A Source-Synchronous Double-Data-Rate Parallel

Source-synchronous double-data-rate (DDR) sig- naling is widely used in electrical interconnects to eliminate clock recovery and to double communication bandwidth. This paper describes the design of a parallel optical transceiver integrated circuit (IC) that uses source-synchronous DDR optical signaling. On the transmit side, two 8-b electrical inputs are multiplexed, encoded, and sent over two high-speed optical links. On the receive side, the procedure is reversed to produce two 8-b elec- trical outputs. The proposed IC integrates analog vertical-cavity surface-emitting lasers (VCSELs), drivers and optical receivers with digital DDR multiplexing, serialization, and deserialization circuits. It was fabricated in a 0.5- m silicon-on-sapphire (SOS) complementary metal-oxide-semiconductor (CMOS) process. Linear arrays of quad VCSELs and photodetectors were at- tached to the proposed transceiver IC using flip-chip bonding. A free-space optical link system was constructed to demonstrate cor- rect IC functionality. The test results show successful transceiver operation at a data rate of 500 Mb/s with a 250-MHz DDR clock, achieving a gigabit of aggregate bandwidth. While the proposed DDR scheme is well suited for low-skew fiber-ribbon, free-space, and waveguide optical links, it can also be extended to links with higher skew with the addition of skew-compensation circuitry. To the authors' knowledge, this is the first demonstration of parallel optical transceivers that use source-synchronous DDR signaling.