Quantum cascade laser based standoff photoacoustic detection of explosives using ultra-sensitive microphone and sound reflector

We report standoff detection of explosives using quantum cascade laser (QCL) and photoacoustic technique. In our experiment, a QCL with emission wavelength near 7.35 mu m was used and operated at pulsed mode. The output light was focused on Trinitrotoluene (TNT) sample in its powder form. Photoacoustic signals were generated and detected by an ultra-sensitive low-noise microphone with one inch diameter. A detection distance up to 8 inches was obtained using the microphone alone. With the increasing detection distance the measured photoacoustic signal not only decayed in amplitude but also delayed in phase, which clearly verified the source location. To further increase the detection distance, a parabolic sound reflector was used for effective sound collection. With the help of the sound reflector, standoff photoacoustic detection of TNT with distance of 8 feet was demonstrated.