Research On Interferometric Φ-OTDR Phase Demodulation Technology For Long-range Vibration Detection

Phase Sensitive-Optical Time Domain Reflection(Φ-OTDR)based distributed fiber optic sensing system is one of the fast developing and widely applicable vibration measurement solutions,which has the advantages of simple structure,easy deployment,corrosion and electromagnetic interference resistance,and long detection distance.By demodulating the phase information of the backward Rayleigh scattered light generated in the optical transmission process,the vibration along the fiber could be located and the vibration signal can be recovered.In this paper,Michelson Interferometer(MI)is used in combination with direct-detectionΦ-OTDR system to reduce the cost and complexity of the system,and at the same time,it could suppress the external environmental noise and solve the problem of phase random fading to a certain extent.In this paper,starting from the basic principle of light scattering in optical fibers,the Φ-OTDR-based vibration detection technology is studied based on a careful analysis of scattering-based fiber-optic distributed sensing systems,as follows:Firstly,the basic principle of Φ-OTDR fiber-optic vibration detection technique combined with Michelson interferometer is studied in depth.The phase information of the fiber optic interferometric signal is demodulated by the phase-generating carrier modulation and demodulation algorithm,which could be used to locate the vibration point and recover the vibration signal accurately.Secondly,the conventional and improved phase-generating carrier modulation demodulation algorithms were simulated and analyzed,and the demodulation results for different light intensities and modulation depths were analyzed and demonstrated in detail,which are applied to the vibration detection system in this paper.Finally,the Φ-OTDR-based vibration detection system was built,and the demodulation and detection experiments of single-point single-frequency vibration signal and multi-point single-slice vibration signal are carried out,and the experimental results were analyzed and discussed in detail.From the experimental results,it is shown that the spatial resolution reaches10 m within the sensing distance of 3km,the signal-to-noise ratio of the vibration signal detected by the single-point single-frequency experiment and the multi-point single-frequency experiment exceeds 20 d B,and the linearity of the whole system is 0.9974 in terms of the correspondence between the demodulated signal frequency and the actual applied frequency.The linearity of the curve corresponding to the amplitude of the sinusoidal signal applied to the PZT and the amplitude of the demodulated vibration signal is 0.995.Therefore,the Φ-OTDR vibration detection system combined with Michelson interferometer reduces the complexity and cost,the coherent fading noise is effectively suppressed,and the vibration signal along the fiber can be effectively detected.