  The unique benefits of Fabry-P\'erot resonators as frequency-stable reference
cavities and as an efficient interface between atoms and photons make them an
indispensable resource for emerging photonic technologies. To bring these
performance benefits to next-generation communications, computation, and
timekeeping systems, it will be necessary to develop strategies to integrate
compact Fabry-P\'erot resonators with photonic integrated circuits. In this
paper, we demonstrate a novel reflection cancellation circuit that utilizes a
numerically optimized multi-port polarization-splitting grating coupler to
efficiently interface high-finesse Fabry-P\'erot resonators with a silicon
photonic circuit. This circuit interface produces spatial separation of the
incident and reflected waves, as required for on-chip Pound-Drever-Hall
frequency locking, while also suppressing unwanted back reflections from the
Fabry-P\'erot resonator. Using inverse design principles, we design and
fabricate a polarization-splitting grating coupler that achieves 55% coupling
efficiency. This design realizes an insertion loss of 5.8 dB for the circuit
interface and more than 9 dB of back reflection suppression, and we demonstrate
the versatility of this system by using it to interface several reflective
off-chip devices.
