Brillouin Optical Correlation-domain Reflectrometry Based On Non-periodical Signal

Distributed optical fiber sensors based on Brillouin scattering have the ability ofdistributed measurement of the physical parameters (such as temperature, stress,strain, and vibration), resulted from the dependence of the Brilouin frequencyshift (BFS) on the physical parameters. In comparison with other sensingtechnologies, Brillouin-based optical fiber sensing has attracting more and moreattention owing to the unique advantages of high resolution, long distance, andaccurate measurement.In this thesis, we propose two new scheme of Brillouin optical correlationreflectometry (BOCDR) to resolve the trade-off between the spatial resolutionand measurement distance, by using noise signal and pseudo random bitsequence (PRBS) instead of the sinusoidal signal to modulate the laser diode.Only one correlation peak can be existed in the fiber under test (FUT) as a resultof the non-periodicity of the noise signal and the PRBS. Thus, the sensing rangecan be arbitrarily long in theory. The thesis is mainly summarized as follows:(1) A technology of Brillouin optical correlation reflectometry using a noise-modulated laser diode is proposed. Due to non-periodicity of the noisesignal, only one correlation peak is synthesized at one point along the FUT.In principle, the measurement range can be arbitrarily long as long as theensemble states are sufficiently large with no repeated state.(2) We propose a scheme to measure the temperature along the FUT based onpseudo-random sequence modulation. Adjusting the code length and coderate of the PRBS, the separation between adjacent correlation peaks isconsiderably long to make sure only one correlation peak existed in the FUT.Thus, the measurement of long distance can be achieved, while retaining thehigh resolution.(3) In theory, we analyze that how to resolve the trade-off between the spatialresolution and the measurement range in the two systems. The principle andexperimental setup of the two schemes are demonstrated. The cm-scalespatial resolutions along a250-m optical fiber are obtained corresponding tothe coherence length of the modulated laser respectively and the temperatureaccuracy are±1.3℃and±0.96℃. The experimental results are in goodagreement with theoretical analysis.(4) Compared with other sensing methods, the proposed two schemes’ virtuesand drawbacks. At the same time, the two systems’ future research contentsare pointed out.