The Study On Digital Processing Of Detected Signals And Signal Demodulation Of Coherent Optical Time Domain Reflectometer

The optical fiber sensors measure environment parameters which modulated the optical signals propagating along the fiber.Among the various implements of optical fiber sensors,coherent optical time domain reflectometer(COTDR),with its advantages of relatively high power and high spatial resolution,has been used widely in the area of distributed temperature or strain detection.This research studies the digital processing of detected signals and signal demodulation of COTDR.For COTDR with an extremely high sensitivity,noise and interference in the environment can severely affected the results of cross correlation of signals.The wavelet threshold denoising method combined with moving average is proposed to remove noise in the COTDR system.In a static temperature detection,the temperature changes slowly,and the temperature variation between two measurements may be too small to identify.To make a more accurate temperature/strain measurement,a weighted fitting method is put forward to improve the detection resolution of COTDR.The denoising method provides a good foundation for the application of the weighted fitting method.Based on the above methods,a signal processing technology of COTDR is proposed.This study researches the technology by theoretical analysis,simulations and experiments.This dissertation is organized as follow:Firstly,we analyze 3 kinds of denoising method using simulation tools and find the combination of wavelet threshold denoising and moving average fits COTDR best.Secondly,we study the statistical nature of the correlation curve theoretically and find a reasonable fitting model sin c2(·).Finally,an experiment measuring temperature by the direct-detecting COTDR at a distance of 26km is carried out,realizing a temperature resolution of 0.034? and a spatial resolution of 42m.