Fiber Grating Couplers for Optical Access via the Chip Backside

For the majority of packaging schemes in silicon photonics, out-of-plane optical access to the waveguide layer is achieved via the chip frontside, away from the substrate. As such, fiber grating couplers have generally been optimized with this in mind. In this article, we present two strategies for designing a fiber grating coupler for coupling to optical fibers placed on the backside of the chip, without the need of superposed reflective layers. An all silicon O-band grating coupler is designed, fabricated, and measured with a peak coupling efficiency to a single mode fiber placed on the backside of the chip of -2.8 dB and a -1 dB bandwidth of ~40 nm. In a second approach, a hybrid silicon nitride/silicon O-band grating coupler is measured in the same configuration with a peak coupling efficiency of -3.2 dB and a -1 dB bandwidth of ~60 nm.