Analysis on Polarization State of the Distributed Optical Fiber Sensor Based on Sagnac/Mach-zehnder Interference Structure

摘要

A new type of distributed optic-fiber sensing system based on Sagnac/Mach-Zehnder has vast prospects in early warning of oil and gas pipeline sabotage, so it is necessary to study the problem of degradation of polarization state which interrupts the application of the sensing system. Firstly, a model of the impacts of polarization state on power transmission coefficient is established in terms of Jones matrix method. With the existence of birefringence, simulations are respectively made: the influence of the changes of equivalent birefringence element parameters on the output signals of the Sagnac interference system, and with the different angles of polarized light, the interruption of Mach-Zehnder interferometer in the response of output to the outside world. A conclusion is drew out: the existence of birefringence will not only lead sensing system to misjudge the outside world's behavior, but it will also affect the interference output signal's phase, and then affect the positioning accuracy. At last, the adjustment of fiber mechanical polarization controller to the system polarization state is selected and polarization control program which uses phase modulation method to evaluate the system-polarization is designed.