, through the surf-zone and onto the beach. The system uses a proprietary integrated scanner, detector, and telescope receiver architecture that tightly couples all receiver components and ladar electronics for small size while providing a large aperture. The system also includes an advanced compact multifunction laser transmitter, a compact 3-D camera, a scanner for wide area search, and temporally displaced

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81 Optech CZMIL. Available at http://www.optech.com/wp-content/uploads/specification_czmil.pdf. Acessed March 14, 2014.

82 G. Tuell, K. Barbor, and J. Wozencraft, 2010, “Overview of the coastal zone mapping and imaging lidar (CZMIL): A new multisensor airborne mapping system for the U.S. Army Corps of Engineers,” Proc. SPIE 7695, Algorithms and Technologies for Multispectral, Hyperspectral, and Ultraspectral Imagery XVI, 76950R.

83 Northrop Grumman, Photo Release-Japan Maritime Self-Defense Force Orders Northrop Grumman’s Airborne Laser Mine Detection Systems, http://www.irconnect.com/noc/press/pages/news_releases.html?d=244673. Accessed on March 14, 2014

84 Ibid.

85 See http://www.arete.com/arete_innovation/streak_tube_imaging_lidar_stil.aspx.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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multiple looks on the fly over the ocean surface for clutter reduction. Additionally, the laser provides time-multiplexed multi-color output to perform day or night multispectral imaging for beach surveillance. New processing algorithms for mine detection in the very challenging surf-zone clutter environment are under development for significant processing gains. The laser-illuminated, multi-spectral imaging system uses three time-stepped spectral bands with a single stabilized and scanned intensified CMOS camera. The 3-D imaging sensor utilizes a single stabilized and scanned 3-D focal plane array to provide a 3-D image of a volume of water with a single green laser.

The primary sensor for use over the beach zone is a range-gated ICMOS camera that is used in conjunction with a multispectral laser illuminator that provides time-interlaced, three-color laser pulses. It generates the multispectral imagery for detecting individual mines, mine lines, and obstacles on the beach. The primary sensor for use over the surf zone is the 3-D camera used in conjunction with the laser operated in the green illumination mode. It provides the 3-D ladar data required to detect single bottom mines, volume mines, and obstacles, as well as mine and obstacle-lines. Both receivers operate simultaneously in all zones; adding the ICMOS camera for surf zone operation provides additional high-spatial-resolution, range-gated imagery to complement the lower transverse resolution imagery generated by the 3-D camera. This helps overcome the high level of clutter associated with the very challenging surf zone environment. The ICMOS camera can also be used in the shallow water zone to provide high-spatial-resolution imagery at specified depths and to assist in 3-D camera automatic gain control. The 3-D camera can also be used on the beach to provide topographic information for obstacle detection, and for detection of targets under camouflage and foliage.86

Finally, the well-known military coastal program Coastal Battlefield Reconnaissance and Analysis (COBRA), is not classified as an active electro-optic system because this Northrop Grumman technology uses passive multi-spectral mine detection.

Sensing of Bubble Laser Scattering

A 2009 publication originating in China saw value in the laser sensing of ocean bubbles.87 The authors described an application to wake-homing torpedoes that are designed to home in on ships by identifying their wake. The basic principle is to use the difference between laser scattering properties of the ship’s wake, which contains a myriad of bubbles, and scattering from seawater in the nonwake region. Signal comparison can identify the ship’s wake and determine the relative position of the ship. Underwater target detection is based on the scattering angle of the scattered light intensity distribution of bubbles in the water.

Conclusion 2-4: Active underwater sensing is likely to be limited to much less than 400 meters distance, reaching this value only in the deep ocean and with a single element detector.

When the technology is applicable, removal of backscatter from the detector by gating becomes important. The technology will undoubtedly have specific applications, but no practical solution to the transparency problem of water seems to be on the horizon.

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86 S.E. Moran, W.L. Austin, J.T. Murray, N. Roddier, R. Bridges, R. Vercillo, R. Stettner, D. Phillips, A. Bisbee, and N. Witherspoon, 2013, “Rapid overt airborne reconnaissance (ROARTM) for mines and obstacles in very shallow water, surf zone and beach,” Proc. of SPIE 5089: 224.

87 S. Liping, Z. Weijiang, R. Deming, Q. Yanchen, and H. Xiaoyong, 2009, “Processing methods in frequency domain for bubble laser scattering signals,” Proc. of SPIE 7382: 73822X.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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FIGURE 2-24 German vibrational imaging system. SOURCE: P. Lutzmann, R. Frank and R. Ebert, 2000, “Laser radar based vibration imaging of remote objects,” In Laser Radar Technology and Applications V, Proc. SPIE 4035: 436.

VIBRATION SENSING

Listening to voices from far away, monitoring the health of machinery bearings, determining the operational status of electrical equipment, detecting underground tunneling activities, fingerprinting a particular operating vehicle—these are some of the applications enabled by remote laser vibrometry. The short wavelength of light, combined with sophisticated processing techniques, enables remote measurement of submicron displacement and sub-micron-per-second velocities. For many industrial applications, this capability is readily available commercially from companies such as Polytec-PI and Ometron. Military and intelligence applications have additional requirements, however, including the ability to operate at long range.

There are a variety of technological approaches to remote laser vibrometry, and this section will cover these alternative approaches and their relative merits for different applications, as well as limitations to performance.

Many laser vibrometer systems are point sensors, and spatial information is derived by scanning the laser spot across a surface. This results in a “data cube” similar to those in hyperspectral imaging, where at each x, y point in an image one associates a complete vibrational spectrum. This data cube can then be processed using algorithms similar to those used in hyperspectral data exploitation, such as creating an image corresponding to vibrational amplitude at a given frequency or even corresponding to a complete vibrational target signature. Laser vibrometers can also be designed and operated in a range-resolved mode, which can be useful in obscurant penetration applications.

Figure 2-24 presents results from a German vibrational imaging system. It shows frequency-resolved processing of the data cube as well as operation in a camouflage penetration scenario.

Beyond scanning spot systems, there is increasing attention being paid to multichannel systems for (a) improved area coverage and finding antinodes of vibrational modes and (b) averaging and common-mode rejection of noise and platform motion.

Commercial applications of laser vibrometers include monitoring of plant machinery, modal analysis of structures (from small parts to gas turbines to buildings), noise reduction, non-destructive analysis (finding manufacturing defects, cracks, and the like). These applications are usually short standoff, as the operator usually has ready access to the target of interest.

Law-enforcement applications include “laser microphones” that can monitor acoustic signals from windows and shades, objects in a room, or directly from a speaker’s body. This even includes listening to both sides of a cellphone conversation by targeting the body of the phone. An important

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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feature is that unlike acoustic microphones that are strongly impacted by background noise, wind, and such, laser vibrometry targets the vibrating surface source directly, and is much less susceptible to such perturbations. This type of application tends to have a moderate standoff range (tens to hundreds of feet).

Military and intelligence applications include vehicle identification and tracking, plant monitoring (industrial intelligence, analysis of enigma facilities, etc.), equipment operational status (including air handlers, transformers, power lines) battle damage assessment, and interrogation of subsurface activity. These applications tend to have a long standoff range, up to many kilometers. One exception is laser vibrometry for buried landmine detection, in which a strong acoustic signal is broadcast from a ground vehicle and a laser vibrometer scans the ground surface for an anomalous response.

Vibrational sensitivity is quantified by both the noise-equivalent vibrational amplitude and the noise-equivalent vibrational velocity, and these quantities are related by the vibrational frequency of interest. (A higher frequency vibration has a higher surface velocity for a given vibrational amplitude). Because most laser vibration sensing technology is based on coherent detection, it can be made to be shot-noise limited in the optical detection by suitable choice of local oscillator power. A limitation (for cases where there is relative motion between the transceiver and the target) is usually laser speckle, which imposes a multiplicative noise on the signal. For long-range operation in the atmosphere, the limitation is atmospheric turbulence. Turbulence-induced “piston” fluctuations impose a phase noise on the optical signal that is indistinguishable from relative distance changes. Both of these limitations can be mitigated through multibeam and multiwavelength approaches that allow rejection of common-mode noise.

Standoff range is limited ultimately by laser power and receiver efficiency and is described by the usual ladar link budget for a shot-noise-limited coherent detection system. Another limitation that must be taken into account in designing system CONOPS is the requirement on the time a vibrating surface must be interrogated to discern the vibrational frequency (or to distinguish a specific vibrational signature from noise). In order to measure a vibrational frequency, one must typically dwell for several vibrational cycles. Bayesian processing techniques have been shown to achieve high frequency precision with very short data records.88

Of course one limitation for laser vibration sensing is that (unlike microphones) in most cases a direct line of sight/optical access to the vibrating surface is required, but acoustic and mechanical transmission of vibrations can be exploited and secondary vibrating surfaces can be used.

The most common commercial laser vibrometer technology is Doppler vibrometry (sometimes called micro-Doppler). In this approach the return signal is mixed with the local oscillator, and the beat frequency between the two electric fields creates a sinusoidally varying photocurrent. The motion of the target surface imposes a modulation of the frequency of that photocurrent. The signal is then processed to convert that FM-modulated signal into the vibrational spectrum. The most common approach is spectrogram processing (see Figure 2-25).

Alternatively, the surface displacement rather than the surface velocity can be measured directly, by measuring the phase of the return signal against that of the local oscillator in a Michelson or Mach-Zehnder interferometer. The local oscillator is often frequency shifted from the transmit wavelength by a modulator, and balanced or double-balanced heterodyne detection retrieves the optical phase. The evolution of this phase then measures the surface motion in time, and the data are processed to produce a vibrational spectrum, as above.

As noted above, there is a relationship between the surface displacement and velocity based on the vibrational frequency. Thus the micro-Doppler technique is best suited for high-frequency vibrations and the direct displacement technique is often best suited to lower-frequency vibrations.

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88 W.F. Buell, B.A. Shadwick, and R.W. Farley, 2000, “Bayesian spectrum analysis for laser vibrometry processing,” Proceedings SPIE The International Society For Optical Engineering, 4035: 444.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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FIGURE 2-25 Block diagram of spectrogram processing. FFT, fast Fourier transform; LO, local oscillator.

SOURCE: A.L. Kachelmyer and K.I. Schultz, 1995, “Laser vibration sensing,” MIT-LL Journal 8 (1): 3.

With both techniques, there is a premium placed on the phase stability (coherence length) of the local oscillator. Both techniques can also be operated with a variety of transmit waveforms, including continuous wave (CW) tones, frequency-modulated continuous wave (FMCW) ramps, and pulse-pair or poly-pulse coherent detection.89

Another technique that has attracted interest in recent years is direct-detection laser vibrometry based on time-resolved imaging of laser speckle patterns. In this approach, no local oscillator is involved, and the detection is performed on a high-frame-rate camera. Surface motions induce small shifts in the speckle pattern at the receiver, and by sizing the spatial sampling at the focal plane appropriately, that small speckle motion can be tracked and processed to derive surface vibration information.90

Coherent detection of vibrometry signals can be accomplished with GM-APDs. As described in Chapter 3 of this report, heterodyne detection typically uses a LO strong enough to ensure that the measurement is limited only by the shot noise of the signal from the object. This heterodyne approach to reaching the shot noise limit (SNL) of photodetection allows the use of low tech p-doped-intrinsic-n-doped (p-i-n) photodiodes while enabling sensitive detection. Some measurement scenarios, however, require a large array of coherent detection channels and substantial LO power across the array, which poses challenges to thermal management of the focal plane. Larger arrays become progressively more unrealistic.

An alternative approach is to reach the SNL using photon-counting detectors, such as Geiger-mode avalanche photodiodes. These detectors of course cannot operate with strong LO powers—they would be immediately saturated and never see the signal light. The solution is to use a weak LO, on the order of the amplitude of the return signal. In this situation, the fields of the LO and signal interfere just as in the strong LO case, and this interference is manifest as a modulation of the photocount rate from the GM-APD. This modulation of the photocount rate occurs at the difference frequency between the LO and signal fields as expected, or is manifest spatially across the array if the signal and LO fields are not coaligned (do not have parallel phase fronts).

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89 J. Totems, V. Jolivet, J.-P. Ovarlez, and N. Martin, 2010, “Advanced Laser Vibrometry In Pulsed Mode Using Poly-Pulse Waveforms And Time-Frequency Processing,” OPTRO 2010, 4th International Symposium on Optronics in Defence and Security, Paris, France.

90 Z. Zalevsky, Y. Beiderman, I. Margalit, S. Gingold, M. Teicher, V. Mico, and J. Garcia, 2009, “Simultaneous remote extraction of multiple speech sources and heart beats from secondary speckles pattern,” Optics Express, 17 (24): 21566.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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There are important limitations to this technique that limit its applicability and the measurement CONOPS. Principal among these are (1) the dynamic range and visibility constraints and requirements on LO power control and (2) the timescale constraints that link photocount rate, beat frequency, and vibrational frequency.

LO power constraints. In this technique it is necessary to keep the LO power as close to the signal power as possible. Naturally the LO power should not be strong enough to saturate the array, or even pose a blocking loss for the signal. An additional constraint, however, is imposed by beat frequency (fringe) visibility. In linear-mode strong-LO coherent detection, the detector is AC-coupled and gain is adjusted to optimize measurement of the beat signal. In weak-LO GM-APD coherent detection, AC coupling does not apply, and the fringe visibility (strength of the beat note) falls off as the LO and signal amplitudes become mismatched. Specifically, the fringe visibility is just the ratio of the field amplitudes.

Timescale considerations. A 100 Hz signal will be detected as a 100 Hz modulation of the interference frequency of the count rate. Clearly the slowest part of the beat frequency needs to be larger than the vibrational frequency. Furthermore, the count rate must be high enough to resolve the beat note but small enough to (1) not saturate the detector and (2) not exceed the maximum readout rate.

As noted above, military and intelligence applications (and to a lesser extent, law enforcement) tend to be longer range than commercial and industrial applications. Thus, key indicators include the development of high-power phase-stable lasers and long-coherence-length LOs.

Further development of very low noise linear-mode APD arrays will reduce the power requirement on LOs, and enable higher-pixel-count “flash imaging laser vibrometers” that will compete with the GM-APD approach.

Future development of very-long-coherence-length LOs will enable extension of laser vibration sensing to even longer ranges. It is also likely that laser vibrometry will be incorporated in multisensor fusion suites to derive more complete information about a scene under observation.

LASER-INDUCED BREAKDOWN SPECTROSCOPY

Laser-induced breakdown spectroscopy (LIBS) uses intense pulses from a laser to induce optical breakdown in a material, typically a solid surface, and then detects the emission spectrum from the breakdown (Figure 2-26).91,92,93 For nanosecond-duration pulses, the initial breakdown creates a plasma (ionized atoms and electrons) that then absorbs a good portion of the energy in the pulse. Shorter pulses in the femtosecond to picosecond range provide weaker signals because they just create surface breakdown and ejected, ionized atoms, and the pulses are over before the plasma absorbs more of the pulse energy. Recent advances in LIBS technology employ two pulses, spaced apart by one to tens of microseconds, where the second pulse acts to efficiently heat the plasma created by the first pulse.94 The detection system then observes the emission created from the irradiation process, with time-gating of the detector to optimize the desired emission spectrum signal. If the pulse or pulses are powerful enough, the emission contains components from the electronic transitions of individual, ionized atomic constituents of the irradiated material. Through spectroscopic analysis of the emission wavelengths, typically in the 170-

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91 L.J. Radziemski and D.A. Cremers, 1989, “Spectrochemical analysis using laser plasma excitation,” in Laser-Induced Plasmas and Applications, L.J. Radziemski and D.A. Cremers, eds., Marcel Dekker, New York, 295.

92 L.J. Radziemski and D.A. Cremers, 2013, Handbook of Laser-Induced Breakdown Spectroscopy, John Wiley, New York, 2006.

93 I. Schechter, A.W. Miziolek, and V. Palleschi, 2006, “Laser-induced breakdown spectroscopy (LIBS): fundamentals and applications,” Cambridge University Press, Cambridge, U.K.

94 R. Ahmed and M.A. Baig, 2009, “A comparative study of single and double pulse laser induced breakdown spectroscopy,” J. App.Phys. 106: 033307.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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1,100-nm range, one can determine the atomic constituents of the material from the characteristic wavelengths of their emission lines. In addition, from the strength of the signals at each line, it is possible to determine their relative concentration and, from this, the chemical makeup of the irradiated material.

LIBS provides a unique capability for in situ remote measurements of materials composition with nonionizing radiation. Although it does not strictly qualify as a non-destructive measurement, the amount of material sampled can be minimal. It is possible, by scanning the beam over a region, to get the spatial distribution of material composition with submillimeter resolution, another unique attribute.

Originally, LIBS was considered a short-range technology (< 1 m), developed for applications such as the detection of lead in paint. With more powerful, and low-divergence lasers, as well as a sufficiently large receiver telescope, it is possible to detect LIBS signals from targets 50-100 m away, thereby making the technology a form of remote sensing.95 The typical laser source is a pulsed, 1-200-mJ, 5-20-ns-duration, Q-switched Nd:YAG laser at 1,064 nm, but other, more eye-safe wavelengths have been considered for long-range systems. These can be produced by third-harmonic generation of the Nd:YAG laser in the UV, or OPO-based shifting of the same laser to the 1,500-1,600-nm region. The typical receiver employs a charge-coupled device (CCD)-array detector, along with a dispersive element such as a diffraction grating, to image the spectrum onto the array. Typical limits of detection for each element are between 0.1 and 200 parts per million, depending on the sample and the element of interest. By employing advances in real-time spectral recognition software (chemometrics), it is possible with LIBS systems to provide a full analysis of the material composition on a 1-s timescale.96 No significant range resolution is required for this technique, although if multiple range returns were in a given pixel’s range resolution capability, this could have an advantage in clutter discrimination. A framing camera is sufficient for LIBs.

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FIGURE 2-26 Generalized schematic diagram of a LIBS system.

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95 A. Miziolek, F. DeLucia, J. Gottfried, and C. Munson, 2010, “Progress in standoff libs detection and identification of residue materials,” in Lasers, Sources and Related Photonic Devices, OSA Technical Digest Series (CD), Optical Society of America: LWD2.

96 Ibid.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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FIGURE 2-27 Schematic of LIBS instrument on Mars rover. SOURCE: See http://msl-scicorner.jpl.nasa.gov/Instruments/ChemCam/.

Figure 2-27 shows a detailed schematic of the LIBS instrument developed and now deployed on the Mars Rover to analyze martian soil. This short-range system employs a 14-mJ, 5-ns pulsewidth, diode-pumped, 1,070-nm, Nd:KGW laser that is mounted along with an 11-cm-diameter receiver telescope, on a rotating mast for beam direction to the desired location on martian soil. An optical fiber transports the received optical signal to a fixed unit in the Rover body, where the signal is optically split into three wavelength bands, each of which is analyzed by a grating/CCD spectrometer. An example of the spectral data product from the device appears in Figure 2-28, with the lines labeled according to the element detected.

The use of femtosecond pulses has shown, as expected, that less pulse energy is required to obtain ablation of material from a surface, and that at the low energies the signal contains much less background continuum emission and line signals from the atmospheric constituents (N and O) surrounding the material. However, the resultant LIBS signal is weaker, and when the pulse energy increases to correct this, the emission signals begin to resemble those from nanosecond pulses.97 One area of evident advantage for femtosecond pulses is in the LIBS detection of biological entities, where it is important to

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97 F.C. De Lucia, J.L. Gottfried, and A.W. Miziolek, 2009, “Evaluation of femtosecond laser-induced breakdown spectroscopy for explosive residue detection,” Opt. Express 17: 419.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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FIGURE 2-28 Spectral data from Mars LIBS instrument. SOURCE: See http://msl-scicorner.jpl.nasa.gov/images/ChemCam_spectra.jpg, http://msl-scicorner.jpl.nasa.gov/Instruments/ChemCam/.

detect the presence of the cyanide group (CN) inherent in the sample rather than the CN created by interactions in the air plasma surrounding the sample.98 The other advantage of femtosecond-pulse LIBS for long-range standoff detection is discussed below.

LIBS technology has sufficiently advanced beyond basic scientific studies to the commercial market, where it finds uses worldwide in manufacturing process control; one example is monitoring the composition of molten metals. Military applications are primarily defensive in nature; they include remote detection of the presence of chemical, biological, nuclear, or explosive material on surfaces. Future advances will include (1) the use of higher-beam-quality lasers to increase the detection range;(2) the application of femtosecond-generated filaments (discussed in Chapter 3), also for extending the range; and (3) more sophisticated double-pulse, multilaser sources that enhance the emission intensity from the ablated materials and improve the minimum detectable constituent level. Although initially developed for short-range (1 m) sensing, advances in the development of high-beam-quality, nanosecond-pulse lasers have extended ranges to 100 m. Advances in array detectors and chemometrics software provide opportunities to develop real-time analysis of LIBS data to identify the material being sensed.

Conclusion 2-5: Laser-induced breakdown spectroscopy provides a unique sensing capability to determine the elemental makeup of the surface of a solid or liquid material.

AEROSOL SENSING

An aerosol is a suspension of liquids or solids in a gas. In Earth’s atmosphere, aerosols are always present at normal altitudes and cause visible laser beams and searchlights to be “seen” as they propagate, due to the scattering of light from the liquid or solid component of the aerosol. Figure 2-29 shows the diameters of a variety of common atmospheric aerosol particles.

In the approximation that the aerosol is a simple sphere with a certain refractive index, one can calculate the fraction of electromagnetic radiation that is reflected directly backwards (“backscattered”) from a beam incident on aerosols of all the same size. The fraction for particles with diameters much smaller than the wavelength of the radiation is proportional to the sixth power of the diameter divided by the fourth power of the wavelength (Rayleigh scattering), while for particles with diameters much larger than the wavelength, the fraction scattered back is simply proportional to the area of the particle. For

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98 M. Baudelet, L. Guyon, J. Yu, J-P. Wolf, T. Amodeo, E. Fréjafon, and P. Laloi, 2006, “Spectral signature of native CN bonds for bacterium detection and identification using femtosecond laser-induced breakdown spectroscopy,” Appl. Phys. Lett. 88: 063901.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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FIGURE 2-29 Distribution of aerosol diameters (µm) in the atmosphere. Water clouds and fogs fall in the 1-100 µm range. SOURCE: By Jisaac9 (Own work) [CC-BY-3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons. See https://en.wikipedia.org/wiki/File:Airborne-particulate-size-chart.jpg.

particles with diameters comparable to the sensing wavelength, the full solution to the scattering problem, based on Maxwell’s equations, must be applied. The solutions, first credited to Gustav Mie in 1908, are referred to as Mie scattering.

The scattering probabilities show, as a function of increasing particle size, an oscillation about the eventual scattering value of the fraction given by the area of the particle. Given that common radar systems work in the 1-to 10-cm wavelength range, it is evident that a large percentage of aerosols fall in the Rayleigh approximation and are weakly scattering, making radar, for the most part, useless for detecting atmospheric aerosols. Given lasers operating at a wavelength on the order of 1 µm—near the scattering peak of a typical aerosol—laser-based sensors can generate a much larger backscattered signal and have provided a unique tool for the detection and study of atmospheric aerosols.

Conventional Aerosol Lidar

The simplest aerosol lidar sensor (Figure 2-30) employs an energetic, short-pulse (nanosecond-range) laser, and a high-speed detector to provide a time-dependent return signal that provides a measure of the spatial distribution of the aerosol concentration. For a monostatic lidar, the signal is essentially proportional to the backscattered fraction of laser light.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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FIGURE 2-30 Schematic of an aerosol lidar.

The lidar equation (see Box 1-3) shows that the expected backscattered signal falls as the inverse square of the distance to the aerosols. The time-dependent detected signal can be corrected to account for this. The return signal as a function of time is determined not only by the intensity of the light returned from a given spatial region, but also by the light lost (from aerosol and other scattering processes, as well as atmospheric absorption) in going to that region and returning, which depends on the distance to the region. If the light lost was due only to the aerosols, and one knew that all the aerosols sensed were of the same diameter, shape and refractive index, it would be possible to accurately determine the aerosol concentrations with a careful calibration of the lidar system parameters and use of the Klett inversion method99 for analyzing the time-dependent signal. Unfortunately, aerosols are rarely this uniform and instead present a distribution of sizes and shapes. Obtaining a reliable measure of the aerosol concentration and makeup from a single-wavelength laser is a challenge. It has been remarked that aerosol lidar provides an accurate measurement, but of what is uncertain.

The aerosol return signal can vary greatly, and for long ranges and/or small aerosol concentrations, lidar systems employing high pulse energies are desirable, to obtain an adequate SNR, especially in the presence of background skylight. For small-diameter aerosols, shorter wavelengths are desirable up to the point at which atmospheric transmission or the scattering loss caused by larger aerosols in the same path becomes a limit. In addition, the same (Rayleigh) scattering of light by molecules in the atmosphere that gives the sky its blue adds to the return signal at short wavelengths. The Klett inversion method was subsequently modified by Fernald100 to include the case where the return signal included Rayleigh-scattered light. More sophisticated single-wavelength aerosol lidar systems employ a single-frequency laser, along with narrow-line filters (etalons) that can discriminate between the relative broad linewidth of the molecular-scattered return signal and the much narrower return linewidth from aerosols.101 This is discussed in greater detail in the “Wind Sensing” section below. An alternative

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99 J.D. Klett, 1981, “Stable analytical inversion solution for processing lidar returns,” Appl. Opt. 20: 211.

100 F.G. Fernald, 1984, “Analysis of atmospheric lidar observations: some comments,” Appl. Opt., 23: 652.

101 S.T. Shipley, D.H. Tracy, E.W. Eloranta, J.T. Trauger, J.T. Sroga, F.L. Roesler and J.A. Weinman, 1983, “A high spectral resolution lidar to measure optical scattering properties of atmospheric aerosols, Part I: Instrumentation and theory,” Appl. Opt. 23: 3716.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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approach employs detection of the Raman-scattered signal from atmospheric molecules, which is discussed in the “Raman Sensing” section below.

Given the limitations in measurement, conventional, single-wavelength aerosol lidars can provide accurate information on the spatial distributions of aerosols but not on their precise makeup. Figure 2-31 shows a photograph of the aerosol lidar system originally developed at Los Alamos National Laboratory102 and subsequently transferred to UCLA as part of the Center for Lidar Environmental and Atmospheric Research (CLEAR). The system was developed to map the spatial characteristics of pollution-generated aerosols in urban environments. It employed a flashlamp-pumped Nd:YAG laser that used an optical parametric oscillator (see Chapter 4) to shift the laser wavelength to an eye-safe 1,570 nm, allowing operation in populated environments. The laser and transmit telescope are mounted to the left of the larger receiver telescope; the laser chiller and power supply are in rack mounts on the left and right cabinets below the telescope; and the signal processing/spatial location electronics are in the top right enclosure, with the system data display. Figure 2-32 presents a data product from this system, showing several scans over the Los Angeles basin; color scales indicate the relative “pollution” level. The distinct elevation boundaries between high and low levels are evident, along with a concentration of pollution in Beverly Hills associated with a cluster of automobile traffic.

China has a severe air pollution problem,103 and may develop lidar technology to monitor aerosol pollutant concentrations. This could provide a technology springboard to other applications.

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FIGURE 2-31 CLEAR aerosol lidar system developed for urban pollution measurements. SOURCE: Courtesy of Q-Peak, Inc.

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102 S. Barr, W. Buttler, D.A. Clark. W.B. Cottingame, and W.E. Eichinger, 1995, “Lidar-observed wind patterns in the Mexico -New Mexico -Texas border region,” Los Alamos National Laboratory Report.

103 See for example http://www.nytimes.com/2013/10/25/world/asia/smoggy-days-in-harbin-prompt-quickreaction.html.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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FIGURE 2-32 Data product from CLEAR lidar system, covering the Los Angeles basin. SOURCE: Courtesy of Q-Peak, Inc.

It is evident that a series of measurements at different times based on aerosol scattering can provide data on aerosol transport in a selected area, and hence some indication of the local winds as well as vertical convection currents. Besides providing useful meteorological information, the aerosol data could track the dispersion of toxic fumes from a fire or industrial accident, since they likely have associated aerosols. In addition, in the case of an intentional chemical or biological agent attack in an urban area, an aerosol lidar could provide information on the immediate spread of the agents, and where they might go in the future.

For defense applications, a similar system was developed in the late 1990s by Schwartz Electro-Optics for the U.S. Army Long-Range Biological Standoff Detection System (LR-BSDS).104 The device was intended to be mounted in a UH-60A (Blackhawk) helicopter platform, for “pop-up” sensing of possible bio-aerosol clouds that would be present in the case of a biological warfare attack. The operational concept employed is sketched in Figure 2-33 and a photograph of the system mounted in the helicopter appears in Figure 2-34.

The LR-BSDS system was specified to detect aerosol clouds up to 50 km away. The transmitter employed a diode-laser-pumped, 1 J/pulse, 20-ns pulsewidth, 100 Hz Nd:YAG laser that was converted to 0.33 J of pulse energy at 1,535 nm by a potassium titanyl arsenate (KTA)-crystal-based optical parametric oscillator (OPO) (see Chapter 4). To be eye-safe according to ANSI standards at the transmit aperture,

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104 L.A. Condatore, Jr., R.B. Guthrie, B.J. Bradshaw, K.E. Logan, L.S. Lingvay, T.H. Smith, T.S. Kaffenberger, B.W. Jezek, V.J. Cannaliato, W.J. Ginley, and W.S. Hungatem, “U.S. Army Soldier and Biological Chemical Command counterproliferation long-range biological standoff detection system (CP LR-BSDS),” Proc. SPIE 3707, Laser Radar Technology and Applications IV, 188 (May 28, 1999).

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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FIGURE 2-33 Drawing of operational concept for LR-BSDS system. SOURCE: Courtesy of Q-Peak, Inc.

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FIGURE 2-34 The LR-BSDS system mounted in a UH-60 helicopter. SOURCE: Courtesy of Q-Peak, Inc.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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the OPO output was coupled to a 7-in. aperture transmit telescope. The receiver employed a 24-in. aperture telescope and a transferred-electron-intensified photodiode (TE-IPD) was the detector. The system did demonstrate detection of aerosols to a distance of at least 45 km, but a change in U.S. Army tactics regarding biological attacks halted further development for operational uses.

As noted above, conventional, single-wavelength aerosol detectors provide ambiguous data on the nature of the aerosol clouds. As an example, the LR-BSDS system could not distinguish a biological agent from a dust cloud, and required additional data, such as the shape of the cloud or other sensor information. The next section discusses more advanced aerosol sensors that can provide less ambiguous data.

In summary, aerosol mapping with lidar has found widespread scientific applications in helping to understand fundamental atmospheric properties such as solar absorption. It can provide information on the three-dimensional nature of pollution, can identify sources of pollution, and can provide micro-scale mapping of pollution transport in urban environments. It has also been developed as one means of detecting and mapping the dispersion of a biological weapons attack. However, the lack of a widespread need beyond science, pollution monitoring, and defense has limited the development of low-cost, high-production-rate aerosol sensors.

Differential Scatter Lidar—Polarization and Multi-Wavelength

More complete data on the composition of the aerosols producing a return signal can be obtained through a combination of multiple-wavelength transmitters and/or polarized sources and polarization-sensitive detection systems. The comparison of the signals from the different channels to obtain better data is sometimes called Differential Scatter (DISC) lidar.

There are several motivations for obtaining improved aerosol data. From an Earth-science standpoint, atmospheric aerosols feature significantly in our planet’s energy balance of incoming solar radiation and energy radiated back to space. Better data on the makeup of the aerosols (including those in water clouds) is necessary for effective atmospheric modeling. As noted above, for biological-agent defense, being able to determine if a cloud is indeed an active agent is crucial to a warning and tracking system.

For aerosols that are very small compared to the laser wavelength, Mie scattering calculations show that there is a very strong variation in backscattered and attenuated energy with wavelength; for large aerosols on the other hand, there is essentially none, and in between, there is a complex relationship. An aerosol lidar employing multiple widely spaced wavelengths provides multiple channels of return data, and the differences in the channels (hence DISC) can be analyzed to better determine the aerosols generating the signal. Figure 2-35 shows atmospheric lidar data obtained with a laser system designed for sensing tropospheric aerosols; the system employs a Nd:YAG laser at 1,064 nm and its second and third harmonics at 532 and 355 nm, along with a Ti:sapphire laser at 750 nm.105 As expected, the strength of the scattering, expressed as an extinction coefficient (loss per km), increases with decreasing wavelength. Analysis of the data showed a broad distribution of aerosol diameters over the range 0.001-1 µm, peaking in the 0.01-0.1-µm range and essentially falling to zero beyond 1 µm.

Aerosols are not always perfectly spherical, and when they are not, they can change the polarization state of the backscattered signal. Lidar systems employing a polarized transmitter and a multichannel, polarization-sensing receiver, can also sense whether or not the aerosols are nonspherical.

One example of a system that includes both wavelength and polarization sensing is the NASA space-based Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) system that was launched into

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105 M. Yabuki, H. Kinjo, H. Kuze and N. Takeuchi, 2001, “Derivation of aerosol optical properties from fourwavelength lidar observations,” Proc. SPIE 4153, Lidar Remote Sensing for Industry and Environment Monitoring, 132, February.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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a sun-synchronous, 705-km altitude orbit in 2006.106 The system includes a diode-pumped Nd:YAG laser transmitter that generates two wavelengths, 1,064 and 532 nm, with 110 mJ/pulse at each wavelength, at a 20-Hz rate. The receiver, employing a 1-m telescope, has three channels, one unpolarized at 1,064 nm with an avalanche photodiode detector and two that are orthogonally polarized at 532 nm with photomultiplier detectors. An example of a CALIOP data product is presented in Figure 2-36.

The inclusion of longer wavelengths would be appropriate for characterization of larger aerosol sizes, such as man-made particles (see Figure 2-29) resulting from industrial production or activities around urban areas. Past work on multiple-wavelength aerosol lidar has included wavelengths in the 1-10-µm range, but systems have suffered from reduced sensitivity due to the lack of both photomultiplier tubes and powerful sources, with the exception of the wavelengths between nine and eleven µm generated by pulsed CO2 lasers. Recent developments in low-noise, high-gain semiconductor detectors as well as high-energy solid state and OPO-based sources make high-sensitivity operation at long wavelengths possible.

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FIGURE 2-35 Atmospheric aerosol data generated by four-wavelength, vertical-pointing lidar system. Some of the data, especially at high altitudes and at short wavelengths, are from Rayleigh scattering from air, rather than aerosols. SOURCE: M. Yabuki, H. Kinjo, H. Kuze and N. Takeuchi, 2001, “Derivation of aerosol optical properties from four-wavelength lidar observations,” Lidar Remote Sensing for Industry and Environment Monitoring, Proc. SPIE 4153: 132.

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106 W.H. Hunt, D. M. Winker, M. A. Vaughan, K. A. Powell, P. L. Lucker and C. Weimer, 2009, “CALIPSO Lidar Description and Performance Assessment,” J. Atmos. Oceanic Technol., 26: 1214.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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FIGURE 2-36 A vertical profile provided by the CALIOP system reveals a thick plume of aerosol haze from fires over a thinner layer of clouds. This image was captured over the southeast Atlantic Ocean, off southern Africa, during the time period shown. SOURCE: P. Lynch, 2009, “CALIPSO sees through the haze,” http://www.nasa.gov/topics/earth/features/calipso-aerosol.html.

The LR-BSDS sensor for long-range detection of bio-aerosol clouds was shown in Figure 2-36; this system provides a limited ability to discriminate threats from other aerosols. There has been considerable interest in, and support for, development of systems that can positively identify bio-aerosols, and modern approaches to this problem combine multiple-sensor approaches, most notably the use of laser-induced fluorescence (LIF), discussed below in the section “Laser-Induced Fluorescence.” Since effective bio-agent aerosols have diameters in the 1-to 10-µm range, DISC systems for this application would be expected to use wavelengths extending further into the infrared than sensors designed for scientific studies of the natural atmosphere.

One such device employed a Nd:YAG laser combined with an OPO to generate 1,064, 1,551 and 3,389 nm and used polarization as well as wavelength diversity to better discriminate bio-aerosols signals from others.107 The discrimination technique employed not only DISC but a technique called wavelength normalized depolarization ratios (WANDER) to compare depolarization results at all three wavelengths. A recent publication reviews work in this area, including the use of CO2-laser-based, multiwavelength DISC sensors in the 9-to 11-µm wavelength range, and suggests an optimal approach would be as follows.

Depolarization measurements in SWIR/MWIR for the detection of unusual concentrations of non-background aerosols; elastic backscattering in SWIR for detecting unusual aerosol concentrations,

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107 J.J. Glennon, T. Nichols, P. Gatt, T. Baynard, J.H. Marquardt, and R.G. Vanderbeek, 2009, “System performance and modeling of a bio-aerosol detection lidar sensor utilizing polarization diversity,” Proc. SPIE 7323, Laser Radar Technology and Applications XIV, 73230T, May.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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and cloud mapping and tracking; UV Laser Induced Fluorescence (LIF) for discrimination of aerosols.108

In summary, applications of DISC technology have been in basic atmospheric studies as well as in systems to identify potential harmful biological agent attacks. DISC system development will likely be done only for scientific and defense applications.

DIFFERENTIAL ABSORPTION LIDAR

Differential absorption lidar (DIAL) seeks to measure the concentration of a gas (or gases) in the region illuminated by the laser transmitter. There are two general varieties, path-averaged, which measures gas over a specific path with no information on the distribution of gas in the path, and range-resolved, where data on the distribution of gas are generated. The measurement relies on the actual absorption of the transmitter light by the gas, not its scattering, where the absorption can be from either electronic transitions, or in the case of molecules, vibrational transitions. In the case of electronic transitions of molecules, they are actually coupled electronic-vibrational transitions, often called vibronic transitions, while the vibrational transitions in molecules are in fact coupled to the rotational states and called ro-vibrational transitions. Both lead to complex structure in the absorption properties.

Path-Averaged DIAL

An optical schematic of a path-averaged DIAL system appears in Figure 2-37. The transmitter power is reflected back to the receiver either by scatter from some available surface (a “topographic” target) or, for a fixed installation, a retroreflector designed to reflect back a large fraction of the incident light. An alternative “bistatic” approach places the receiver at the desired end of the path. A key element of the system is a tunable-wavelength transmitter that provides at least one wavelength absorbed by the gas or gases of interest (“on-line”), and one wavelength that is not (“off-line”). The key element of the measurement is to evaluate the difference between the received signals for the on-line and off-line wavelengths, which accounts for “differential” in the DIAL acronym. The measurement provides data on the presence of the gas in the path between the source and the reflecting surface but no data on where the gas may be in the path, only its average absorption level (hence the descriptor “path-averaged”). With enough tuning in the source, it is possible to detect multiple gases (“a” and “b” in the figure) along the path.

Conventional, path-averaged gas spectroscopy is often done with incoherent broadband sources, where the receiver employs a dispersive element to do spectral discrimination needed to detect the desired absorption lines of the gas or gases sensed. For industrial fence-line monitoring, incoherent-source systems that use Fourier-transform infrared spectroscopy (FTIR) have been deployed, operating over paths as long as 1 km. The outstanding feature of the FTIR system is the ability to cover a very broad wavelength range, 0.7-30 µm. This range covers the strong vibrational transitions of all molecules exhibiting vibrational absorption features. Defense applications have included detection of chemical weapons, with most of the important distinguishing features of chemical agents falling in the atmospheric window between 8 and 12 µm. Disadvantages of the FTIR system include the relative mechanical complexity and high capital cost of the device; as a result, they have found limited commercial success in recent years for atmospheric measurements, although they are widely used for laboratory work.

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108 S. Lambert-Girard, N. Hτ, B. Bourliaguet, P.-F. Paradis, M. Piché, and F. Babin, 2012, “Proposal for a standoff bio-agent detection SWIR/MWIR differential scattering lidar,” Proc. SPIE 8358, Chemical, Biological, Radiological, Nuclear, and Explosives (CBRNE) Sensing XIII, 835805, May 1.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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FIGURE 2-37 Schematic of path-averaged DIAL system.

The advantages of lasers for the path-averaged sensing application are as follows:

1. Much higher spectral brightness than incoherent sources, which allows longer paths or, for some systems, the use of available surfaces for reflection of the source back to the receiver rather than purposely located retroreflectors. For defense applications, such as the detection of chemical weapons, the ability to use an available surface rather than a retroreflector is a major advantage.

2. The higher SNR that is possible in the system is important in detecting the small changes with wavelength exhibited by small concentration of gases. With coherent sources, a variety of techniques to extend detection sensitivity have been devised, such as wavelength modulation spectroscopy, and the ultimate detection sensitivity of laser-based systems is many orders of magnitude higher than with incoherent-source systems. In applications involving measurements in the atmosphere, the high sensitivity is often unusable because of absorption from other gases.

3. The higher spectral resolution that is possible is useful when there are multiple interfering absorption features from other gases in the path that interfere with the gas to be sensed.

The main disadvantages of coherent sources have included the limited wavelength coverage, a problem when either the gases to be sensed are unknown or a wide range of gases are to be detected, and the relative cost and complexity of the coherent source. Past work on path-averaged DIAL systems has included the use of tunable, pulsed, high-energy CO2 lasers in the 9-to 11-µm region to measure vibrational transitions, where the high available powers allowed multikilometer ranges against topographic targets. As for spectral coverage, there have been recent developments in broadband sources based on nonlinear optics that provide significant power over much of the IR region, and future advances in path-averaged systems may be able to exploit these sources, which provide broadband power from a diffraction-limited source. This is discussed in more detail in Chapter 3, in the section on femtosecond lasers.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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For scientific applications, one dramatic application of path-averaged DIAL is in space-based measurements of Earth’s atmosphere, where the satellite contains the tunable transmitter as well as the receiver and the signal is reflected from Earth’s surface. As an example, systems for measurement of the spatial distribution of CO2 throughout the planet are under development by NASA, probing absorption by CO2 vibrational levels in the 1,500- and 2,000-nm wavelength regions. By tuning through the absorption lines and analyzing the absorption lineshape, it may be possible to get some understanding of the vertical distribution of the CO2 concentrations, since the spectral linewidths are strongly dependent on temperatures and hence altitude. Given the long ranges involved, the NASA programs plan to use erbium- or thulium-doped fiber lasers to generate the required power levels.

While path-averaged DIAL does not directly provide spatial information (with the exception of high-altitude or space-based, down-looking sensors, which may be able to take advantage of the absorption lineshape change with altitude), it is possible to employ tomographic techniques to build up spatial information, when it is possible to compare data from a variety of different path angles. The simplest example would be a ground-based system that could make use of multiple reflecting surfaces at different angles or ranges. Another example is an aircraft-based, down-looking sensor that obtains signals from reflections off the ground and can provide wide-area data that can be analyzed to localize sources of the gases sensed. Given advances in source and detector technology, it should also be possible to develop space-based sensors for a variety of gases beyond those, such as CO2, that are of interest for science.

When semiconductor diodes are employed as sources, the technology is sometimes called tunable diode laser absorption spectroscopy (TDLAS), and a variety of sensitive gas measurement techniques have been developed to leverage the ability to rapidly amplitude- or frequency-modulate the diode output. One important application for long-path measurements is detection of very weak atmospheric absorption features, of interest to basic science spectroscopy as well as possible effects of atmospheric absorption on directed energy laser systems. Figure 2-38 shows a schematic of one TDLAS system, designed to measure atmospheric absorption due to weak electronic transitions of O2 in the 895-nm region. This is the region where the diode-pumped cesium alkali directed energy laser (DPAL) operates.109 The system employs 100-kHz amplitude modulation along with electronic coherent detection to improve the SNR. Spectral data from the system for 150- and 1,000-m paths appear in Figure 2-39, which shows the location of the cesium laser transitions as well as the weak absorption lines due to the vibronic O2 transitions. Owing to the very weak absorption levels, the measurements would not be possible with conventional incoherent sources.

The relatively recent development of cascade semiconductor lasers (Chapter 4) has led to much wider wavelength coverage for TDLAS of the infrared molecular absorption bands, and development of a “killer app” for these devices in gas sensing could lead to a major reduction in price. At present, many commercial TDLAS systems make use of more conventional interband lasers, originally developed for telecommunications applications. These are readily available and relatively low in cost. Table 2-2 lists the detection sensitivities for a variety of gases for TDLAS sensors, primarily based on near-IR diode lasers (wavelengths less than 2.2 µm) which sense the overtone vibrational levels of the molecules. For two gases there is greatly increased sensitivity when one employs longer wavelength QCL-based sources, which sense the much more strongly absorbing fundamental vibrational absorption bands. The sensitivities are expressed in parts-per million meters (ppm-m), so for a path length of 1 km the sensitivity would be greater than listed by a factor of 1,000. For example, for sensing of CO gas, one can detect 40 parts per billion (ppb) over a 1 km path with a near-IR source, while a sensor at 4.8 µm could, in principle, detect levels well below a ppt. (In practice, the ambient levels for CO in a normal atmosphere are well over this level.)

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109 C.A. Rice, G.E. Lott, and G.P. Perram, 2012, “Open-path atmospheric transmission for a diode-pumped cesium laser,” Appl. Opt. 51: 8102.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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FIGURE 2-38 Schematic of path-averaged TDLAS system operating at 895 nm. SOURCE: C. Rice, G. Lott, and G. Perram, 2012, “Open-path atmospheric transmission for a diode-pumped cesium laser,” Applied Optics 51: 8102.

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FIGURE 2-39 Data from 895-nm TDLAS system for 150-(black) and 1,000-m (gray) paths. The broad lines, with level identifications, are from O2 atmospheric absorption. SOURCE: C. Rice, G. Lott, and G. Perram, 2012, “Open-path atmospheric transmission for a diode-pumped cesium laser,” Applied Optics 51: 8102.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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TABLE 2-2 Detection Sensitivities of TDLAS Systems to Various Gases. SOURCE: Mark Allen, Physical Sciences Inc., June 5, 2013.

ppm-m
Near-IR(<2.2 µm) 2 to 3 µm 4 to 8 µm
Gas 300 K 1 atm Gas 300 k 1 atm 300 k 1 atm 300 k 1 atm
HF 0.2 HCN 1.0
H2S 20.0 CO 40.0 0.02 0.0001
NH3 5.0 CO2 1.0
H20 1.0 NO 30.0 0.6 0.03
CH4 1.0 NO2 0.2
HCI 0.15 O2 50.0
CH3CN 10.0 CH3CHOHCH3 20.0
CH2CI2 10.0 CH3CH2OH 20.0
CH3OH 20.0

For defense applications, as noted above, the 8-12µm wavelength range is well understood and documented for its usefulness as the “fingerprint region” for chemical weapons, generally heavy molecules with strong fundamental vibrational-rotational absorption bands in the region. It is likely that FTIR-based sensors will be replaced in the future by arrays of QCL sources that provide full coverage of the region and allow longer range and more precise path-averaged sensing, with localization provided via tomographic approaches that could employ UAV-based sensors based on reflection from the ground.

Another defense application is explosives detection, where the goal would be detect gases given off by the solid explosives or those associated with explosives production. Here the challenges are detection sensitivity, as the vapor pressures for most explosives themselves are very low.110 The production of explosives, particularly low-technology, so-called homemade explosives (HMEs) such as urea nitrate (UN), ammonium nitrate (AN) and nitromethane (NM) may involve materials such as ammonia that produce more detectable gases. Remote explosives detection by path-averaged DIAL is clearly an area of interest for both defense and homeland security.

Compared to sensors based on incoherent sources, DIAL systems have been able to detect much smaller concentrations of gas but, until recent developments in broadband lasers, they could not match the broad wavelength ranges of the incoherent sensors. The development of practical, room-temperature QCLs is set to provide much higher gas-detection sensitivities through operation in the fundamental vibrational absorption bands of molecular gases. Thus far, military applications of DIAL have been primarily defensive in nature and include the detection of chemical agents in the so-called molecular fingerprint region of 8-to 12-µm wavelength. The DIAL technology is well known throughout the world. Although there are differences in development of QCL-based sources, any gaps are likely to narrow, especially if industrial sensors find wider deployment.

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110 J. Steinfeld and J. Wormhoudt, 1998, “Explosives detection: A challenge for physical chemistry,” Annu. Rev. Phys. Chem. 49(1): 203.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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Conclusion 2-6: Path-averaged differential absorption lidar will see a large increase in sensitivity through the use of quantum cascade lasers.

Conclusion 2-7: With lightweight differential absorption lidar systems enabled by semiconductor lasers and emerging broadband coherent sources (see Chapter 3), the ability to place sensors on unmanned aerial vehicles should allow development of three-dimensional data from relatively simple sensors.

Range-Resolved DIAL

While path-averaged DIAL systems can provide significantly improved performance in terms of gas-detection sensitivity compared to systems based on incoherent sources, or allow measurements from space, range-resolved DIAL technology (Figure 2-40) provides a truly unique capability. It generally utilizes a pulsed, tunable source (or sources) and light backscattered by aerosol/Rayleigh scattering and compares the range-gated return signals as a function of wavelength to measure the absorption of the light by a gas or gases in the region probed by the laser. With enough tunability in the transmitter one can detect different gases (“a” and “b” in the drawing) as a function of range, but many DIAL systems are designed to detect only one gas. The simplest system can use one online and one off-line measurement wavelength, but more sophisticated DIAL systems employ multiple off- and online wavelengths.

The sensitivity of range-resolved DIAL to sensing gases depends on the signal-to-noise levels of the returned signals, which must be subtracted from one another. The presence of atmospheric fluctuations from the aerosol distributions and variations in atmospheric refractive index provides another source of noise, and more advanced DIAL systems will send off- and online pulses closely spaced together to minimize changes in the atmosphere, with a spacing less than 1 ms sufficient to “freeze” the atmosphere. Even with this technique, it is difficult for operating DIAL systems to detect less than a 1 percent difference in the strength of the returned signals. The need to reduce fluctuations in the returned signals generally limits DIAL receivers to the use of direct detection, as the pulse-to-pulse speckle fluctuations from coherent detection are large.

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FIGURE 2-40 Schematic of range-resolved DIAL system.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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Given the need to employ atmospheric backscatter, range-resolved DIAL systems for long-range use are generally limited to shorter wavelengths and thus probe absorption due to electronic transitions. Many gases of special interest, such as CH4 and a variety of related hydrocarbons, do not have electronic transitions low enough in energy to be accessed within the UV transmission range of the atmosphere and thus can only be sensed by range-resolved DIAL systems operating in the infrared. Operating ranges are limited compared to the short-wavelength systems, and the system performance may vary depending on the aerosol content of larger-diameter particles.

The applications of range-resolved DIAL can be divided into scientific and applied, the latter including the detection and monitoring of pollution and the detection of leaks in gas pipelines.

Basic atmospheric science programs include monitoring of global distributions of water vapor and ozone. Water vapor is by far the most important greenhouse gas, and understanding its distribution in the atmosphere is of great importance. The NASA LASE system111 employs a pulsed Ti:sapphire laser at 815 nm sending out pulse pairs separated by 0.4 ms in time and 70 picometer in wavelength to generate the DIAL data, with a pulse energy of 150 mJ. The receiver employs a 40-cm-diameter telescope and a silicon avalanche photodiode. Initial missions were flown based on an ER-2 platform, at altitudes of 16-21 km. The data product from one mission appears in Figure 2-41, showing the complex nature of water vapor distributions in the atmosphere. Before development of the DIAL system, equivalent data, with much less global coverage, had to be generated by balloon-born radiosondes. A ground-based water-vapor DIAL system, situated on the Zugspitze, (2.7-km ASL) a mountain in Germany, also employs a high-energy (250 mJ, 2 ns pulsewidth) Ti:sapphire laser around 817 nm and provides range-resolved data up to an altitude of 12 km.112

DIAL operating in the UV wavelength region can take advantage not only of aerosol backscatter but also of the more predictable Rayleigh backscatter. Ozone is of interest from both a scientific and an applied standpoint. At high altitudes, (stratospheric) ozone is crucial in absorbing harmful deep-UV light from the sun, while at low altitudes, (tropospheric) ozone is a pollutant that affects heath and damages vegetation and is one of the components of smog in cities. Ozone absorption has a very broad, generally unstructured band. Operational ozone DIAL systems have been based on the ground for providing data on tropospheric ozone, while aircraft-based systems have pointed the DIAL system downward for tropospheric measurements or upward for stratospheric measurements. For detection in the stratosphere, where ozone concentrations are high, typical on/off wavelengths used are in the range 308-355 nm, while for lower concentrations in the troposphere, 289 and 299 nm are used as on/off wavelengths. For systems to date, detection sensitivities are in the 10 ppbv range, and typical data show ozone levels in a large range around 100 ppbv.113 In the future, NASA and other agencies are planning to build space-based ozone DIAL systems to provide full global coverage.

The other gases—O3 (ozone), SO2, and NO2—are of interest for monitoring pollution created by burning fossil fuels, various industrial activities, internal combustion engines, or volcanoes. SO2 resulting primarily from burning coal, is toxic, with death possible for prolonged exposure at 1 ppm and was reportedly one of the gases that caused more than 10,000 deaths during the London Great Smog of 1952. More recently, SO2 has been identified as one of the major causes of “acid rain.” With the development of range-resolved DIAL systems operating around 300 nm it became possible to track the generation and distribution of SO2. A variety of mobile, ground-based DIAL systems were deployed in the United States and Europe starting in the 1970s to better understand how SO2 was dispersed from sources, and early results showed that the gas spread over a wider region (100-1,000 km) than expected. This led to major changes in the regulation of emissions from power plants. Recently, SO2 lidar systems have been

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111 See http://www.nasa.gov/centers/langley/news/factsheets/LASE.html.

112 H. Vogelmann and T. Trickl, 2008, “Wide-range sounding of free-tropospheric water vapor with a differential-absorption lidar (DIAL) at a high-altitude station,” Appl. Opt. 47: 2116.

113 A.O. Langford, 2012, “Differential Absorption Lidar (DIAL) for characterizing ozone distributions and transport,” WESTAR Conference on Western Ozone Transport, Boulder, Colorado, October 11, http://www.westar.org/12percent20Techpercent20Conf/Presentations/Langford.pdf.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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FIGURE 2-41 Measurement of water vapor by LASE onboard the ER-2 aircraft that flew from near Bermuda to Wallops Island, Virginia. The horizontal scales indicate the geographical coordinate of the measurement (at the Universal Time, UT) and the vertical scale indicates the vertical location. The color scale from pink (light detection) to black (heavy detection) indicates the concentration of water vapor in the atmosphere. White areas in the image indicate measurements below instrument threshold or a lack of data. SOURCE: NASA Langley Research Center, Lidar Atmospheric Sensing Experiment (LASE): Measuring Water Vapor, Aerosols and Clouds, http://www.nasa.gov/centers/langley/news/factsheets/LASE.html, accessed September 6, 2013.

developed in China to better understand the causes of severe smog events in cities such as Beijing, exacerbated by the Chinese use of coal-fired power plants. With a 10-mJ-energy, 10-ns source around 300 nm, reported sensitivities are 15 ppb for a 300-m averaging path, with a range extending to 3 km.114

Nitrogen dioxide and its related molecule NO are created by cars, trucks, and other combustion sources; reacting with hydrocarbons and sunlight, they lead to the formation of tropospheric ozone. The mix of gases forms smog, with the visible-wavelength absorption for NO2, leading to the brownish atmospheric color evident in major smog events. DIAL systems have also been used to track the formation and dispersion of NO2 and operate in the 450-nm wavelength region. With a 150-mJ source at 450 nm, sensitivities are 10 ppb for a 300-m path, and the range is up to 6 km.115 NO can be sensed as well, but the detection wavelength, around 227 nm, has very limited transparency in a sea-level atmosphere, owing to the absorption of O3 and other molecules, and the sensing range is limited to 0.5 km.116,117

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114 W. Staehr, W. Lahmann, and C. Weitkamp, 1985, “Range-resolved differential absorption lidar: Optimization of range and sensitivity,” Appl. Opt. 24: 1950.

115 Ibid.

116 M. Aldén, H. Edner, and S. Svanberg, 1982, “Laser monitoring of atmospheric NO using ultraviolet differential-absorption techniques,” Opt. Lett. 7: 221.

117 Z.G. Guan, P. Lundin, L. Mei, G. Somesfalean, and S. Svanberg, 2010, “Vertical lidar sounding of atomic mercury and nitric oxide in a major Chinese city,” Appl. Phys. B101: 465.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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Other entities measured by UV DIAL include air toxics such as atomic mercury at 254 nm,118 generated primarily by volcanoes, coal-fired power plants, and waste incineration; Cl2 gas around 300 nm,119 often released by industrial accidents; and the aromatic hydrocarbons benzene, toluene, ethylbenzene, and the three isomers of xylene (collectively referred to as the BTEX compounds) around 220-270 nm.120 Benzene has long been considered a carcinogen. Emission sources for BTEX compounds include refineries, gasoline service stations, and vehicle exhaust. In areas where mobile emissions are significant, ambient concentrations of BTEX are tens to perhaps 200 ppb C and represent the majority of total toxic organic compounds. Because, with the exception of benzene, these aromatics are moderately reactive in photochemical smog formation, their chemistry in the atmosphere is of great interest.

As noted above, mid-IR range-resolved DIAL systems cannot provide the range of shorter-wavelength systems, and their performance is more dependent on the variable concentrations of larger atmospheric aerosols. However, there is interest in sensing of the wide variety of hydrocarbons that have fundamental, intense vibrational transitions in the 3,300-3,400-nm wavelength region, both for science and for applications, which include not only pollution monitoring but also detection of gas leaks over pipelines and, possibly, exploration activities to discover new sources of oil and gas.121 Probably the most utilized mid-IR DIAL system has been developed by the U.K. National Physical Laboratory (NPL).122 The mid-IR source was used in combination with a UV source to make a variety of ground-based range-resolved DIAL measurements, and Table 2-3 summarizes the results. The technology employed in the NPL dates back to before 2000; for DIAL systems built now, a variety of improvements in sources and, for the mid-IR range, detectors, would allow longer ranges and, perhaps, higher detection sensitivities.

Many of the ground-based pollution measurements have been from mobile platforms that have allowed measurements at a variety of locations throughout the world. One of the most notable systems appear in Figure 2-42, from NPL, which has pioneered in the mid-IR sensing of hydrocarbons. Most recently, another such system was shipped to China, to measure Hg and NO in the city of Hangzhou.123 In terms of systems engineering, these mobile sensors employ commercial lasers designed for operation in scientific laboratories; they are essentially like a large laboratory that has been given wheels, without any attempt at miniaturization or ruggedization. The helicopter-based aerosol sensor (Figure 2-37) is an example of a much more engineered sensor, with a source more powerful than those in the mobile lidar trucks.124

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118 H. Edner, G. W. Faris, A. Sunesson and S. Svanberg, 1989, “Atmospheric atomic mercury monitoring using differential absorption lidar techniques,” Appl. Opt. 28: 921.

119 H. Edner, K. Fredriksson, A. Sunesson, W. Wendt, 1987, “Monitoring Cl2 using a differential absorption lidar system,” Appl. Opt. 26: 3183.

120 M.J.T. Milton, P.T. Woods, B.W. Jolliffe, N.R.W. Swann, and T.J. McIlveen, 1992, “Measurements of toluene and other aromatic hydrocarbons by differential-absorption LIDAR in the near-ultraviolet,” Appl. Phys. B55: 41.

121 U.S. Patent US 6509566 B1, “Oil and gas exploration system and method for detecting trace amounts of hydrocarbon gases in the atmosphere,” Publication date Jan 21, 2003, and references cited therein.

122 M.J.T. Milton, T.D. Gardiner, F. Molero, and J. Galech, 1997, “Injection-seeded optical parametric oscillator for range-resolved DIAL measurements of atmospheric methane,” Opt. Commun. 142: 153.

123 Z.G. Guan, P. Lundin, L. Mei, G. Somesfalean, and S. Svanberg, 2010, “Vertical lidar sounding of atomic mercury and nitric oxide in a major Chinese city,” Appl. Phys. B101: 465.

124 M. Sjöholm, P. Weibring, H. Edner, and S. Svanberg, 2004, “Atomic mercury flux monitoring using an optical parametric oscillator based lidar system,” Opt. Exp. 12: 551.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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TABLE 2-3 Detection Sensitivities and Maximum Ranges of NPL Range-Resolved DIAL System for Various Gases

Infrared DIAL System UV/Visible DIAL System
Species Sensitivity Max. Range Species Sensitivity Max. Range
CH4 50 ppb 1 km NO 5 ppb 500 m
C2H2 40 ppb 800 m NO2 10 ppb 500 m
C2H4 10 ppb 800 m SO2 10 ppb 3 km
C2H6 20 ppb 800 m O3 5 ppb 2 km
higher alkanes 40 ppb 800 m Hg 0.5 ppb 3 km
HCI 20 ppb 1 km Benzene 10 ppb 800 m
N2O 100 ppb 800 m Toluene 10 ppb 800 m
CH3OH 200 ppb 500 m Xylene 20 ppb 500 m

NOTE: NB. The sensitivities apply at a range of 200 m for a 50 meter plume.

SOURCE: M.J.T. Milton, T.D. Gardiner, F. Molero and J. Galech, 1997, “Injection-seeded optical parametric oscillator for range-resolved DIAL measurements of atmospheric methane,” Opt. Commun. 142: 153.

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FIGURE 2-42 NPL (U.K.) mobile lidar truck. SOURCE: Courtesy of NPL.

Range-resolved DIAL would be of great use for a number of defense applications, requirements, including the detection of chemical and biological weapons and buried explosives. Unfortunately, the signature absorption bands for chemical weapons fall in the 8-12 µm atmospheric window, where aerosol scattering is weak and highly variable. This means that detection is better done with path-averaged DIAL, employing tomographic techniques if spatial information is needed. Overtone transitions of chemical agents in the 3-5-µm window better suited for range-resolved DIAL tend to exhibit very broad and indistinguishable absorption features, making agent identification difficult. Biological agents, effectively very large molecules, do not exhibit identifiable absorption features. As noted above, the low vapor pressures of explosives125 make detection even by path-averaged DIAL a challenge. The production of

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125 J. Steinfeld, and J. Wormhoudt, 1998, “Explosives detection: A challenge for physical chemistry,” Annu. Rev. Phys. Chem. 49(1): 203.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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HMEs might generate NH3 but the most intense absorption bands are at around 10 µm, and the weaker overtone transitions overlap with common atmospheric components such as water vapor and CO2.

DIAL systems have been considered for some law enforcement applications, such as the detection of drug production facilities. As an example, kerosene, diethyl ether, and acetone are all used in the production of cocaine, and a range-resolved DIAL system design might be able to locate production operations normally hidden in a jungle or in some industrial areas. Methamphetamine production involves a number of volatile chemicals, likely detectable with DIAL techniques, although the number of different production techniques suggests there may not be one chemical that could be used as a clear indicator.

Conclusion 2-8: Operation of differential absorption lidar at long ranges is easiest with the use of ultraviolet and visible wavelengths, which provide the strong aerosol and Rayleigh backscatter needed. An important, but limited, set of gases can be sensed at these wavelengths.

Conclusion 2-9: The range of differential absorption lidar systems is reduced at longer wavelengths and is severely limited out in the 3-5 and 8-12 µm atmospheric window regions, though improvements in sources and detectors are countering this.

Conclusion 2-10: Range-resolved differential absorption lidar is an important tool for scientific studies and will find further deployment on airborne and space-based platforms.

RAMAN SENSING

The Raman effect is the inelastic (non-wavelength-preserving) scattering of photons from a species, through excitation of electronic or vibrational states. A Raman lidar system (shown schematically in Figure 2-43) senses the inelastic scattered photons in a receiver tuned to a different wavelength, or wavelengths, from that of the transmitter; it can develop range-resolved data (gases “a’ and “b” in Figure 2-45) by analysis of the time-resolved return signals at different wavelengths.

image

FIGURE 2-43 Schematic of Raman lidar system.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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FIGURE 2-44 Comparison of scattering processes from atmospheric species: (1) elastic, (2) conventional (Stokes) Raman, and (3) anti-Stokes Raman.

Figure 2-44 shows the conventional Rayleigh scattering process, which leaves the photon going out with essentially the same energy (wavelength) as the photon coming in, but with a different propagating direction (i.e., momentum). From a quantum-mechanical treatment, the incoming light induces a transition from the ground state to a “virtual” electronic state in an atom or molecule of the species, which very rapidly (on the order of 10-16 s) reemits the light with the same wavelength, based on the same ground electronic state. The Raman process (2) differs in that the reemitted light terminates on a different ground state, most typically another vibrational or rotational level for a molecular species, and is lower in energy and thus longer in wavelength. The probability of a Raman scattering processing is typically less than 10-3 that of Rayleigh scattering,126 with same (1/λ)4-dependence, favoring the use of short wavelengths for detection. When the photon scatters off a molecule that is already vibrating, the anti-Stokes process (3) yields a higher energy scattered photon with a shorter wavelength. At the higher energy of vibrational levels, very few molecular vibrations are already excited, so the fraction of anti-Stokes scattering is negligible. For rotational levels, however, the energies are small enough to allow a reasonable level of anti-Stokes scattering.

When the excitation wavelength approaches the energy of the upper electronic level, the probability of the Raman process greatly increases (resonance Raman), much more than the (1/λ)4 dependence, although for many species the wavelengths needed to observe a significant resonant enhancement are too short to be transmitted through the atmosphere.

The details of the spectral nature of the Raman scattering process depend on the nature of the species. This discussion considers only the sensing of molecules. Simple diatomic molecules like N2 and O2 at normal atmospheric temperatures have a single strong Raman line from the vibrational level with weak sidebands that result from the addition of rotational levels to the interaction, as well as purely rotational Raman scattering. More complex molecules with a number of vibrational modes have number of Raman lines, again with rotational structure except for spherically symmetric molecules. As with absorption spectra, molecules have unique Raman spectra, or fingerprints, that allow identification. For complex molecules some vibrations lead to the absorption of light while others only show up in Raman scattering.

A typical Raman lidar has a fixed-wavelength transmitter, with a receiver that may be (1) tuned to a specific Raman-shifted longer wavelength when only one species is to be sensed or, (2) capable of

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126 See http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19730009019_1973009019.pdf.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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spectrally resolving the returned signal with, say, a detector array, when multiple species are to be detected.

Advantages of Raman lidar include the following:

• No atmospheric scattering by entities other than the one to be sensed is needed for measurement.

• Each species produces a distinct Raman line (or lines) displaced in wavelength from the source, and one lidar system with only one wavelength laser can sense multiple species.

• Since the expected return signal is shifted in wavelength from the source laser, the appropriate filters can make the detection system immune to (elastic) backscatter of the transmitted light.

• The lidar source does not need to be tunable.

• Because Raman scattering has a weak probability, there is no appreciable attenuation of the source beam by the species. Thus large, concentrations can be measured over a long distance.

Problems with Raman lidar include these:

• Except for resonance Raman, the scattering from species is a weak process, much weaker than Rayleigh.

• Compared to DIAL the sensitivity is many orders of magnitude lower.

• When short wavelengths are employed for detection, fluorescence (see the next section) provides interference from species where the short-wavelength light creates real rather than virtual electronic transitions. This is especially true with resonance Raman, which employs wavelengths close to, if not actually overlapping with, real electronic transitions.

The last problem can be mitigated by the use of multiple transmitted wavelengths, since the fluorescence spectrum is generally unchanged with small changes in excitation wavelength, whereas the Raman signal shifts linearly with the transmitter wavelength.

The weakness of the Raman process generally limits Raman lidar to the detection of atmospheric species that have high concentrations, and deployed systems are typically used for scientific applications in atmospheric probing, such as detection of N2, O2, and water vapor as a function of altitude. For each of the purely vibrational (Q-branch) Raman lines there are weaker lines from vibrational-rotational Raman scattering, and a determination of the relative intensities of the lines allows the determination of the molecular temperature. In addition to the gas Raman lines, there is Raman scattering from liquid- and ice-phase water. Clearly, a Raman lidar is able to measure not only gas density and temperature, but also the phase state of atmospheric water.

An example of a Raman lidar used for atmospheric measurements is a facility run by the U.S. Department of Energy (Figure 2-45) at its Lamont, Oklahoma site; another, similar facility is in operation at Darwin, Australia. “The lidar uses a commercial lamp-pumped, tripled Nd:YAG laser, operating at 30 Hz with 300-400 mJ pulses, to transmit light at 355 nm. A 61-cm diameter telescope collects the light backscattered by molecules and aerosols at the laser wavelength and the Raman scattered light from water vapor (408 nm) and nitrogen (387 nm) molecules.”127 “These signals are detected by photomultiplier tubes and recorded using photon counting with a vertical resolution of 7.5 m.”128 A range-resolved data product on water vapor mixing for the vertically directed lidar appears in Figure 2-46.

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127 R.K. Newsom, 2009, Raman Lidar (RL) Handbook, March, DOE/SC-ARM/TR-038.

128 Ibid.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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FIGURE 2-45 The Raman lidar (RL) at Lamont, OK, operated by the U.S. Department of Energy’s Atmospheric Radiation Measurement (ARM) Climate Research Facility. SOURCE: U.S. DOE Atmospheric Radiation Measurement Program.

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FIGURE 2-46 Data product as a function of time over 4 days showing the water mixing ratio in the atmosphere as a function of altitude. SOURCE: US DOE Atmospheric Radiation Measurement Program.

Similar systems are in use worldwide. To name a few, NASA Goddard has built a mobile-trailer based system, the Scanning Raman Lidar (SRL), and deployed it at a variety of locations: the Max Planck Institute for Meteorology in Hamburg, Germany, has a system in operation in Barbados; the Atmospheric Science Programme office of the Department of Space in India has built a system that is in operation in India; and Chinese-built Raman lidars have been reported to be operating in Beijing, Xi’an, and Wuhan.

For defense applications, other than providing localized meteorological data, Raman lidar systems do not provide the sensitivities required for the standoff atmospheric detection of chemical, biological and explosives at other than trivial ranges.

A variation on remote Raman detection is designed to sense the presence of an entity on a surface rather than in the atmosphere. One application that has been investigated is detection of surface-deposited chemical agents by 266-nm, UV illumination of surfaces at ranges up to 500 m,129 and there is continuing work on employing short-range Raman sensors for surface contamination measurements, for both defense and homeland security uses. However, it appears that to detect low levels of chemical agents the system sensitivity is not sufficient to work beyond meter-level ranges.

Since the discovery of surface enhanced raman scattering (SERS),130 where the Raman scattering signal for an entity on a particular metallic surface may be increased by a factor of 106-1014 with near- to visible-wavelength laser excitation, there have been applied studies of the technique to detect very small levels of entities on surfaces, especially chemical and biological agents and explosives. To date, most of the work has been for very short range detection, but in the future the detection range could be extended with suitable sources. Considerable challenges remain, since the enhancement varies over many orders of magnitude depending on the metal and its surface condition.

Conclusion 2-11: Raman detection provides data on range-resolved concentration of gases, but the low sensitivity of the technique limits its use to major atmospheric constituents such as nitrogen and water vapor.

Conclusion 2-12: Applications of Raman sensing technology are likely to be limited to scientific research.

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129 See http://www.ecd.bnl.gov/pubs/BNL69444AB.pdf.

130 M. Fleischmann, P.J. Hendra, A.J. McQuillan, 1974, “Raman spectra of pyridine adsorbed at a silver electrode,” Chem. Phys. Lett. 26: 163.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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LASER-INDUCED FLUORESCENCE

Laser-induced fluorescence (LIF) lidars (see Figure 2-47) are similar to Raman lidars in that they require only that the entity to be detected be present in the atmosphere or on the surface of a target of interest. Unlike Raman lidars, however, LIF systems seek to create real transitions in the entity through excitation to higher electronic levels. In some cases, the entity returns to the ground electronic level by, in part, emitting light (fluorescing), and a LIF lidar system employs a receiver tuned to the wavelength region of the fluorescence. Through time analysis of the fluorescence, it is possible to localize the detected entity in the atmosphere, thus provided range-resolved data, and by employing a tunable receiver, or multiple receivers, one can detect multiple entities (“a” and “b”) as a function of range. It is also possible to detect fluorescence from entities on a surface. Typical systems employ pulsed UV sources for the required excitation of the electronic levels as well as range/species resolution. The decay time of the returned signal, as well as the fluorescence spectrum itself, can provide some ability to distinguish different entities.

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FIGURE 2-47 Schematic of LIF lidar.

Advantages of LIF systems include the following:

• Like Raman lidar no need for scattering by atmospheric constituents.

• The ability to detect some entities where no other long-range methodology is available, including biological compounds, oil films on water, and the condition of different forms of vegetation.

• The ability to use spectral and temporal resolution to distinguish multiple species with overlapping fluorescence-excitation bands.

Disadvantages include these:

• Complex analysis when the fluorescence decay time is long, confounding range resolution.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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• The typical “quenching” or low quantum efficiency of fluorescence for many species, reducing the return signals and the ultimate sensitivity. DIAL may be more sensitive in many cases.

• Typical fluorescence emission spectra are broad, requiring a matching broad detector response, leaving the system susceptible to background light, especially from the sun during daytime.

• Interference by Raman scattering from major atmospheric constituents.

Ways to mitigate some of the disadvantages of fluorescence lidar include the use of multiple or tunable excitation wavelengths, which can provide additional means to discriminate among multiple species as well as the strong Raman signals from the atmosphere. In addition, one can detect the fluorescence signal with an array of wavelength-dispersed detectors, to generate a single-shot spectrum for further analysis. In fact, with a tunable source and detector array, one can generate a hypercube of data to aid in unraveling the returns.

To enhance the signal strength from a given entity, it may be possible to inject fluorescent taggants into the atmosphere—basically, chemicals that selectively bind to the entity to be detected—and greatly enhance the return. For detection of entities on surfaces, many materials in solid or liquid form have much higher fluorescent quantum efficiencies than in gas form, since their fluorescence is not quenched by transferring energy to atmospheric molecules. In contrast to Raman scattering (with the exception of SERS), fluorescence-based sensing of entities on surfaces may provide a sensitive means for detection of small quantities of the desired species.

LIF lidar is possible for the gaseous forms of numerous aromatic hydrocarbons, which all have broad fluorescence spectra in the region of 250 to 400 nm, peaking around 290 nm. Atmospheric plumes (a few meters thick) of benzene, toluene, xylenes, naphthalene, and other aromatics have been remotely detected with LIF at less than ppm-m levels at significant ranges.131 The BTEX compounds, which have subtle distinguishing fluorescence features, were identifiable individually, but some mixes (e.g., p-xylene, o-xylene, and m-xylene) are not readily quantitatively separable by remote sensing at a single wavelength. However, the more complex aromatics remotely detected (e.g., chlorobenzene, benzoyl chlorine, and piperdine) have prominent fluorescence features, simplifying discrimination. Models show that concentrations of multicompound mixtures could be determined at the 1-ppm-m sensitivity over ranges of 1-2 km for laser pulse energies on the order of 10 mJ.

Fluorescence of hydrocarbons in water is used in situ to detect low levels of contamination, such as from oil spills, and can be done remotely as well. One commercial system, the FLS Lidarsupplied by Laser Diagnostic Instruments in Estonia, has been adapted for airborne, downward-looking applications, particularly for mapping organic pollution (including oil and petroleum products) in water and on soil at altitudes between 50 and 500 meters.132 The system employs a 308-nm XeClexcimer laser; and Figure 2-48 shows typical spectra from several different measurements over water.133 Recent work has looked at the use of onshore or ship-based UV-LIF systems for a lower-cost approach to spill detection.134

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131 W. Eichenger, private communication on results from unpublished work in 1992 at Los Alamos National Laboratory is support of a DEA remote sensing for a drug interdiction demonstration.

132 S. Babichenko, 2008, “Laser remote sensing of the European marine environment: LIF technology and applications,” Remote Sensing of the European Seas, 189, Springer.

133 See https://spie.org/x43265.xml?ArticleID=x43265.

134 R. Karpicz, A. Dementjev, Z. Kuprionis, S. Pakalnis, R. Westphal, R. Reuter, and V. Gulbinas, 2006, “Oil spill fluorosensing lidar for inclined onshore or shipboard operation,” Appl. Opt. 45: 6620.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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FIGURE 2-48 Sample fluorescence spectra generated by the airborne FLS (in arbitrary units, a.u.). Curves (a) and (b) represent clean Baltic Sea water and a sample polluted by ship motor oil (single laser shot, flight altitude 200 m). Curves (c) and (d) represent clean Atlantic Ocean water and a sample polluted by fuel oil (five combined laser shots, flight altitude 500 m). The peak at 344 nm is from Raman scattering by water. SOURCE: https://spie.org/x43265.xml?ArticleID=x43265.

LIF from surfaces has been applied to the study of vegetation to determine the presence of chlorophyll fluorescence from plankton, via 532-nm excitation135 and, in another experiment, the physiological state (health) of trees in a forest through excitation of chlorophyll fluorescence with a 397-nm source and detection of fluorescence in the 450-800-nm region.136

Perhaps the most investigated LIF technology has been applied to the remote detection of biologically active aerosols, which could be deployed in biological warfare. As noted in the section “Aerosol Sensing,” while aerosol lidar can detect the potential existence of a biologically active aerosol release, confirmation that it is truly a threat is best done through the fusion of data from multiple sensors. Biologically active molecules have inherent fluorescence (autofluorescence) resulting from several components: the amino acids tryptophan, tyrosine and phenylalanine; the reduced nicotinamide adenine dinucleotides (NADH, NADPH); and the flavin compounds.137Figure 2-49 shows the fluorescence spectra for each of the components, when driven with lasers at the appropriate peak excitation wavelength.138 For local, or “point detectors” of biomolecules, one can employ multiple lasers to generate all of the fluorescing components and, by appropriate data analysis based on test aerosols, strive

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135 F.E. Hoge and R. N. Swift, 1981, “Airborne simultaneous spectroscopic detection of laser-induced water Raman backscatter and fluorescence from chlorophyll a and other naturally occurring pigments,” Appl. Opt. 20: 3197.

136 H. Edner, J. Johansson, S. Svanberg, and E. Wallinder, “Fluorescence lidar multicolor imaging of vegetation,” Appl. Opt. 33, 2471 (1994).

137 S.C. Hill, R.G. Pinnick, P. Nachman, G. Chen, R.K. Chang, M.W. Mayo, and G.L. Fernandez, “Aerosolfluorescence spectrum analyzer: real-time measurement of emission spectra of airborne biological particles,” Appl. Opt. 34, 7149 (1995).

138 T.H. Jeys, W.D. Herzog, J.D. Hybl, R.N. Czerwinski, and A. Sanchez, “Advanced Trigger Development,” Lincoln Laboratory Journal 17, 29 (2007).

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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FIGURE 2-49 The fluorescence cross sections for tryptophan, NADH, and riboflavin are broad functions of emission wavelength for excitation at 280 nm, 340 nm, and 450 nm, respectively. SOURCE: T. Jeys, W. Herzog, J. Hybl, R. Czerwinski, and A. Sanchez, 2007, “Advanced trigger development”, Lincoln Laboratory Journal 17(29).

to develop a sensor that can distinguish threats from benign biologically active or other species. Unfortunately, it has been found that there are wide variations in the intensity and spectra even for the same nominal bio-aerosol, and sensors may use other detection schemes in combination, such as LIBS, to provide a better detection probability.

For LIF lidar detection of bio-aerosols, tryptophan within the biological entities fluoresces and would seem to provide the best sensing. However, the excitation wavelength (<300 nm, and usually the fourth harmonic of Nd-doped lasers, around 260-266 nm) suffers significant atmospheric attenuation, especially under low-visibility conditions. In addition, as discussed above, use of the short wavelengths is likely to generate substantial interference signals from aromatic and other hydrocarbons that may be present in the atmosphere. NADH, while having weaker emission, can be excited by the Nd-doped laser third harmonic, which is better transmitted through the atmosphere and has been employed in several bio-aerosol LIF lidar systems. (While riboflavins might provide LIF, the signal from them does not appear to be significant in bio-aerosols, even if they have high fluorescence cross sections, and their emission spectra fall in a region where solar emission would produce major interference.)

Sandia National Laboratories (SNL) has developed the Ares lidar for bio-aerosol detection.139Figure 2-50 shows this system mounted in a van. The device employs a third-harmonic, pulsed, lamp-pumped Nd:YAG laser with a 10-ns pulsewidth. Schmitt et al. describes the specifics of

Backscattered and fluorescent light collected by an 18.75-cm-diameter Maksutov telescope is collimated and directed to a long-pass dichroic beamsplitter, which reflects light with wavelengths shorter than ~360 nm, effectively separating the elastically backscattered laser light from the LIF light. The elastically scattered light is focused onto a photomultiplier tube (PMT), where it is detected and subsequently digitized to provide a record of the aerosol backscatter intensity as a function of range with 1.5 m resolution. Laser-induced fluorescence collected by the telescope passes through the long-pass filter and is focused onto the entrance pinhole of an imaging spectrometer. The LIF spectrum is detected by a gated, intensified charge coupled device (ICCD).

Figure 2-51 shows a data product from the ICCD detection channel with bio-aerosol fluorescence detected, along with atmospheric Raman components.140

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139 See http://www.sandia.gov/pcnsc/research/research-briefs/2004/Ares_Ulatraviolet_Laser_Induced_Fluorescence_(UV_LIF)_Standoff_System_Development_and_Testing_by_R._L._Schmitt,_et_al..pdf.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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FIGURE 2-50 Photograph of the SNL-developed Ares aerosol and LIF lidar mounted in a van. SOURCE: http://www.sandia.gov/pcnsc/research/researchbriefs/2004/Ares_Ulatraviolet_Laser_Induced_Fluorescence_(UV_LIF)_Standoff_System_Development_and_Testing_by_R._L._Schmitt,_et_al..pdf.

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FIGURE 2-51 Bio-aerosol LIF signal detected by Ares LIF channel, with Raman signals from the atmosphere present as well. SOURCE: R. Schmitt and W. Seng, “UV Laser Induced Fluorescence Remote Sensing Technology and Applications,” presentation to the committee, May 10, 2013.

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140 R. Schmitt and W. Seng, “UV Laser Induced Fluorescence Remote Sensing Technology and Applications,” presentation to the committee, May 10, 2013.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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FIGURE 2-52 Photograph of Canadian-developed, bio-aerosol sensor (SR-Biospectra) designed for 100-m ranges. SOURCE: http://www.ino.ca/media/134174/srbiospectra.pdf.

The Defence Research and Development Canada (DRDC) in Valcartier has also been developing LIF bio-aerosol detection systems,141 including the Standoff Integrated Bio-aerosol Active Hyperspectral Detection (SINBAHD), a van-mounted system employing a 130-180 mJ/pulse, XeFexcimer laser at 351 nm with a 125-Hz pulse rate, 12-inch collecting optics, and a range-gated, intensified CCD covering 370-600 nm, with 5 nm spectral resolution. In addition, DRDC, working with INO in Quebec, has developed a short-range LIF lidar (SR-BioSpectra) designed to detect and classify bio-aerosol events in enclosed, semienclosed, and wide open spaces, over a 100-m range. A picture of that device appears in Figure 2-54, where the laser employed is a diode-pumped, frequency-tripled Nd:YAG laser.

Work on LIF-based bio-agent lidars in Europe is reported in Norway, the U.K., and Germany.142 A French (CILAS)-led, European-funded consortium, Biological Optical Defence Experiment (BODE), developed a prototype short-range, 280- and 355-nm-based LIF lidar for civil applications.143 A recent publication shows development, at least at the modeling stage, of a LIF-based lidar for biological warfare agent detection at the Beijing Institute of Technology in China,144 while a recent Asian consortium employed a 355-nm LIF system to detect the presence of fluorescence from Asian dust and air-pollution aerosols transported from urban and industrial areas, indicating the scientific interest in LIF.145

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141 See http://cbdstconf.sainc.com/pdfs/Wednesday_5_0950_Simard.pdf.

142 “Laser Based Stand-Off Detection of Biological Agents,” NATO RTO Technical Report TR-SET-098, http://ftp.rta.nato.int/public//PubFullText/RTO/TR/RTO-TR-SET-098///$$TR-SET-098-ALL.pdf.

143 O. Meyer, C. Jacquelard, J.M. Melkonian, P. Chardard, P. Lanson, and D. Petitgas, 2010, “Stand-off biological detection by LIF (laser induced fluorescence) lidar,” OPTRO 2010 – 4th International Symposium Optronics In Defence And Security, Paris, France, February 3-5.

144 P. Liu, Y. Zhang, S. Chen, T. Lan, Y. Wang, Z. Qiu, W. Kong, and G. Ni, 2009, “Simulation of ultraviolet laser-induced fluorescence LIDAR for detecting bio-aerosol,” SPIE Proceedings 7511, 2009 International Conference on Optical Instruments and Technology: Optoelectronic Measurement Technology and Systems, S. Ye, G. Zhang, and J. Ni, eds., 75111C.

145 N. Sugimoto, Z. Huang, T. Nishizawa, I. Matsui, and B. Tatarov, 2012, “Fluorescence from atmospheric aerosols observed with a multi-channel lidar spectrometer,” Opt. Exp. 20: 20800.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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In Chapter 3 the use of femtosecond-duration, high-energy lasers to create atmospheric filaments for use in creating LIF from bio-aerosol clouds is discussed.

One variant of LIF lidar is photodissociation (PD)-LIF, followed by laser-induced fluorescence, which is being investigated for detection of trace amounts of solid explosives on surfaces.146,147 The technique employs a UV laser, in the references cited operating at 236 nm, which dissociates the nitrogenrich explosive materials on surfaces, generating NO molecules. These are then excited by the same laser pulse and generate fluorescence at a shorter wavelength than the exciting laser, in the 224-227-nm region. The short-wavelength LIF allows discrimination from long-wavelength fluorescence from other species as well as from atmospheric molecules, and the technique claims sensitivities high enough to be of practical use in, say, tracking sources of explosives. The main drawback for sensing is the poor atmospheric transmission at the wavelengths used, which may limit the working range to around 10 m.

In summary, operational LIF systems have been deployed to detect the presence of oil and other hydrocarbon pollutants on the oceans, as well as detect biologically active aerosols that could be deployed in biological-agent warfare. LIF can provide long-range detection and/or identification of some entities not easily sensed by any other techniques.

Conclusion 2-13: Laser-induced fluorescence can be used for range-resolved detection of a limited number of entities that exhibit fluorescence, and it is an important tool for detection of biologically active aerosols as well as small quantities of certain solids and liquids on surfaces.

WIND SENSING

Wind sensing makes use of Rayleigh-scattered returns from molecules, which become more intense as the wavelength decreases. If one employs a single-frequency source and examines the returned signal with GHz-level spectral resolution, the lidar returns at short wavelengths will show (Figure 2-53) a very narrow spectral line from the aerosol returns and a much broader linewidth from molecular Rayleigh scattering. In a normal atmosphere the broad linewidth is due to a mix of dephasing of the Rayleigh scattered light due to collisions with other molecules, leading to a Lorenztian lineshape, and the Doppler shift from molecular thermal motion, leading to a Gaussian lineshape. The Doppler shift is large due to the small mass of the molecules and their resultant high velocities. While aerosol particles are also in motion—primarily Brownian motion created by collisions with atmospheric molecules—the velocities are much slower and so is the resultant Doppler shift, leading to a spectrally narrow scattered return. As noted above, it is possible to employ narrow-linewidth spectral filters to distinguish aerosol from molecular scattering given the large difference in spectral character. One approach employs placing an etalon or a gas with a narrow absorption line in front of the detector to eliminate the aerosol signal.

The spectra shown in Figure 2-53 assume that there is no wind present in the volume probed by the laser. When there is, the bulk Doppler shift for both aerosols and molecules leads to a displacement in the center frequency of the returned signal, proportional to the Doppler shift along the direction of the beam, which can be either positive or negative depending on the wind direction. Lidar systems that can sense this shift can generate data on wind, a unique function available from no other remote-sensing system. Clearly, to generate full wind data the sensing beam must be scanned in different directions, since data from a single beam direction cannot separate out wind speed and wind direction.

Two types of lidar wind sensors can be built, one based on scattering from aerosols, the other from molecular species, and both are discussed in the subsections below.

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146 C.M. Wynn, S. Palmacci, R.R. Kunz, K. Clow, and M. Rothschild, 2008, “Detection of condensed-phase explosives via laser-induced vaporization, photodissociation, and resonant excitation,” Appl. Opt. 47: 5767.

147 C.M. Wynn, S. Palmacci, R.R. Kunz, and M. Rothschild, 2010, “Noncontact detection of homemade explosive constituents via photodissociation followed by laser-induced fluorescence,” Opt. Exp. 18: 5399.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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image

FIGURE 2-53 Spectra of the atmospheric backscattered Rayleigh and aerosol signals along with three etalon transmission functions used for incoherent wind-sensing lidars. SOURCE: B. M. Gentry, H. Chen and S.X. Li, 2000, “Wind measurements with 355-nm molecular Doppler lidar,” Opt. Lett. 25: 1231.

Coherent Detection of Aerosol Scatter

In coherent wind-sensing (Doppler) lidar (Figure 2-54), narrow-spectral-bandwidth light is emitted from the transmitter laser and backscatters off aerosol particles in the atmosphere.

The relative speed of the particles with respect to the detector results in a Doppler shift, Δf, of the backscattered radiation compared to the transmitted radiation:

image

where Vrad is the radial component (along the line of site) of the velocity and λo is the transmitted wavelength. The backscattered radiation is mixed with a source (local oscillator) having a frequency near to that of the transmitted frequency. The mixing process leads to sum and difference frequencies, and for a Doppler lidar the difference (beat) frequency is detected and processed by RF electronics. As an example, for a 1,500-nm laser, the Doppler shift is 1.3 MHz for a 1 m/s velocity.

The majority of coherent wind sensors use pulsed lasers. A limit to the ability to resolve small velocities is the spectral width of the laser pulse. For a Gaussian-shape pulse the spectral width (full width at half maximum, FWHM) Δν = 0.44/Δτ, where Δτ is the FWHM pulsewidth, independent of the laser wavelength. Thus, a 10-ns-duration pulse has a spectral width of 44 MHz and, for a 1,500-nm source it would be a challenge to resolve 1-m/s velocities from the signal returns. Typically, one would want to use pulses of several hundred nanoseconds to achieve this type of velocity resolution. There is also a trade-off between the laser pulsewidthand the range resolution Δr of the lidar via 2 /cr Δτ=Δ, where c is the speed of light. For example, a 200-ns pulsewidth results in a 30-m range resolution. While a longer pulsewidth is indicated for more precise Doppler frequency measurement, increasing the pulsewidth proportionally decreases the range resolution. More sophisticated schemes can be used to

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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image

FIGURE 2-54 Schematic of coherent-detection wind-sensing (Doppler) lidar based on aerosol scattering.

accurately measure both position and velocity, such as using a series of laser pulses, called a pulse doublet or poly pulse measurement. They have a high time × bandwidth product, as will be discussed below.

To measure horizontal wind from a ground-based system, several strategies have been developed. In the velocity-azimuth Doppler (VAD) technique, the beam is directed at an angle from zenith and scanned 360 degrees in a cone. Returns are collected and the velocity components {u, v, w} are calculated. Here, {u, v, w} are the streamwise, cross-stream, and vertical components of the velocity. Another scan strategy is the range-height, or RHI, technique in which the beam is aligned with the average u vector and scanned in a small range of angles to probe the related range of heights downstream.

As is evident from the numbers, wind measurements require very-frequency-stable, narrow-linewidth laser sources. For a 1,500-nm laser, the center frequency is 2 × 1014 Hz, and a source with a 1-MHz linewidth must have a purity of 5 × 10-9. Even if one locks the source laser to the local oscillator, the time delay in the returned signal places constraints on how much absolute drift of the operating frequency is allowable.

Historically, CO2 gas lasers were first used to demonstrate coherent wind-sensing lidar systems, due to the relative ease of obtaining, for use in the local oscillator, stable, single-frequency operation from low-pressure, CW-discharge CO2 lasers, given the narrow gain linewidth for the CO2 vibrational-rotational transitions and the relatively minor perturbation of the laser resonator by the gas medium. The main challenge was to make stable optical cavities, which was accomplished through the use of good engineering design and the use of low-thermal-expansion materials such as Invar to set the cavity length. Unfortunately, as noted above, the aerosol backscatter level around the 10.6-µm wavelength of CO2 lasers is low and varies widely with atmospheric conditions, requiring multi-Joule-level pulsed transmitters based on high-pressure, pulsed CO2 lasers for even modest ranges.

Early solid state lasers were not good candidates for coherent lidars, owing to the considerably higher frequency instability from lamp-pumping-induced fluctuations in the optical path length of the laser medium compared to CO2 lasers. With the development of diode-laser pump sources, it became

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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possible to make much more frequency-stable, single-frequency, low-power lasers used for the local-oscillator function, and through development of injection-seeding techniques based on the low-power lasers it became possible to make high-energy sources for use as the transmitter. Although the first solid state systems employed Nd:YAG lasers at 1,064 nm, concern over eye-safety has led to the recent development of sources based on Er-doped solid state lasers around 1,600 nm, and Tm and Ho-doped lasers for use around 2,000 nm. While fiber-format solid state lasers cannot yet generate the high-energy pulses needed for very long-range sensors, they have been employed for shorter-range systems and leverage the development of fiber components used for the telecom industry.

One of the most sought-after coherent wind sensors has been a space-based system that would provide global coverage and mapping of all atmospheric winds. Such a system would be of major value to science as well as of significant practical use for weather forecasting and commercial aviation routing, which could take advantage of favorable winds at different altitudes to speed up flights and save on fuel consumption. For military applications, such a system would provide greatly improved theater weather forecasts. It would also help with the determination of improved mission flying routes and support precision reconnaissance instruments and precise missile launch and reentry calculations, as well as improved ballistic wind estimates for conventional munitions.148

NASA began developing plans for space-based systems that would have required 20-J/pulse CO2 lasers but in the 1990s moved to development of the 2,000-nm region with Ho-doped solid state lasers and now has system designs that can function from space with 250 mJ of energy.149 At present, NASA is testing a diode-pumped, 10-Hz, 250-mJ, Tm, Ho:LuLiF4 solid-state-laser-based system, the Doppler Aerosol Wind Lidar (DAWN) instrument, on aircraft platforms and has generated data over a 10-km vertical range from a DC-8, with a modest 15-cm-aperture receiver. Development of laser technology for space-based missions is in the planning stage. In the next section molecular-scattering-based wind sensing is discussed. This is also being developed for space-based sensors, generating data from atmospheric regions where aerosol densities are low.

Ground-based coherent wind sensors have been developed for a number of applications. Following several crashes of airplanes on takeoff or landing approaches, there was interest in fielding sensors to detect wind-shears around airports, particularly so-called “dry” wind shear not associated with rainstorms and thus undetectable by conventional radar systems. Other aviation applications have included detection of wake vortices generated by large aircraft on takeoff, a hazard to smaller aircraft in particular. Interest has lessened somewhat with improvements in air-traffic control procedures, but, in general, having a wind sensor as an adjunct to other meteorological sensors is a useful asset at an airport. (Attempts to put wind sensors directly on aircraft have met with resistance due to cost considerations, not the least of which is finding a location for the sensor.)

An example of a ground-based wind sensor for airport applications is the WindTracer system developed by Lockheed Martin Coherent Technologies. The device employs a pulsed, milliJoule-level solid state laser, which has evolved from a Tm-doped, 2,000-nm region laser to the present Er:YAG-based source at 1,617 nm. Specifications for the WindTracer product appear in Table 2-4, and a photograph of the shed-size unit is presented in Figure 2-55. Notable is the operating range of unit, which extends to as much as 33 km. According to the vendor, since the initial deployment in 2002, WindTracer successfully operates at Hong Kong, Tokyo, Osaka, London, New York, San Francisco, and Las Vegas airports, and is planned for installation at airports in Bangkok and Sicily. In addition, systems are in use at the Charles de Gaulle International Airport near Paris and at the Frankfurt International Airport, primarily for wake-vortex detection and ultimately determination of safe timing for departing aircraft. The goal is to attempt to speed up departure rates. A sample of the data product from the WindTracer system appears in Figure 2-56.

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148 S.B. Alejandro, “Application of LIDAR Against Soft Targets,” presentation to the committee, March 5, 2013.

149 U.N. Singh, “Emerging and enabling lidar technologies and techniques for NASA’s space-based active optical remote sensing missions,” presentation to the committee, April 17, 2013.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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TABLE 2-4 Specifications for WindTracer Doppler Wind Sensor.

Measurement
Typical range 400 m to 18 km
Maximum range 33 km
Radial wind velocity range ±35 m/s
Minimum range resolution 100 m
Scanner  
Azimuth range 0 to 360 degrees
Elevation range -5 to 185 degrees
Resolution 0.001 degrees
Pointing accuracy ±0.1 degrees
Optical clear aperture 12 cm
Transceiver  
Laser wavelength 1,617 nm
Pulse energy 2.5 mJ ±0.5 mJ
Pulse duration 300 ns ±150 ns
Pulse repetition frequency 750 Hz
Beam diameter 9.6 cm (e-1 intensity width)
Shelter  
Environment All weather
Weight 2600 kg
Dimensions 197 × 244 × 329 (h) cm (clearance with lightning rods)
Power specification 200-240 VAC single phase, 50 or 60 Hz (specified at time of purchase) 50 A service required.

SOURCE: See © 2013 Lockheed Martin Corporation. All Rights Reserved.

image

FIGURE 2-55 WindTracer system. SOURCE: © 2013 Lockheed Martin Corporation. All Rights Reserved.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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image

FIGURE 2-56 Sample WindTracer color-coded radial wind velocity map at a 1-degree elevation tilt and out to a range of ~11 km. Overlaid on the radial velocities are wind barbs depicting the vector velocity field. SOURCE: S. Hannon, K. Barr, J. Novotny A. Oliver, J. Bass, and M. Anderson, “Large scale wind resource mapping using a state-of-the-art 3-D scanning lidar,” WINDPOWER 2008, Houston, June 1-4, 2008, Available at http://www.resamericas.com/media/918770/awea-2008_large-scale-wind-resource-mapping-using-a-state-of-the-art-3-D-scanning-lidar.pdf.

Mitsubishi Electric (Japan) claims manufacture of a similar-sized Doppler wind sensor, employing a 1,500-nm-region pulsed solid state laser150 and lists a 20-km horizontal range. Based on a recent publication, the laser final stage may use a state-of-the-art, Yb, Er:glass planar waveguide amplifier, which has been shown capable of generating an output power of 3.3 kW with a pulse width of 580 ns, 1.9 mJ of pulse energy, a repetition rate of 4 kHz, and a high beam quality, with an M2 factor of 1.3.151 Applications of the device are listed as follows:

• Meteorological measurements (3-D profile of atmospheric wind, turbulence).

• Air traffic safety (low-level wind shear around airports, sudden gusts, and aircraft wake turbulence).

• Air environment consultation (urban heating, monitoring of substances released in emissions from factories, application in wind power generation).

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150 See http://www.mitsubishielectric.com/bu/lidar/products/coherent/index.html.

151 S. Kameyama, T. Yanagisawa, T. Ando, T. Sakimura, H. Tanaka, M. Furuta, and Y. Hirano, 2013, “Development of wind sensing coherent Doppler lidar at Mitsubishi Electric Corporation from late 1990s to 2013,” CLRC 2013, 17th Coherent Laser Radar Conference, Barcelona, Spain, June 17-20. Conference papers are available at http://www.tsc.upc.edu/clrc/.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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image

FIGURE 2-57 (a) Calculations of operating range for Doppler wind-sensing lidars as a function of visibility, with 1,550-nm laser pulse energy as a parameter. (b) Leosphere system in field installation at wind-turbine farm.

SOURCE: (a) See http://esamultimedia.esa.int/docs/technologytransfer/Leosphere-Windcube.pdf. (b) See http://www.upwind.eu/media/576/D6.1.1.pdf.

The emergence of a significant number of wind-turbine power systems has led to the development of a number of commercial short-range wind sensors. The economics of first determining good locations for turbines and then developing sensors that provide look-ahead data for turbine control justifies expenditures for laser-based systems rather than simple anemometers. The devices tend to be based on 1,550-nm region, all-Er:fiber architectures, given that only 0.1- to 1-km ranges are needed, and systems are advertised from Mitsubishi Electric, HALO Photonics (U.K.), Leosphere (France), and ZephIR (U.K.).

Leosphere employs technologies initially developed at the Office National d’Etudes et RecherchesAérospatiales (Onera), the French national aerospace research center. Figure 2-57a shows data developed by Onera for a 1,550-nm based system with a 30-m range resolution, showing operating range under different atmospheric visibility conditions, with laser pulse energy as a parameter.152 The LeosphereWindcube product for wind-turbine power generation facilities runs with 10-µJ (20-kHz-rate, 200-ns-duration) pulse energies and provides condition-dependent ranges of 40-200 m.153 A photograph of the device appears in Figure 2-57b. The unit emits four separate beams at an angle to the vertical, spaced 90 degrees apart to develop wind-direction data.

The ZephIR wind sensor is unique since, in contrast to the sensors just discussed, it employs a CW, 1,550-nm laser provided by NKT Photonics (Denmark). The source is based on a distributed feedback (DFB) single-frequency Er:fiber laser, amplified to approximately a 1-W power level.154 According to the ZephIR manufacturer,155 the device measures wind speeds at five different heights at altitudes in the 10-200 m range. The beam is projected at 30 degrees to the vertical and rotated to form a cone. The beam is focused to different heights in that cone and measures 50 points of data every second at each height. Speed measurements fall in the <1 to 70 m/s range, with <0.5 percent speed accuracy

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152 See http://esamultimedia.esa.int/docs/technologytransfer/Leosphere-Windcube.pdf.

153 Ibid.

154 See http://esamultimedia.esa.int/docs/technologytransfer/Leosphere-Windcube.pdf.

155 “Fibre lasers stretch from cells to wind farms,” Optics and Lasers Europe, March 2009, 15, optics.org/ole.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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image

FIGURE 2-58 The ZephIR unit in the field. SOURCE: ZephIR 300 installed, image courtesy of ZephIRLidar.

variation and <0.5 degrees of direction accuracy variation. The manufacturer claims that “ZephIR has also now amassed more than 3 million hours of operation across 650+ deployments globally spanning a decade of commercial experience.”156 A photograph of the trash-can-sized unit appears in Figure 2-58.

Conclusion 2-14: Wind sensor system complexity has been an issue for long-range systems but has been addressed and solved for short-range sensors, based on a market pull.

Conclusion 2-15: Any country that develops a space-based wind sensor could use it for commercial and military advantages.

Incoherent Detection of Molecular Scatter

Figure 2-59 shows a schematic of an incoherent-detection, wind-sensing lidar system. In contrast to the coherent-detection system, the device employs narrow-line filters and multiple detector channels to determine the shift in the center wavelength of the scattered light. Figure 2-55 shows the positioning and response of three band-pass filters (Edge1, Edge2, and Locking Filters) employed in the so-called double-edge detection technique, which has evolved from the original edge-detection concepts for incoherent-detection wind sensing.157,158,159 According to Gentry, Chen, and Li,

The separation of the two edge filters is chosen to be 5.1 GHz, so the sensitivity of the broader molecular signal is equal to that of the narrower aerosol signal. The locking etalon peak is located 1.7 GHz from the Edge1 filter peak, so the crossover point of the two edge-filter bandpasses is aligned to the half-height point of the Locking Filter band-pass. Actively locking the laser and the etalon at this point on the Locking Filter ensures symmetry of the edge-filter channels for wind measurement.

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156 Fred Olson Ltd. Ten years at the top, measuring right to the top, with ZephIR Lidart. http://www.fredolsen.co.uk/news/ten-years-top-measuring-right-top-zephir-lidar. Accessed on March 14, 2014.

157 C.L. Korb, B.M. Gentry, and C.Y. Weng,” Edge technique: Theory and application to the lidar measurement of atmospheric wind,” Appl. Opt. 31 4202 (1992).

158 C.L. Korb, B.M. Gentry, S.X. Li, and C. Flesia,” Theory of the double-edge technique for Doppler lidar wind measurement,” Appl. Opt. 37 3097 (1998).

159 B.M. Gentry, H. Chen, and S.X. Li, “Wind measurements with 355-nm molecular Doppler lidar,” Opt. Lett. 25, 1231 (2000).

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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image

FIGURE 2-59 Schematic of incoherent-detection wind-sensing (Doppler) lidar based on molecular scattering.

image

FIGURE 2-60 Profiles of wind speed and direction measured by the molecular lidar system compared with data from collocated balloons on November 17, 1999. SOURCE: B. M. Gentry, H. Chen, and S. X. Li, 2000, “Wind measurements with 355-nm molecular Doppler lidar,” Opt. Lett. 25: 1231.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
×

Differential comparison of the signals from the two-edge filters allows determination of the Doppler shift from the backscattered signal. Although, in principal, one could apply the edge technique to sense aerosol Doppler shifts, current efforts center on the use of the scheme for molecular backscatter, since it provides data for regions of the atmosphere having low aerosol content and thus not suited for coherent sensing.

Figure 2-60 shows validation data160 taken by NASA from a ground-based, double-edge lidar that used a 45-degree elevation and four azimuthal angles for wind-direction determination. The system employed a single-frequency, tripled Nd:YAG laser that produced 15-ns pulses at a 10-Hz repetition rate, with 70 mJ of energy and a 80-MHz linewidth. The receiver telescope had a 45-cm aperture. Lidar data were compared with a conventional balloon-borne wind sensor and show excellent agreement in terms of both wind velocity and direction, with evident larger scatter at the highest altitudes.

Currently, NASA is fielding the TWiLiTE (Tropospheric Wind Lidar Technology Experiment) designed for airborne platforms, and flight tests have been carried out on DC-8, WB57, and ER-2 aircraft, with plans for long-duration tests on a Global Hawk UAV.161

The European Space Agency (ESA) has been funding development of the Atmospheric Dynamics Mission (ADM-Aeolus) effort, to employ an incoherent wind-sensing lidar (Aladin) to sense the lowermost 30 km of the atmosphere. The system was originally set for launch in October 2007 but experienced significant issues with performance of the 355-nm laser due to optics contamination and damage issues, and the launch date is now set for 2015.

An alternative approach to incoherent wind sensing is under NASA-funded development, the Optical Autocovariance Wind Lidar (OAWL),162 which uses a double-edge sensor for the molecularbackscatter channel but is able to sense the Doppler shift of the aerosol channel through the use of a high-spectral-resolution interferometer in another detection channel. The advantage of the technology is that one sensor, at one wavelength, would be able to carry out the entire wind-sensing mission from space. Ground-based testing validation of the technology has been carried out, and aircraft-based measurements are in the planning stages.

COMMERCIAL LASER/LADAR PRODUCTS

As discussed in this chapter, there is a wide variety of uses for ladars, ranging from mapping and imaging to velocimetry.163 While there is no rigid dividing line between scientific, military, and commercial applications, the first two domains are characterized by small numbers of high-performance devices, while the third is performance-constrained by marketplace affordability. Nonetheless, the compelling advantages of ladar in terms of distance and velocity measurement have created an active marketplace with a variety of products in two principal application areas: (1) speed detection for law enforcement and (2) distance sensing for positioning and obstacle avoidance.

Police use ladar because it precisely measures target distance and velocity within a small field of view. For example, the illuminator (laser) can be used to select a specific vehicle (even in traffic) from which reflections can be measured. The devices (one is illustrated in Figure 2-61) have measurement filters to overcome errors introduced by surface irregularities such as mirrors and may rely on features on license plates (e.g., retroreflective backing materials) intended to support their use. The police ladar

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160 B.M. Gentry, H. Chen, and S.X. Li, 2000, “Wind measurements with 355-nm molecular Doppler lidar,” Opt. Lett. 25: 1231.

161 See http://twilite.gsfc.nasa.gov/index.php.

162 C.J. Grund, S. Tucker, R. Pierce, M. Ostasziewski, K. Kanizay, D. Demara, and J. Howell, 2010, “Development and demonstration of an optical autocovariance direct detection wind lidar,” Paper B3P5, 2010 Earth Science Technology Forum (ESTF2010), June 22-24, Arlington, Virginia. http://esto.nasa.gov/conferences/estf2010/papers/Grund_Chris_ESTF2010.pdf.

163 See http://www.lidar-uk.com/usag EOf-lidar/ for a listing.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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devices are very affordable. Online auction sites have them for $1,500-2,000. Interestingly, the devices have stimulated active countermeasures by motorists, such as so-called “ladar jammers” which emit light in the same frequency range as the speed guns. The effectiveness of these countermeasures is unclear.

The marketplace for the second application area, distance sensing, also seems primarily oriented toward automotive applications. In this case, rather than law enforcement, the primary motivations are safety and navigation. The devices are characterized by multiple lasers and detectors organized in an array format to achieve a wide field of view; the array can be cyclically rotated to achieve 360 degree swaths of real-time information.

These devices can in principle be used to provide greater situational awareness or even driver assistance for a human driver. A more intriguing application with substantial commercial investment is underway that has a potentially much larger market than speed guns: driverless automobiles. In this application, the ladar serves as part of a package of sensory apparatus needed for the autonomous vehicle to avoid stationary obstacles as well as moving objects such as pedestrians and other vehicles.

The sensors used in this application (e.g., for the Stanley164 self-driving car, which won the 2005 DARPA Grand Challenge) include those from SICK, examples of which are shown in Figure 2-62 were also used atop an autonomous vehicle that completed the DARPA Urban Challenge in 2007.

image

FIGURE 2-61 UltraLyte LTI 20-20 speed enforcement LiDAR. SOURCE: http://www.lasertech.com/UltraLyte-Laser-Speed-Guns.aspx.

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164 S. Thrun, M. Montemerlo, H. Dahlkamp, D. Stavens, A. Aron, J. Diebel, P. Fong, J. Gale, M. Halpenny, G. Hoffmann, K. Lau, et al., 2006, “Winning the DARPA Grand Challenge,” Journal of Field Robotics.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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image

FIGURE 2-62 Ladars from SICK atop “Little Ben,” an experimental self-driving car developed for the 2007 DARPA Urban Challenge. Credited to Lehigh University.

The ladar type highlighted by the ovals in Figure 2-62 is intended for short-range (e.g., 20-50 m) outdoor operation. It uses a 905 nm light source, has a 270 degree field of view, fog correction, a 25-50 Hz scanning frequency, and an operating range (depending on model) of 20-50 m, with an error of about 1 cm. Retail price ranges from $4,000 to $6,000.

Significant indicators would include dramatic reductions in cost and/or performance. The thousand dollar range of commercial ladar products listed above does not cover integration into a system vehicle. The integration is a barrier to entry but one that many are striving to overcome commercially. The achievable performance could be improved by signal processing methodologies such as algorithmic advances or advances in signal processing hardware such as a higher degree of parallelism or integration. Any advances that are applicable in the commercial lidar domain are equally useful in more specialized systems, such as those for defense applications. Also, improvements in size, weight, and power are generally indicative of manufacturing capabilities that are applicable across technology applications, including defense.

Incentives for the commercial lidar technologies include (for the producer) general advantages that stem from active and profitable instrument manufacturing capabilities, including the ability to allocate some profits to further research and development. Disincentives for the commercial ladar technologies include trade-offs that come from commercialization, such as trading away performance to hit an attractive price point, or overoptimizing for a particular problem domain such as speed detection. Improvements in laser range finder technologies will enhance advanced manufacturing capabilities such as autonomous, flexible manufacturing and sensor-actuator models.

The key technology improvements will increase performance (including measurement accuracies, scan rates, and system reliability) while decreasing cost. The likely means of achieving desirable new positions in the cost/performance trade space is integration (and the associated miniaturization) of components rather than a particular component technology such as a better laser, detector, or processor. Experience with both applications of the technologies and the processes used to manufacture them will present opportunities for technology advances.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
×

There are significant foreign capabilities in commercial ladar. Vendors exist abroad, such as SICK in Germany and Hokuyo in Japan as well as in the United States (e.g., Velodyne).

Conclusion 2-16: Commercial applications for active EO sensing will become widespread, dramatically increasing the use of active EO sensing and significantly lowering the cost.

The performance and precision of active EO sensors are compelling, but there is currently a cost trade-off that may limit commercial penetration in very cost-sensitive settings. For example, ultrasonic sensors that aid in automobile parking exist and only cost a few dollars; an active EO sensor in this application could be considered overkill. However, the importance of the human lives involved in autonomous vehicles will put pressure on the sensor technologies for accuracy, and with sensor fusion, diversity and redundancy. The “overkill” of an active EO sensor could in fact be a desirable attribute if the sensor is multifunctional and can be made cheaply.

Conclusion 2-17: Robotics—for example, autonomously navigated vehicles—is likely to be a dominant commercial and military application for active EO sensing.

Conclusion 2-18: Active EO sensing is poised to significantly alter the balance in commercial, military, and intelligence operations, as radar has done over the past seven decades.

Conclusion 2-19: Most active EO sensors used for military and security application will fall into one of two categories: (1) adaptation of inexpensive commercial devices to military application or (2) development of unique ladar systems for the military. The first category is accessible to nonstate actors and resource-poor countries. The second category is only accessible to countries with significant resources.

Conclusion 2-20: Development programs for high-end active EO sensors will involve observable activities or indicators of the direction these programs are taking.

Conclusion 2-21: Widespread, commercially available active EO technology offers a low barrier to entry for asymmetric adversaries with limited resources as well as to those seeking to adapt the technology for large-volume applications.

Recommendation 2-1: Analysts assessing states of technological development and projected timelines for developments of military significance should pay close attention to activity in the commercial sector.

Suggested Citation:"2 Active Electro-Optical Sensing Approaches." National Research Council. 2014. Laser Radar: Progress and Opportunities in Active Electro-Optical Sensing. Washington, DC: The National Academies Press. doi: 10.17226/18733.
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In today's world, the range of technologies with the potential to threaten the security of U.S. military forces is extremely broad. These include developments in explosive materials, sensors, control systems, robotics, satellite systems, and computing power, to name just a few. Such technologies have not only enhanced the capabilities of U.S. military forces, but also offer enhanced offensive capabilities to potential adversaries - either directly through the development of more sophisticated weapons, or more indirectly through opportunities for interrupting the function of defensive U.S. military systems. Passive and active electro-optical (EO) sensing technologies are prime examples.

Laser Radar considers the potential of active EO technologies to create surprise; i.e., systems that use a source of visible or infrared light to interrogate a target in combination with sensitive detectors and processors to analyze the returned light. The addition of an interrogating light source to the system adds rich new phenomenologies that enable new capabilities to be explored. This report evaluates the fundamental, physical limits to active EO sensor technologies with potential military utility; identifies key technologies that may help overcome the impediments within a 5-10 year timeframe; considers the pros and cons of implementing each existing or emerging technology; and evaluates the potential uses of active EO sensing technologies, including 3D mapping and multi-discriminate laser radar technologies.

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