Tunable Multi-mode Diode Laser Spectroscopy For Methane Gas Monitoring

Gas detection is of great importance in industrial control and biomedical applications, especially in environmental monitoring field. Optical technique has the advantages of large range, long life and non-contact. Tunable diode laser absorption spectroscopy (TDLAS) is one of the most widely-used optical techniques for the high spectral resolution and narrow linewidth. The detection technique of CH4 and relative physics questions was investigated by employing multi-mode diode laser.The emitting spectrum parameter of diode laser was measured. The temperature and current coefficient are 0.02nm/mA and 0.35nm/°C by changing the current and temperature separately. We gave a novel approach to make sure the location of multiple absorption lines, which scanned by multi-mode diode laser. By comparing the modulated spectroscopy of diode laser with the line intensity of CH4 at 1318nm from HTRAN database, and one absorption line at 1318.2nm was scanned by multi-mode diode laser only.Wavelength modulation spectroscopy was performed to measure the CH4, A concentration calibration profile with high linearity was obtained by measuring the peaks of second harmonic signal. The discrepancy between the measured and calculated value was less then 2%. A sensitivity of 640ppm was obtained. Multi-peaks of the second harmonic signal appeared at low pressures, Multi-peaks fitting of direct absorption signal indicate that it was composed by four small absorption lines, and the result was proved by HTRAN database. Because of broadening effective, Better signal will be obtained at high pressure. After temperature correction, the center frequency shift coefficient -0.017cm-1/atm was obtained, it's close to the result -0.015nm/atm from HTRAN database.Simulated experiment of CH4 was demonstrated by computer process base on lineshapes theoretics. Absorption lineshapes of CH4 at 1atm and 0.01atm was obtained by simulation. Direct absorption signal and second harmonic signal with high correlation was obtained between simulation and experiment. The simulation not only proved the experiment but also guide the future work.