Large area Si3N4 grating-assisted Ge photodetectors on a photonic integrated circuit for free-space coupling

In a free-space optical communication system, the transmitter and receiver pair must complementarily orient to one another to establish an efficient link. If the receiver can determine the incident angle of arrival, this initial acquisition pointing procedure can be accomplished quickly. We explore a means of determining angle of arrival using grating assisted photodetectors compatible with commercial silicon integrated photonics foundry services. By creating an array of photodetectors, each with a complementary acceptance angle, a receiver can efficiently determine the incident beam angle. The available germanium (Ge) photodetector layer thickness is designed for lateral light absorption and would be too thin to absorb surface normal light. The two layers of silicon nitride available in the process offered an opportunity to efficiently couple 1550nm free-space light into the Ge photodetectors. Our designs significantly increase photodetector responsivity without increasing physical size of the Ge, which avoids additional junction capacitance, maintains high bandwidths, while limiting noise and dark current. We designed and measured a collection of grating assisted photodetectors capable of sensing incident light across -2 to +30 degrees from normal. Simulations show can achieve between -1.0 and -2.5 dB (55- 80% directivity) output coupling efficiency. A technique to adapt the grating-assisted photodetectors for optically coherent detection is also introduced, which could further extend communication link distances.