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Numerical Optimization of Grating Coupler Parameters for InP-based Photonic Circuits

We have performed 2D finite-difference time-domain (FDTD) simulations of fiber to chip coupling. The experimental semiconductor structure of the waveguide based on In-GaAsP /InGaAIAs compounds was used in the calculations. This structure is intended for use in electro-optical modulators constructed according to the Mach-Zehnder scheme. Our numerical calculations showed that a simple uniform grating couplers have extremely low coupling efficiency of about 5% for InP-based waveguides. We performed a global optimization of the grating coupler parameters such as etch depth, thickness, period, angle of the grating in relation to the waveguide, etc. We demonstrated a maximal coupling efficiency of about 60% which can be further increased by optimizing a number of other parameters.