14 May 2018 Advances in fast tunable laser spectroscopy in the infrared
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The invention and development of all-electronically tuned quantum cascade lasers has made it possible to obtain spectral information, covering over 1 μm in the long wave infrared region, regarding absorbers in less than fraction of a millisecond. The electronic tuning is achieved through the use of a acousto-optically generated phase grating in a crystal. As described previously, the acousto-optic modulator (AOM) tuned QCL is capable of switching lasing wavelengths in time of the order of 0.5 μs, regardless of the size of the wavelength step. The wavelength tuning is achieved via change in the acoustic wave RF frequency. Thus, a complete spectrum covering < 1 μm tuning (for example from ~8.5 μm to ~ 9.5 μm) can be obtained in less than 20 μs, when the RF frequency is changes in response to an analog drive. For experimental reproducibility of spectra, we have implemented a digital scanning system that permits selection of step size and step speed.

Using the above system, we have carried out a number of studies to explore the usefulness of rapid scanning QCL systems. One study involves spectroscopy of highly absorbing liquids. Almost all liquids absorb very strongly in the long wave infrared region. We have used attenuated total reflection technique to study liquids such as isopropyl alcohol (IPA), ethanol, water, alcoholic beverage such as vodka, gin and scotch and 2,2,2-trifluroethanol. In each case a complete spectrum from ~8.5 μm to ~ 9.5 μm is recorded in a single shot 500 μs scan.

In another study, we have explored transient mixing of gases (R134A HFC introduced into a fast flowing stream of air) in a flow tube where time dependent spectra of mixing gases are obtained in <600 consecutive shots during a 300 ms time span. I will describe both studies in detail.

The fast spectroscopic study of liquids now opens up the potentially exciting area of real time process monitoring in chemical, biological and food and wine industry. The transient flow spectroscopy study clearly indicates that the AOM tuned QCL system will be of immediate use in supersonic flow studies and in the study of combustion and explosion dynamics.
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C. Kumar N. Patel, C. Kumar N. Patel, Rodolfo Barron-Jimenez, Rodolfo Barron-Jimenez, Ilya Dunayevskiy, Ilya Dunayevskiy, Mariano Troccoli, Mariano Troccoli, "Advances in fast tunable laser spectroscopy in the infrared", Proc. SPIE 10639, Micro- and Nanotechnology Sensors, Systems, and Applications X, 1063920 (14 May 2018); doi: 10.1117/12.2306940; https://doi.org/10.1117/12.2306940

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