Fiber-Optic Acoustic Sensing Based Trace Gas Analyzer By Using Photoacoustic Spectroscopy Technology

With the continuous high-speed development of the national economy,the electrical power demanded in all industries is constantly increasing.Electric system is becoming high-capacity,ultra-high-voltage and intelligent.Insulation condition and potential accident in the transformers can be realized in time by continuously online monitoring the electrical equipment.Photoacoustic?PA?spectroscopy?PAS?technology has been the first choice for online monitoring in electric system on account of high sensitivity and high resolution.In PAS system,traditional condenser microphones have some disadvantages,such as mismatching with the PA cell and vulnerability to magnetic interference.Aiming at these problems,this thesis focuses on development of the fiber-optic acoustic sensor based trace gas analysis system by using PAS technology.Theoretical analysis is carried out on the principles of fiber-optic Fabry-Perot?F-P?acoustic sensor.We fabricate a cantilever-based fiber-optic F-P acoustic sensor for resonant PA system.The experiment results show that the sensitivity and detection limit of the presented sensor are 256.7 mV/Pa and 8.5 ?Pa/Hz^1/2,respectively,at frequency 1400 Hz.For a non-resonant PA system,low-frequency acoustic sensor is necessary.Considering its good compactness and small Young's modulus of Parylene-C,we propose a Parylene-C-based fiber-optic F-P low-frequency acoustic sensor.At the frequency of 30 Hz,the sensitivity of the sensor is 1044 mV/Pa and the detection limit is 9.5 ?Pa/Hz^1/2.Compared with traditional condenser microphones,fiber-optic acoustic sensor exhibits the advantages of small size,high sensitivity and immunity from electromagnetic interference.A resonant PA system based on fiber-optic acoustic sensor is developed,which consists of distributed feedback?DFB?lasers,erbium-doped fiber amplifier?EDFA?,resonant PA cell,fiber-optic F-P acoustic sensor and signal processing unit.By optimizing the system structure and operating parameters,the detection limit of acetylene gas is achieved to be 0.84 parts per billion?ppb?.Moreover,a half-open-type resonant PA cell is presented for demand of less gas and faster response time when online monitoring.Without increasing the length of PA tube,the half-open-type resonant PA cell exhibits lower resonant frequency,which increases the response time and PA signals,meanwhile decreases the gas sample as well.Experiment results show that the detection limit of acetylene gas is reached to be 0.81 ppb.The Parylene-C-based fiber-optic F-P low-frequency acoustic sensor,together with infrared thermal radiation source and non-resonant PA cell,constitutes a PA detection system for multiple dissolved gases in transformer oil.The detection limits of acetylene?C₂H₂?,methane?CH₄?,ethane?C₂H₆?,ethylene?C₂H₄?,carbon monoxide?CO?and carbon dioxide?CO₂?are 0.11,0.21,0.13,0.16,0.14 and 0.48 parts per million?ppm?,respectively.The experiment results of the gas mixture demonstrate that the measured concentrations of the six gases are very close to the real concentrations,and the average deviations are less than 5%.This thesis can lay the theoretical and experimental basis for the wide application of optical fiber sensing technology in the electric system.