Differential board/backplane optical interconnects for high-speed digital systems. Part I: Theory

Switching noise is one of the major performance bottlenecks in a dense optical interconnect system. In this paper, we show that differential configuration and balanced operation at both the transmitter and the receiver sides are essential to achieve low switching noise (≤5%) with large array size (≥16). A fully differential configuration is proposed in this paper to minimize the possible switching noise. Several candidate structures for differential optical interconnects are investigated. Based on these structures, the signal-to-noise ratio (SNR) and the system penalty due to channel mismatch are analyzed and the results are compared to those of a single-ended interconnect with similar driver and receiver structures. From this analysis, we show that (1) The SNR of differential optical interconnects is similar to that of single-ended structures, (2) System penalty due to mismatch is negligible if there exists a slight channel mismatch (≤1 dB). However, a power penalty (≈1.52 dB) exists when the mismatch between differential channels is significant (≥2 dB). A prototype based on this fully differential interconnect concept has been designed and fabricated and its performance is reported in [54].