Abstract
We report the theoretical analysis of a quasi-distributed sen- sor system for absorption measurements based on the frequency- modulated continuous wave (FMCW) technique combined with fre- quency modulation spectroscopy (FMS). The laser diode injection current is sawtooth modulated to provide a linear scan of the output over a certain frequency/wavelength range and the output is also externally modulated at radio frequencies. The sensor units consist of a series of open-path micro-optic cells constructed from graded index (GRIN) lenses, each with a unique beat frequency. By arranging for only one sideband of the modulation to be attenuated by the absorption feature, a new signal, proportional to the absorbance, appears in the output spec- trum at a frequency corresponding to the difference between the rf- modulation frequency and the beat frequency of a cell. The method is highly sensitive and applicable to a variety of chemical species with nar- row absorption lines, such as in trace gas analysis. We present the mathematical analysis of the proposed method for single and multiple
Published Version
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