Research Of Brillouin Distributed Optical Fiber Sensing System Based Mixed-frequency Method

Today, our country is developing rapidly, including economy and transportation,development and utilization of energy, transportation systems such as railways and bridges,communications media such as optical cable and electric cable, oil field and mine. At the sametime of developing and utilizing environment, one important thing is to make sure the security ofworking environment. This requires a very precise safety monitoring system, and we can use thissystem to monitor a variety of safety risks, get early risk warning of what will happen and avoidthe losses of personnel and property. Therefore, using optical fiber to do distribute sensingmeasurement is of great social significance in terms of monitoring environment. The applicationsabout Brillouin fiber sensor are always some important construction sites, so the request formeasurement accuracy should be very high.This article puts forward a method that demodulating frequency shift signal of Brillouinsystem by mixing method. In the frequency of12GHz photodetector, back Brillouin scatteringlight and reference light interact to produce interference signal, namely the Brillouin frequencyshift signal, and then do mixing processing. Reduce the signal frequency from11GHz tomiddle-frequency band below1GHz, and then signal processing again. This can avoid lowdegree of stability brought by the high frequency signal processing, and the subsequentamplification and filter devices, do not have to use high frequency microwave devices, whichreduces the cost.Considering that the effect of birefringence in optical fiber will cause unstable polarizationstate of light, this article adds a scrambler to Brillouin fiber sensor system, and we select theposition of the scrambler in this system, we also verify that the scrambler can really reduce thesystem error and improve system accuracy by doing several experiments.This article also describes how to improve signal-to-noise ratio (SNR) of the system byusing wavelet method. Considering a large number of random noise existing in the system, wecompared the traditional denoising with the wavelet denoising, on the basis of the preliminaryuse of accumulative average. We choose the db5Wavelet to do denoising processing. First weneed to decompose the signals to5layers, do denosing processing for each layer by thresholdvalues. And then, we need regroup signals from layer to layer, and reconstruct a complete signal which is the result of original signal after denoising. By this way, the SNR of the system is raisedabout12dB then before.Brillouin distributed optical fiber sensing system, which is based on the mixing-frequencymethod, not only reduces costs, decreases the difficulty of subsequent signal processing, but alsotakes a step toward practicability again, and it is important to actually improve the performanceof Brillouin optical time-domain system.