Research On Signal To Noise Ratio Of The BOTDR System Based On Coherent Detection

The BOTDR sensor is a distributed optical fiber sensor.It can simultaneouslyachieve the distributed time and spatial information of measured field along thesensing fiber.However, the low SNR in signal,which limits the spatial resolution andtemperature or strain resolution,results that the system is insufficient to satisfy thetechnology requirement and limited its applications in engineering.Hence,the paperemploys narrow bandwidth filter such as the bragg gratings by researching on theprinciple of BOTDR and analyzing the characteristics of Brillouin scattering light torealize the stability of the reference light.The stability cannot be realized well,in thissense, is inherent to optical amplification such as distributed Raman amplification toimprove the SNR and enhance the measuring resolution and length of BOTDR system.Firstly,the paper analyzes the study status on the sensors based on BOTDR at homeand abroad.The Brillouin scattering in fiber is introduced in detail from severalaspects,including the relation of Brillouin frequency shift and power versustemperature and strain and coherent heterodyne detection technology of Brillouinsignal,which laid a foundation for designing the whole system..Secondly, the paper studys the instablity of reference light in detail in opticalcoherent heterodyne detection and analyzes the working principle and effect factors ofLiNbO3electro-optic intensity modulator to obtain reference light of the Brillouin shift.This paper proposes a new method of filtering reference light by fiber bragggrating.The experimental results show that the stable reference light in a long time canbe obtained.Then this paper describes Raman amplification principle,classification and Ramanpump configuration.The various configurations are compared and their relativeadvantages are highlighted and discussed.The experimental results show that the SNRof the signal in the BOTDR system can be enhenced.Finally,the paper designs a system of the combination of Raman amplification withFBG filtering to enhance BOTDR system performance.The feasibility of the system is vertified by measuring the temperatue along the fiber.