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AOTF-based Spectral Imaging for Enhanced Stand-off Chemical Detection


OBJECTIVE: Build an AOTF Imaging System for Enhanced Standoff Chemical Detection in the Long-wave Infrared Region. DESCRIPTION: Acousto-optics can be defined as the study of the interactions between sound waves and light waves. In particular it is the study of diffraction of light by ultrasound or sound in general. Acousto-optic effects are usually based on the change of the refractive index of a medium due to the presence of sound waves. The sound waves can produce an effective refractive index grating in the material which influences the propagation of the light beam. There is a growing interest in acousto-optical devices for the deflection, modulation, signal processing and frequency shifting of light beams. Recent progress in crystal growth and high frequency piezoelectric transducers have enabled this technology. The Chemical and Biological Defense community has the need for better methods of standoff detection of chemical and biological agents. Infrared absorption spectroscopy has proven to be a very useful tool in the detection and identification of airborne chemicals and aerosols. Pattern recognition is used to compare the infrared spectrum of library molecules against the infrared spectra of airborne contaminants. In particular, chemical warfare agents and Toxic Industrial Chemicals (TICs) have distinctive absorption lines in the infrared region. Infrared spectroscopy has been used to detect chemicals at very low concentrations. Infrared spectroscopy also holds the promise of low false alarm rates due to the spectral pattern matching over a large number of spectral bins. The size, weight, and power requirements of current infrared spectrometers have limited their utility in field environments. Chemical agent infrared absorption/emission is largely confined to the 8 to 12 micron region of the EM spectrum. Tunable filters such as Acousto-Optic Tunable Filters (AOTF) are just becoming available in this wavelength region. In an AOTF-based sensor, selected wavelengths of light can be deflected onto a focal-plane-array, providing spectral imaging capabilities. The resulting imaging system would have a number of advantages over conventional standoff systems. The proposed system would contain no mechanical moving parts, making it inherently rugged and precise. The system could be made compact and thus easily integrated into a variety of configurations. Inexpensive infrared longwave focal-plane-arrays are now becoming available allowing for low cost imaging capabilities. AOTF technology also allows for the simultaneous detection of two or more wavelengths of light. This effect could be used to provide better methods of optical pattern matching for standoff chemical/biological detection. Methods of compressed sensing could be utilized to reduce data acquisition times and improve detection probability. AOTF technology may also provide polarameteric imaging capabilities. Current standoff capabilities for aerosol detection and tracking could be significantly enhanced using polarization information. PHASE I: Develop and design an AOTF-based spectral imager using a longwave infrared focal plane array. Design a lightweight, low-power, inexpensive hyperspectral imaging sensor for wide area standoff detection of chemical agents. The ability to also use the technology to detect biological agents would be advantageous. The spectral region of the sensor should be chosen to interrogate spectral signatures of chemical plumes. Traditionally the 8 to 12 micrometer region of the electromagnetic spectrum has been used for standoff chemical detection. The system should have sufficient spectral and spatial resolution to detect and discriminate chemical agent plumes. The detection and discrimination capabilities of the sensor in this region should be comparable to existing HSI chemical/biological sensors. The goal is to passively detect small chemical plumes (25 meters or smaller) of a chemical agent such as sarin at relevant concentrations (less than 10 ppmv) at a distance of 5 kilometers or more under ambient conditions. PHASE II: Build and test an AOFT spectral imaging system. Construct a standoff hyperspectral imaging sensor designed for the detection of chemical plumes. Utilize the best methods and technologies for reducing the size and weight of HSI systems while maintaining required sensitivities. Test and characterize the performance of the new HSI sensor. Based on the test results, refine the design of the new standoff chemical imaging sensor. PHASE III: DUAL USE APPLICATIONS: Further research and development during Phase III efforts will be directed towards a final deployable design, incorporating design modifications based on results from tests conducted during Phase II, and improving engineering/form-factors, equipment hardening, and manufacturability designs to meet U.S. Army CONOPS and end-user requirements. Further, demonstrate the technology"s applicability to stand-off detection of chemical and biological threat materials. There are many environmental applications for a small chemical standoff sensor. A rugged, sensitive and flexible chemical detector will benefit the manufacturing community by providing finely tuned monitoring of chemical processes. Also, first responders such as Civil Support Teams (CST) and Fire Departments have a critical need for a rugged, relatively inexpensive but versatile and rugged sensor that can be transported to the field to test for possible contamination by CW agents and other toxic chemicals. REFERENCES: 1. Neelam Gupta,"Investigation of a mercurous chloride acousto-optic cell based on longitudinal acoustic mode", Applied Optics, volume 48, issue 7, pages C151-C158, 2009. 2. I. C. Chang,"Acousto-optic tunable filters", Optical Engineering, volume 20, page 824-829, 1981. 3. N. B. Singh, D. Kahler, D. J. Knuteson, M. Gottlieb, D. Suhre, A. Berghmans, B. Wagner, J. Hedrick, T. Karr, and J. J. Hawkins,"Operational characteristics of a long-wavelength IR multispectral imager based on an acousto-optic tunable filter", Optical Engineering, volume 47, issue 1, page 013201, 2008. 4. Vitaly B. Voloshinov, Neelam Gupta, Gregory A. Knyazev, and Nataliya V. Polikarpova,"An acousto-optic XY deflector based on close-to-axis propagation of light in the single Te crystal", Journal of Optics, volume 13, number 1, page 015706, 2011. 5. David J. Knuteson, Narsingh B. Singh, Milton Gottlieb, Dennis Suhre, Andre E. Berghmans, David A. Kahler, Brian Wagner, and Jack Hawkins,"Crystal growth, fabrication, and design of mercurous bromide acousto-optic tunable filters", Optical Engineering, volume 46, issue 6, page 064001, 2007. 6. Arnaldo D'Amico, Corrado Di Natale, Fabio Lo Castro, Sergio Iarossi, Alexandro Catini and Eugenio Martinelli,"Volatile Compounds Detection by IR Acousto-Optic Detectors", Unexploded Ordnance Detection and Mitigation, NATO Science for Peace and Security Series B: Physics and Biophysics, pages 21-59, 2009. 7. Joo-Soo Kim, Sudhir B. Trivedi, Jolanta Soos, Neelam Gupta, and Witold Palosz,"Growth of Hg2Cl2 and Hg2Br2 single crystals by physical vapor transport", Journal of Crystal Growth, volume 310, issue 10, pages 2457-2463, 2008. 8. N. Gupta,"Hyperspectral and Polarization Imaging with Double-Transducer AOTFS for Wider Spectral Coverage", International Journal of High Speed Electronics and Systems, volume 17, number 4, pages 845-856, 2007. 9. N.B. Singh, D. Suhre, D., N. Gupta, W. Rosch, and M. Gottlieb,"Performance of TAS crystal for AOTF Imaging", Journal of Crystal Growth, volume 225, pages 124-128, 2001. 10. N. Gupta,"Acousto-optic tunable filters for Infrared Imaging,"Proceedings of the SPIE, volume 5953, pages 59530O/110, 2005.
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