Laser Radar Progress and Opportunities in Active Electro-Optical Sensing (2014) / Chapter Skim
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3 Emerging Electro-Optical Technologies
3 Emerging Electro-Optical Technologies
Pages 107-153
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From page 107... ...
The focus is on several coherent systems such as temporal and spatial heterodyning, synthetic aperture ladar, multiple-input, multiple-output (MIMO) imaging, and speckle imaging.
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From page 108... ...
108 LASER RADAR FIGURE 3-1 Reflectivity versus wavelength and angle for six different materials.
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From page 109... ...
TEMPORAL HETERODYNE DETECTION: STRONG LOCAL OSCILLATOR As described in Chapter 2, laser radar systems using direct detection (as in 3-D flash imaging) can be limited by the noise in the detector.
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From page 110... ...
TEMPORAL HETERODYNE DETECTION: WEAK LOCAL OSCILLATOR Traditional heterodyne detection with a strong LO has an added challenge if arrays of detectors are desired. While a high-resistance receiver, such as a focal plane array with relatively low bandwidth, can operate with low LO power, other standard high-bandwidth IR detectors used in heterodyne detection systems, such as a linear GHz-bandwidth photodiode, can require as much as 1 mW of LO power or more to reach the shot noise limit.
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From page 111... ...
With large enough macropixels, beat frequencies can be sampled up to the detector bandwidth rather than being limited by the detector array frame rate. In experiments, the return signal for each pulse is coherently integrated -- that is, a fast Fourier transform of the photon arrival times for each pulse is performed -- then incoherently averaged over LO and transmit signals are offset in frequency by π ππππ ππ‘ , the beat signal is located at π ππππ ππ‘ .
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From page 112... ...
Unlike direct detection receivers, the dominant noise source in heterodyne or coherent receivers is the shot noise generated by the local oscillator beam. For a matched filter receiver, that effective noise is equal to one detected photon per resolution element (in both time and space)
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From page 113... ...
Conclusion 3-2: Heterodyne detection can be used with a weak local oscillator if detectors are already sensitive enough so that a strong local oscillator is not required as a method of increasing the receiver sensitivity. SYNTHETIC-APERTURE LADAR According to Voxtel "Conventional optical imagers, including imaging.
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From page 114... ...
Marechal, and T Wright, 2005 "Syntheticaperture imaging laser radar: Laboratory demonstration and signal processing," Appl.
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From page 115... ...
Marechal, and T.J. Wright, 2005, "Synthetic aperture imaging laser radar: laboratory demonstration and signal processing," Appl.
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From page 116... ...
Ashby, 2011, "Synthetic aperture ladar flight demonstration," in CLEO:2011, Laser Applications to Photonic Applications, OSA Technical Digest (CD) (Optical Society of America, 2011)
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From page 117... ...
Ashby, 2011, "Synthetic aperture ladar flight demonstration," in CLEO:2011 - Laser Applications to Photonic Applications, OSA Technical Digest (CD) (Optical Society of America, 2011)
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From page 118... ...
Of course the SWaP advantages of modest aperture sizes come at the price of not collecting as many photons on receive from each image resolution element, making synthetic aperture ladar a rather power-hungry technique, placing a premium on laser and detector efficiency, and driving laser power requirements. This is a fundamental consideration that whenever resolution is increased, it means photons are scattered from a smaller resolution cell.
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From page 119... ...
The LO must remain phase coherent over the round trip of the range to target, unless a sample of the transmitted waveform is stored for use in beating against the return signal. One storage method to reduce coherence length requirements is to input a sample of the master oscillator into a long fiber delay and use the output of that fiber as an LO rather than creating the LO from the master oscillator as it exists when the signal is returned from the target.
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From page 120... ...
23 It is reasonable to expect that the advantages of SAL and related advanced coherent active imaging techniques will drive the research, development, and deployment of such systems in a variety of countries. This will drive development of high power coherent laser systems capable of achieving wide bandwidth waveforms, as well as long coherence length LOs.
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From page 121... ...
Although detectors record only intensity and do not directly preserve the phase profile of the electric field, digital holography provides a means to extract the spatial phase variation across an optical aperture using the spatial beat frequency between the signal and the LO. As previously discussed in the section on synthetic aperture ladar, having access to both the amplitude and phase of the optical field enables capabilities not readily possible with intensity-only imaging; with digital postprocessing, the exact electric field at any point can be calculated from spatial heterodyne phase extraction.
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From page 122... ...
In additional to digital holography, spatial heterodyne detection and lensless imaging, scientists and engineers in the field also refer to this technology as "holographic aperture ladar" 30 and "spatially processed image detection and ranging (SPIDAR) ." 31 In addition to performing single-wavelength digital holography, multiple-wavelength holography is a subset that allows for fine-resolution 3-D imaging.
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From page 123... ...
McManamon, and D Shemano, 2011, "Digital holography for coherent imaging for multi-aperture laser radar," Conference paper, Digital Holography and Three-Dimensional Imaging, Tokyo Japan, May 9-11, Optical Society of America, p.
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From page 124... ...
This technology requires coherent illumination with narrow linewidths in order to record the interference between the local oscillator and object return signal. Narrow linewidths and coherence length requirements are inherent issues for all types of coherent imaging; the section on synthetic aperture ladar previously discussed provides details regarding these issues.
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From page 125... ...
However, many serious technical issues such as the conformal focal planes technology itself, as well as analysis and implementation of local oscillator illumination of a curved surface, would need to be researched and proven before such designs could be considered. In the broad field of digital holography, researchers worldwide are actively involved in this field of research.
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From page 126... ...
55 S.M. Beck et al., 2005, "Synthetic-aperture imaging laser radar: Laboratory demonstration and signal processing," Appl.
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MIMO techniques as described here can be implemented using either temporal or spatial heterodyne (digital holography) techniques.
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From page 128... ...
Work in high-bandwidth detector arrays suitable for use in temporal heterodyne sensors would be another indicator. MIMO technology will allow imaging with high angular resolution using much lighter and more compact aperture arrays than a monolithic aperture.
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From page 129... ...
Interference between the various contributions to the optical field produces a speckle pattern of bright and dark intensity regions in the receiver. In many optical applications, speckle is considered a nuisance -- it degrades target images obtained when conventional microwave-radar imaging techniques are applied to laser radar.
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From page 130... ...
61 The addition of a local oscillator phase and frequency modulation using an optical phase-locked loop to the ESPI system (OPL-ESPI) allows for the generation of Doppler speckle contours of a vibrating surface even from unstable sensor platforms.
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From page 131... ...
This can be done statistically (called wavelength decorrelation) to obtain the laser radar cross section of an object, which has been demonstrated with submillimeter range resolution in the laboratory.
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From page 132... ...
71 "The reflective reference point near the scattering object causes bright voxels to appear in certain regions of the 3-D array that represent the location in space of scattering cells on the surface of the object" this is similar to digital holography, with a glint near the object acting as the local oscillator. 72 The 3-D image is "formed by recording the location of these bright voxels." 73 The speckle frames are stacked to form a 3-D array.
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From page 133... ...
The larger the range extent of the target, the smaller the frequency step must be. Laser requirements are an important consideration for all speckle imaging techniques.
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From page 134... ...
Since the 3-D imaging technique described above requires stepping through multiple wavelengths during the scan, rapidly tunable lasers or multiwavelength lasers will reduce the scan time necessary. Speckle imaging techniques such as LASCA and ESPI are widely described in the open literature and new developments in these areas would also likely be widely published.
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From page 135... ...
There are a number of potential applications of wavelengthdependent speckle in advanced manufacturing, machine vision, industrial inspection, robotics, and dimensional metrology. These methods can also be used to obtain laser radar cross sections, surface slope maps, or even 2-D and 3-D images of targets.
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From page 136... ...
. The heterodyne detection provides shot noise limited performance so that even weak return signals can be detected and the information in the carrier phase is retained.
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Newbury, 2009, "Rapid and precise absolute distance measurements at long range," Nature Photonics 3(6)
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From page 138... ...
As is discussed in more detail below, if the single-pulse peak intensity from a femtosecond laser exceeds a certain level in the atmosphere, self-focusing due to the nonlinearity of the molecules in the atmosphere leads to the formation of a "filament" of light, where the beam size remains constant over a wider range of distance than allowed by diffraction. Laser-induced breakdown spectroscopy (LIBS)
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From page 139... ...
Zhiwen Liu, and C Philbrick, "Long-path supercontinuum absorption spectroscopy for measurement of atmospheric constituents," 2008, Optics Express 16 (12)
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From page 140... ...
94 Another approach to measurements with SC sources has employed a Fourier-transform spectrometer to analyze the return signals with a processing time of several seconds. 95 Even faster data processing would be possible through the combination of a grating or other dispersive element along with a multielement detector, although the ultimate spectral resolution would still be less than possible with Fourier-transform techniques.
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101 S.A. Diddams, 2010, "The evolving optical frequency comb [Invited]
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From page 142... ...
A key concept behind filament formation is the self-focusing effect 103 for laser beams, where propagation in a medium with a positive value of nonlinear refractive index leads to a collapse of the beam diameter from the intensity-created positive lens in the medium. Analysis of the effect shows that the threshold for it to occur is a function of the peak power in the beam, not the intensity.
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From page 143... ...
Wille, and L WΓΆste, 2000, "Infrared extension of the supercontinuum generated by femtosecond terawatt laser pulses propagating in the atmosphere," Opt.
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From page 144... ...
Wille, and L WΓΆste, 2000, "Infrared extension of the supercontinuum generated by femtosecond terawatt laser pulses propagating in the atmosphere," Opt.
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Used with permission. FIGURE 3-22 Nighttime photograph of SC light generated by a vertically directed beam from the Teramobile system.
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146 LASER RADAR FIGURE 3-23 "(A) Schematic of the Teramobile lidar experimental setup.
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From page 147... ...
has provided significant improvement in the measurement accuracy of path-averaged DIAL sensors. Continuum sources that are the result of coherent generation processes -- and are thus precision frequency combs -- have provided greatly enhanced gas detection sensitivities over other active sensors through the dual-comb technique, and have allowed broad spectral scans to be taken in under 100 microseconds.
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From page 148... ...
125 Even in the very low light limit, "photon-counting," shot noise limit, etc., the physics is that of a classical electromagnetic field interacting with matter. In this section, however, potential new sensing modalities that can be realized by exploiting the truly quantum nature of the optical field are considered.
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From page 149... ...
. Squeezed light was first demonstrated in the late 1980s and has been used to demonstrate phase measurement below the shot noise limit, 129 absorption spectroscopy below the vacuum state limit, 130 and a variety of other measurements beyond the standard quantum limit.
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From page 150... ...
Kaushik, "Squeezing the local oscillator does not improve signal-to-noise ratio in heterodyne laser radar," 2007, Optics Lett.
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From page 151... ...
(Notionally, this would mean far field spatial resolution of Ξ»R/ND, where D represents either the transmitter diameter in a flying spot lidar or the receive aperture in a focal plane lidar.) Another application of entangled photon states is measurement below the shot noise limit, reaching the Heisenberg limit.
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From page 152... ...
Thus, neither beam produces a target image by itself -- the beam interacting with the target provides no spatial resolution, and the beam falling on the detector array has not interacted with the target. The correlations between the two beams can be either classical or quantum in nature, and ghost imaging can use either direct or phase-sensitive coherent detection.
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From page 153... ...
Conclusion 3-15: High-level, active EO emerging technologies will most likely be pursued through funding at university, government, or industry laboratories, with indicators given by publications and presentations. Conclusion 3-16: Coherent active EO systems will continue to develop for applications that require access to the optical field (not just intensity)
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