Research On Key Techniques Of Enhanced Coherent Optical Time Domain Reflector

Optical time domain reflectometer(OTDR)realizes detection of optical fiber system by detecting the light pulse propagation in optical fiber backward Rayleigh scattering signal.In order to improve the dynamic measurement range of OTDR,researchers applied coherent detection technique in OTDR,and this is coherent optical time domain reflectometer(COTDR).COTDR transfers the power of Rayleigh scattering signal to by using the method of coherent detection to the intermediate frequency signal,and filters out most of the noise signal to achieve high dynamic range through band-pass filter.Although COTDR can detect loss events of an optical fiber such as broken points,curve and attenuation,it could not precaution the potential threat and avoid the damage before it actually happens.These potential threat events such as broken points caused by engineering construction usually disturb the fiber through vibration field even if the threat event is still far away from the fiber.This means it is possible to make an early warning if the disturbance on the fiber could be identified,and avoid huge economic losses.The phase-sensitive OTDR(Φ-OTDR)has been applied in many fields in recent years for its ability in distributed vibration sensing.However,narrow linewidth and small frequency drift laser source must be utilized to generate high interference visualization because the Φ-OTDR system measure vibration by using coherent effect,which will introduce strong fading noise to the measurement result of accurate loss event measurement.In COTDR,we can improve the dynamic measurement range by increasing number of measurement or detect light power.Increasing the number will increase measurement time,this will reduce the measuring efficiency of system;Increasing power of injection detection optical can be achieved by increasing the peak power or width.In fact the peak power of the laser is limited.Therefore we can only increase the detection of pulse width,which in turn will lower the spatial resolution of the system.In order to overcome the above defects,we propose a fully distributed fusion system combining Φ-OTDR and COTDR for synchronously vibration and loss measurement based on the frequency sweep of laser.A tradeoff has been analysed between frequency sweep rate and inject pulse width.And when the vibration duration is long enough,the dead zone of Φ-OTDR can be effectively eliminated with this approach because every point at the fiber changes quickly between dead zone and sensitive zone.At the same time,Interval time Φ-OTDR curve correlation degree is low,the coherent fading noise of OTDR traces can be suppressed by accumulation.In order to simplify the structure of fusion system,we used direct detection structure in the experiment.In the experiment,the joggle of OTDR curve is less than 0.45dB and the signal-to-noise ratio(SNR)of vibration event at 9.1km of the 35km sensing fiber is higher than 10 dB while the injected pulse width is 100ns and period is 400ms,which corresponding to 10m spatial resolution,and the frequency sweep rate is 40M/S according to the tradeoff we give.In order to further improve system dynamic measurement range of COTDR,and reduce the jitter of COTDR decline curve,this paper proposed applying the cyclic encoding technology to the COTDR system.Compared with simple code and gray code needing many groups of encoded data to achieve the decoded result,the cyclic code as the special case of simple encoding,is decoded by sending a set of code result.Firstly,we have accomplished the loop code COTDR simulation,and then carry on the loop code COTDR experiment according to the simulation result.Simulation.results show that the jitter of linear detector COTDR system curve is lower than using logarithmic detector COTDR system,and when COTDR curve jitter is higher than 0.02 dB,loop coding technology can’t reduce the jitter of COTDR curve,improve COTDR curve dynamic range.In experiment,even while the joggle of COTDR curve are the ministry we can achieve(0.02dB),loop code can’t reduce the joggle of COTDR curve,and the dynamic range of COTDR can’t be improved which are consistent with the simulation results.Compared with ordinary COTDR,coding COTDR needs functions of encoding and decoding,and has the strict request for sequential control of the system.When coding COTDR curve can’t be aligned,we will get decoding COTDR curve of many sharp spines,and this will result in error detection.In order to avoid the errors of detection results,this paper design the special-purpose acquisition card for coding COTDR system.The acquisition card integrates code generation,cumulative average,data storage and transmission,and other functions.The acquisition card master clock drives the whole acquisition card,thereby eliminating the aligning of coding COTDR curve,which will improve the coding COTDR system stability.In the aspect of hardware design,this paper designed acquisition card which achieves the real-time control of acquisition card by means of collaborative work with FPGA and MCU.In terms of software design,this article realizes acquisition card of each function module through Verilong HDL and assembly language step by step,and the decoding function of embedded in PC control software.In conclusion,this paper proposes and implements an enhanced COTDR system.The system achieved loss and disturbance events synchronous measurement by frequency sweeping.So we can realize the early warning and monitoring of optical networks.In addition,this paper proposes to apply cyclic encoding to COTDR system,in order to improve dynamic COTDR system measurement range.In order to avoid the encoding COTDR decoding error,this paper also designs the special-purpose acquisition card COTDR coding system.