Power minimization and technology comparisons for digital free-space optoelectronic interconnections

This paper investigates the design optimization of digital free-space optoe lectronic interconnections with a specific goal of minimizing the power dis sipation of the overall link, and maximizing the interconnect density. To t his end, we discuss a method of minimizing the total power dissipation of a n interconnect link at a given bit rate. We examine the impact on the link performance of two competing transmitter technologies, vertical cavity surf ace emitting lasers (VCSEL's) and multiple quantum-well (MQW) modulators an d their associated driver-receiver circuits including complementary metal-o xide-semiconductor (CMOS) and bipolar transmitter driver circuits, and p-n junction photodetectors with multistage transimpedance receiver circuits. W e use the operating bit-rate and on-chip power dissipation as the main perf ormance measures. Presently, at high bit rates (>800 Mb/s), optimized links based on VCSEL's and MQW modulators are comparable in terms of power dissi pation. At low bit rates, the VCSEL threshold power dominates. In systems w ith high Bit rates and/or high fan-out, a high slope efficiency is more imp ortant for a VCSEL than a low threshold current. The transmitter driver cir cuit is an important component in a link design, and it dissipates about th e same amount of power as that of the transmitter itself. Scaling the CMOS technology from 0.5 mu m down to 0.1 mu m brings a 50% improvement in the m aximum operating bit rate, which is around 4 Gb/s with 0.1 mu m CMOS driver and receiver circuits. Transmitter driver circuits implemented with bipola r technology support a much higher operating bandwidth than CMOS technology ; they dissipate, however, about twice the electrical power. An aggregate b andwidth in excess of 1 Tb/s-cm(2) can be achieved in an optimized free-spa ce optical interconnect system using either VCSEL's or MQW modulators as it s transmitters.