| The mode separating coupler is a key component for mode division multiplexing (MDM) techniques and two-mode fiber sensor systems. The existing mode separating couplers are made of a polished fiber stacked on a prism, which is unstable and has high insertion loss. The aim of the work described in this dissertation was to develop an all-fiber mode separating coupler that has good mechanical and thermal stability, low loss and compatible in optical fiber systems.;Using scalar supermode formalism as the basis, the finite difference and fiber mode expansion methods were used to study the feasibility of the fabrication of the all-fiber mode separating coupler with polishing, fusion and tapering method, and other various methods. For the polishing method, the coupler structure could be uniform or parabolic according to the fiber polishing ways and the fiber could have a circular or an elliptical core. The simulation results show that it is possible to make a mode separating coupler with both uniform and parabolic structures. However, using circular core fibers requires precise polished depth and coupling length that may be difficult to realize in practice. Elliptical core fibers have certain advantages and make the fabrication of the mode separating couplers relatively easy. For the fusion and tapering method, both the circular and elliptical core fiber couplers were modeled and the numerical results show that both circular and elliptical core fused couplers are able to act as mode separating couplers if their structure is properly designed. In order to relax the fabrication requirements and realize adiabatic couplers, some improvements such as pre-etching and prepolishing techniques as well as thermally expanded core fibers and highly elliptical core fibers are proposed.;An all-fiber mode separating coupler was realized by the fusion and tapering method for the first time. The LP11 mode transfer efficiency reaches as high as 92.3% and the excess loss is as low as 0.2 dB. A numerical simulation that takes into account the variation of core shape and relative refractive index is in very good in agreement with the experimental results, which confirms our theoretical understanding of the mode separating coupler. |
