Control of quantum cascade laser sources in stand-off detection of trace explosives

摘要

We pursue the development of an eye-safe stand-off technique suitable for the detection of trace explosives. As theactive illumination sources, tunable quantum cascade lasers (QCLs) are employed in Mid-LWIR (long-wave infrared) inthe range of 6 to 11 μm, which contains many spectral features from analytes of interest. Any fluctuation of the laserbeam direction and/or beam profile is amplified at the sample position, which would lead to diminished performance ofthe detection technique, both in sensitivity and selectivity. Several beam stabilization approaches were conducted toovercome this challenge: 1) Using a KBr/diamond pellet as a diffuser in combination with a multimode fiber 2)Feedback stabilization of quantum cascade laser beam steering. The purpose of the first method is to make a temporallyand spatially incoherent laser beam source through the multimode fiber and KBr/diamond pellet. The second approach isto stabilize the beam position by using an active feedback loop. We have demonstrated that beam wander and specklenoise were successfully suppressed by these approaches. Independently, we have developed a custom-built broadbandlaser source in the Mid-LWIR range consisting of several high power Fabry Perot (FP)-QCLs. The FP-QCLs wereoperated in both CW and pulsed modes at different diode temperatures, and the emission spectra were collected by aFTIR. For our future work, the output beams will be collimated to spectrally combine multi-QCLs and aligned towardthe same target. Also, a spatial heterodyne spectrometer (SHS) will be applied to discriminate spectral and spatialinformation from a single snapshot.