Abstract
Silicon photonic integrated circuits have received a lot of attention recently. Due to high-refractive-index contrast between silicon and silicon dioxide, silicon-on-insulator photonic devices usually can be made very compact and easily extend to a large-scale system. However, the light coupling between silicon photonic wire and fiber is a critical issue. Here we present a two-step coupler which can be used to efficiently couple the light from fiber to silicon photonic wires. Because the difference between the waveguide core (about 1um) and fiber core (about 10um) dimension is very large, we utilize SU8 photoresist to fabricate a 3D adiabatic taper for reducing the coupling loss between the fiber and an intermediate waveguide as the first-step coupling. After coupling to the intermediate waveguide, the optical wave is further transferred to the silicon photonic wire through a grating-assisted directional coupler. We use FDTD to analyze the grating period for wavelength around the communication band at 1550nm and the grating period was calculated to be 1.15um and 50nm in height. We use PECVD(plasma enhanced chemical vapor deposition) to deposit SiON for the intermediate waveguide. In the measurement, we have fiber end-to-end insertion loss about -14.6db at 1552nm. Polarization extinction ratio between TE and TM is around 20dB. The misalignment tolerance of 3dB decay is 4m in horizontal direction and 6um in vertical direction.