Sensing Technology Based On Polarization Information Of The Fiber Rayleigh Scattering

Distributed optical fiber sensors(DOFS)have penetrated into various fields,such as railway transportation,pipeline transportation and seismic wave detection,etc.,because the fiber itself is a passive device and has a small size,light weight,could be winded into any shape and has the advantages of a series of traditional electrical sensors such as antielectromagnetic interference and corrosion resistance.At present,most widely used fiber optic sensors are based on scattered light intensity information or phase information sensing,and polarization information is rarely used.Even many systems will add additional equipment to eliminate the effects of polarization.However,polarization information can also be used for sensing,the most typical of which is a polarization-sensitive optical time-domain reflectometer(POTDR),but this type of system can only measure the polarization state change of one point at a time.Because when the polarization state of a certain point changes,every point after that will be affected,so POTDR cannot achieve a real distributed measurement,making such a system could not play a role in pipeline monitoring and bridge monitoring and other important occasions,but usually this occasion is really urgent for the perception of changes in polarization state.However,birefringence is an intrinsic property of the optical fiber.If the birefringence of the optical fiber is used as the amount of comparison before and after the disturbance change,a truly distributed measurement can be achieved,because the birefringence of each point on the fiber will not be changed by other points.Therefore,the first problem to be solved is how to accurately measure the birefringence distribution of the fiber.Aiming at the problem of how to accurately measure distributed birefringence,this paper expands the theoretical analysis,method research and simulation verification.The main contents of the work are as follows:(1)Summarizes several current systems that use polarization information for sensing,including point measurement systems,quasi-distributed measurement system,and distributed measurement systems,including the working principles introductions of different systems.(2)The principle of scattering in optical fiber is analyzed in detail.First,the basic information of the light polarization state is introduced,and several representations of the light polarization state are introduced.Then,the representation method of Stokes vector is used to analyze the transmitted light is analyzed,followed by the analysis of scattered light in the fiber,and two common methods for measuring the polarization state of the fiber are also introduced.(3)According to the principle of light propagating in optical fiber,combined with the analysis of the currently known optical fiber distributed measurement system,a suitable measurement system-pulse compression phase-sensitive optical time domain reflectometry(PC-ΦOTDR)is selected.Then it analyzes the working principle of the system and the reasons for choosing the system,built the corresponding experimental device,performed the measurement of birefringence and analyzed the corresponding result.Verification experiment is 2 km traditional single-mode fiber,which achieves a spatial resolution of 8.6 cm.In addition,the characteristics obtained by this distributed measurement system are discussed in detail,and the results point out that the phenomenon existing in the current system: the measured birefringence is related to the bandwidth(spatial resolution)of the incident light.(4)The phenomena related to the measured birefringence and spatial resolution found in the experiment were studied.Due to the nature of the system,the spatial resolution corresponds to the bandwidth of the incident light,so it can also be said that the effect of different incident bandwidths on the measurement of birefringence is studied.First,compare several known simulation models to choose a simulation model suitable for the current system,then use the simulation model to simulate the impact of different bandwidth incident light on the measurement of fiber birefringence,and then experimentally verify a series of different bandwidth pulsed light Measurement results.Through simulation and experiment,a consistent conclusion is obtained: when the bandwidth is less than a certain value,the measured birefringence and the bandwidth change linearly,and after the bandwidth reaches a certain size,the measured birefringence will not change with the incident bandwidth variety.