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An essential milestone in the development of lidar for biological aerosol detection is accurate characterization of agent, simulant, and interferent scattering signatures. MIT Lincoln Laboratory has developed the Standoff Aerosol Active Signature Testbed (SAAST) to further this task, with particular emphasis on the near-and mid-wave infrared.Spectrally versatile and polarimetrically comprehensive, the SAAST can measure an aerosol sample's full Mueller Matrix across multiple elastic scattering angles for comparison to model predictions. A single tunable source covers the 1.35-5 m spectral range, and waveband-specific optics and photoreceivers can generate and analyze all six classic polarization states. The SAAST is highly automated for efficient and consistent measurements, and can accommodate a wide angular scatter range, including oblique angles for sample characterization and very near backscatter for lidar performance evaluation. This paper presents design details and selected results from recent measurements.

Standoff LIDAR detection of BW agents depends on accurate knowledge of the infrared and ultraviolet optical elastic scatter (ES) and ultraviolet fluorescence (UVF) signatures of bio-agents and interferents. MIT Lincoln Laboratory has developed the Standoff Aerosol Active Signature Testbed (SAAST) for measuring polarization-dependent ES cross sections from aerosol samples at all angles including 180° (direct backscatter) [1]. Measurements of interest include the dependence of the ES and UVF signatures on several spore production parameters including growth medium, sporulation protocol, washing protocol, fluidizing additives, and degree of aggregation. Using SAAST, we have made measurements of the polarization-dependent ES signature of Bacillus globigii (atropheaus, Bg) spores grown under different growth methods. We have also investigated one common interferent (Arizona Test Dust). Future samples will include pollen and diesel exhaust. This paper presents the details of the apparatus along with the results of recent measurements.

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Laser-based remote sensing is undergoing a remarkable advance due to novel technologies developed at MIT Lincoln Laboratory. We have conducted recent experiments that have demonstrated the utility of detecting and imaging low-density aerosol clouds. The Mobile Active Imaging LIDAR (MAIL) system uses a Lincoln Laboratory-developed microchip laser to transmit short pulses at 14-16 kHz Pulse Repetition Frequency (PRF), and a Lincoln Laboratory-developed 32x32 Geiger-mode Avalanche-Photodiode Detector (GmAPD) array for singlephoton counting and ranging. The microchip laser is a frequency-doubled passively Q-Switched Nd:YAG laser providing an average transmitted power of less than 64 milli-Watts. When the avalanche photo-diodes are operated in the Geiger-mode, they are reverse-biased above the breakdown voltage for a time that corresponds to the effective range-gate or range-window of interest. The time-of-flight, and therefore range, is determined from the measured laser transmit time and the digital time value from each pixel. The optical intensity of the received pulse is not measured because the GmAPD is saturated by the electron avalanche. Instead, the reflectivity of the scene, or relative density of aerosols in this case, is determined from the temporally and/or spatially analyzed detection statistics.There are several advantages to sensor architectures that use direct detection and arrays of photon-counting detectors. Perhaps the most significant advantage is a reduced requirement for power-aperture product of more than an order of magnitude. In this paper, we describe the LIDAR sensor system used in MAIL, and our experimental results showing system sensitivity, and temporal and spatially resolved releases of aerosol clouds within a controlled chamber.

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