Research Of Polarization Optical Time Domain Reflectometry (POTDR)

Distributed optical fiber sensors make use of the changing of optical signals to measure the space and time distribution information of the physical parameter along the fiber. Polarization-sensitive optical time domain reflectometry (POTDR) is a kind of distributed optical fiber sensors, POTDR can measure the external physical changes by detecting the evolution of state of polarization (SOP) of Rayleigh scattering light. In 30 years after POTDR presented, the successful application of POTDR is difficult although many studies have been done. This dissertation focuses on some key issues of POTDR; the improving and perfecting of POTDR can exactly measure the distribution of SOP (local birefringence); the relationship between birefringence and stress or bending is studied, which connect the measurement results of POTDR and sensing parameters. This work has important academic value for improving the sensing theory of POTDR, and important guidance for promoting the application of POTDR. The main innovative points of this dissertation are presented as following:1. A piezoelectric-polarization-controller-assisted POTDR (PPC-assisted POTDR) system is proposed. In this system, PPC-SOP generator which can accurately control the SOP is used to change the input SOP of fiber under test, the input and scattering light all pass through the PPC-SOP generator, and the fiber polarizer ensures the consistency of measurement system. Many POTDR traces can be got by changing the driving voltage of PPC, then the polarization parameters can be analyzed; the spatial distribution of fiber birefringence is calculated.2. Sensing experiment of stress and bend have been done based on PPC-assisted POTDR. The location of stress can be detected exactly. Correlation coefficient is proposed to calculate the bend radius. The results show that the correlation coefficient is approximately a constant if the bend radius is determined, the correlation coefficient exponentially changes when the bend radius changes. This method is effective for few sampling points, and conducive to promoting the practical POTDR.3. A fiber waveplate model is proposed for the superposition of intrinsic birefringence and induced birefringence under stress and bend. The local birefringence contains intrinsic and induced birefringence when the fiber is bend, the superposition is derived by matrix multiplication. Simulation results show that when fiber length is small enough, the superposition of two kinds of birefringence shows a'8'with the change of axis angle; if intrinsic birefringence and induced birefringence are close, the superposition with a small'8'fluctuate in a very small range; when induced birefringence is large enough, the effect of intrinsic birefringence is very small for superposition, the superposition will form a large'8'on the Poincare sphere.4. Because of low signal to noise ratio (SNR) and poor resolution of previous POTDR, a coherent POTDR system is proposed. The evolution of SOP is detected utilizing the coherent Rayleigh scattering light in two fibers (Reference fiber and fiber under test). The birefringence is calculated based on Jones matrix, and the trace of Jones matrix can describe the magnitude of birefringence. Compared with the previous POTDR in the same pulse, the resolution and SNR of coherent POTDR is significantly improved.