The detection sensitivity of trace gas concentrations using optical fiber gas sensor based on wavelength modulation spectroscopy is very high.In the discrete sampling process

it is difficult to sampling in the center frequency of absorption line.Frequency deviation would cause serious deviation of test result.Fourier series expansion is used to analyze wavelength modulation spectroscopy.WMS signal

the first harmonic baseline is detected by the average value of maximum and minimum to restrain the influence of frequency deviation on measurement precision of the system

which has nothing to do with gas volume fraction.The ratio of second harmonic peaks and first harmonic baseline as system output which eliminate the interference of optical intensity fluctuations.