Study On Polarization Of Single-Mode Fiber Optic Interferometer

Interferometric optical fiber sensors have drawn much attention for its high sensitivity in sensor technology, and have been applied in many fields. While, polarization fading of optical signal induced by enviromental fluctuation is still a problem. Though Polarization maintaining optical fiber(PMF) can be used to overcome the fading, it is still costly. So it is significant to study the polarization problem in single mode optical fiber(SMF) and realize dynamic compensation technology for polarization fading in all optical fiber interferometric system.After introduced the polarization theory and influencing factors of polarization status's variation in SMF, the birefrigent effect induced by optical fiber twist was analyzed emphatically. Firstly, the geometric effect and stress effect in twisted optical fiber was solved using the coupled mode theory, and the dielectric constant variation under twist stress was discussed. Afterwards, using the Poincare sphere method characterized the linear, circular and elliptical retarder, and analyzed the retard and rotation effects in twisted optical fiber. Then we further studied the evolution of the state of polarization along a twisted SMF considering the twist rate, different azimuth angle of input polarized signal, fiber length and the ellipticity angle of the fiber. Then considering the Mach-Zehnder SMF interferometer, the transmission matrixes of optical devices were introduced briefly and the output intensity was deduced. Based on the intensity equation, the influence of the azimuth angle of input light on the output signal's visibility and extra phase shift were researched by simulation.Based on above analysis, we designed a device to compensate the polarization fading dynamically in all fiber system based on fiber twisting. A stepping motor was used to control the fiber twist in a low velocity, a mechanical device achieved the twist. The circuit control part consisted of optical signal processing and stepping motor control. In the program control of single-chip machine, the stepping motor will not stop work until find a suitable work condition by detection the signal status of the optical fiber in real time. The system is testified to be feasible by experiment and its visibility can achieve to be 0.707.