In the detection of trace gas based on tunable diode laser absorption spectroscopy (TOLAS) , 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.%在基于可调谐二极管激光吸收光谱技术的痕量气体检测中,波长直接调制时剩余振幅调制也会伴随发生,并对检测信号的线型和系统噪声产生严重影响.为了探讨剩余振幅调制对波长调制光谱二次谐波信号线型的影响,采用同时考虑振幅调制和波长调制两种影响因素的计算方法,理论分析出二次谐波信号的计算公式,与仅考虑波长调制的信号计算方式进行同条件线型计算比对,取得了二次谐波信号基线和正负峰值随剩余振幅调制的变化数据.结果表明,剩余振幅调制的大小对检测信号的线型和信号基线有直接影响,采用这种方法计算得出的二次谐波信号线型更贴近实际检测.
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