Abstract
This article presents new spectroscopic results in standoff chemical detection that are enabled by monolithic arrays of Distributed Feedback (DFB) Quantum Cascade Lasers (QCLs), with each array element at a slightly different wavelength than its neighbor. The standoff analysis of analyte/substrate pairs requires a laser source with characteristics offered uniquely by a QCL Array. This is particularly true for time-evolving liquid chemical warfare agent (CWA) analysis. In addition to describing the QCL array source developed for long wave infrared coverage, a description of an integrated prototype standoff detection system is provided. Experimental standoff detection results using the man-portable system for droplet examination from 1.3 meters are presented using the CWAs VX and T-mustard as test cases. Finally, we consider three significant challenges to working with droplets and liquid films in standoff spectroscopy: substrate uptake of the analyte, time-dependent droplet spread of the analyte, and variable substrate contributions to retrieved signals.
Highlights
Identifying chemical residues in a standoff situation can mitigate potential hazards in suspected cases of chemical warfare agent (CWA) or explosives contamination
This article presents new spectroscopic results in standoff chemical detection that are enabled by monolithic arrays of Distributed Feedback (DFB) Quantum Cascade Lasers (QCLs), with each array element at a slightly different wavelength than its neighbor
The standoff analysis of analyte/substrate pairs requires a laser source with characteristics offered uniquely by a QCL Array. This is true for time-evolving liquid chemical warfare agent (CWA) analysis
Summary
Identifying chemical residues in a standoff situation can mitigate potential hazards in suspected cases of CWA or explosives contamination. The development of a practical instrument capable of identification against a large library of chemicals requires widely tunable LWIR sources that are rugged enough to be deployable This is because (1) the linewidths of absorption features can be broad, for the condensed phase, and (2) vibrational modes from multiple chemical targets tend to overlap intermittently throughout the LWIR, known as the spectral fingerprint region In addition to broadband tunability, the laser source must quickly scan its emission wavelength while maintaining high reproducibility in power and wavelength, allowing for efficient signal averaging through multiple measurements This is especially important for the standoff detection of chemicals in handheld systems where available averaging time is often short due, for example, to hand movements and aiming drift when training a laser beam onto a small target of sample from distances exceeding one meter. To achieve broadband tunability and measurement speed we have developed a monolithic LWIR light source consisting of four 32-element DFB QCL arrays (QCLAs) that cover a spectral region between 6.5 and 10.5 μm
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