Explosive detection and identification using a wide-area, hyperspectral Raman imaging sensor

Raman hyperspectral imaging (HSI) is a valuable technique for the detection of threat materials (i.e. explosives and/or narcotics), especially if those materials are located in a complex area with varied background constituents. Raman spectroscopy can provide a unique molecular fingerprint of a threat material, which allows it 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 limitations include low optical throughput, larger size/weight requirements, and area of interrogation size limited to the size of a focused laser spot. These limits are typically due to a system’s spectrometer, commonly a dispersive grating based approach that requires a narrow entrance slit width and long focal length optics to accurately resolve and pass the collected scattered light onto the detector. In addition, using focused laser excitation creates eye-safety concerns that can restrict the usage of Raman sensors for most real-world applications. To address these issues, ChemImage Corporation 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 slit-less grating-based Michelson interferometer with no moving parts. The entrance aperture to the SHS can be orders of magnitude larger than a traditional spectrometer’s entrance slit, which provides an etendue gain of equal magnitude. This feature also allows the laser to be utilized in a defocused configuration, providing an area of coverage up to centimeters in diameter. The sensor also comprises a fiber-array spectral translator (FAST) bundle, a 2-D hyperspectral imaging fiber composed of dozens of smaller fibers, which gives the sensor the ability to spatially discriminate the area of interrogation. The combination of these two technologies is termed FAST-SHS. This paper will provide the background of spatial heterodyne spectroscopy and Raman hyperspectral imaging, the setup and design of a breadboard FAST-SHS, and provide initial results.