Analysis of multi-component broadband absorbers based on wavelength modulation united absorption spectroscopy

Wavelength Modulation United Absorption Spectroscopy (WM-UAS), which is virtually an extension of Wavelength Modulation Spectroscopy (WMS), for measuring multi-component broadband absorbers is proposed and demonstrated in this paper. "Broadband absorbers" indicates matters that do not exhibit resolved absorption spectra within a certain wavelength range. WM-UAS requires the assistance of a kind of probe substance which exhibits a series of narrow absorption profiles to measure broadband absorbers. An improved WMS technique is first introduced, with which the limitation of low absorbance in conventional WMS is broken through. On this basis, the mathematical expression for WM-UAS is deduced step-by-step. Simulations and experiments are conducted so that the procedures as well as the validity of WM-UAS is presented. In the experiment, water vapor is selected as the probe substance while methanol vapor and ethanol vapor are simultaneously measured. According to Allan deviation analysis, the optimal average time of the system is 150.18 s and the corresponding detection limit is 19.38 ppm·m for ethanol and 14.85 ppm·m for methanol.