Research On Fiber Nonlinearity And Optimization Of Pulse Peak Power In Distributed Optical Fiber Sensing

Distributed fiber sensing is a sensing technology that uses optical fiber as a carrier.The optical fiber is not only a sensitive element for acquiring information,but also a channel for transmitting information in the sensing system.Therefore,compared with traditional electrical sensors,distributed fiber sensor systems have the advantages of high resolution,small error,and strong electromagnetic interference resistance.Moreover,they can continuously monitor physical parameters such as strain,temperature,and vibration by a long-distance continuous distributed manner.At present,this technology has been widely used in the monitoring field of large structures such as rails,bridges and extremely high voltage(EHV)transmission lines.It has broad application prospects.In a distributed sensing system,stronger pulse power at the incident port,stronger power at the receiving port and hence better signal-to-noise ratio(SNR)and longer sensing distance.However,due to nonlinear effects in fiber,especially self-phase modulation and stimulated Brillouin scattering,the pulse power injected into fiber cannot be increased without limit.Because of the existence of such performance degradation factors and motivated by the needs of practical applications,this dissertation has carried out relevant optimization studies on the effects of self-phase modulation and stimulated Brillouin scattering,and striven to find a suitable pulse injection optical power to increase the SNR of a system by increasing the pulse power as much as possible,while avoiding the performance degradation caused by nonlinearity as much as possible.The research work is as following:1)A distributed optical fiber sensing system for pulse peak power optimization is designed.An ultra-narrow linewidth laser is used and gated by a switching semiconductor optical amplifier to obtain pulsed light,and then the pulsed light is input into a tunable erbiumdoped fiber amplifier(EDFA)to generate different pulse peak powers before being injected into an optical fiber.Based on this system,the waveform data of the incident and the output pulse light of the sensing fiber,and the spectral data of the received scattered light are carefully measured for the study of fiber nonlinearity.2)Pulse power optimization under the constraints of self-phase modulation has been studied.First,a theoretical numerical simulation was carried out to simulate the evolution of pulses of different powers in a standard single-mode fiber with distance,and an empirical formula for the upper limit of pulse power under self-phase modulation is obtained through the simulation.Then,an experimental system for studying pulse power optimization was built,and experimental data related to self-phase modulation was obtained.Through the analysis and processing of the data,an empirical formula for the upper limit of pulse power under self-phase modulation is obtained.3)Pulse power optimization under the constraints of stimulated Brillouin scattering has been studied.According to the experimental system constructed above,the spectral data of scattered light is collected.Then the data is analyzed to deduce an empirical formula of the upper limit of pulse power under stimulated Brillouin scattering.In addition,cases with stimulated Brillouin scattering suppressed has also been studied,in which a coding scheme is investigated and compared to those cases without coding.4)Through the analysis of two groups of empirical formulas for the upper limit of the pulse power,it shows in a dispersion environment self-phase modulation impose more stringent limit on the peak incident power of pulse,which gives us an estimation of the maximum incident pulse power in distributed optical fiber sensing systems.For a typical 25 km distributed optical fiber sensing system,the upper limit of the pulse peak incident power is estimated to be about 1 W.