调制相位差对TDLAS信噪比的影响

用可调谐二极管激光吸收光谱技术检测气体时,不同的调制相位差结果不同。为了提高谐波检测灵敏度,采用同时考虑频率调制与振幅调制的方法分析和提取二次谐波信号,推导了相位差与二次谐波信号光电流和残余幅度调制噪声光电流的函数关系表达式,通过模拟实验分别取得了二次谐波信号和残余幅度调制噪声随相位差的变化趋势,并重点分析了相位差对信噪比的影响。结果表明,振幅调整和频率调制之间的相位差对信噪比的影响很大,与典型值/2时相比,最大降幅为59.18%;调制相位差是继残余幅度调制之外,影响信噪比的一个重要因素。

Effect of modulated phase difference on TDLAS signal-to-noise ratio

When using tunable diode laser absorption spectroscopy(TDLAS) for gas detection, modulation phase difference would make the result different. In order to improve harmonic wave detection sensitivity, frequency modulation and amplitude modulation were simultaneously chosen to analyze and extract the second harmonic wave signal. The functional relationship expression between the phase difference and the photo-electron current of the second harmonic wave and the residual amplitude modulation noise was also derived. From the simulation experiments, the variation trend of the second harmonic wave signal and the residual amplitude modulation noise with the phase difference were obtained. And then, the effect of phase difference on signal-to-noise ratio(SNR) was analyzed. The results show that the phase difference between amplitude modulation and frequency modulation has a significant impact on SNR. The largest drop is 59.18% when compared with the typical value of phase difference of /2. Modulation phase difference is another important affecting factor of SNR besides residual amplitude modulation.