Research Of The Gas Concentration Measurement Based On Continuous Wave Cavity Ring-Down Spectroscopy

It is very important for the safety production and safety technology of coal industry and human life to detect gas. For many traditional gas concentration measurement methods measuring accuracy is not high. Cavity ring-down spectroscopy is a novel spectroscopic technique with both high sensitivity and high spectral resolution. Too long since it emerged, CRDS all have used pulse laser as light source. Compared to pulse-CRDS (P-CRDS),continuous wave (CW-CRDS) can obtain spectral resolution and detection sensitivityIn this paper, the new gas concentration measurement system based on CW -CRDS was introduced. The primary content includes:First, the measurement principle of the multi-type gas concentration is analyzed, characteristic parameters of ring-down cavity and basic concept of laser and cavity mode is introduced. And the factors that affect the gas concentration measurement precision, sensitivity and resolution are researched. Two methods of coupling between laser mode and cavity mode is described. We deduce the variation of output light electric field with the scan speed,incident light frequency and gas absorption in two methods.Second, a new gas concentration measurement system based on CW -CRDS was introduced. This method permits us to use CRDS without switching the laser off or/and interrupting the beam, mechanically, for instance AOM. The structure of the gas concentration measurement system is presented, and the performance and working principle of each device are analyzed.Last, we numerically calculate the coupling efficiency of our CW-CRDS device with respect to the scanning speed of the cavity and the reflectivity of the mirrors. On the one hand, it have been investigated that how photons are coupled to the cavity and how build-up in intensity and leak out of the cavity as a function of time. It is noted that both ring-up and decay times decrease, but oscillations' amplitude increase as the reflectivity increases. On the other hand, relative intensity increases with decreasing velocity. Additionally, simulations show that a non-detuned cavity displayed transmitted signals which are highly dependent on the mirrors' speed and reflectivity. We have analyzed the behavior of the novel detuning process. Simulations also display a difference between switching the laser off and detuning it. Then, the experimental analysis of the gas concentration measurement that uses the designed system is performed, the relation ring-down time and gas concentration is analyzed.