Jonathan M. Richardson

Optical Techniques for Detecting and Identifying Biological-Warfare Agents

Rapid and accurate detection and identification of biological agents is an objective of various national security programs. Detection in general is difficult owing to natural clutter and anticipated low concentrations of subject material. Typical detection architectures comprise a nonspecific trigger, a rapid identifier, and a confirming step, often in a laboratory. High-confidence identification must be made prior to taking action, though this must be traded against regrets stemming from delay. Sensing requirements are best established by positing plausible scenarios, two of which are suggested herein. Modern technologies include the use of elastic scatter and ultraviolet laser-induced fluorescence for triggering and standoff detection. Optical and nonoptical techniques are used routinely in analyzing clinical samples used to confirm infection and illness resulting from a biological attack. Today, environmental sensing serves at best as an alert to medical authorities for possible action, which would include sample collection and detailed analysis. This paper surveys the state of the art of sensing at all levels.

Polarimetric lidar signatures for remote detection of biological warfare agents

Polarimetric Lidar has been recently proposed as a method for remote detection of aerosolized biological warfare agents. Accurate characterization of the optical signatures for both biological agents and environmental interferents is a critical first step toward successful sensor deployment. MIT Lincoln Laboratory has developed the Standoff Aerosol Active Signature Testbed (SAAST) as a tool for characterizing aerosol elastic scattering cross sections.1 The spectral coverage of the SAAST includes both the nearinfrared (1-1.6 μm) and mid-infrared (3-4 μm) spectral regions. The SAAST source optics are capable of generating all six classic optical polarization states, while the polarization-sensitive receiver is able to reconstruct the full Stokes vector of the scattered wave. All scattering angles, including those near direct backscatter, can be investigated. The SAAST also includes an aerosol generation system capable of producing biological and inert samples with various size distributions. This paper discusses the underlying scattering phenomenology, SAAST design details, and presents some representative data.

The MIT Lincoln Laboratory optical systems test facility

The Optical Systems Test Facility was established at MIT Lincoln Laboratory to support a broad scope of program areas, encompassing tactical ground-based sensors through strategic space-based sensors. The Optical Systems Test Facility comprises several separate ranges developed as a coordinated set of test sites at MIT Lincoln Laboratory. There are currently four separate ranges in the facility, an active range (Laser Radar Test Facility), a passive range (Seeker Experimental System), an aerosol range (Standoff Aerosol Active Signature Testbed) and an optical material measurements range. The active range has optical and target facilities for evaluating elements of laser radar sensors as well as complete ladar systems. It has facilities for simulating long range wavefronts and for dynamic target motions. The passive range concentrates on evaluating passive infrared sensors, with capabilities for static and dynamic scene generation in both cryogenic and room temperature environments. The aerosol range is currently configured for the measurement of both particulate and bio-agent aerosol dispersion characteristics. The optical materials measurements range started with measurement capabilities for laser radar target materials and is currently being expanded to measure both emissivity and reflectance of materials from the visible through the infrared.

The Standoff Aerosol Active Signature Testbed (SAAST) at MIT Lincoln Laboratory

The Standoff Aerosol Active Signature Testbed (SAAST) is the aerosol range within the MIT Lincoln Laboratorys Optical System Test Facility (OSTF). Ladar and Lidar are promising tools for precise target acquisition, identification, and ranging. Solid rocket effluent has a strong Lidar signature. Currently, calculations of the Lidar signature from effluent are in disagreement from measurements. This discrepancy can be addressed through relatively inexpensive laboratory measurements. The SAAST is specifically designed for measuring the polarization-dependent optical scattering cross sections of laboratory-generated particulate samples at multiple wavelengths and angles. Measurements made at oblique angles are highly sensitive to particle morphology, including complex index of refraction and sample shape distribution. With existing hardware it is possible to re-aerosolize previously collected effluent samples and, with online and offline diagnostics, ensure that these samples closely represent those found in situ. Through comparison of calculations and measurements at multiple angles it is possible to create a realistic model of solid rocket effluent that can be used to extrapolate to a variety of conditions. The SAAST has recently undergone a dramatic upgrade, improving sensitivity, flexibility, sample generation, sample verification, and level of automation. Several measurements have been made of terrestrial dust and other samples.

Photon-Counting Lidar for Aerosol Detection and 3-D Imaging §

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.

INEXPENSIVE CHEMICAL DEFENSE NETWORK FOR A FIXED SITE

The Inexpensive Chemical Agent Detection System (ICADS) consists of a network of affordable line-of-sight sensors, each designed to detect chemical threats passing between two points with high sensitivity and a low false-alarm rate. Each leg of the ICADS system is composed of two devices, a broadband IR transmitter, and a receiver containing a long-wave-IR spectrometer. The spectrometer continually measures the spectrum of the radiation emitted by the transmitter, which is separated from the receiver by up to several hundred meters, forming a line of protection. A chemical vapor or aerosol plume with sufficient long-wave-IR absorption causes a characteristic change in the spectrum of light collected by the receiver as the plume crosses the protected line, signaling a threat. Background measurements were conducted to determine background-limited performance. Additionally, a sensor composed of a long-wave-IR fixed-grating spectrometer and a hot-filament transmitter was designed and built. Measurements of the signal-to-noise ratio (SNR) and resolution agree with our analytical model and meet sensor requirements.

The seeker experimental system at MIT Lincoln Laboratory

The Seeker Experimental System (SES) is the passive range within MIT Lincoln Laboratorys Optical System Test Facility (OSTF). The SES laboratory focuses on the characterization of passive infrared sensors. Capable of projecting static and dynamic scenes in both cryogenic and room temperature environments, SES supports sensors that range from tactical ground based systems through strategic space-based architectures. Optical infrared sensors are a major component of military systems, having been used to acquire, track, and discriminate between potential targets and improve our understanding of the physics and phenomenology of objects. This paper delineates the capabilities of the SES laboratory and describes how they are used to characterize infrared sensors and develop new algorithms and hardware in the support of future sensor technology. The SES Cryogenic Scene Projection System vacuum chamber has recently been upgraded to allow dynamic projection of radiometrically accurate two-color infrared imagery. Additional capabilities include the ability to combine imagery from multiple sources, NIST traceable radiometric calibration, and dynamic scene projection in an ambient environment using a combination of high speed mirrors, point source blackbodies, and resistive array based dynamic infrared scene projectors.

Active Range of the Optical Systems Test Facility at MIT Lincoln Laboratory

The Active Range of the Optical Systems Test Facility was established in 2003 to allow for rapid development and demonstration of active electro-optic technology in a system context, investigate critical phenomenology issues in a repeatable environment, and enhance expertise in electro-optic technology. The test facility consists of four major parts: a control room, a 50-m range with installed ladar systems, a far-field emulator comprising of a 1-m primary mirror and zoom optics, and a dynamic target manipulator for full-scale (1-m) targets. This paper will focus on the capabilities of the Optical Systems Test Facility and present some examples of laser radar experiments and data taken in the range.

THE STANDOFF AEROSOL ACTIVE SIGNATURE TESTBED (SAAST) AT MIT LINCOLN LABORATORY

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.