Research On Performance Improvement Of Phase Sensitive Optical Time Domain Reflectometry Based On Optical Pulse Coding

Optical fiber sensing technique is born in the 1970 s,and the industry is booming.Phase-sensitive optical time domain reflectometer(?-OTDR)is one of the most important branches of distributed optical fiber sensing(DOFS).It has the advantages of high sensitivity and instantaneous response property and has always been a concern.With the development of large-scale infrastructures,communication networks and power grids,higher requirements for the performance parameters of sensors are proposed when they are used in online detection.Optical pulse coding(OPC)technique can break the tradeoffs between key parameters of DOFS,such as spatial resolution,signal to noise ratio(SNR)and sensing range.However,previous researches mostly focused on the sensing systems based on direct detection.In recent years,coherent detection has been widely considered a promising detection method in DOFS.Therefore,the research on OPC ?-OTDR based on coherent detection is of great significance.Nowadays,the OPC ?-OTDR still has some limitations: the measurement time is relative long,usually several times of that in a single pulse;the performance is affected by interference fading and laser frequency drift but there is no relevant research.This paper focuses on three aspects: measurement time cut,interference fading elimination,and frequency drift compensation.A bipolar Golay coding ?-OTDR is proposed,and the corresponding interference fading elimination and frequency drift compensation methods are proposed.The main work is summarized as follows:1.A mathematical model of ?-OTDR with heterodyne detection is derived,and a spectrum extraction and remix(SERM)method with single pulse for fading elimination is proposed: first,a corresponding simulation platform is built according to the mathematical model.It is helpful to analyze the characteristics of backscattering signals comprehensively and systematically;second,a method for interference fading elimination in the digital domain is proposed,which will not increase the system/modulation complexity;finally,taking the ?-OTDR with single pulse as an example,in simulation and experiment,the phase demodulation error is effectively avoided.2.A bipolar Golay coding ?-OTDR is proposed: the theoretical analysis of the bipolar coding ?-OTDR is carried out;the system design,simulation and experiment in the aspects of encoding,detection and decoding processes are completed.The proposed system breaks the trade-offs between the key parameters in ?-OTDR based on direction detection.Compared with unipolar coding ?-OTDR,not only the measurement time is halved,but also the coding gain is greatly improved,enabling the fastest measurement speed in OPC ?-OTDR.3.In bipolar coding ?-OTDR,the frequency drift is compensated and the interference fading is eliminated: a real-time distributed frequency drift compensation method in digital domain is proposed,which can solve the problem of intensity distortion after the decoding and significantly increase the coding rejection ration and the SNR in retrieved perturbation.The SERM method is combined with coding system,which can eliminate the interference fading after decoding and avoid the phase demodulation errors.For the first time,the ability of distributed detection in OPC ?-OTDR is demonstrated,and the system performance is improved from several aspects.