Modulation coefficient analysis in optical fiber methane gas sensor based on wavelength modulation spectroscopy

The detection sensitivity of methane trace-gas concentration using optical fiber gas sensor based on tunable diode laser absorption spectroscopy (TDLAS) and wavelength modulation spectroscopy (WMS) is very high, the absorption line is detected using a laser diode wavelength-sweep technique, and Fourier series is used to analysis WMS signal. The ratio of the second harmonic peak and the first harmonic baseline as output is used to eliminate the influence of optical intensity fluctuations. The fluctuations of temperature in the gas cell cause absorption coefficient and modulation coefficient to change, a simple piece of equipment with temperature acquisition devices is developed for measuring methane gas concentration. Under the different modulation amplitudes, the deviations of system output caused by temperature fluctuations are different, and the optimal modulation coefficient is chosen to suppress the influence of temperature fluctuations.