"Snapshot" stand-off detection of target chemicals using broadband infrared lasers

We present further development of an eye-safe, invisible, stand-off technique designed for the detection of target chemicals (such as explosives) in a single "snapshot" frame. Broadband Fabry-Perot quantum cascade lasers (FP-QCLs) are employed as active illumination sources, in the Mid-LWIR (long-wave infrared) in the range of 7 to 12 mu m, to interrogate the spectral features from analytes of interest. We have developed a custom-built broadband laser source utilizing an OEM FP-QCL. This "white" broadband laser source enables stand-off detection in a single snapshot frame. Light from this source was collimated and aligned toward the target several meters away. The "backscatter" and absorption signals from target chemicals are spectrally extracted by an LWIR spectrometer based on the spatial heterodyne spectroscopy (SHS) technique. The SHS offers high throughput and full spectral coverage in each single frame from an IR imaging array. This manuscript will cover the implementation and optimization of FP-QCLs for this broadband spectroscopic application. We will also discuss the operation and processing of SHS images to extract spectral information. Finally, we will present results of measurements using specific analytes to demonstrate the application of the method to stand-off detection of targets such as explosives and other chemical threats.