Design of a mode filter consisting of two dual-mode highly elliptical-core fibers

Coupling characteristics of directional couplers consisting of two dual-mode highly elliptical-core fibers have been studied in detail using a first-order perturbation approach, and two different designs of a mode filter are proposed. The study shows that the fibers should be polished normal to the minor axis of the core ellipse if the first design principle is used, whereas they should be polished normal to the major axis if the second design principle is used. It is also observed that the latter design is much easier to realize in practice.<
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