Study On The Manufacturing Theory And The Key Techniques Of Fused Polarization Maintaining Fiber Couplers

At present, high performance and low cost Polarization-maintaining Fiber (PMF) couplers are demanded more extensively in the military and civil fields. There are many difficulties in the Fused-biconical Taper (FBT) fabrication process, such as the slow speed, low yield, poor rate of finished products, bad device consistency and high cost. The manufacturing theory of FBT approach and the key techniques are studied in this dissertation. The main research contents include the following four aspects: the precise auto-alignment technique of PMF, coupling theory of PMF couplers, high performance manufacturing equipment, and the relation between process parameters of fusion-elongation and the coupler's performance. The major research efforts include:Based on the polarization observation by lens-effect-tracing technique (POL), a new method using five characteristic points, which are more sensitive to the azimuthal angle of PMF, is presented to complete the auto-alignment of Matching Refractive-index Panda Fibers. After theoretical analysis and computer simulations, experiments are conducted on the auto-alignment platform. The results indicate that the method achieves higher measurement accuracy and better resolution. It is easier to determine the angular position of the main birefringent axes of PMF.The propagation properties and coupling coefficient in the fused-tapered zone of PMF couplers are analyzed according to Heat transfer-Structure mechanics-Electromagnetics multiphysics coupling fields. The coupling mode equation is solved and the power distribution of the linearly polarized light through the coupler is obtained. Then, the performance parameters such as coupling ratio, extinction ratio and excess loss are studied theoretically. The major research includes the following three points. First, using finite element method, the influence factors on propagation properties for PMF couplers are analyzed, including the shape of coupler cross-section, the taper of coupler bipyramidal and the angle deviation of the birefringent axes. Next, based on the coupling mode theory of optical waveguide, a new formula for coupling coefficient of PMF couplers is derived, which is applicable to any PMF with circular core and nonaligned birefringent axes. Third, based on the real deformation when the PMF is fusion-elongated, the numerical computation and theoretical analysis of excess loss mechanism is presented. The influence factors are discussed, such as the shape of the Stress Applying Parts (SAP), refractive index difference between the SAP and the cladding, the angle deviation between birefringent axes.A fusion-elongation platform is built for PMF couplers, based on the fusion-elongation device for single-mode fiber couplers. An auto-alignment mechanism is adopted to realize the key technique for PMF coupler fabrication. After the alignment, the intermediate sections of the fibers are close and parallel with one another by an approaching mechanism. Accordingly the light coupling and splitting is possible. The fusion-elongation platform is controlled accurately by the master computer and the step motors. Hence the conical contour is improved and the excess loss is reduced. The fusion-elongation equipment is compact, convenient and of low cost. It can complete the auto- fabrication of PMF couplers.Lots of fusion-elongation experiments are conducted, focusing on the relation between optical performance and process parameters, and aiming to improve the fabrication performance. The influence rule of the process parameters on the couplers' performance is disclosed and then used to find optimal process parameters for high performance fabrication of PMF couplers.