We demonstrate the integration of vertical-cavity surface-emitting laser (VCSEL) arrays with silicon photonics chip using flip-chip bonding technique, with bidirectional vertical-coupled grating coupler for light coupling.
One of the most important remaining issues faced by the silicon photonics community is the on-chip laser source. The potential solution falls into two categories. The direct solution for this issue is making lasers based on epitaxial growth on silicon, and the hybrid solutions is realized by the the integration of silicon photonics chips with III-V epitaxy on laser chips. The direct solution has challenges of material limitations, while the hybrid solution can take advantages from the mature fabrication processes for the high quality silicon photonics chips and the VCSELs. 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 -. The flip-chip bonding technique has been used to bond both single VCSEL  and VCSEL arrays to CMOS chips . 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 and the output from the VCSEL need to be aligned to the vertical grating coupler on the silicon photonics chip. In this paper, we demonstrate the integration of 2x4 VCSEL  array with 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.
A vertical grating coupler is required to couple the light from the VCSEL onto the silicon photonics chip. Such vertical grating couplers have been designed to couple light from optical fibers onto silicon photonics chips . However, the coupling efficiency of the single-side coupled vertical grating coupler can be further improved, and a coupling efficiency limit of 50\(\%\) applies when the structure is symmetric. In addition, the near-field mode size of the output from the VCSEL is about 6 um, smaller than the 9 um mode from an optical fiber. So a smaller grating coupler is required to couple the light from the VCSEL, which makes high-accuracy alignment more difficult to achieve during the bonding process. A bidirectional vertical grating coupler is designed to couple light from the VCSEL as shown in Fig. 1. The grating coupler is a symmetric structure with uniform gratings. Output from the VCSEL diffracts at the center of the grating coupler and couples equally into the waveguides on both sides of the grating. The top view of a microscope image of the bidirectional grating coupler is shown in Fig. 2(a). Two adiabatic tapers are used on both sides of the grating coupler to convert the mode from the VCSEL into the fundamental mode of the sub-micron waveguides. The coupled light in the two arms of the vertical grating coupler can be either used separately or recombined, depending on the application. Such bidirectional vertical grating couplers can be also made into arrays to couple light from VCSEL arrays, like shown in Fig. 2(b).