Simultaneous real-time spectroscopy using a broadband IR laser source.

In this study, we have developed a simultaneous grating spectroscopy using a broadband IR laser source capable of detecting moving targets in real time. The broadband IR laser source operated in pulsed mode provides a broad spectral range, which covers absorption bands of many chemical analytes. The laser operating conditions were optimized to cover the broadest wavelength range spanning spectral features for the analytes of interest, based on a detailed understanding of the broadband source. This measured the signal from two samples, a 1% acetaminophen KBr pellet sample and toluene in a gas cell. These samples were characterized by illuminating them with the IR broadband source and collecting the transmitted or reflected signal through a grating spectrometer and onto an IR focal plane array (FPA). The results clearly show discrete peaks comparable to the FTIR reference spectra and the spectral features of the samples were successfully discriminated. We believe that the proof of concepts presented here are of broad applicability and will aid advanced real-time standoff detection research.

Time resolved characterization of Fabry-Perot quantum cascade lasers for use in a broadband "white light" source.

We report the time resolved characterization of Fabry-Perot quantum cascade lasers (FP-QCLs). We are developing a custom-built broadband laser source in the Mid-LWIR range by combining several high power FP-QCLs for a single snap shot application. This white light source would enable not only stand-off detection applications in a single snapshot but also new data collection modalities such as live, real-time chemical imaging, requiring extremely rapid measurements. In this study, the two FP-QCLs were operated in CW and pulsed modes with varying applied currents and diode temperatures to optimize the best laser operation condition to cover a broad spectral range including spectral features for the analytes of interest. To understand mode behavior of the FP-QCLs in a short period of time, the spectral output for each test condition was temporally resolved. Under most of the conditions, FP mode hopping was observed during the time evolution through the pulse length (3000 ns). Based on the time-resolved spectra, the ideal spectral characteristics for a single snap shot application are discussed, with respect to a broad spectral bandwidth, a flat-top power profile, and high spectral power density.

Reduction of speckle noise and mitigation of beam wander in tunable external cavity quantum cascade lasers using rotating diamond/KBr pellet coupled with multimode fiber.

We experimentally demonstrate speckle noise reduction and beam wander mitigation by using a rotating diamond/KBr pellet and a multimode fiber (MMF). As the diamond/KBr diffuser is rotated, the reflected speckle images that are captured by an infrared camera are temporally averaged. We demonstrate 78% speckle noise reduction by averaging 25 frames, which is within 80% of the theoretical contrast reduction. Large beam position fluctuations are also significantly suppressed by adding the MMF. This combination of beam wander mitigation and speckle reduction offers significant benefits for emerging optical technologies that use quantum cascade lasers as illumination sources.

Infrared photothermal imaging spectroscopy for detection of trace explosives on surfaces.

We are developing a technique for the standoff detection of trace explosives on relevant substrate surfaces using photothermal infrared (IR) imaging spectroscopy (PT-IRIS). This approach leverages one or more compact IR quantum cascade lasers, which are tuned to strong absorption bands in the analytes and directed to illuminate an area on a surface of interest. An IR focal plane array is used to image the surface and detect increases in thermal emission upon laser illumination. The PT-IRIS signal is processed as a hyperspectral image cube comprised of spatial, spectral, and temporal dimensions as vectors within a detection algorithm. The ability to detect trace analytes at standoff on relevant substrates is critical for security applications but is complicated by the optical and thermal analyte/substrate interactions. This manuscript describes a series of PT-IRIS experimental results and analysis for traces of RDX, TNT, ammonium nitrate, and sucrose on steel, polyethylene, glass, and painted steel panels. We demonstrate detection at surface mass loadings comparable with fingerprint depositions ( 10µg/cm2 to 100µg/cm2) from an area corresponding to a single pixel within the thermal image.