Research On Demodulation Technology Of Temperature Self-compensating Optical Fiber Sound Pressure Sensor Based On EFPI-FBG

Acoustic detection is essential in many practical applications,such as environmental noise measurement,non-destructive structure monitoring,photoacoustic spectrum detection,and sound source localization.Compared with the traditional electronic acoustic sensors,optical fiber acoustic sensors have many advantages such as compact structure,light weight,high sensitivity,wide frequency response,large dynamic range,electromagnetic immunity and easy multiplexing,having attracted great attention and been widely used in many fields.Among them,the extrinsic Fabry-Perot interferometer(EFPI)optical fiber sound pressure sensor based on the sensitive diaphragm,which has the merits of high sensitivity and simple structure,is one of the research hotspots of acoustic sensors.However,the EFPI fiber optic sound pressure sensor has cross sensitivity of sound pressure and temperature which will lead to inaccurate sound pressure measurement.Therefore,it is necessary to perform temperature compensation in practical applications.Fiber Bragg gratings(FBG)is a common structure to measure temperature owing to its characteristics of simple structure,high temperature resistance,and easy multiplexing.FBG is usually cascaded with EFPI to constitute hybrid sensors,but due to the trade-off between the accuracy and speed of the existing demodulation technologies,these combined sensors are mainly used to measure static parameters,such as temperature and pressure/strain.For the above problems,a synchronization demodulation algorithm based on the white light interference(WLI)system is proposed for the EFPI-FBG composite sensors in this paper,which enables EFPI-FBG type sensors to achieve dynamic signal measurement by a single demodulation system.In which the F-P cavity length and the center wavelength of FBG are acquired by fast white light interferometry demodulation algorithm and quadratic fitting method respectively.In order to verify the feasibility and performance of the proposed algorithm,a temperature-compensated fiber-optic sound pressure sensor with a series structure of diaphragm-based EFPI and FBG is constructed here,where EFPI is used to measure the sound pressure,and the FBG measures the temperature while performing temperature compensation for EFPI.This article starts with the structural principle and spectral characteristics of the EFPIFBG hybrid sensor,and focuses on the demodulation technology of the EFPI-FBG temperature self-compensating optical fiber acoustic sensor based on the white light interference system.The details are as follows:1.In-depth research on the acoustic sensing principle of EFPI and the temperature sensing principle of FBG,focusing on the analysis of the sensing principle and composite reflection spectrum expression of the EFPI-FBG composite sensor,which is ready for the design of the demodulation algorithm ready.2.Aiming at the EFPI-FBG composite sensor,a synchronization demodulation algorithm based on the white light interference system has been designed in this paper,and the methods to accurately separate the EFPI reflection spectrum and the FBG reflection spectrum,as well as the fast white light interferometry demodulation algorithm for the low-finesse F-P cavity and quadratic fitting method for FBG are introduced in detail.Finally,the synchronization demodulation algorithm is implemented in Lab VIEW.3.In this paper,the demodulation error of the proposed demodulation algorithm is carried out by simulation,and the influencing factors on demodulation accuracy are studied,including the wavelength resolution of the spectrum,FBG bandwidth and the light source bandwidth.When the wavelength resolution is 39 pm,FBG width is 0.1nm,and the light source bandwidth is 1525?1605nm,the simulation indicates that within the F-P cavity length range of100?1000?m,the demodulation accuracy of F-P cavity of EFPI-FBG composite sensor almost is the same with the single F-P cavity's,and the offset error of FBG is within ±1.5pm.In view of the ±2? phase jump error in the fast white light interference demodulation algorithm,this article analyzes the reason for the jump and considers the attempted correction method.Because the center wavelength of FBG will also be modulated by EFPI spectrum,this article takes the averaging method to reduce the demodulation error of FBG.Besides a simulation demodulation speed test is performed,the speed can arrive to 3.93 k Hz on the Intel i5-10210 U laptop,and20 k Hz is expected to be achieved on the computer with higher configuration.4.In this paper,a temperature self-compensating optical fiber sound pressure sensor based on EFPI-FBG is constructed to verify the proposed demodulation algorithm.The experimental results show that the temperature accuracy of the synchronization demodulation algorithm is±1.3?,compared with no temperature compensation,temperature compensation reduces the sound pressure measurement error from 100% to ±10%,and the demodulation speed after temperature compensation reaches 3.2k Hz without optimization.The resolution of F-P cavity length and FBG center wavelength under static condition are 276 pm and 0.334 pm,respectively,indicating that the proposed algorithm has high demodulation resolution.The synchronous demodulation algorithm proposed in this article has the ability to measure the length of the F-P cavity and the center wavelength of FBG at high speed,high accuracy,high resolution and large dynamic range.A simple white light interference system can be used to realize the dynamic demodulation of the EFPI-FBG composite sensor,realizing the practical application of temperature self-compensating fiber optic sound pressure sensor based on EFPI-FBG.