| Compared to electronic sensors,fiber Bragg grating（FBG）sensors are less susceptible to electromagnetic interference,have lower manufacturing costs,higher sensitivity and reliability,and are important in distributed measurement of optical fiber sensing due to their superior multiplexing networking capabilities.Therefore,the study of FBG sensor multiplexing technology has become extremely important.However,the traditional FBG multiplexing technologies include wavelength division multiplexing（WDM）,time division multiplexing（TDM）,space division multiplexing（SDM）and optical frequency domain reflection multiplexing（OFDR）,which have the problems of high cost and low spatial resolution.In this paper,FBG multiplexing technology based on single-arm frequency-shifted interferometry（SA-FSI）is studied.It is a simple,compact,practical and versatile optical fiber sensing technology.It has many advantages over traditional techniques:does not require pulsed light source and fast detector,allows the FBGs to overlap spectrally,demodulates quickly,does not require reference signals,etc.The main contents of this thesis are as follows:（1）The basic principle of SA-FSI is introduced.The expression of the interference signal of the SA-FSI system multiplexing FBGs is deduced.The performance parameters of SA-FSI system are analyzed and discussed.The formulas for calculating the spatial resolutionδL and the sensing rangeLmax of the system are deduced,and the system crosstalk is classified and discussed.（2）A SA-FSI system is constructed,and position information of three strong FBGs is measured.By adjusting the modulation depthαand DC bias working pointεof the electro-optic modulator（EOM）to a suitable value,the maximum intensity of interference from the first-order sideband signal can be obtained.In the case of the overlap of the reflection spectrums of the gratings,serial and hybrid multiplexing of 3strong FBGs with a spatial resolution of 0.1 m are still achieved.（3）A new SA-FSI system is obtained by improving the EOM structure,and the weak grating array is multiplexed.Since the EOM modulation efficiency is not high due to the velocity mismatch between the propagation of the RF wave and the light wave in the conventional EOM,the number of the multiplexing FBGs is limited,and the spatial resolution of the multiplexing system is difficult to be further improved.By removing the terminal impedance of the EOM,there is a strong reflection of RF signal inside the EOM.This method greatly improves the forward and backward modulation efficiency of the EOM,and the interference signal intensity is significantly enhanced by about 10dB.Based on the improvement of EOM modulation efficiency,setting the scan range of RF frequency from 2-8 GHz,achieving the multiplexing of 15 weak FBGs with spatial resolution of a few centimeters and sensing range over 1 km.Besides,reflection spectrum of 15 weak FBGs is constructed through a tunable filter for the first time in a SA-FSI system.This paper uses a method based on SA-FSI to multiplex FBG sensors.Through theoretical analysis,experimental verification and data analysis of the experimental results,the feasibility of high spatial resolution multiplexing FBGs in a SA-FSI system was verified.This technology has important research value and potential application prospects in distributed measurement of fiber optic sensing. |
