The length of a directional coupler, including the straight and curved parts, is strongly polarization dependent, especially for use in waveguide tap monitoring applications. Three types of curved structure in the coupled regions are presented to demonstrate the different phase contributions in a directional coupler. A thick silicon-on-insulator waveguide single-mode region was theoretically verified by using the beam propagation method, thereby significantly improving the polarization dependence and coupling loss with a conventional fiber. The Mach–Zehnder directional coupler made of a thick silicon-on-insulator waveguide could minimize the severe polarization dependence on the optical tap port and achieve a flattened wavelength response by implementing the coupled phase effect from the directional coupler’s curved structures. The results demonstrated that the optical waveguide tap port, carrying a portion of the light signal, showed a 0.024 coupling ratio and for the polarization-dependent loss at a wavelength. The wavelength variation in the tap splitting ratio and polarization was less than 1% and , respectively, across the entire C-band. A per interface coupling loss was also achieved between the thick silicon-on-insulator waveguide and SMF-28 fiber.
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