We report on the theoretical investigation of using an amorphous Ge0.83Si0.17 lateral taper\nto enable a low-loss small-footprint optical coupling between a Si3N4 waveguide and a low-voltage\nGe-based Franz-Keldysh optical modulator on a bulk Si substrate using 3D Finite-Difference\nTime-Domain (3D-FDTD) simulation at the optical wavelength of 1550 nm. Despite a large\nrefractive index and optical mode size mismatch between Si3N4 and the Ge-based modulator,\nthe coupling structure rendered a good coupling performance within fabrication tolerance of\nadvanced complementary metal-oxide semiconductor (CMOS) processes. For integrated optical\nmodulator performance, the Si3N4-waveguide-integrated Ge-based on Si optical modulators could\nsimultaneously provide workable values of extinction ratio (ER) and insertion loss (IL) for optical\ninterconnect applications with a compact footprint.
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