Design And Implementation Of Coherent OTDR System

The Rayleigh scattering-based distributed optical fiber sensing technologies have the advantage of continuous distribution,anti-electromagnetic interference,long distance and high accuracy.So they have a bright future in the structural health monitoring(SHM)of long distance and high danger such as tunnel traffic,high-tension transmission line,submarine optical fiber cable,large civil engineering and so on.Optical time-domain reflectometry(OTDR)is an important Rayleigh scatteringbased distributed optical fiber.OTDR with the virtue of one-end access is one of the most successful nondestructive diagnostic tools for a fiber link.However,with the development of information technology,the requirement of sensing distance becomes more and more longer.For example,the required sensing distance for the SHM of submarine optical cable is up to hundreds of kilometers.The development of OTDR is seriously restricted by the shorter sensing distance.Therefore,the coherent-OTDR(COTDR)based on OTDR is developed.At present,the research on COTDR are mainly in the laboratory.There is a few standard assembly form and supporting case.What's more,the rare COTDR products with high performance are all produced by foreign companies,which can't be applied to areas related to confidentiality and restrict the application of COTDR in real cases seriously.In view of the above shortcomings of OTDR,an in-depth study is made and a complete COTDR system with supporting case is realized.In the optical unit,continuous sweep-frequency method is adopted,which not only suppress the fading noise,but also save the time of a single measurement and make the COTDR more suitable for actual measurement.Considering the jitter of laser power brought by frequency sweeping,a constant-power EDFA was used to eliminate the jitter.In the electrical unit,a small signal conditioning circuit board and single chip control circuit board are designed,which are integrated conveniently.The variable gain amplifier(VGA)used in the signal conditioning circuit board can amplify the electrical signals to the biggest range of acquisition card.And the single chip control circuit board can set the gain value of the VGA.In the software unit,we write supporting computer software for COTDR case.The user can monitor the events along the sensing fiber through simple software operating.The professional mechanical structure design software Solid works is used to design the case of COTDR which can be placed in a standard 4U cabinet.After designing and building the COTDR system,we make some experiments on the performance of the system.Under the laboratory environment,the system has a wide dynamic measurement range of 32dB and spatial resolution of 100m is realized.Besides,the system can identify the 0.2dB attenuation of events.We perform the single-span and multi-span experiments in the field.In the single-span experiment,the dynamic measurement range of 33.5dB(which corresponds to the sensing length of 165km)is realized.In the multi-span experiment,the sensing length is up to 500km,which can be significantly enhanced.Experimental results show that the Coherent OTDR system designed in this article has a good performance,and it has a wide dynamic measurement range and high spatial resolution.The chassis with corrosion protection and electromagnetic shielding makes the system more stable and portable.The instrument of COTDR designed in this paper achieves a sensing length of 500km in an ultra-long communication optical cable,making up for the vacancy in such applications in china and providing a feasible solution in the localization of COTDR instrument research.