Expanded Beam Backside Coupling Interface for Alignment-Tolerant Packaging of Silicon Photonics

We demonstrate an alignment-tolerant backside coupling interface in the O-band for silicon photonics by generating an optimized through-substrate (downward) directionality beam from a TE-mode grating coupler and hybrid integrating the chip with backside silicon microlenses to achieve expanded beam collimation. The key advantage of using such an expanded beam interface is an increased coupling tolerance to lateral and longitudinal misalignment. A 34 $\mu$ m beam diameter was achieved over a combined substrate thickness of 630 $\mu$ m which was then coupled to a thermally expanded core single-mode fiber to investigate the tolerances. A 1-dB fiber-to-microlens lateral alignment tolerance of 14 $\mu$ m and an angular alignment tolerance of $1^{\circ }$ was measured at a wavelength of 1310 nm. In addition, a large $\pm$ 2.5 $\mu$ m 1-dB backside alignment accuracy was measured for the placement of microlens with respect to the grating. The radius of curvature of Si microlens to achieve a collimated beam was 480 $\mu$ m, and a 1-dB longitudinal alignment tolerance of 700 $\mu$ m was measured for coupling to a single-mode expanded core fiber. The relaxation in alignment tolerances make the demonstrated coupling interface suitable for chip-to-package or chip-to-board coupling