Advances in standoff detection of trace explosives by infrared photo-thermal imaging

A technique for stand-off detection of trace explosives using infrared (IR) photo-thermal (PT) imaging, remote explosives detection (RED), is under development at the Naval Research Laboratory. In this approach, compact IR quantum cascade lasers (QCLs) tuned to strong absorption bands of trace explosives illuminate a surface of interest. An IR focal plane array is used to image the surface and detect any small increase in the thermal emission upon laser illumination. The technique has been previously demonstrated at several meters of stand-off distance indoors and in field tests with sensitivity to explosive traces as small as a single grain (~1 ng), while operating the lasers below the eye-safe intensity limit (100 mW/cm 2 ) at the tested wavelengths. By varying the incident wavelength slightly, selectivity between TNT and RDX has been achieved. A complete test and analysis can be performed in less than 1 second. This manuscript critically examines components used with RED and demonstrates several improvements. These include QCL drive electronics for narrower spectral emission linewidth, fixed wavelength QCL packaging that optimizes spectral and spatial output, fiber-optic coupling for QCL beam steering and spatial filtering, cooled IR sensors that increase sensitivity and speed, tunable QCL sources that increase selectivity and extend the library of possible analytes, and dynamic PT signal processing that can increase sensitivity and speed. When considered in combination with the capabilities previously demonstrated for RED, and its capability to operate within eye-safety limits, this technology offers the potential for a wide area of applications relating to the detection of trace explosives on surfaces in both non-contact and stand-off configurations.