Research On Gas Concentration Detection Based On Off-Axis Integrated Cavity

Laser absorption spectroscopy in gas detection has the advantages of high sensitivity,high resolution and fast response,and has a wide application prospects in the fields of atmospheric science,environmental science and industrial process.Methane(CH4),an important greenhouse gas,affects the climate of the environment,and accurate measurement of CH4 concentration is very important for atmospheric research.This dissertation focuses on the detection of CH4 gas concentration by off-axis integrated cavity output spectroscopy(OA-ICOS).An experimental device of detection of gas concentration based on OA-ICOS was constructed.A distribution feedback(DFB)laser with a central wavelength of 1392 nm was employed as light source.The light emitting from the DFB laser was coupled into an optical cavity with an optical collimator by off-axis manners.The optical cavity was formed by a pair of mirrors separated by a distance of 61 cm with high reflectivity of ~99.996% and 1m radius of curvature.The light transmitted from the optical cavity was focused to the photodetector by the focusing lens,and the data from photodetector were acquired and stored in the computer by data acquisition system(DAQ).In OA-ICOS,the cavity mode affects the signal to noise ratio(SNR)of the system.Adjusting the mode of the laser incident on the optical cavity and the excitation of the high-order mode to suppress cavity mode noise,the SNR was improved.In the near-infrared region,CH4 has a rich absorption spectrum.According to the HITRAN 2012 database,the spectral lines at 7185.87cm-1 was selected as the absorption line for CH4.There is no interference of water vapor and other major gases near this wavelength,and CH4 has a strong absorption.In this paper,the concentration of CH4 was retrieved by measuring the absorption spectrum of CH 4 near 7185.87cm-1.The minimum detectable absorption coefficient ?min=1.29×10-9cm-1 with 80 s average time and the corresponding detection limit(1?)is about 228.8ppbv.In order to further improve the detection sensitivity of OA-ICOS,wavelength modulation technique(WMS)combined with OA-ICOS technique was developed wavelength-modulated off-axis integrated cavity output spectroscopic technique(WMS-OA-ICOS).The WMS-OA-ICOS technique was used to measure the absorption spectrum of CH4 near the center wavelength of 7185.87 cm-1,and the measurement results were compared with the results of OA-ICOS measurements.The experimental results show that compared with the traditional OA-ICOS technique,WMS-OA-ICOS provided an 8-fold improvement in detection sensitivity.The minimum detectable absorption coefficient is ?min=9.4?10-11cm-1Hz-1/2,and the corresponding CH4 detection sensitivity is 28.9ppbv.The CH4 concentration measurements were obtained by the following two methods: the second harmonic(2 f)peak-height and the first harmonic normalized second harmonic(2f/1f).The results show that 2f/1f can reduce laser power fluctuations and have better linearity than 2f.The linearity of the 2f peak and CH4 concentration is 0.992,and the linearity of the 2f/1f and CH4 concentration is 0.993.