The Effect Of FM-AM Phase Difference On Line Shape Of TDLAS Second Harmonic

Tunable Diode Laser Absorption Spectroscopy (TDLAS) technology has the advantages of high spectral selectivity, high sensitivity, high precision, real-time and sustainable on-line detection etc. It is essential for us to research the real-time and on-line detection of pollution gases based on near infrared TDLAS technology in the atmosphere.In harmonic signal detection based on TDLAS, wavelength modulation can be performed by a modulation of injection current directly. According to the characteristics of semiconductor laser, it will arise residual amplitude modulation when the frequency of laser is modulated, and the phase difference between these two types of modulation has effect on both the harmonic signal and line shape.In this paper, the wavelength modulaiton spectroscopy is used in consideration of residual amplitude modulation in order to make clear the effect of phase difference on harmonic line shape. The second harmonic signal is selected as the detection signal by comparing the harmonic signals of the first six, the mathematical derivations for wavelength modulation spectroscopy are presented and then the theoretical calculation formulas of considering FM-AM phase difference have been deduced. In addition, the impact of FM-AM phase difference on harmonic signal line shape are also simulated and calculated under the condition of different modulation, and they are also fitted. Finally, the degree of influence of FM-AM phase difference on second harmonic signal shape are calculated and analyzed. The calculation results show that the second harmonic signal positive peak and the left negative peak and right negative peak as well as the signal baseline are all changed with the FM-AM phase difference in the trend of periodic change, meanwhile, the distance between left negative peak and right negative peak are not affected by it, also remaining constant. Compared with the condition that phase difference of FM-AM is π/2,,the degree of variation are large for left negative peak and right negative peak, the difference of left negative peak and right negative peak, the left peak-to-valley difference, the left peak-to-valley ratio, signal baseline. However, the degree of variation of positive peak and the peak-to-valley difference as well as the peak-to-valley ratio are rather small.