Raman hyperspectral imaging (RHSI) is an emerging chemical imaging technique that provides spectral and spatial information simultaneously in one measurement, and therefore can be a valuable tool for the detection and analysis of targets located in complex backgrounds. In particular, RHSI is useful for the detection and identification of threat materials (i.e., homemade and military-grade explosives) on the surfaces, where the concentration of target of interest could be very low and is typically found within complex scenery. Raman spectroscopy has the capability to provide a distinct molecular fingerprint of a threat material for unambiguous identification, can work at standoff distances (up to 100+ meters), and is capable of being conducted remotely, which makes it beneficial for installation into a stationary vehicle screening assembly. In spite of its numerous advantages, the implementation of Raman instrumentation for hyperspectral imaging is rather challenging owing to the low Raman scattering efficiency, potentially high SWaP constraints, and the need for a tightly focused laser spot that may result into the photo or thermal degradation of the sample. Some limitations can be overcome by utilizing the deep UV laser excitation, as the Raman cross section increases exponentially, i.e., ν4 with laser frequency. However, current generation UV Raman spectrometers require very narrow slit widths and long focal length optics, which means they have very low optical throughput, can be physically large and heavy, and can only probe an area the size of a tightly focused laser beam, eliminating the option to investigate large areas with defocused excitation. In addition, the use of focused laser excitation creates eye-safety concerns that restricts the usage of Raman sensors for most real-world applications To address these issues, ChemImage Sensor Systems (CISS) is developing a hyperspectral Raman imaging system capable of yielding high spectral resolution in a small form factor while using eye-safe, defocused laser excitation. This innovation combines a spatial heterodyne spectrometer (SHS), a slit-less spectrometer that operates similar to Michelson interferometer without requiring moving parts, with a fiber array spectral translator (FAST) fiber array, a twodimensional imaging fiber that provides spatial information of the target area. This combination of technologies, known as FAST-SHS that is compatible with deep UV excitation and creates a high throughput Raman hyperspectral imager capable of yielding very high spectral resolution measurements while simultaneously providing expanded area coverage and a faster search rate than traditional Raman systems. Recently, we have developed a FAST-SHS system consisting of fiber bundles with numerous fibers. FAST-SHS is the first spatial heterodyne Raman spectrometer to incorporate FAST technique that is capable of performing hyperspectral measurements at various standoff distances using defocused laser excitation. This paper will discuss the background of FAST-SHS technology for Raman hyperspectral imaging, the initial setup and design of the sensor, and provide initial detection results for wide area detection capabilities for the identification and analysis of threat targets.
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.