Recently, silicon-on-insulator (SOI) has become one of the most promising platforms for integrated optics.Different application needs waveguides with various width, which are generally connected using linear tapers. However, to realize a high efficiency, the taper should be long enough. This obviously runs counter to the integration of photonic devices.
To realize a short and efficient mode-size converter, the group of Prof. Xinliang Zhang present an efficient segmented-stepwise method to design a short and low-loss mode-size converter. An SOI platform based converter with 20μm 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 12μm wide waveguide and a 0.5μm wide single mode waveguide, with comparable performance of a regular scheme using 150μm long linear taper. For demonstration, the converter was fabricated by electron-beam-lithography and inductively-coupled-plasma etching. A conversion loss of -0.62 ± 0.02dB at 1550nm was experimentally measured.
The result is published in Optics Letters (Vol. 39, Issue 21, pp. 6273-6276 (2014)), and this work was supported by the National Basic Research Program of China (Grant No. 2011CB301704), the National Natural Science Found for Distinguished Yong Scholars (61125501) and NSFC Major International Joint Research Project (61320106016), NSFC (Grant No. 61275072), the New Century Excellent Talent Project in Ministry of Education of China (NCET-13-0240), and Huawei Technologies Co. Ltd.