Research On Multi-Parameter Hybrid System Based On ?-OTDR

The distributed optical fiber sensing(DOFS)system has a wide range of potential applications in large infrastructure health monitoring,geological exploration,perimeter security and other fields due to its characteristics such as long range/large scale sensing ability,fast response of sensing,high accuracy and multiple parameter sensitivity.The traditional distributed optical fiber sensing system often detects only a single parameter,such as loss,temperature and disturbance.For example,when an abnormality occurs in the oil pipeline,there will be several factors such as oil leakage,corrosion and cracking.Therefore,single parameter detection will result in a higher false alarm rate.If multiple parameters can be acquired simultaneously,the false alarm rate can be significantly reduced.Therefore,the multiple parameters sensing gradually becomes the developing trend of distributed optical fiber sensing due to its importance.At present,the researches on multiple parameters measurement are concentrating on stress and temperature.The most representative one is BOTDR?However,the problem of cross-sensitivity between stress and temperature exists in BOTDR system.Furthermore,BOTDR system cannot detect external disturbances.Furthermore,BOTDR system can only detect static parameter.?-OTDR has been widely used in vibration detection because of its high sensitivity and fast response ability.However,the traditional ?-OTDR cannot detect static parameters.At present,there are rare researches on measuring dynamic and static parameters simultaneously without increasing the complexity of the system.According to these shortcomings,the main works of this thesis are as follows:At first,in order to realize the distributed sensing of vibration and temperature at the same time,a hybrid system has been proposed in this thesis base on Rayleigh and Raman scattering light.The whole system shares a common optical structure,which can realize distributed vibration and temperature sensing without increasing system complexity.The experimental results have shown that this designed hybrid sensing system could realize fully distributed vibration and temperature sensing along a 12km long sensing fiber.The hybrid system could achieve a 500Hz frequency response range with a spatial resolution of 10m,and could accurately reconstruct the spectral characteristics of the knocking event.And heterodyne coherent detection method and unwrapping algorithm were introduced to the hybrid system to rebuild dynamic electric field of RBS by analyzing the heterodyne beat signal.The error degree of measured value and actual value of different frequencies reached 0.00004 and the signal-to-noise ratio of vibration is higher than 20dB.In terms of temperature measurement,the temperature uncertainty at 50? was only 0.29?.Secondly,we have proposed a hybrid system combining ?-OTDR and COTDR system based on the frequency sweep of laser.The system could not only monitor the loss of the fiber with high precision but also measure external vibration events.Due to the introduction of frequency sweep technology,the dead zone problem in ?-OTDR could be avoided.By researching the principle of frequency sweep,a tradeoff has been analysed between frequency sweep rate and inject pulse width and period,the appropriate frequency sweeping speed was determined.So that the hybrid system could monitor both high-precision loss curve and external disturbance.The experimental result showed that the fading noise value of COTDR curve is less than 0.15dB and the dynamic range was 23dB.The vibration event detected at 9.5km of the sensing fiber was higher than 10dB.The vibration event detected at 9.5km of the sensing fiber was higher than 10dB.The hybrid system can real-time monitor the external vibration signal along the optical cable and warn the failure of optical cable caused by illegal intrusion and destruction.In general,through the research work in this paper,feasible multi-parameter distributed optical fiber sensing solutions are proposed,which can simultaneously detect temperature,loss and disturbance simultaneously.These solutions can provide technical support for improving the reliability of DOFS technology in field applications.