Tunable Laser-based Phase-shifting Interferometry And Its Application In Multi-point Fiber-optic Fabry–Perot Acoustic Wave Sensing And Detection

Optical fiber acoustic sensors play an important role in many fields due to their small size,anti-electromagnetic interference,wide frequency response range,and adaptability to harsh environments.The diaphragm-type extrinsic Fabry-Perot interference(EFPI)acoustic sensor has aroused extensive research interests due to its high sensitivity and probe-type sensor structure.The intensity detection method based on quadrature point(Q-point)is one of the most widely used demodulation techniques for EFPI acoustic sensors,but the intensity demodulation detection method has a limited detection dynamic range,and signal distortion occurs when detecting strong acoustic signals.In addition,the Q points of different acoustic sensors are susceptible to change due to fluctuations in the external environment,which increases the difficulty of multiplexing multiple arrays.Phase generated carrier(PGC)demodulation scheme is a mature phase demodulation technology,which usually requires piezoelectric ceramic transducer(PZT)to generate phase carrier.The mechanical characteristics of PZT lead to limited frequency response range,which makes the scheme more suitable for relatively low frequency acoustic signal detection.In this paper,the phase-shifting interferometry with tunable laser-based wavelength or frequency modulation is used to achieve phase demodulation of diaphragm type fiber optic EFPI acoustic sensor,and its application in the simultaneous demodulation of multi-point acoustic sensor array is studied.The main work of this paper is summarized as follows:Firstly,the operational principle of multi-beam interference and phase-shifting interferometry technique are illustrated and discussed.The relationship between reflected and transmitted light intensity and phase difference of F-P interferometer under different reflectance is simulated by MATLAB software.The sensing principle of EFPI acoustic sensor is analyzed and the fabrication process of EFPI sensor based on polyethylene terephthalate(PET)diaphragm is introduced in detail.Through the basic principle of phase-shifting interferometry,the fixed-step phase-shifting algorithm(PSA)is analyzed,and the method of wavelength shifting interferometry is proposed to recover phase information.Summarizes the demodulation idea of optical fiber acoustic sensor for tunable laser-based phase-shifting interferometry.We build a tunable laser-based phase-shifting interferometry system in this work.Five current sources are driven and controlled in parallel by FPGA to realize the control of laser output wavelength for the purpose of phase shifting.The laser output wavelength range is1527?1567 nm,The FPGA is controlled by the host computer Lab VIEW software to realize the quasi-continuous wavelength scanning or high-speed switching of specified working wavelength.Then,the multiplexing technology of acoustic sensor based on self-calibration wavelength shifting interferometry is studied.The high-speed wavelength switching of the MG-Y laser is used to introduce phase shifts in the wavelength domain and demodulate the phase information of multiple sensing channels in real time,achieving a cavity-length sampling frequency of 100 k Hz.By building a four-channel acoustic sensing detection system,it is verified that the system can demodulate the EFPI acoustic sensor array simultaneously and has strong stability under environmental fluctuations.Finally,we propose frequency-modulated quasi-continuous phase-shifting interferometry(FMQC-PSI)demodulation technique with improved phase sampling rate.Four laser frequencies with ?/2 phase difference are modulated sequentially to generate quasi-continuous phase shift signal.Combining with the data collected in the previous sequence,a phase measurement value can be obtained at each sampling point.The phase sampling rate of cavity length is increased to 600 k Hz.This technology can achieve the same phase sampling rate(600 k Hz)as the laser frequency modulation speed.The demodulation error caused by the time delay between phase shifts is analyzed by numerical simulation.A compact four-sensor FMQC-PSI demodulation system was constructed for two-dimensional sound source localization,achieving a localization accuracy of 1.98 cm,which proves the effectiveness of the system for simultaneous demodulation of the acoustic sensor array.In conclusion,in this thesis,we investigate the acoustic sensor demodulation method based on tunable laser-based phase-shifting interferometry and builds a measurement sensing system that simultaneously demodulates the acoustic sensor array,which realizes the fast,large dynamic range and high stability demodulation of the acoustic sensor array.It is expected to be used in practical applications such as sound source localization in harsh environments or multi-point photoacoustic spectroscopy gas detection.