Research Of Polarization Coupling Detection In Polarization-Maintaining Fiber Based On White Light Interferometry

The principle of detecting polarization coupling in Polarization-Maintaining fiber based on white light interferometry is systematically analyzed in this dissertation. The whole testing system of polarization coupling in PM fiber had been designed and achieved. The PM fiber coil is an important part of fiber gyro. The polarization coupling caused by stress and twist in coil of fiber gyro can be found effectively by using this system. So the quality of fiber gyro could be controlled. The test system using white light interferometry has great signification for the research and production of fiber gyro. The main polarization mode and coupling mode in PM fiber has some optical path difference because of internal birefringence. The optical path difference between the main polarization mode and coupling mode could be compensated through the scanning mirror in Michelson interferometer. The interference between the main polarization mode and coupling polarization mode could be realized. By checking the distance between two zero grade interferential fringe, the position of polarization coupling along the detected PM fiber could be computing. There are two main structures in the test system: polarization state adjustment and Michelson interferometer. In the structure of polarization state adjustment, the polarization state of input linear polarization light can be adjusted by the rolling half-wave plate, polarization prism and optical-electronic detecting module. In the Michelson interferometer, the compensation of optical path difference can be achieved by high precise linear motion rail and high precise step-motor. The interferential signal is captured by an optical-electronic detecting module. Then, the acquisition data is transferred to a computer by PCI data acquisition card. The final test result is gained after the data processing. The system could test a 1000m long PM fiber and the coupling coefficient that could be detected is -33dB. The spatial resolution of testing system is 57.2mm. The following are innovations of this dissertation. 1. The system spatial resolution had been improved by using broadband white light source. White light interferometer had been realized in this dissertation. The principle of testing polarization coupling based on white light interferometry had been analyzed. The theory computing model of testing system had been founded according to the test system structure. 2. By using white light source with low coherence length, the white light interferometry had been realized. The optical path difference was compensated by moving scanned mirror 300mm. The coupling along 1000m PM fiber could be detected, and the spatial resolution of system is 57.2mm. 3. The polarization state of linear polarization light is adjusted by the rotational half-wave plate and polarization prism. The best results of linear polarization light interference had been implemented by adjusting the polarization state. The contrast ratio and the test sensitivity had been improved. The problem of PM fiber alignment of PM fiber was resolved. 4. In the mechanical design, the high accuracy ball screw has been used to reduce the friction between the motive parts. The load of motion block had been decreased through using less size scanning mirror and adjustment mechanism. The high level micro motion has been achieved by linear motion rail and step motor. In the full scanning range of 300mm, the 0.2μm motion precision was implemented to scan the compensated optical path difference. 5. The interferential data was processed, and the noise level had been reduced by filter. Because of system characteristic, the maximum points of interferential curve are dense. The wavelet transform had been applied in the process of interferential data to check the maximum points because wavelet transform has the advantage of zooming in data details.