Detection of multi absorption lines for CH4 using broadband laser beam modulation

Abstract

In this work we implemented a setup to modulate wavelength of a broad band beam of low laser power to detect multi absorption lines of multi gas species simultaneously, using long absorption path and monochromator. This setup consists mainly of tunable diode Laser built with an External Cavity designed by Littrow configuration, typical Herriott multipass cell with absorption path length of 30 m, monochromator with photomultiplier tube as detector, function generator, digital oscilloscope, and a photodiode. The proposed setup gives us ability to apply two techniques: direct absorption and wavelength modulation, in addition to changing some parameters for tuning diode laser. Using the proposed setup with low laser power we were able to detect multi absorption lines of a small concentration of CH4 simultaneously, based on spectroscopic databases we believe these detected lines belong to weak absorption NIR region of CH4. Most of the detected lines were listed recently in HITRAN 2012 but some of them are not listed, which merits further theoretical and experimental study which we are currently perusing.