Short and efficient mode-size converter designed by segmented-stepwise method.

We present an efficient segmented-stepwise method to design a short and low-loss mode-size converter. A silicon-on-insulator platform-based converter with 20 μm length and 95.2% conversion efficiency is acquired by taking only 10 optimization generations using 2D-FDTD method. A 3D-FDTD simulation is performed to verify the calculated results, returning an efficiency of 92.1%. The proposed device can be used to connect a 12-μm-wide waveguide and a 0.5-μm-wide single-mode waveguide, with comparable performance of a regular scheme using 150-μm-long linear taper. For demonstration, the converter was fabricated by electron-beam-lithography and inductively-coupled-plasma etching. A conversion loss of -0.62±0.02  dB at 1550 nm was experimentally measured.