Research On Optical Noise Characteristics Of Brillouin Optical Time Domain Analyzer

Distributed optical fiber sensors(DOFS)based on optical reflection technology measure the backscattering of light in optical fibers to obtain the spatial distribution of physical parameters(loss,temperature,stress or liquid density,etc.).It has important application value in border security,fire warning,oil pipeline detection and smart city construction,due to its durability,stability,small size and anti-electromagnetic interference.Among them,the Brillouin Optical Time Domain Analyzer(BOTDA)based on Stimulated Brillouin Scattering(SBS)has the outstanding advantages of long sensing range,high-precision and distributed temperature strain detection,and is a hot spot of scientific research and industry.In BOTDA sensor,the performance metric of sensing depends on the signal-to-noise ratio(SNR)of the detected signal,while the signalrelated optical noise is a key factor limiting the optimization of sensing performance.Therefore,this paper focuses on the optical noise in the system,and optimizes the sensing performance by deeply studying the mechanism of such noise and proposing corresponding innovative solutions to improve the SNR of BOTDA,optimize the sensing performance and promote the development of distributed Brillouin sensing technology and application.The related work and innovations of this paper are as follows:1)In terms of long-distance sensing,the traditional optical pulse coding theory does not consider the influence of optical noise on the theoretical coding gain,and the real coding gain brought by the coding technology needs to be studied.For this issue,the noise model of single-pulse and coded BOTDA system is theoretically established,and the analysis points out that the optical noise related to Brillouin gain,such as polarization noise and spontaneous Brillouin scattering(SpBS)to signal beating noise could penalize the theoretical coding gain,depending on the code type and the interrogated fiber position.Experiments have verified the accuracy of this model with optimized single-pulse,periodic codes and aperiodic codes.This noise model quantifies the actual SNR enhancement provided by pulse coding at any fiber position and in any operating condition,and also provides a method to optimize the coded BOTDA system for the best sensing performance under the given conditions.2)In terms of fast measurement,the phase to intensity noise of laser limits the dynamic sensing performance of slope-assisted BOTDA technology(SA-BOTDA),and in-depth research and suppression schemes for this noise have not been reported.For this issue,the generation mechanism,distribution characteristics and influencing factors of the phase to intensity noise are analyzed in detail,and a method for postprocessing balance detection is proposed,which can effectively suppress the laser phase noise without increasing the measurement time.In addition,a chirp compensation method is proposed to overcome the inherent asymmetry of Brillouin gain spectrum(BGS)and Brillouin loss spectrum(BLS),further improving the effectiveness of post-processing balance detection,and the SNR of SA-BOTDA system is close to the optimal theoretical value.This technology uses a 1 MHz linewidth laser,and achieves 6.1 dB and 8.3 dB improvement in SNR at a spatial resolution of 2 m and 4 m,respectively,compared to the traditional SA-BOTDA technology.3)In terms of engineering applications,the loop configuration consisting of a leading fiber and a sensing fiber is a common approach to realize BOTDA,which is however impractical to implement for some field scenarios where only a single fiber is physically available.Moreover,the single-fiber-based BOTDA technology cannot achieve long-distance sensing due to the Rayleigh beating noise.For this issue,a single-fiberbased BOTDA with far-end modulation is proposed,in which an auxiliary light and a pulse are injected from the near-end of the fiber,and the counterpropagating probe light is generated by modulating the auxiliary light from the far-end.By dedicatedly designing the frequency and temporal profile of each lightwave,Brillouin interaction takes place only between the pulse and the probe light,while stray effects such as four wave mixing(FWM)and Rayleigh beating noise are avoided,so that a long-distance single-fiber sensing can be realized.The experiment verifies the feasibility of this technology and a 80-km-long sensing range,2 m spatial resolution and 1.73 MHz frequency accuracy is experimentally demonstrated with 4000 trace averages.The sensing performance is far superior to the traditional single-fiber based BOTDA technology,and is similar to the performance of the traditional BOTDA technology.