Research On The Polarization Multiplexing POTDR

The polarization optical time domain reflectometry(POTDR)is one kind of distributed optical fiber sensors(DOFS)based on Rayleigh backscattering.This technology measures the external physical changes by detecting the state of polarization(SOP)of Rayleigh scattering along a fiber.Because the local birefringence of a normal single mode fiber(SMF)distributes randomly,the SOP of the Rayleigh backscattering evolves randomly along the fiber.But the SOP of Rayleigh backscattering returning from each position of fiber is fixed until the fiber is disturbed.When an external disturbance acts on the fiber,SOP of the probe light will be modulated by the disturbance.Then after the disturbance point,SOP of backscattering from each position of the sensing fiber will change.Therefore,the disturbance can be located at the starting point of the changing SOP.POTDR has the advantage of good sensitivity,short response time,low cost and so on.It has shown a great potential in applications of vibration detection,intrusion monitor and so on,such as health monitoring of building structures,safety monitoring of communication link or live wire,perimeter security.However,traditional POTDR still has some shortcomings.With the help of a polarizer,the change of SOP of the Rayleigh backscattering induced by disturbance can be transformed into the change of intensity which can be detected by an optical detector.There are two situations causing small changes of signal at many positions,which is called "single fading":(1)The first one is that the change direction of the SOP on the Poincare sphere is perpendicular to the direction of the polarizer,so the component of the backscattering signal in the direction of the polarizer keeps constant which induces signal fading.(2)The second one is that when the SOP of the backscattering signal is on a special position,the change of birefringence caused by disturbance may not affect the SOP.So it doesn't change after the fiber is disturbed.To overcome the shortcomings of the traditional POTDR system,this paper proposes a polarization multiplexing system based on a multi-wavelength laser to suppress the signal fading.The main work of this paper is as follows:(1)Evolution of SOP during the sensing process is simulated by utilizing Mueller matrix.SOP's and birefringence of the fiber can be simulated and calculated,and the simulation results are combined with Poincare sphere representation.They are depicted on Poincare sphere,and the two reasons for the signal fading phenomenon are analyzed in detail.(2)Polarization multiplexing is realized by a multi-wavelength laser and the characteristic of polarization maintaining fiber.The initial SOP's with even separation on the big circle of Poincare sphere are used to suppress the signal fading caused by a special position of the backscattering SOP.A method to approximate the length of polarization maintaining fiber satisfying the requirement is proposed.Meanwhile,a polarization measurement method is proposed to verify the SOP's output the polarization maintaining fiber.(3)A polarization multiplexing POTDR system based on a multi-wavelength laser is proposed.Lightwaves with three wavelengths pass through a 1.5 m polarization maintaining fiber to make the SOP's evenly separated on the big circle.A dense wavelength division multiplexer(DWDM)is used to demultiplex the signals for different probe lightwaves.Then three polarizers with orthogonal analyzing directions are used to analyze the signals respectively.In the experiment,the system achieves location and measurement of 10.8 Hz vibration,and suppresses the signal fading greatly.Compared with the traditional POTDR system,the signal-to-noise ratio is improved by about 1 dB,and the inaccurate location probably occurring in POTDR,which is caused by the signal fading,is avoided.