Research On Acetylene Near Infrared Photoacoustic Spectroscopy Detection Technology Based On Tunable Laser

Acetylene(C₂H₂)is a very active alkyne compound.It is easy to burn,decompose and explode in the air.The explosion concentration range is 2.5%-80%.Compared with the explosive range of other flammable and explosive gases,acetylene explosion range is larger and the lower limit of explosion is lower.In the detection of transformer fault type,it is one of the important characteristics.By detecting the content of acetylene gas,we can judge whether partial discharge or arc occurs in transformer.Therefore,the detection of trace acetylene gas is of great significance to the life prediction and online evaluation of transformer,safety production,air quality detection and underground gas environment detection.Photoacoustic spectrum gas detection technology is a spectroscopic technology developed based on the photoacoustic effect.When the gas to be measured in the photoacoustic cell is illuminated by a modulated monochromatic light,the light energy will be absorbed in a non-radiative relaxation manner.Or all of it is converted into thermal energy,and the gas is heated to expand and generate a pressure wave.A high-sensitivity sound sensor can receive the photoacoustic signal,and the gas concentration can be obtained by analyzing the photoacoustic signal.Photoacoustic spectrum gas detection technology has the advantages of high detection resolution,high sensitivity,strong anti-interference ability and wide detection range.It is an effective method to realize gas real-time and accurate detection at present.Based on the theory of photoacoustic spectrum,the relationship between photoacoustic signal and sensitivity of microphone,gas concentration and power of light source is deduced.An experimental device of gas photoacoustic spectrum detection based on erbium ytterbium Co-doped DBR fiber laser is built to detect the concentration of trace acetylene.According to the absorption spectrum of acetylene,the structure and mode selection characteristics of DBR fiber laser are analyzed theoretically,and a setup of laser with center wavelength near acetylene absorption peak is built.Based on the temperature sensitive characteristic of Bragg grating,the central wavelength of the laser can be adjusted by adjusting the temperature,and the central wavelength of the laser basically corresponds to the acetylene absorption line.Photoacoustic cell is the most important part of the photoacoustic spectrum detection system.In this paper,the principle of the first-order resonant photoacoustic cell is studied,the transmission model of the first-order longitudinal resonant photoacoustic cell is established,and the simulation analysis is carried out.According to the simulation results,the basic structure and geometric size of the photoacoustic cell are determined,and the design and fabrication of the photoacoustic cell are completed by using quartz glass tube as the material and basic structure.The DBR fiber laser is used as the light source,and the cylindrical first-order longitudinal resonance photoacoustic cell is used as the acoustic signal generator.The resonant frequency of the cavity is 1665 Hz,which matches the theoretical calculation.At the same time,the noise detection of the photoacoustic spectrum system is explored.Finally,the relationship between photoacoustic signal,acetylene concentration and laser power is studied.The linear regression equation is obtained by linear fitting,and the inversion test of 100 ppm acetylene gas for one hour is carried out.The standard deviation of the system measurement result is obtained to be 7×10^-6.