Design of an extra-low-loss broadband Y-branch waveguide splitter based on a tapered MMI structure

An optimal design for an extra-low loss buried waveguide Y-branch splitter is presented over a broadband wavelength range from 1500~1600 nm. A tapered multimode waveguide section, earlier used to reduce the excess loss, is optimized when the light distribution at the end of the multimode waveguide section is matched to the profile of the symmetric supermode for the structure of the two branching waveguides. An optimization that combines genetic algorithm and a gradient-based search method are used to obtain the optimal geometrical parameters for the multimode waveguide section as well as the widths for the input and branching waveguides. An excess loss of 0.015 dB@1550 nm was obtained after the proposed optimization. For these optimized parameters, even when packaging induces a typical offset between input standard singlemode fibre and waveguide, the wavelength dependence of the output ratio between the two branches is small (less than 0.03 dB).