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 acoustooptically 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 changed 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 AOM tuned QCL, we have carried out a number of studies to explore the usefulness of rapid scanning QCL systems. One study involves spectroscopy of liquids. Since almost all liquids absorb very strongly in the long wave infrared region, we have used attenuated total reflection (ATR) to study liquids such as isopropyl alcohol (IPA), ethanol, water, various alcoholic beverage such as vodka, gin and scotch, 2,2,2-trifluroethanol and Epsom salt dissolved in water. In each case a complete spectrum from ~8.5 µm to ~ 9.5 µm is recorded in a single shot 500 µs scan. From these studies, we can verify the claimed “proof” of the alcoholic beverages. In another study, we have explored mixing of gases (R134A HFC bolus injected 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, including a video showing the flow mixing evolution on millisecond time scale. 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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