| Spatial resolution is one of the most important parameters for a BOTDA system.In this dissertation,a pulse subdivision method was developed to improve the spatial resolution of a BOTDA system.The research includes detailed theoretical analysis,simulation and experimental verification of the method.The details are as follows:(1)Based on the relationship between pump pulse width,temperature/strain disturbed fiber segment length and Brillouin gain spectrum,pulse subdivision method was developed to improve the spatial resolution of a BOTDA system.Firstly the long pulse was divided into a number of sub-pulses.According to the relationship between pulse width and Brillouin signal,Brillouin spectrum generated by the long pulse could be subdivided into equal numbers of sub-spectra.Each sub-spectrum could provide detailed sensing information about a fiber sub-segment.Thus spatial resolution was determined by those subdivided sub pulses,not the original long pulse.With this method cm-level spatial resolution could be achieved using a single long pulse with a pulse width as large as dozens of nanosecond.This method avoids the limit in spatial resolution of a long pulse BOTDA system,but retains the advantage of high signal-to-noise ratio and Brillouin frequency shift(BFS)accuracy supported by long pulses.The method was theoretically simulated and experimentally verified.For experimental verification,recognition capability on a melting point between two different fiber sections,and sensing performance on discrete strain distribution were tested.For melting point recognition,thanks to five-multiple subdivision a 1 m spatial resolution over 31 km sensing fiber was realized using a 50 ns pump pulse.For strain sensing test,two 0.5m stretched fiber sections separated by a 0.2m loose fiber was recognized using a 20 ns pump pulse with ten-multiple subdivision analysis.The spatial resolution is 0.2m,which is ten times improvment compared with that before subdivision analysis.(2)In BOTDA distributed detection,when a same event was detected using different pump pulses,Brillouin signals with different line width were obtained,but their starting points located at a same point.This phenomenon was analyzed using pulse subdivision method,based on which a bidirectional BOTDA detection technology with high-resolution was proposed.The location method was experimentally tested.A 1 m long strained section on a 25 km sensing fiber was tested using different pump pulses.The starting and ending points of the hot spot locating at 41 m and 40 m from the fiber tail are recognized,respectively.Location precision is determined by the sampling interval of the system.Lorentz bimodal fitting of the Brillouin spectrum of 50 ns pulse was carried out,giving a strain measurement error of 36??.By increasing BFS interval and reduce Brillouin bandwidth,measurement accuracy can reach up to 2??.(3)Finally pipeline crack detection was carried out based on pulse subdivision BOTDA system.Single and double-crack with 50 cm spacing were detected on a 6 km fiber using a pulse of 30 and 50 ns,respectively.For double cracks signals,after pulse subdivision analysis spatial resolution of the system was increased from 5 m to 50 cm.Compared with short pulse,differential pulse pair and dark pulse technology,pulse subdivision BOTDA system is characterized by its simplicity and high signal-to-noise ratio.It avoids the limit in spatial resolution of a long pulse BOTDA system,but retains the advantage of high signal-to-noise ratio and excellent BFS accuracy supported by long pulses.In addition,the method could be integrated with other BOTDA technologies since it uses a traditional BOTDA system.The method is believed to have extensive application prospects in distribution sensing. |
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