Research On TDLAS Gas Detection System Based On Calibration-free Wavelength Modulation Spectroscopy

Tunable Diode Laser Absorption Spectroscopy(TDLAS) is a common monitoring method for the detection of gas concentration, temperature, pressure and velocity. With the invention of the reliable and narrow-linewidth tunable lasers at room temperature, TDLAS has developed rapidly recently and is widely used for gas concentration detection.Two main methods of TDLAS sensing has been used: direct absorption(DA) and wavelength modulation spectroscopy(WMS). DA is simple and do not need to be calibrated, but brings a larger error in low absorption. In WMS, the wavelength is modulated by a scan signal at low frequency superimposed on a modulation signal at high frequency, isolating the detected signal from low-frequency noises to improve the system sensitivity. However, the common calibration strategy is not suitable for lots of practical environments. With the development of TDLAS, ―calibration-free‖ WMS techniques have been the research highlight gradually.In this paper, the methane concentration sensor is established based on calibration-free TDLAS and the main research topics are as follows:First of all, based on Beer-Lambert law, the line strength and lineshape function of gas absorption spectral are introduced. With the help of the database of HITRAN, the transition band of methane is chosen and the related parameters are calculated. The basic knowledge of DA and WMS is studied and harmonic signals are obtained by Fourier expansion. The potential sources of uncertainty in the process of harmonic signal detection are summarized and analyzed.Second, the methane gas detection system based on TDLAS is introduced and built up. The modulation parameters are analyzed and optimized based on the experiments. DA and WMS techniques are separately used for the concentration monitor and calculation. The influence of temperature on the harmonic signal is investigated theoretically and experimentally.Furthermore, the ―calibration-free‖ WMS method is realized by using the fitting measurement and simulation of second harmonic signal. Using Matlab, the model of methane concentration sensor is built up and simulated. The WMS-2f is normalized by the fitted transmitted laser intensity, eliminating the influence of laser intensity fluctuation. The influence of parameters in the process of curve fitting are analyzed respectively and the least-squares fit for fitting strategy is also introduced.Finally, the WMS-2f spectral-fitting is demonstrated and the key performance factors of the whole system are analyzed with the measurement of methane absorption spectroscopy in laboratory, and the gas concentration can be calculated directly. The ―calibration-free‖ WMS method agrees well with the traditional WMS method.