Research On The Effect Of Modulation Phase Difference On TDLAS SNR

As reposition the relationship between the development of productive forces and environmental protection, environmental monitoring is increasingly important, non-contact optical detection as one of the monitoring methods is developing rapidly. Tunable Diode Laser Absorption Spectroscopy (TDLAS) has become a powerful technique for real-time and on-line gas analysis, with features of high sensitivity, high selectivity, fast response, high Signal-to-Noise Ratio (SNR) and good environmental adaptability etc, by which a variety of gases can be detected on-line together.The SNR is the most important scale to measure performance of TDLAS trace gas detection system. Direct current modulation of diode lasers induces frequency modulation (FM) simultaneously superimposed on amplitude modulation (AM). A second harmonic detection method is usually utilized to measure trace gas concentration in a TDLAS trace gas detection system. As there is a phase difference between the AM and FM of the diode laser, it is important to research on the effect of modulation phase difference on the second-harmonic signal, residual amplitude modulation (RAM) noise and SNR. In this study, a FM-AM method which includes modulation phase difference is applied to analyze and extract the second harmonic signal. Calculating expressions containing modulation phase difference of the second harmonic signal and residual amplitude modulation noise are deduced respectively. Some typical detection parameters and Lorentzian line-shape function are selected as the calculating condition. Meanwhile, the left peak-to-valley difference and right peak-to-valley difference of second harmonic signal value are used respectively as detection signals. The corresponding SNR of the detection system is calculated finally.The results show that signal, noise and SNR are different with the different modulation phase difference, and these differences can not be ignored. In conclusion, the modulation phase difference is an important factor which has an effect on SNR. And it is an impact factor of TDLAS detection limit.