Differential Board/Backplane Optical Interconnects for High-Speed Digital Systems Part II: Simulation Results

Abstract

In dense parallel optical chip-to-chip interconnects, the switching noise arising from the power supply will be a limiting factor on the receiver performance. In principle, this problem can be overcome by using a fully differential transmission scheme. However, laser nonlinearities as well as variations in the coupling/transmission losses and in the laser/photodetector quantum efficiencies will produce nonideal complementary photodetector signals. In this paper we analyze the performance of a dc-coupled fully differential interconnect in terms of the deterministic vertical and horizontal eye closure at 1 Gb/s for different optical power levels and extinction ratios in the two complementary arms. Two types of transmitters are investigated; one based on an ideal optical modulator switch (IOMS) and another based on conventional modulation of two separate lasers. In general, we find that the timing jitter increases 1) when the imbalance between the power levels in the two arms increases, 2) when the difference between the laser turn-on delay in the two arms increases, and 3) when the receiver and transmitter bandwidth is reduced. To achieve low latency and to accept a maximum power imbalance of 4–5 dB at the receiver, the IOMS and the lasers should be modulated to produce an extinction ratio of at least 10 dB, and the lasers should be biased above threshold. If low latency is not the main concern, an improvement in the maximum tolerable imbalance can be obtained with regular laser modulation by biasing the lasers below threshold, where the additional laser turn-on delay actually helps to reduce the jitter. The jitter can be reduced and the maximum tolerable imbalance be increased if faster receivers and transmitters are employed, however, at the expense of tighter requirements to the transmitted optical waveforms. © 1993 IEEE