| With the widespread application of optical fiber technology in the fields of communication,measurement,control,energy,and other fields,the constant evolution of fiber industry raises the level of testing,accuracy,and precision required on the fiber infrastructure to support these applications.Fiber testing plays an important role in the design,deployment,and maintenance of optical fiber systems.It continues to promote the high-quality development of fiber industry as the emergence of new technologies and the expansion of old technologies.Optical time-domain reflectometry(OTDR),as everyone knows,is one of the earliest researched and applied fiber testing technology.It has become the most basic and widely used way in the field of optical fiber.However,traditional OTDR always suffers from the trade-off between its spatial resolution and dynamic range.Fortunately,photon counting optical time-domain reflectometry(PC-OTDR)-a new fiber testing technology derived from the combination of OTDR and quantum detection technology,can solve this contradiction.Therefore,it is of great importance to research on PC-OTDR for the improvement of testing ability in fiber.In this thesis,the testing principles and characteristics are analyzed based on the studying of PC-OTDR,and then the technical difficulties in achieving higher performance are discussed.By optimizing the detection mode,device performance and data processing algorithm,the research focuses on removing the limitation of detector saturation counting on the improvement of dynamic range,eliminating the effect of dispersion on the deterioration of spatial resolution,and overcoming the technical problems in the application of aeronautic fiber testing.The main works and innovations of this thesis are as follows.(1)A novel scheme to improve the dynamic range of PC-OTDR by using external time-gates is proposed and demonstrated.For PC-OTDR system,further improvement of the dynamic range is limited by the dead time of single photon detector(SPD).In other words,the dynamic range for PC-OTDR system must not be improved by arbitrarily increasing the back-propagation signal,due to the maximum count rate of SPD limited the level of back-propagation signal.To solve this problem,a high-speed optical switch is used to modulate the back-propagation signal,and to allow that only a certain part of the back-propagation signal-defined by the gate-is detected by the following SPD.By doing so,for each segment of OTDR trace,we can increase the corresponding back-propagation signal to the maximum count rate of the SPD,by improving the intensity of probe signal,thus increasing the dynamic range for the PC-OTDR trace.In the demonstration,by using the 50 ns external gates,a dynamic range of 30 dB is achieved for a PC-OTDR test of 70-meter long aeronautic fiber,showing an improvement of 11 dB in dynamic range comparing with the no gating case,in good agreement with the theoretical results.With the improved dynamic range,successful identification of a 0.37 dB small bend loss event is detected with 30-second accumulation measurement,which is not identified for the no gating case with the same accumulation time.(2)A scheme of dispersion independent PC-OTDR based on an infinite backscatter technique is proposed and demonstrated.In the process of long-distance optical fiber testing,the pulse of the OTDR system is broadened due to the influence of dispersion,which results in the limitation of spatial resolution.A detection method using long laser pulses related to the fiber length is designed to solve this problem.The back-propagation OTDR signals along the fiber are obtained by differential analysis of a long laser pulse that illuminates or darkens the fiber under test(FUT)gradually.Experimental results show that this scheme drastically eliminates the need for high performance short-pulse lasers,and can develop a PC-OTDR system with its spatial resolution independent with dispersion property of the FUT.In addition,to improve test efficiency,a hybrid externally time-gated technique and infinite backscatter technique PC-OTDR is proposed,which has remarkable results on shortening the testing time and improving the signal-to-noise ratio(SNR)for long-haul fiber links.The experimental results show that when the same SNR is obtained,the testing time can be reduced by about 80% by using 5 external time gated signals to scan a 25 km long FUT.(3)A prototype of aeronautic fiber testing equipment based on PC-OTDR is carried out and has been successfully applied to a certain aircraft manufacturer.The existing OTDR technology can not meet the measurement requirements of short distance and high spatial resolution of aeronautic fiber.Aiming at this problem,the single-photon detection technology is used to design a method for the high spatial resolution automatic testing for multi-channel optical cables.Firstly,to meet the demand of the system for cost-effective short-pulse light sources,a novel approach is presented which provides a highly effective modulated method for a gain-switched semiconductor laser by using step-pulse signal modulation.If the modulation current is suddenly increased when the carrier density reaches the threshold,the carrier accumulation rate will be accelerated,therefore obtaining higher power and shorter width pulse output.Secondly,a novel approach based on wavelet packet analysis using a support vector machine(SVM)is proposed for the automatic identification of fiber defects,so that the data processing process is simplified and the dependence on the designer’s experience is reduced.The event signs are decomposed by the optimal basic wavelet packet after the events are located,and the normalized energy features of the event signs as eigenvectors are extracted as the input of training and testing based on the results of signal reconstruction.Finally,the SVM model is built,and fiber defects can be identified with the eigenvector as input.The test results of the system show that,with the home-made gain switching pulsed laser at 850 nm,the dynamic range of about 18 dB and the spatial resolution of less than 9 cm are achieved,and the event recognition rate of 99% is reached.It is of great significance to realize the localization of high-precision aviation optical cable testing equipment. |
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