Research On Mechanism And Methods Of Pulse-width Multiplexing ?-OTDR System With Asynchronous Sampling Structure

Phase-sensitive optical time-domain reflectometer(?-OTDR)as a new type of distributed optical fiber sensor,not only has the advantages of conventional optical fiber sensors,such as anti-electromagnetic interference,insulation,corrosion resistance and so on,but also has high precision,long detection distance and multipoint simultaneous detection.?-OTDR can be widely used in oil pipeline,the health monitoring of the power grid,and defence border perimeter security,nuclear power plants and other important facilities.However,?-OTDR exists in the process of practical application of high leakage alarm rate and false alarm rate.The reasons mainly caused by the system noise inside device,optical fiber complex background noise in the different environment,laser frequency drift,state of polarization.According to the mechanism of pulse-width affect the performance of the system,the pulse-width multiplexing ?-OTDR method was proposed.By using pulse width multiplexing ?-OTDR with asynchronous sampling optical path structure for the study of lower the nuisance alarm and missing alarm rate of the ?-OTDR as well as optimize the positioning accuracy.In the pulse-width multiplexing ?-OTDR method,the optical pulse sequence with different width is injected into the sensing fiber,then Rayleigh-backscattered data collected under different pulse width is regarded as the output of different sensors and finally the multi-sensor information fusion algorithm is used to process all the alarms of sensors.In this paper,pulse-width multiplexing ?-OTDR with asynchronous sampling optical path structure is adopted,the optical pulses with fixed width in the conventional direct detection ?-OTDR system was replaced by the optical pulse sequence with width changes according to a certain rule.In a pulse width multiplexing ?-OTDR system,data collected under different pulse widths exhibit significant correlations and differences.By making full use of these correlations and differences among data,richer and more comprehensive information about the disturbance signal can be obtained.In the validation experiment,eight pulse width multiplexing ?-OTDR was built,and its capabilities to reduce the nuisance alarms and missing alarm rate by more than 90% have been verified.Positioning accuracy of conventional ?-OTDR system is determined by the pulse width,and limitied to the sampling rate of acquisition equipment.There exists multiple pulse width in pulse width multiplexing ?-OTDR system,so the positioning accuracy can not be determined by the pulse width of arbitrary.Therefore,a method by making use of the inclusion relationship and cross relationship among multiple positioning intervals was put forward to determine the positioning accuracy of the pulse width multiplexing ?-OTDR with asynchronous sampling optical path structure.The experimental results show that the average positioning accuracy of the pulse width multiplexing ?-OTDR with asynchronous sampling structure is equivalent to positioning accuracy corresponds to the minimum pulse width in the optical pulse sequence,and the minimum positioning accuracy is far better than average.The positioning accuracy of pulse width multiplexing ?-OTDR with asynchronous sampling structure can be optimized by using the relationships among multiple positioning interval.In this paper,the proposed method of pulse width multiplexing ?-OTDR with asynchronous sampling structure reduces the system high nuisance alarm and missing alarm rate,optimizes the positioning accuracy of the system,simple structure,lower cost at the same time.This provides a guarantee for ?-OTDR system working reliability and stability in practical application.