Research On Gas Sensing Method Based On Fiber Cavity Ring-Down Technique

Fiber cavity ring-down spectroscopy(CRDS)technique has attracted great attention among researchers in recent years due to its high sensitity and high stability,which has been applied in the detection of trace gas concentration.However,in many practice applications,it is often need to detect the gas composition and concentration simultaneously or require multi-point gas detection.Therefore,it is of great practical significance to carry out synchronous detection of gas composition and concentration and multi-point gas detection based on CRDS technique.In order to reduce the cost of gas sensing system,this paper proposes a fiber cavity ring-down gas sensing method based on frequency shift interference technology.In order to improve the sensitivity of gas sensing system,this paper also studies the effect of spontaneous emission noise on the characteristics of the amplified fiber cavity ring-down gas sensing system.The main contents and results of the thesis are as follows:1.A method for synchronous detection of gas composition and concentration of fiber cavity ring-down based on frequency-shifted interference technique is studied.The experimental results show that the absorption loss spectrum of the target gas can be obtained by combining this technology with the laser wavelength scanning method.Then,the peak absorption algorithm is used to extract the characteristic absorption wavelength of the target gas from the loss spectrum.Finally,the target gas can be identified by comparing the extracted characteristic absorption wavelength with the standard characteristic absorption wavelength of the gas.Using the above method,an acetylene gas is identified accurately and its concentration is also detected synchronously.What is more,the sensitivity was as high as 263.48 ppm.In addition,the experimental results show that the stability of the sensing system is greatly improved due to the differential detection explored by our proposed sensing method.The first-order standard deviation of the gas concentration is ±0.03%,which is superior to that of traditional fiber cavity ring-down gas sensing system.2.The multi-point fiber cavity ring-down gas sensing method based on frequency shift interference technology is studied.The releted formula of the sensing method is derived and its sensing mechanism is analyzed.On the basis of theoretical analysis,a multi-point gas sensing scheme with series structure was designed.The simultaneous detection of acetylene gas concentration in two positions was realized experimentally,and the sensitivity of the two sensing channels reached 289.69 ppm and 508.25 ppm respectively.The research results show that the sensitivity of the second channel is worse mainly due to the larger insertion loss of the gas cell in the second channel.In addition,the repetitive experimental results show that the first-order standard deviation of concentration for detection of 1% concentration of acetylene gas is 0.035% and 0.059% respectively,which indicate the gas sensing system has good stability.3.The effect of spontaneous emission noise on the characteristics of the amplified fiber cavity ring-down gas sensing system is studied.The relationship between the spontaneous emission noise of erbium-doped fiber amplifier(EDFA)and the cavity loss,gas sensitivity of gas sensing system are derived and simulated.The simulation results show that the baseline drift of the cavity ring-down curve can be greatly improved by using band-pass filter to filter the spontaneous emission noise of EDFA.The narrower the bandwidth of the band-pass filter is,the smaller the baseline drifts is.Thersfore the detection accuracy and stability of the gas sensing system can be improved by using the narrow bandwidth filter.In addition,the simulation results also show that the gas sensing system has better detection accuracy and stability when the EDFA is placed after the gas cell instead of in the front of the gas cell.The greater the gain of the EDFA is,the higher the sensitivity of the gas sensing system is.