Monolithic integration of microlenses on the backside of a silicon photonics chip for expanded beam coupling

To increase the manufacturing throughput and lower the cost of silicon photonics packaging, an alignment tolerant approach is required to simplify the process of fiber-to-chip coupling. Here, we demonstrate an alignment-tolerant expanded beam backside coupling interface (in the O-band) for silicon photonics by monolithically integrating microlenses on the backside of the chip. After expanding the diffracted optical beam from a TE-mode grating through the bulk silicon substrate, the beam is collimated with the aid of microlenses resulting in an increased coupling tolerance to lateral and longitudinal misalignment. With an expanded beam diameter of 32 μm, a ±7 μm lateral and a ±0.6^° angular fiber-to-microlens 1-dB alignment tolerance is demonstrated at the wavelength of 1310 nm. Also, a large 300 μm longitudinal alignment tolerance with a 0.2 dB drop in coupling efficiency is obtained when the collimated beam from the microlens is coupled into a thermally expanded core single-mode fiber.