Research On Long-range Chaotic Brillouin Optical Correlation Domain Analysis Technology

Distributed optical fiber sensing technology based on Brillouin scattering has been widely used in the fields of large building structure monitoring,perimeter security monitoring,transportation,aerospace and artificial intelligence.However,the traditional Brillouin distributed optical fiber sensing technology suffers from a trade-off between the sensing range and spatial resolution in principle.Due to its δ-like function and auto-correlation properties,chaotic laser has been proposed as the signal source in the distributed optical fiber sensing system to solve this problem,that is,chaotic Brillouin optical correlation domain analysis(Chaotic BOCDA).However,the inherent time delay signature(TDS)and non-zero background of chaotic laser introduce noise into the system,making the sensing range severely limited.In addition,controlling a single correlation peak through an optical delay line to scan the entire sensing fiber is time-consuming and difficult to achieve highspeed distributed strain measurement.Therefore,the development direction of the chaotic BOCDA system is to realize massive sensing points(range/spatial resolution)and highspeed real-time distributed strain monitoring based on a long sensing range.In this paper,a chaotic BOCDA sensing scheme based on the optimized time-gated configuration and differential denoising technique is proposed to extend the sensing range,and a chaotic Brillouin optical correlation/time domain hybrid analysis(Chaotic BOC/TDA)sensing scheme is introduced to realize multi-point simultaneous monitoring.The specific research work is carried out as follows:(1)A chaotic BOCDA distributed sensing scheme based on the time-gated configuration and differential denoising technique is proposed to suppress the background noise and improve the sensing range.The feasibility of the scheme is theoretically verified.Firstly,the basic principle of stimulated Brillouin scattering and the sensing mechanism based on stimulated Brillouin scattering are described.Secondly,the working principle of the basic chaotic BOCDA is introduced,and the limitation of background noise on the sensing range is detailed and simulated.Then,in order to extend the sensing range,a chaotic BOCDA sensing scheme based on the time-gated configuration and differential denoising technique is proposed.The working principle of the system is detailed,and the performance improvement for suppressing the background noise is analyzed and simulated.Finally,the simulation results demonstrate the elimination of TDS by the time-gated configuration with high extinction ratio pulse and the suppression of noise floor by the differential denoising technique,which is important for enhancing the sensing range of the chaotic BOCDA system and laying a foundation for the experimental research of long-range chaotic BOCDA distributed strain sensing.(2)A long-range chaotic BOCDA distributed strain sensing scheme based on the timegated configuration and differential denoising technique is experimentally investigated to achieve a sensing range of 27.54 km,where the number of effective sensing points is largely increased to more than 1,020,000.Firstly,by optimizing the time-gated configuration,a chaotic pump pulse with high extinction ratio and high peak power is modulated,and the improvement of the signal-to-background ratio(SBR)of the chaotic Brillouin gain spectrum(BGS)is experimentally verified.Secondly,the differential denoising technique is introduced into the distributed sensing experiment,which effectively suppresses the noise floor accumulated along the fiber and further improves the SBR of the chaotic BGS.Then,the pure chaotic BGS is obtained by Lorentz fit,reducing the standard deviation of the Brillouin frequency shift(BFS)measurement at the fiber end to 2.27 MHz.Finally,the distributed static strain sensing along a 27.54 km long sensing fiber with a spatial resolution of2.69 cm is experimentally demonstrated,where the number of effective sensing points is more than 1,020,000.(3)A chaotic BOC/TDA distributed sensing scheme is proposed to solve the problem of point-by-point positioning and measurement time-consuming in the chaotic BOCDA system,which theoretically proves the feasibility of simultaneous monitoring of multiple points along the fiber.Firstly,a sensing model of the chaotic BOC/TDA is established for simulation.Then,the acoustic field,chaotic BGS,and position scanning of the chaotic BGS are simulated.Finally,the simulation results prove that the chaotic BOC/TDA system can simultaneously realize the rapid reconstruction of BGS at multiple locations along the fiber,greatly reducing the number of distributed positioning,and can realize high-speed distributed sensing.