Contents I. Introduction
One of the most important remaining issues faced by the silicon photonics community is the on-chip laser source. Potential solutions fall into two categories. One is heterogeneous integration by direct wafer bonding or epitaxial growth, the other is hybrid integration of silicon photonics chips with III - V epitaxy on laser chips. Heterogeneous integration has challenges related to crystal properties and fabrication process, whereas the hybrid solution can integrate the optimized silicon photonics chips to best in breed III-V epitaxial laser chips. Due to their surface-normal emission, VCSELs are of great interest for hybrid integration with grating-based off chip couplers in silicon, enabling simple, direct bonding schemes. Enormous progress has been made in VCSEL technology in the past two decades, with various successful attempts at hybrid integration of VCSELs to various carrier substrates [1], [2]. The flip-chip bonding technique has been used to bond both single VCSEL [1] and VCSEL arrays to CMOS chips [3]. However, flip-chip bonding of VCSELs with silicon photonics chips is more challenging than the bonding of VCSELs with CMOS chips. High alignment accuracy is required during the bonding process as the output from the VCSEL needs to be precisely aligned to the vertical grating coupler on the silicon photonics chip. In this paper, we demonstrate the integration of a VCSEL [4] with a silicon photonics chip using the flip-chip bonding technique, with custom designed vertical grating couplers as the input/output interfaces. The schematic of the bonding structure is shown in Fig. 1.
