At MIT Lincoln Laboratory, avalanche photodiodes (APDs) have been developed for both 2-μm and 3.4-μm detection using
the antimonide material system. These bulk, lattice-matched detectors operate in Geiger mode at temperatures up to 160 K.
The 2-μm APDs use a separate-absorber-multiplier design with an InGaAsSb absorber and electron-initiated avalanching
in the multiplier. These APDs have exhibited normalized avalanche probability (product of avalanche probability and
photo-carrier-injection probability) of 0.4 and dark count rates of ~150 kHz at 77 K for a 30-μm-diameter device. A 1000-
element imaging array of the 2-μm detectors has been demonstrated, which operate in a 5 kg dewar with an integrated
Stirling-cycle cooler. The APD array is interfaced with a CMOS readout circuit, which provides photon time-of-arrival
information for each pixel, allowing the focal plane array to be used in a photon-counting laser radar system. The 3.4-μm
APDs use an InAsSb absorber and hole-initiated avalanching and have shown dark count rates of ~500 kHz at 77 K but
normalized avalanche probability of < 1%. Research is ongoing to determine the cause of the low avalanche probability
and improve the device performance.
MIT Lincoln Laboratory has developed a concept that could enable remote (10s of meters) detection of trace
explosives' residues via a field-portable laser system. The technique relies upon laser-induced photodissociation of
nitro-bearing explosives into vibrationally excited nitric oxide (NO) fragments. Subsequent optical probing of the first
vibrationally excited state at 236 nm yields narrowband fluorescence at the shorter wavelength of 226 nm. With proper
optical filtering, these photons provide a highly sensitive explosives signature that is not susceptible to interference from
traditional optical clutter sources (e.g., red-shifted fluorescence). Quantitative measurements of trace residues of TNT
have been performed demonstrating this technique using a breadboard system, which relies upon a pulsed optical
parametric oscillator (OPO) based laser. Based on these results, performance projections for a fieldable system are made.
Mohan Vaidyanathan, Steven Blask, Thomas Higgins, William Clifton, Daniel Davidsohn, Ryan Carson, Van Reynolds, Joanne Pfannenstiel, Richard Cannata, Richard Marino, John Drover, Robert Hatch, David Schue, Robert Freehart, Greg Rowe, James Mooney, Carl Hart, Byron Stanley, Joseph McLaughlin, Eui-In Lee, Jack Berenholtz, Brian Aull, John Zayhowski, Alex Vasile, Prem Ramaswami, Kevin Ingersoll, Thomas Amoruso, Imran Khan, William Davis, Richard Heinrichs
KEYWORDS: Sensors, LIDAR, 3D image processing, 3D acquisition, Target detection, Imaging systems, Image processing, Control systems, Image sensors, Data processing
Jigsaw three-dimensional (3D) imaging laser radar is a compact, light-weight system for imaging
highly obscured targets through dense foliage semi-autonomously from an unmanned aircraft. The
Jigsaw system uses a gimbaled sensor operating in a spot light mode to laser illuminate a cued
target, and autonomously capture and produce the 3D image of hidden targets under trees at high 3D
voxel resolution. With our MIT Lincoln Laboratory team members, the sensor system has been
integrated into a geo-referenced 12-inch gimbal, and used in airborne data collections from a UH-1
manned helicopter, which served as a surrogate platform for the purpose of data collection and
system validation. In this paper, we discuss the results from the ground integration and testing of the
system, and the results from UH-1 flight data collections. We also discuss the performance results
of the system obtained using ladar calibration targets.
Coherent solid-state optical sources based on Nd:YAG/Cr4+:YAG passively Q-switched microchip lasers cover the spectral range from 5000 to 200 nm, producing multikilohertz pulse trains with pulse durations as short as 100 ps and peak powers up to 1 MW. The wavelength diversity is achieved through harmonic conversion, parametric conversion, Raman conversion, and microchip-laser-pumped miniature gain-switched lasers. In all cases, the optical heads have been packaged in a volume of less than 0.5 liters. These compact, robust devices have the proven capability to take what were complicated laser-based experiments out of the laboratory and into the field, enabling applications in diverse areas. The short pulses are useful for high precision ranging using time-of-flight techniques, with applications in 3-dimensional imaging, target identification, and robotics. The short pulse durations and ideal mode properties are also useful for material characterization. The high peak powers can be focused to photoablate material, with applications in laser-induced breakdown spectroscopy and micromachining. Ultraviolet systems have been used to perform fluorescence spectroscopy for applications including environmental monitoring and the detection of biological aerosols. Systems based on passively Q-switched microchip lasers, like the lasers themselves, are small, robust, and potentially low cost, making them ideally suited for field applications.
Brian Aull, Andrew Loomis, Douglas Young, Alvin Stern, Bradley Felton, Peter Daniels, Debbie Landers, Larry Retherford, Dennis Rathman, Richard Heinrichs, Richard Marino, Daniel Fouche, Marius Albota, Robert Hatch, Gregory Rowe, David Kocher, James Mooney, Michael O'Brien, Brian Player, Berton Willard, Zong-Long Liau, John Zayhowski
Lincoln Laboratory has developed 32 x 32-pixel ladar focal planes comprising silicon geiger-mode avalanche photodiodes and high-speed all-digital CMOS timing circuitry in each pixel. In Geiger mode operation, the APD can detect as little as a single photon, producing a digital CMOS-compatible voltage pulse. This pulse is used to stop a high-speed counter in the pixel circuit, thus digitizing the time of arrival of the optical pulse. This "photon-to-digital conversion" simultaneously achieves single-photon sensitivity and 0.5-ns timing. We discuss the development of these focal planes and present imagery from ladar systems that use them.
MIT Lincoln Laboratory is actively developing laser and detector technologies that make it possible to build a 3D laser radar with several attractive features, including capture of an entire 3D image on a single laser pulse, tens of thousands of pixels, few-centimeter range resolution, and small size, weight, and power requirements. The laser technology is base don diode-pumped solid-state microchip lasers that are passively Q-switched. The detector technology is based on Lincoln-built arrays of avalanche photodiodes operating in the Geiger mode, with integrated timing circuitry for each pixel. The advantage of these technologies is that they offer the potential for small, compact, rugged, high-performance systems which are critical for many applications.
We have recently conducted a series of laboratory and field test to demonstrate the utility of combining active illumination with hyperspectral imaging for the detection of concealed targets in natural terrain. The active illuminator, developed at MIT Lincoln Laboratory, is a novel microlaser-pumped fiber Raman source that provides high- brightness, subnanosecond-pulse-length output spanning the visible through near-IR spectral range. The hyperspectral- imaging system is comprised of a compact, grating-based spectrometer that uses a gateable, intensified CCD array as the detector element. The illuminator and hyperspectral imaging system are mounted on a small platform that is itself mounted on a tripod and scanned in azimuth to build an image scene of up to several hundred spectral bands. The system has been deployed under a variety of environmental conditions, including night-time illumination, and on a variety of target scenes, including exposed and concealed plastic and metallic mine-like targets. Targets have been detected and identified on the basis of spectral reflectance, fluorescence signatures, degree of polarization, and range-to-target information. The combination of laser-like broadband illumination and hyperspectral imaging offers great promise in concealed or obscured target detection. On-going developments include the incorporation of broadband illuminators in the 1 to 2 micrometers and 3 to 5 micrometers spectral bands, with corresponding increases in spectral coverage of the imaging and detection systems.
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