Research On Coherent Rayleigh Noise Reduction Method In Single-Ended BOTDA System

In recent years,with the rapid development of aerospace,bridges,civil construction and other fields,higher requirements have been placed on performance indicators such as measurement accuracy and sensitivity of distributed optical fiber sensors,so it has become particularly important to develop higher-quality optical fiber sensors.Among them,because the advantages of convenient application,superior reliability,and can still detect when the fiber breaks.the single-ended structure of the distributed optical fiber sensor has received extensive attention and research from scholars at home and abroad.This paper firstly introduced the background and significance of the research,including the development and classification of BOTDA sensing technology for single-ended structures,and focused on the research progress and status of interference fading and its reduction methods.The relationship between the Brillouin frequency shift in the optical fiber and the temperature and strain of the environment was theoretically analyzed,and the functional expression between them was established.The relationship between the Brillouin scattering spectrum and the pulse width of the incident light was promoted and discussed,laying a foundation for subsequent simulations and experiments.Aiming at the influence of interference fading on the signal-to-noise ratio and measurement accuracy of single-ended BOTDA systems based on Rayleigh scattering,this paper started with the statistical distribution of Rayleigh scattering and used MATLAB to establish the model and analyz the mechanism of interference fading noise generation.Mainly include the influence of optical fiber refractive index distribution,light source frequency and light pulse width on the interference fading waveform.The simulation results show that the uneven distribution of the refractive index on the fiber link is an important factor leading to the problem of interference fading;By changing the frequency of the light source to increase the sample of the detection curve to reduce the repeatability of the detection curve is an effective way to decrease the interference fading;Increasing the light pulse width can reduce the amplitude fluctuation of the detection curve caused by interference fading,but this comes at the cost of reducing the system spatial resolution.Based on the generation mechanism of the interference fading problem and its impact on the signal-to-noise ratio and measurement accuracy of the system,this paper proposed a noise reduction scheme based on the phase modulation of light waves—single-ended BOTDA local heterodyne detection system based on QPSK modulation.The principle of reducing interference fading noise and the system signal-to-noise ratio were analyzed and deduced,and the system model was established and simulated by MATLAB software.The results show that the amplitude fluctuation of its Rayleigh detection signal and the CRN percentage component are significantly reduced by 1.77%,1.99%,and 2.32%,which effectively reduce the percent component of CRN and power fluctuations of the Brillouin signal after adopted QPSK modulation technology;The system signal-to-noise ratio is 20.35d B when the PRBS bit length set to 11th order(N=2¹¹-1).Compared with the single-ended Rayleigh BOTDA system without QPSK modulation technology,the system signal-to-noise ratio increased by 15.1d B.