This presentation introduces the spectroscopic concepts and results enabled by arrays of Distributed Feedback (DFB) QCLs, with each element at a slightly different wavelength than its neighbor. In portable optical systems, such as standoff threat detectors and in situ gas analyzers, this increases analyte sensitivity and selectivity by broadening spectral source coverage while also allowing for extremely fast all-electronic wavelength tuning with no moving parts.
This talk will first present the QCL array and its packaging, then move into the description of an integrated prototype standoff detection system, and finally show condensed phase standoff threat detection results from a handheld system from over 1 meter. These data are each compared with legacy contact-based methods to ensure that the technique can be reliably deployed to handheld chemical analysis using suitable chemometric algorithms.
The data show how monolithic and all-electronic tuning enables next-generation spectroscopes that are not only more robust and miniature than those that utilize external cavity-tuned lasers, but that are inherently more stable in terms the shot-to-shot amplitude and wavelength parameters. This enhanced stability increases signal to noise for a given configuration (pathlength, averaging time, concentration, etc…). Some discussion of how to maximize the benefits of high speed, highly reproducible tuning is presented, including detector, preamplifier, and digitization considerations.
Quantitative laser spectroscopic measurements of complex molecules that have a broad absorption spectra require broadly tunable laser sources operating preferably in the mid-infrared molecular fingerprint region. In this paper a novel broadband mid-infrared laser source comprising of an array of single-mode distributed feedback quantum cascade lasers was used to target a broadband absorption feature of benzene (C6H6), a toxic and carcinogenic atmospheric pollutant.
The DFB-QCL array is a monolithic semiconductor device with no opto-mechanical components, which eliminates issues with mechanical vibrations. The DFB-QCLs array used in this work provides spectral coverage from 1022.5 cm-1 to 1053.3 cm-1, which is sufficient to access the absorption feature of benzene at 1038 cm-1 (9.64 μm).
A sensor prototype based on a 76 m multipass cell (AMAC-76LW, Aerodyne Research) and a dispersive DFB-QCL array beam combiner was developed and tested. The Allan deviation analysis of the retrieved benzene concentration data yields a short-term precision of 100 ppbv/Hz1/2 and a minimum detectable concentration of 12 ppbv for 200 s averaging time. The system was also tested by sampling atmospheric air as well as vapors of different chemical products that contained traces of benzene.