| Distributed optical fiber sensing technology has attracted intensive attention throughout the world for the past several decades due to its unique performance.The fiber is used as the medium for transmission and sensing in distributed optical fiber sensors,which can realize the temporal and spatial measurement of environmental quantities such as temperature,vibration and strain.Compared with traditional electronic sensors,distributed optical fiber sensors possess excellent characteristics of resistance to electromagnetic interference,anti-corrosion,electronics insulation,high sensitivity and low cost.Among them,the optical frequency domain reflectometry(OFDR)sensing system based on Rayleigh scatter has attracted increasing scientific interests due to high sensitivity and high spatial resolution.Up to now,it has been applied in many fields such as aerospace,health care,high-precision instrument detection.However,there are still many problems and challenges in OFDR based distributed optical fiber sensing,such as the tradeoff between the tuning rate of the laser source and the frequency response range,the contradiction between the sensing distance and the spatial resolution,the difficulty of high spatial resolution and large strain measurement since the spatial mismatch caused by the large strain stretch between the reference signal and the measurement signal,which limit the applications of sensing system.In order to solve the above-mentioned problems,some researches and experiments are proposed to improve the OFDR performances in this thesis.The main contents are as follows.(1)The research progresses of OFDR distributed optical fiber sensing technology based on Rayleigh scattering in terms of sensing distance,spatial resolution,strain measurement range,vibration sensing,nonlinear error compensation are reviewed.The sensor theory model of OFDR distributed optical fiber sensing technology is analyzed.An experimental platform for distributed optical fiber sensing measurement based on OFDR is established to provide theoretical support for subsequent experimental research.(2)In order to break the contradiction between the tuning speed of lasers and the vibration frequency response range,the compressive sensing OFDR distributed optical fiber vibration system based on time-resolved method for measuring high-frequency sparse vibration signals is proposed.The vibration signal applied on the interrogation fiber is sampled by a multi-frequency sub-Nyquist sampling method and reconstructed by the compressive sensing technique.Experiment result shows that the measurable frequency of sparse signal is achieved up to 200 Hz with a sweeping rate of 40 nm/s.(3)In order to reduce the influence of the reference signal effected by the environment in the actual vibration measurement application and improve the measurement accuracy in distributed dynamic strain sensing by using time-resolved optical frequency domain reflectometry,a single-shot vibration measurement method without reference signal based on polarization-maintaining fiber is proposed.The vibration signal information along the fiber can be obtained by monitoring the side peak shifts through the autocorrelation.In addition,a moving time-gating method is proposed to measure the high frequency vibration signal in single-shot OFDR system.The sampling time between sampling points can be reduced so that the effective sampling rate of the sampling system is increased and the maximum frequency response range can be improved.Compared with the time-resolved method,the maximum frequency response range is improved by 4 times by the moving time-gating method in single-shot OFDR distributed optical fiber vibration sensing system.(4)Aiming at the tradeoff between sensing distance and spatial resolution,the image processing technology is proposed in OFDR distributed optical fiber sensing system.Two-dimensional image or two-dimensional matrix information constructed by the spectrum shift information and sensing distance can be obtained by the cross-correlation results after processing the collected distributed Rayleigh scattering signals.Then,the constructed image information is processed by two-dimensional image processing technology so that the strain gradient information along the fiber is analyzed clearly.By using the two-dimensional image processing technology,random noise can be eliminated and the measurement accuracy of OFDR sensing system can be improved.Experimental result shows that the spatial resolution of 1.3 mm can be realized along the sensing fiber of 52 m by using the image processing technology in OFDR distributed sensing system.(5)In order to solve the problem that it is difficult to achieve high spatial resolution and large strain measurement due to the spatial mismatch between the reference signal and the measurement signal caused by large strain stretching,firstly,two-dimensional image processing technologies are used to smooth the outliers caused by the spatial mismatch.The identification and analysis of large strain gradient information can be effectively realized by this scheme.The strain gradient information up to 7000με can be analyzed and identified with a 4mm spatial resolution by using image processing technologies.In addition,in order to improve the spatial resolution and strain measurement range further,a distance domain compensation method is proposed in OFDR system,the spectrum shift information generated by the cross-correlation result is converted into the distance domain length to compensate distance domain information of the reference signal so that the similarity between the reference signal and measurement signal can be improved.Compared with the traditional processing method,measurements range of strains up to 10000με can be realized along the sensing fiber with a spatial resolution of 2mm,which greatly improves the performance parameters of the OFDR system and broadens the application fields of the OFDR system.In conclusion,this thesis researches several methods to improve the performance of the OFDR distributed optical fiber sensing system based on Rayleigh scattering.By analyzing the main limitation factors of key parameters,the effective method is found to improve the system performance,which provides new ideas for high-performance OFDR distributed optical fiber sensing system. |
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