Research On Non-Cross Sensitive Measurement Of Temperature And Strain Based On Chaotic Bocda Distributed Optical Fiber Sensing

Brillouin distributed optical fiber sensing technology has important applications in the fields of civil engineering,electric power,chemical industry,and aerospace.This technology usually uses pulsed light,continuous light whose frequency or phase is sinusoidal modulated as the detection signal,However,the correlation domain systems suffer from a trade-off between the measurement range and the spatial resolution.In order to solve this contradiction problem,chaotic laser has been proposed as a detection signal to be used in distributed optical fiber sensing systems because of its noise-like and self-correlation function as a pin-shaped characteristic,namely chaotic BOCDA.The sensing mechanism is essentially based on the linear relationship between the Brillouin frequency shift in the optical fiber and the strain or temperature.Therefore,the chaotic BOCDA system faces the problem of cross-sensitivity between temperature and strain.In this paper,using the two Brillouin gain peaks of a large effective area non-zero dispersion-shifted fiber(LEAF)or the different temperature and strain coefficients of the two modes in a few-mode fiber(FMF),Successfully realized the simultaneous measurement of temperature and strain of the chaotic BOCDA system.The specific research work is carried out as follows:(1)A chaotic BOCDA distributed optical fiber sensing experimental system based on LEAF fiber was built;the scattering characteristics of chaotic laser in LEAF fiber were measured experimentally,and the first and third peaks of the Brillouin gain spectrum(BGS)in LEAF fiber were measured.The frequency shift has a different linear relationship with strain and temperature.By analyzing the Brillouin frequency shift of the two peaks in the Brillouin gain spectrum of the chaotic laser,the simultaneous measurement of temperature and strain can be preliminarily achieved by analyzing the two Brillouin frequencies of the chaotic laser in a LEAF.A temperature resolution of 1 °C and a strain resolution of 20 ?? can be obtained with a spatial resolution of 3.9 cm.The actual temperature and strain measurement errors are 0.37 °C and 10 ??,respectively,which are within the maximum measurement errors.(2)The theoretical model of chaotic BOCDA distributed optical fiber sensing based on few-mode fiber is established,Numerically calculated the chaotic Brillouin gain spectrum(BGS)and phase shift spectrum(BPS)of different mode combinations of LP01-LP01,LP01-LP11,LP11a-LP11 a,LP11a-LP11 b in the few-mode fiber,as well as the coupling efficiency between chaotic BGS and chaotic BPS of different mode combinations.The different temperature coefficients and gage coefficients of the two modes are combined LP01-LP01 and LP11a-LP11 a to decouple the temperature and strain in FMF.It is calculated theoretically that (?)T and(?)e are 24.8? and 1003.5?? respectively.Their temperature and strain errors are 0.2°C and 3.5?? respectively.