Research On Phase Demodulation Technology Of Interferometric Fiber-Optic Acoustic Sensors

Acoustic sensing technology plays a very important role in many industries.In the power industry,it is often used to detect the acoustic signal generated by the photoacoustic effect of the gas in the transformer oil,so as to judge and detect the fault types of the transformer.Traditional electrical acoustic sensors,by converting acoustic wave signals into voltage signals,are easily affected by electromagnetic interference.In contrast,fiber-optic acoustic sensing technology,as an optical detection technology,can achieve high-precision,broadband acoustic signal measurement,and is small in size,light in weight,resistant to electromagnetic interference,and easy for large-scale multiplexing and networking.This thesis focuses on the study of fiber optic acoustic sensing technology and its phase demodulation algorithm.The design and preparation of sensing film,the design and preparation of interferometric fiber optic acoustic sensors,the design of phase demodulation algorithm and the configuration of the system are carried out.The innovative research has realized the measurement of acoustic signals with large dynamic range and broadband response.The specific research contents and innovative achievements are as follows:(1)Based on the theory of film vibration,the research on the preparation process of the composite film is carried out,and the high-performance optical acousto-sensitive titanium-aluminum-titanium composite film is developed using three technological processes.The sensing film with a diameter of 11 mm has an amplitude sensitivity of up to 123 nm/Pa.Based on the sensing film,extrinsic Fabry-Perot Interferometric(EFPI)fiber optic acoustic sensors and Michelson Interferometric(MI)fiber optic acoustic sensors are designed and fabricated.(2)To overcome drawback of quadrature point demodulation that its dynamic range is limited,based on the Ellipse-Fitting Differential-Cross-Multiplication(EF-DCM)phase demodulation algorithm,a new type of dual-EFPI fiber optic acoustic sensor is developed.Through two EFPIs with different cavity lengths,two interference signals whose phase difference is not integer times of π are obtained and EF-DCM algorithm is used to realize phase demodulation.A dual EFPI acoustic sensing system based on EF-DCM is built,whose linearity of phase amplitude and sound pressure reaches 0.999 from 0.5 rad to 3.16 rad.Compared with quadrature point demodulation,upper limit of the dynamic range improves by 3.7 times experimentally and can improve by 17.3 times theoretically.(3)To overcome drawback of EF-DCM that it is not suitable for small signal demodulation,a Linear-Fitting Trigonometric-Identity-Transformation Differential-Cross-Multiplication(LF-TIT-DCM)phase demodulation algorithm is proposed to decrease lower limit of the dynamic range.The phase demodulation is realized by using the LF-TIT-DCM algorithm through two interference signals whose phase difference is odd times of π.Based on the LF-TIT-DCM algorithm,a dual-wavelength acoustic sensing system based on LF-TIT-DCM is built by constructing two interference signals whose phase difference is odd times of π by wavelength.Linearity of the phase amplitude and the sound pressure reaches 0.9998 from0.02 rad to 3.26 rad and lower limit of the dynamic range decreases by 1/25 compared with EF-DCM.Since phase difference of interference signals at the two input ends of the mode coupler is π,a mode-coupled anti-phase acoustic sensing system based on LF-TIT-DCM is built.Linearity of the phase amplitude and the sound pressure reaches 0.9999 from 0.03 rad to 3.18 rad.(4)To overcome drawback of LF-TIT-DCM that it is not able to demodulate aperiodic signal and it requires large amount of computation,a Linear-Fitting Differential-Cross-Multiplication(LF-DCM)phase demodulation algorithm is proposed,whose demodulation speed improves by 43 times.The LF-DCM algorithm is used to achieve phase demodulation through two groups of interference signals whose phase difference is odd times of π.Based on the LF-DCM algorithm,two groups of interference signals whose phase difference is odd times of π are constructed by wavelength,and a four-wavelength acoustic sensing system based on LF-DCM is built.Linearity of the phase amplitude and the sound pressure reaches 0.9997 from 0.09 rad to 2.45 rad and it can demodulate acoustic wave signals in the frequency range from 1 Hz to 100 k Hz.In addition,based on the mode-coupled anti-phase mechanism,a mode-coupled anti-phase dual-wavelength acoustic sensing system based on LF-DCM is constructed.Linearity of the phase amplitude and the sound pressure reaches 0.9997 from0.11 rad to 4.10 rad.