Effect of RAM on WMS signal line shape

In the detection of trace gas based on tunable diode laser absorption spectroscopy(TDLAS),residual amplitude modulation(RAM) might happen simultaneously when wavelength modulation is performed directly by input current.This RAM will affect line-shape of measurement signal and system noise.In order to study the influences of RAM on the line-shape of the second harmonic signal of wavelength modulation spectroscopy(WMS),a calculation method of the second harmonic signal was put forward simultaneously considering amplitude modulation and frequency modulation(AM-FM) factor.After analyzing the method,the corresponding expression was derived.Quantitative calculation results were compared between AM-FM method and traditional FM method.The baseline,positive and negative peak of the second harmonic signal were obtained.The results show that the RAM has direct influences on detection signal line shape and signal baseline.The signal line shape calculated with the AM-FM method more close to detected results.