| Online gas analysis with high-sensitivity, high-precision is great significance to industrial production process and environment pollution. Tunable diode laser absorption spectroscopy (TDLAS) is a new spectral technique developed in recently years. TDLAS has been widely applied in the field of industrial production process and pollution gas monitoring due to its advantages.In trace gases detection based on second harmonic signal of TDLAS, line-shape of the signal contain a lot of measurement information. Because the signal line-shape are related to modulation conditions, so there must be an optimum modulation condition, under this condition the maximum of signal with special characteristics of line-shape can be obtained. On the other hand, the optimal condition of modulation can be obtained through research on the signal profile, and according to the best modulation conditions to guide practical detection.According to the characteristics of diode lasers, in the detection of trace gases based on TDLAS,residual amplitude modulation (RAM) could be produced simultaneously when wavelength modulation is performed directly by input current. This RAM will affect signal line-shape of measurements and system noise. In order to study the influences of RAM on the line-shape of wavelength modulation spectroscopy second harmonic signal, an expression of second harmonic signal for simultaneously considering amplitude modulation and frequency modulation (AM-FM) factor is derived. Using the expression quantitative calculation comparisons between AM-FM method and traditional frequency modulation method are performed. The variation of second harmonic signals baseline, positive and negative peak are also calculated in this work. The results show that the RAM has direct influences on detection signal line shape and signal baseline. Calculated signal line shape by the AM-FM method more close to detected results. |
PDF Full Text Request