| With harmonic detection, tunable diode laser absorption spectroscopy because of the characteristics of low detection limits, high signal to noise ratio and high sensitivity becomes a powerful in-situ and real-time method for monitoring trace gases.In this paper, the basic theory of gas absorption spectroscopy, the basic structure and the characteristics of major components of tunable diode laser absorption spectroscopy detection system were described. Frequency modulation spectroscopy and wavelength modulation spectroscopy and different points between two methods were described. Based on it, an innovative second harmonic signal analysis and extraction method of wavelength modulation spectroscopy with taking into account the wavelength and amplitude modulation was proposed. Residual amplitude modulation noise that neglected in traditional wavelength modulation spectroscopy method was proposed as an important factor affecting the sensitivity. The difference between peak and valley of second harmonic signal was utilized for the system detection signal to calculate the SNR and detection limit.The impact of incidental amplitude modulation to SNR and detection limit were discussed and calculated in detail. The results of calculating in the case of Lorentz and Gaussian line-shape show that residual amplitude modulation noise is an important and non-ignorable factor to affect the SNR and detection limit. |
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