Research On Photoacoustic Spectroscopy Gas Detection Based On High-finesse Fiber Optic EFPI Microphone

Nowadays,trace gas detection technology has been widely used in industrial production,medical diagnosis,and environmental protection.In the condition monitoring of power system,the change of the gas composition and concentration in the transformer insulating oil reflects the operating condition of the equipment.Therefore,the study of gas detection technology is of great significance to ensure the proper functioning of the power system.Compared with traditional electrochemical gas detection methods,photoacoustic spectroscopy（PAS）gas detection has attracted more and more attention due to its fast response,low background noise,high sensitivity,and the ability for online monitoring.The core detection component of PAS is microphone,which were mainly the electrical microphones in the past.With the development of optical fiber sensing technology,the extrinsic optical fiber Fabry-Perot（EFPI）acoustic sensor has the advantages of small size,light weight and anti-electromagnetic interference,more suitable for the application in PAS detection system.In this paper,facing the high sensitivity requirements of the microphone in PAS system,an optical sensitivity-enhanced method based on the multi-beam interference effect is proposed,and two EFPI sensitizing sensors are designed and prepared,and they are applied in the photoacoustic spectrum detection system.Specific research content is as follows:（1）According to the theory of optical interference,based on the multi-beam interference effect,a sensing scheme that uses high-order optical interference to amplify the interference phase difference to achieve sensitivity enhancement is proposed,and the Fourier transform white light interferometric demodulation method（WLI）is used.The high-order spatial frequency peak after the Fourier transform corresponds to the high-order interference light.At this frequency,the amplified phase can be obtained by arctangent method.Considering the influence of interference light intensity and contrast,a comprehensive evaluation function is constructed to optimize the parameters of the sensitization sensing system according to the requirements of different sensitization multiples.When the expected sensitization multiple is 10 times,the reflectivity of R1 needs to reach 80%.（2）According to the optimization results,to further reduce air loss,a high-finesse EFPI-enhanced sensing scheme based on a fiber collimator is proposed,which uses a collimator to transform divergent light into collimated light.And the output end of the collimator lens is plated with a 50%anti-reflection coating to improve the reflectivity.Due to the coupling efficiency of the collimator is sensitive to angle tilt,a sensor package solution with adjustable pitch angle is designed and processed to achieve high-efficiency coupling of reflected light,and its interference spectrum characteristic contrast and FSR can be freely designed according to requirements.A high-precision EFPI sensor with the diaphragm diameter of 4mm is successfully prepared.Experimental results shows that the second-order and third-order peak demodulation signals has achieved gain of 6d B and 10d B,respectively.At the same time,the signal-to-noise ratio of the second-order peak is improved,up to 2d B.Using second-order peak demodulation,the minimum detectable sound pressure of the system is 0.3m Pa/Hz^1/2@16Hz.（3）Based on the electron beam evaporation process,a new 550 nm Cr/Ag/Au composite sound-sensitive diaphragm is prepared.20 nm Cr is used as the adhesion layer to improve the adhesion between the photoresist and the metal.500 nm Ag is used a deformable layer because it can be fabricated at low temperature to minimize the accumulation of diaphragm stress caused by temperature changes,improving sensitivity and transfer success rate.Au is coated as an anti-corrosion layer,which also provides high reflectivity.A high-precision EFPI acoustic sensor with a large diaphragm diameter of 10mm based on a 35%reflectivity coated fiber is prepared.The response is flat in the range of2～250Hz,the first-order peak sensitivity exceeds-105d B re 1rad/μPa,the second-order Peak sensitivity exceeds-100 d B re 1rad/μPa.（4）A non-resonant photoacoustic spectroscopy gas detection system is built,and the components of the system such as light source,filter and photoacoustic cell are introduced in detail.The prepared high-precision EFPI acoustic sensor is used to detect the concentration of C₂H₂ and CO₂ gas.For C₂H₂,the response sensitivity of the first-order peak and second-order peak are 4.5×10^-4 rad/ppm and 9.1×10^-4rad/ppm,respectively.The detection limits are 9.6 ppm and 7.5 ppm,respectively,and the detection limit of the second-order peak is increased by 25%.The response sensitivities of the measured first-order and second-order peaks of CO₂ are 3.16×10^-3 rad/ppm and 6.43×10^-3 rad/ppm,respectively,the detection limits are 2.5 ppm and 2 ppm,and the detection limit of the second-order peak is increased by 20%.