Laser Radar Progress and Opportunities in Active Electro-Optical Sensing (2014) / Chapter Skim
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2 Active Electro-Optical Sensing Approaches
2 Active Electro-Optical Sensing Approaches
Pages 25-106
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From page 25... ...
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.
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From page 26... ...
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)
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A series of signals along the range resolution coordinate produces a reflective projection of the object that can be used to reconstruct the object. 8 Figure 2-1 shows one such 1-D image.
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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)
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"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.
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From page 30... ...
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. FIGURE 2-3 A gated active EO imaging system can eliminate clutter and obscurants that might otherwise degrade the image.
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From page 31... ...
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.
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From page 32... ...
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.
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The scanner repositions the interrogation point and the process is repeated. FIGURE 2-6 Block diagram of a typical bistatic, scanned 3-D imaging laser radar.
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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.
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From page 35... ...
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.
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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)
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. 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)
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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.
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SOURCE: MIT Lincoln Laboratory. some scenarios, operation at lower altitudes is acceptable, but high altitude operation is desirable where survivability is a concern.
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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.
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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 213, but uses linear-mode APD arrays instead of Geiger-mode APDs.
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43 P McManamon, 2012, "Review of ladar: A historic, yet emerging, sensor technology with rich phenomenology," Optical Engineering 51(6)
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. 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)
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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)
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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. FIGURE 2-16 Flash ladar image showing penetration through dust.
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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.
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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)
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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.
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From page 49... ...
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.
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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.
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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.
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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.
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From page 53... ...
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 73 See http://www.deepocean.net/deepocean/index.php?
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From page 54... ...
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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Fu, 2009, "Analysis of characteristics of blue-green laser propagation through ocean water," Proc.
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79 The SHOALS technology is marketed today by Optech. 80 It uses green laser pulses (second harmonic of the Nd:YAG laser)
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From page 57... ...
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.
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From page 58... ...
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.
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From page 59... ...
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.
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From page 60... ...
. Because most laser vibration sensing technology is based on coherent detection, it can be made to be shotnoise limited in the optical detection by suitable choice of local oscillator power.
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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.
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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.
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From page 63... ...
, 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.
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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.
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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)
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From page 66... ...
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.
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The Klett inversion method was subsequently modified by Fernald 100 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)
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From page 68... ...
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.
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From page 69... ...
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 ElectroOptics for the U.S.
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70 LASER RADAR FIGURE 2-33 Drawing of operational concept for LR-BSDS system. SOURCE: Courtesy of Q-Peak, Inc.
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From page 71... ...
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)
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From page 72... ...
Past work on multiple-wavelength aerosol lidar has included wavelengths in the 1-10µm 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 CO 2 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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ACTIVE ELECTRO-OPT EL TICAL SENSIN APPROACH NG HES 73 FIGUR 2-36 A ver RE rtical profile prrovided by the CALIOP syste reveals a th plume of a em hick aerosol haze from fires over a thin f nner layer of cllouds. This ima was captur over the so age red outheast Atlanti Ocean, off ic southe Africa, duri the time pe ern ing eriod shown.
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From page 74... ...
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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From page 75... ...
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 2.
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Figure 2-38 shows a schematic of one TDLAS system, designed to measure atmospheric absorption due to weak electronic transitions of O 2 in the 895-nm region. This is the region where the diode-pumped cesium alkali directed energy laser (DPAL)
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paths. The broad lines, with level identifications, are from O 2 atmospheric absorption.
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From page 78... ...
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.
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From page 79... ...
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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Initial missions were flown based on an ER-2 platform, at altitudes of 1621 km. The data product from one mission appears in Figure 2-41, showing the complex nature of water vapor distributions in the atmosphere.
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Lahmann, and C Weitkamp, 1985, "Range-resolved differential absorption lidar: Optimization of range and sensitivity," Appl.
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From page 82... ...
Sunesson and S Svanberg, 1989, "Atmospheric atomic mercury monitoring using differential absorption lidar techniques," Appl.
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From page 83... ...
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.
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From page 84... ...
Conclusio 2-9: The range of diffe on r erential abso orption lidar systems is re educed at lon nger wavelen ngths verely limited out in the 3-5 and 8-12 μm atmosph and is sev d 3 heric window regions, tho w ough improvem ments in sourrces and deteectors are countering this s. Conclusio 2-10: Ran on nge-resolved differential absorption li a idar is an important tool for scientific c studies an will find further deplo nd f oyment on ai irborne and sspace-based platforms.
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From page 85... ...
Simple diatomic molecules like N 2 and O 2 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.
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From page 86... ...
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 N 2 , O 2 , and water vapor as a function of altitude.
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From page 87... ...
, 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.
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From page 88... ...
• 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.
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From page 89... ...
• 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.
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From page 90... ...
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.
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From page 91... ...
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 bioaerosol LIF lidar systems.
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From page 92... ...
Seng, "UV Laser Induced Fluorescence Remote Sensing Technology and Applications," presentation to the committee, May 10, 2013.
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From page 93... ...
, nted mploying a 1 30-180 mJ/pu ulse, XeFexci imer laser at 3 351 nm with a 125-Hz puls rate, 12-inc collecting optics, and a r se ch o range-gated, intensified CCD covering 370600 nm, with 5 nm spe w ectral resolutio In additio DRDC, wo on. on, orking with IN in Quebe has develo NO ec, oped a short-rang LIF lidar (S ge SR-BioSpectr designed to detect and classify bio-a ra)
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From page 94... ...
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.
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From page 95... ...
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 ∆r of the lidar via ∆r = ∆τ c / 2 , where c is the speed of light.
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From page 96... ...
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, CO 2 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 CO 2 lasers, given the narrow gain linewidth for the CO 2 vibrationalrotational transitions and the relatively minor perturbation of the laser resonator by the gas medium.
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From page 97... ...
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.
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From page 98... ...
98 LASER RADAR 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)
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From page 99... ...
Mitsubishi Electric (Japan) claims manufacture of a similar-sized Doppler wind sensor, employing a 1,500-nm-region pulsed solid state laser 150 and lists a 20-km horizontal range.
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From page 100... ...
, 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.
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From page 101... ...
Conclusio 2-14: Win sensor sys on nd stem complex has been an issue for long-range systems but has xity n r been addressed and solved for sho nsors, based on a market pull. ort-range sen Conclusio 2-15: Any country tha develops a space-based wind sensor could use it for commer on y at d r t rcial and milit tary advantag ges.
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From page 102... ...
X Li, 2000, "Wind measurements with 355-nm molecular Doppler lidar," Opt.
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From page 103... ...
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.
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104 LASER RADAR devices are very affordable. Online auction sites have them for $1,500-2,000.
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From page 105... ...
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.
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From page 106... ...
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.
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