Research Of Optical Fiber Methane Sensor Based On Photothermal Effect

With the development of industry,toxic and harmful gases have become the source of danger that people must take seriously.For example,as flammable and explosive gas,methane is an important kind of industrial raw material and fuel gas in people's daily life.It's important to carry out effective and eal-time concentration detection.Compared withtraditional electrical gas sensor,the optical fiber gas sensor has many advantages including intrinsic safety,high sensitivity and anti-interference ability,so it has always been the focus of foreign scholars.As typical fiber gas sensor,the theory basis of specturm absorptive optical fiber gas sensor is the absorption effect of gas in the near infrared band,which can be diviede into direct absorption effect(light intensity attenuation)and indirect effect(photothermal or photoacoustic effect).In this paper,we use the photothermal-induced phase change to detect gas concentration,and the linear Sagnac interference structure is proposed for the first time as the structure of gas sensing system.The main work of this paper include the following aspects:1.The motion form of gas molecules and its absorption spectra theory are studied from microcosmic view,the typical absorption line of gas molecules is depicted and we select the suitable absorption peak for methane detection,then we focus on theory analysis of the photothermal-induced phase modulation effect of gas molecules,the relationship between phase and gas concentration is deduced,which provides a theoretical basis for gas sensing system in this paper.2.The advantages and disadvantages of the Michelson interferometer,the Mach-Zehnder interferometer,the Sagnac interferometer and the Febry-Perot interferometer are compared and finally we select the linear Sagnac inferometer as gas sensing stucture which is improved based on conventinal Sagnac inferometer,the theory of system is analyzed,including the principle of phase detection and the output formula,the various devices of the sysytem are introdued in detail,and the experimental system is builted manually.3.The single-point concentration detection experiment is demonstrated with methane as target gas,The relationship between the system output and the DFB laser modulation signal and the gas concentration is analyzed,and good measurement result is obtained.Under certain experimental conditions,the system's noise equivalent concentration of frequency domain and time domain is 0.13%and 0.5%,respectively.It is proved that the average method of frequency domain superposition can effectively improve the detection capability of weak gas-absorbing signal?the scheme of the distributed detection system is put forward and analyzed theoretically,On this basis,we demonstrate the feasibility of the two-point detection sysstem.