Threat detection using a standoff, wide-area hyperspectral Raman imaging sensor

Raman hyperspectral imaging (HSI) can be an advantageous technique for the detection and identification of threat materials (i.e. homemade and military grade explosives), especially if those materials are located in a complex scene. Raman spectroscopy has the capability to provide a distinct molecular fingerprint of a threat material, which gives it the ability to provide near unambiguous threat identification. Unfortunately, the current generation of Raman sensors have numerous limitations that hinder their performance and limit their ability to be applied in real world scenarios. These systems offer low optical throughput, have larger size/weight requirements, and can only interrogate an area of interest the size of a focused laser spot. These limitations are typically due to a system's spectrometer, which traditionally utilizes a dispersive grating and requires a narrow entrance slit width and long focal length optics to accurately accept and pass the collected scattered light onto the detector. In addition, the use of focused laser excitation creates eye-safety concerns that restrict the usage of Raman sensors for most real-world applications. With these issues in mind, ChemImage Sensor Systems (CISS) is developing a next generation Raman sensor capable of providing a wide-area of coverage and improved eye-safety using defocused laser excitation. This is made possible by utilizing a spatial heterodyne spectrometer (SHS), a slitless grating-based Michelson interferometer with no moving parts. The entrance aperture to the SHS can be centimeters in diameter, which provides the SHS an etendue orders of magnitude greater than a traditional spectrometer. This feature also allows the excitation laser to be defocused to centimeters in diameter. In addition, the sensor utilizes a fiber-array spectral translator (FAST) bundle, a 2-D hyperspectral imaging fiber composed of hundreds of smaller fibers, which gives the sensor the ability to spatially distinguish the area of interrogation. The combination of these two technologies is termed FAST-SHS. This paper will discuss the background of spatial heterodyne spectroscopy and Raman hyperspectral imaging, the initial setup and design of the sensor, and provide initial detection results.