Distributed Frequency-drift Compensation Technique For Bipolar Golay Coding ?-OTDR

In recent years,phase-sensitive optical time-domain reflectometer(?-OTDR)has attracted attention because of its high sensitivity and fast response speed.As people's requirements for their performance parameters become higher,optical pulse coding technology has gradually developed.As early as the 1970 s,researchers proposed the optical pulse coding technique for overcoming the constraints of parameters in radar.Using different coding methods,it is possible to increase the pulse energy without sacrificing the spatial resolution of the system,or to increase the signal-to-noise ratio signal without sacrificing the sensing distance.It can be seen that it can break the constraint relationship of key parameters.With continuous research in recent years,the performance of optical pulse coding ?-OTDR has been greatly improved.However,laser frequency drift remains a key factor affecting its performance.In order to avoid the impact of laser frequency drift on the sensing system,on the one hand,it is possible to stabilize the light source frequency from the source by selecting a laser with good frequency stability or adding a frequency stabilization device,but it requires complex system design and is very costly.On the other hand,the impact of frequency drift can be compensated by improving the sensing system and signal processing method,which is more flexible and can obtain a better compensation effect.At present,the performance effect of laser frequency drift on the optical pulse coding ?-OTDR is mainly reflected in the fact that it reduces the relative ratio of peak and side lobes.There have been previous compensations for associated frequency drift.However,because it isn't the distributed compensation or the compensation method is too complex,it is not suitable for the system.Starting from the distributed compensation frequency drift,this thesis proposes a novel and simple distributed frequency drift compensation method,and the main work is summarized as follows:1.First,the optical fiber sensing system is briefly introduced.The advantages and disadvantages of each system are explained,and the development of ?-OTDR and the influence of laser frequency drift are introduced in detail.Then,the principle of heterodyne detection ?-OTDR,the principle of Golay coding,and the principle of unipolar and bipolar Golay coding ?-OTDR that have been completed by this research group are introduced.2.The influence formula of frequency drift on single pulse ?-OTDR system and Golay coding ?-OTDR system is first derived,and then a simple and effective distributed compensation scheme is proposed.Through theoretical analysis and experimental verification,the frequency drift of the reference fiber and the sensing fiber are measured under the condition of injecting the same pulse and injecting the single pulse and the code pulse respectively,which proves the consistency of the frequency drift and illustrates the feasibility of the compensation scheme.3.Finally,in order to verify the distributed compensation effect,the perturbation measurement experiment of two points was performed in the 9.7 km sensing range.The signal-to-noise ratio(SNR)of the demodulated perturbation is increased by 2.8 d B and18.9 d B respectively compared to the case without frequency drift compensation.In addition,the solution can be used for other complex coding systems providing more possibilities for system design.