A high-performance SOI grating coupler with completely vertical emission

Abstract

Silicon-on-insulator grating coupler is a crucial passive device that enables the coupling of light into and out of a submicron, silicon-based photonic integrated circuit. In particular, a grating coupler with completely vertical emission is vital for interfacing surface-emitting/receiving optoelectronic devices, and can largely reduce the packaging cost and complexity due to off-normal configuration. Unfortunately, for a grating coupler with completely vertical emission, it inevitably induces the second-order diffraction that significantly degrades the coupling efficiency and enhances the backreflection. In this work, we propose and study a new concept for making a high-performance grating coupler with completely vertical emission. Following our design strategy, we numerically show that a 1.1 dB total coupling loss, a - 24.4 dB backreflection, and a 20 nm spectral linewidth can be achieved at the same time (without full optimization), when our grating coupler is butt-coupled to a standard single-mode fiber operating around 1310 nm wavelengths. Compared to the previous proposals such as slant grating, polymer wedge, and entrance mirror made by slit or chirped grating, our approach requires only CMOS process compatible elements and does not involve complex numerical methods to reach the desired performances. The design methodology and optimization procedure will also be discussed.