Board Level H-Tree Optical Clock Distribution with Substrate Mode Holograms

This paper describes a design for a board-level free-space optical clock distribution system implemented with substrate mode holograms. The design uses an H-tree distribution scheme to balance propagation delays for different clock paths on each board. The H-component provides four diffracted output beams with ~11% efficiency for each output, and the required laser power for the clock distribution system is calculated. To provide 80 clock fan-out signals for a five-board system, the clock distribution system requires 4.7 mW of laser power with 10μW of received optical power at each node. The misalignment analysis for the free-space optical system is presented and design considerations are given. The factors causing clock skew and timing jitter in the optical clock distribution system are also examined. An experimental system was demonstrated with a 622-MHz clock signal distributed with 36 ps of timing jitter and less than 10 ps of clock skew.