High Speed And High Spatial Resolution Distributed Brillouin Optical Fiber Sensing Technology

Distributed Brillouin fiber-optic sensing technology has many advantages such as continuous measurement without blind zone,large number of sensing points and long sensing range.It is widely used in the fields of structural health monitoring for large-scale infrastructure and geological geophysics.However,due to some limitations,the traditional distributed Brillouin fiber sensing has suffered from the problems of a long measurement time(in the order of minutes)and a low spatial resolution(meter level),and thus its applications and practical value are seriously restricted.Therefore,how to improve the measurement speed and sensing spatial resolution is the research difficulty and hot issue of distributed Brillouin sensing technology.This thesis develops high-speed and high spatial resolution distributed optical sensing technologies to achieve high-performance distributed fiber sensing with measurement time as short as ?s scale and spatial resolution up to mm level.In the field of high-speed Brillouin fiber sensing,to solve the problem of narrow slope bandwidth for traditional Brillouin gain spectrum,this thesis proposes a fast distributed optical fiber sensing based on a slope,a ratio of Brillouin phase shift to Brillouin gain,which has a larger available bandwidth than conventional Brillouin gain slope.In the case of 30 ns pulsed pump wave,the available bandwidth of the proposed slope is 200 MHz,which is four times larger than the bandwidth of the conventional Brillouin gain slope,enabling the maiximum strain dyanmic range of 2467 ?? and the maximum sampling rate of 101 k Hz.Moreover,this thesis proposes a multi-slope-assisted sensing technology,which uses frequency agile modulation to construct cascaded multiple slopes,increasing the strain dynamic range to 5000 ?? with a sampling rate up to 1 k Hz.In order to further improve the measurement speed,this thesis proposes a Brillouin optical time domain analysis based on optical chirp chain to realize ultra-fast distributed measurement,which can resolve the problem of the time-consuming frequency scanning process.The technology firstly modulates the probe light into an optical chirp segment by optical frequency agile modulation,and then connects optical chrip segments to form an optical chirp chain.A single pump pulse enters the sensing fiber and sequentially interacts with each optical chrip segment resulting in a fast scanning of Brillouin gain spctra along the entire sensing fiber.Furthermore,this thesis proposes a twice correlation algorithm,which can realize a corrected Brillouin gain spectrum and improve the signal-to-noise ratio(SNR).In the experiment,for a 10 m sensing fiber,the proposed optical chirp chain based fiber sensing achieves distributed vibration measurements with the sampling rate up to the order of MHz corresponding to a single measurement time of less than 1 ?s.In the aspect of high spatial resolution sensing,this thesis proposes a rising edge demodulation(RED)algorithm to address the limitations on spatial resolution,which can effectively improve the spatial resolution of Brillouin optical time-domain analysis without increasing the hardware cost.Due to the fact that the measured Brillouin signal can be regarded as superposition of sub-Brillouin signals generated by subdivision of the pump pulse,the RED algorithm can obtain a nonlinear coefficient matrix by deriving the resing edge of Brillouin signal,which can reconstrut the Brillouin gain spectra distribution with a much sharper boundary.Theoretically,the spatial resolution after reconstruction corresponds to t he width of the sub-pulse after subdivision.In the experiment,by performing the RED algorithm on the measurement result of an 8 ns pump pulse,the spatial resolution reachs up to the order of mm scale resulting in 100 times improvement.In addition,the algorithm uses frequency domain information to perform multiple demodulation,and to realize a high SNR via a unique "self-averaging denoising".Considering that Brillouin dynamic grating sensing can effectively avoid the limitation of phonon lifetime on spatial resolution,this thesis proposes a phase-shifted Brillouin dynamic grating based on single-pump phase modulation and discusses its potential for a high spatial resolution sensing.Generally,two pulsed pump waves can generate a Brillouin dynamic grating at a fixed position in an optical fiber,and then a phase modulation of one of the pulsed pump wave can generate a phase-shifted Brillouin dynamic grating(PS-BDG).Both of the simulation and the experiment verify that the reflection spectrum of the PS-BDG has notch structure which is the typical feature of the phase-shifted grating.For distributed fiber sensing,the phase-shifted point determines the spatial resolution,which provides a potential for a high spatial resolution.