| Raman distributed optical fiber sensing can realize large-scale and high-precision temperature detection.It has significant social needs and application prospects in the field of large-scale infrastructure structure health monitoring such as bridges,tunnels,oil and gas pipelines,and smart grids.However,the detection signal of Raman distributed optical fiber sensing is spontaneous Raman scattering signal which is very weak,and the sensing cable is mostly laid in the field harsh environment with high optical fiber loss,which has the technical bottleneck of low Signal-to-Noise Ratio(SNR).The increasing of the detection pulse width can improve the SNR of the system,thereby the temperature accuracy and temperature resolution performance of the system will improve,but it will reduce the spatial resolution.Therefore,how to effectively improve the SNR of the Raman distributed optical fiber sensing while taking into account the spatial resolution performance is an important scientific issue for the performance improvement of the system.In addition,Raman distributed optical fiber sensor can only detect temperature as a single parameter,but cannot realize the cooperative detection of other physical parameters.At present,the system cannot effectively realize the accurate positioning and prediction of the location of the temperature mutation in the early stage of the disaster,which is also the key technical bottleneck that needs to be solved urgently in the application of the Raman distributed optical fiber sensor.In response to the above-mentioned scientific problems and technical bottlenecks,with thefunding of major national scientific research instrument development,Shanxi Province scientific and technological research projects,the whole chain of research work of "basic theory and methods-instrument development and development-engineering research and application"has been carried out.In this thesis,based on the theoretical mechanism research of the improvement of the fiber Raman transmission equation,several key methods and techniques are proposed to improve the temperature accuracy,temperature resolution and spatial resolution performance of the system.Besides,a dual-parameter detection scheme is proposed to realize the coordinated monitoring of distributed temperature and structural cracks along the optical fiber.A pre-warning model based on distributed optical fiber sensor is proposed,which solves the problem that the current distributed optical fiber sensor cannot predict and alarm in the field of disaster safety monitoring.Based on the above-mentioned new sensing solutions,with the goal of "Improving Energy Security Capabilities",the high-precision Raman distributed optical fiber sensor and dual-parameter Raman distributed optical fiber detector have been developed and successfully applied in Qinshui Gas Pipeline in Shanxi Province and the spontaneous combustion safety monitoring field of Xishan Coal Field in Shanxi Province providing solutions for the accurate determination of the location of spontaneous combustion hidden danger points in governance areas such as gas leakage in the gas pipeline network,coal mined areas,and coal mine roadways.The main research content and results of this thesis are as follows:(1)In the field of Raman distributed optical fiber sensing with high temperature accuracy,the system temperature accuracy is limited by the photoelectric response gain of the avalanche photodetector(APD),fiber temperature sensitivity,fiber mode dispersion and fiber mutation loss.This article firstly establishes the optical fiber Raman scattering temperature control model,and theoretically reveals the influence of the above factors on the temperature accuracy of the system.Then based on the mechanism research of the improved Raman transmission equation,a variety of new sensor demodulation schemes are proposed to improve the system temperature accuracy.First,a dynamic gain calibration method is proposed to calibrate the problem of the temperature drift of the APD causing the temperature accuracy of the system to decrease.The temperature accuracy of the system is increased from 6.4? to 1.2? at a sensing distance of 10.0 km.Then a differential temperature sensitivity compensation method is proposed to solve the problem that the temperature sensitivity of the sensing fiber gradually deteriorates with the increase of the sensing distance,and the system temperature accuracy is further improved to 0.36?.In addition,in the field of practical engineering applications,the fiber mode dispersion differential method and the double-ended loop demodulation method are also proposed to solve the problem that the temperature accuracy of the system decreases due to the Raman dual-wavelength difference and the sudden loss of the fiber.(2)In the field of high temperature resolution Raman distributed optical fiber sensing,in view of the technical bottleneck of the system temperature resolution is limited by SNR,this thesis proposes a dynamic differential attenuation identification method to solve the influence of the traditional system calibration channel optical interference noise on the temperature resolution performance.In addition,this method does not require all-fiber calibration before temperature measurement,which simplifies the system demodulation process.The experimental results show that the SNR of the system can be increased to 13.32 dB by suppressing the interference noise of the calibration light.The temperature resolution performance of the system is 0.18? at a sensing distance of 17.0 km.Finally,in the large temperature measurement range,this thesis simulates the influence of SDP system,DDP system and fiber attenuation on the temperature resolution performance.(3)In the field of high spatial resolution Raman distributed optical fiber sensing,in view of the scientific problem that the spatial resolution of the system is limited by the pulse width of the light source,this thesis proposes a new mechanism and a new scheme based on(Amplified Spontaneous Emission)ASE correlation time domain compression and demodulation.This scheme uses ASE source instead of pulsed laser as the detection signal,and firstly establishes a temperature control model of optical fiber ASE Raman scattering transmission.The time-domain differential reconstruction of the backward Raman anti-Stokes scattering signal excited by the sensing fiber is used to extract the Raman anti-Stokes signal with the timing characteristics of the ASE source at each position point.Then based on the time-domain compression demodulation of the correlation function,the correlation between the spatial position of the ASE Raman scattering temperature modulated light field and the ASE reference signal is revealed,and the demodulation equation of the relationship between the fiber mutation temperature and the correlation peak-to-peak value is proposed.The simulation results show that the system can increase the traditional meter-level spatial resolution performance to 7.5 mm at a sensing distance of 10.0 km.The most important thing is that the spatial resolution performance achieved by this scheme has nothing to do with the sensing distance.(4)In the field of infrastructure structural health and safety monitoring,Raman distributed optical fiber sensing is required to simultaneously monitor structural cracks and environmental temperature changes.In response to this application requirement,this thesis proposes a new dual-parameter optical fiber sensing scheme based on Raman Stokes optical loss analysis and temperature synergistic effect to realize distributed temperature and structural fissure coordinated monitoring along the optical fiber.The solution is based on the optical fiber loop temperature demodulation method to extract distributed temperature information along the optical fiber,and based on the optical fiber Raman Stokes optical loss analysis method to detect the range of structural cracks.The experimental research shows that this solution can also achieve a crack detection range of 1.6 mm to 5.6 mm and a crack detection resolution of 0.4 mm based on the fitted Raman Stokes optical loss intensity while ensuring high-precision temperature measurement along the optical fiber.(5)In the field of energy development and operation safety monitoring,Raman distributed optical fiber sensing is required to quickly and accurately measure the location information of temperature mutations along the optical fiber before various disasters occur.In response to this problem,this thesis proposes a heat transfer function advanced warning technology based on the temperature change rate of the sensor optical cable and the environmental difference,which solves the problem of the system's response time deterioration caused by the lag effect of the temperature transmission of the sensor optical cable.Experimental results show that the temperature sensor response time of the system can be optimized from 23.4 seconds to 1.3 seconds.Furthermore,a data mining and fusion technology based on multi-stage real-time moving average method is proposed,which conducts data mining and analysis on historical temperature sensor data along the optical fiber and establishes a prediction model.This model realizes the accurate prediction of the temperature change along the sensing optical cable nearly 60 seconds in advance,and solves the problem that the current distributed optical fiber sensor cannot predict and alarm in the field of disaster safety monitoring.(6)With the goal of "Improving Energy Security Capabilities",in order to meet the requirements of high-precision and dual-parameter coordinated monitoring in the field of gas transmission pipelines and coal field spontaneous combustion safety monitoring in Shanxi Province,based on the key methods proposed above,this thesis carried out the integration and instrumentation research of a new type of Raman distributed optical fiber sensing system,and developed high-precision Raman distributed optical fiber sensor and the dual-parameter Raman distributed optical fiber detector respectively,and they have been successfully applied to safety monitoring of the Qinshui Gas Pipeline in Shanxi Province and the Xishan Coal Field in Shanxi Province providing a solution for the accurate determination of the location of spontaneous combustion hidden danger points in the gas pipeline network gas leakage,Xishan coal field goaf,coal mine roadway and other governance areas. |
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