Heuristic inverse design of integrated mode converter by directly reshaping silicon waveguide

Abstract

Compact photonic devices for mode manipulation in silicon waveguide are of importance as fundamental building blocks in on-chip photonic integrated circuits. In this paper, we present a heuristic inverse design scheme based on directly reshaping the silicon waveguide for TE-polarized mode conversions. No additional complicated internal nanoscale structures are introduced to the waveguide. The boundary deformations of the 6 μm-long functional region are described by Bernstein polynomials and the optimal design is searched and determined by the gradient-based method of moving asymptotes. The TE0-to-TE1 (TE1-to-TE2) mode converter can be efficiently obtained after 19 (52) iterations from a straight waveguide. Quasi-3D simulations show the high mode purity (>0.99) at center wavelength of 1550 nm, while the lowest mode purity keeps higher than 0.97 within the operational bandwidth of 60 nm for all designs. Relatively high transmission efficiency is maintained as well. 3D simulations with SOI configuration validate the functionality and the satisfactory performances. For both 340 nm and 220 nm waveguide thicknesses, the conversion efficiencies respectively keep higher than 0.98 and 0.90 for TE0-to-TE1 and TE1-to-TE2 mode converters at center wavelength. By simply cascading the two proposed mode converters, reciprocal TE0-to-TE2 mode conversion can be achieved. Moreover, we demonstrate the robustness of proposed designs by adding ±10 nm geometric deviations.