Research On Diaphragm Based EFPI Fiber Optic Acoustic Sensing Technology

Acoustic sensor is a type of detector that detects,recognizes,and locates targets by detecting acoustic signals.Acoustic detecting at different frequency bands has broad application prospects.In the past few decades,electroacoustic sensors have dominated the field of acoustic detection.However,electroacoustic sensors are not suitable in some special environments.Compared with traditional electroacoustic sensors,fiberoptic acoustic sensors have inherent advantages of fiber-optic systems such as the immunity of electromagnetic interference,high sensitivity,compact structure,longdistance transmission loss,easy multiplexing,and multi-parameter measurement.Nowadays,fiber optic acoustic sensors are used in many fields,such as medical imaging,underwater monitoring,seismic monitoring,and non-destructive detection of structures.The extrinsic Fabry-Perot interferometer(EFPI)combines the advantages of optical fiber and interferometer technology,which has both high resolution and compact structure,and has gradually become one of the hot spots in the research of fiber optic acoustic sensor technology.This paper focuses on the key technical issues of the diaphragm based EFPI fiber optic acoustic sensor.The main work includes:(1)The theoretical model of the EFPI fiber optic sensor is analyzed in detail from the two aspects of optical and mechanical performance.In terms of optics,the difference between the interference spectrum of the standard F-P interferometer and the fiber F-P interferometer is deeply analyzed.The propagation and coupling characteristics of the light beam in the fiber F-P cavity are theoretically analyzed,and the Gaussian mode coupling model is used to analyze the influence of the loss caused by the inclination and misalignment of the fiber end face on the interference fringes;In terms of mechanical properties,the vibration distribution of the diaphragm is derived from the vibration equation,and the analytical formula of the diaphragm vibration is obtained.At the same time,the acoustic response characteristics of the diaphragm for forced vibration are analyzed.This part of the work has laid a theortical foundation for the structuall design of high-sensitivity fiber opitc acoustic sensor in this papar.(2)A highly sensitive EFPI fiber optic acoustic sensor based on gold film is proposed.The ultra-thin gold film with a thickness of only 140nm and the end face of the fiber collimator are used as two reflectors.The optical fiber collimator is first applied in the field of acoustic wave detection,which can effectively reduce the loss of the transmitted beam in the F-P cavity,improve the contrast of the EFPI interference spectrum,and increase the sensitivity of the EFPI acoustic sensor.In this paper,we describe in detail the fabrication process,experimental equipment,and important acoustic parameters of the proposed EFPI acoustic sensor.The experimental results show that the gold film based EFPI acoustic sensor has a wide frequency response range of 45 Hz～12 kHz,and the flat frequency response range extends from 400 Hz to 12 kHz,covering almost all audible sound frequency bands.The sensitivity and minimum detection sound pressure of the proposed sensor are measured to be-175.7 dB re 1rad/μPa and 470/μPa/Hz1/2@150Hz,95.3μPa/Hz1/2@2kHz,respectively.(3)Aiming at the environmental noise contained in the acoustic signal collected by the EFPI acoustic sensor,this thesis introduces the basic speech processing flow,and analyzes the denoise principle of the spectral subtraction algorithm and the Wiener filter algorithm in detail.In the experiment,using the spectral subtraction denoise algorithm and the Wiener filter denoise algorithm,the signal collected by the proposed has been processed for speech noise reduction,and the algorithm has been tested both subjectively and objectively.The signal-to-noise ratios of the collected signal after the noise reduction by the spectral subtraction and Wiener filter algorithm have been increased by 6.4 dB and 5.3 dB,respectively,and the speech quality evaluation scores have also been increased by 0.29 and 0.4.Experimental results show that the denoise algorithm has a good effect in suppressing the environmental noise in the acoustic signal collected by the EFPI fiber optic acoustic sensor.