Optical interconnections: status and prospects

Abstract

Optical interconnections are of interest as a high speed replacement for electrical interconnections in digital computers. Optical fiber technology between mainframes or subsystems is readily incorporated without adaptation. At the backplane or board level, the technology must be adapted. The use of optical waveguides is currently being studied by several groups at the board level. Free-space interconnections have been made between boards with only a laser, two inexpensive lenses, and a detector. The interconnections operated with GaAs logic at rates of 1 Gbit/s with 18.8% differential electrical current efficiency and an estimated 93% optical coupling efficiency (<0.5-dB loss). The energy and power dissipation for intrachip interconnections have been calculated by several groups and found to be lower than that of electrical interconnections for distances of the order of several millimeters. Techniques to obtain fanouts of up to 10 at high speeds with no optical splitting loss are described. Even larger lossless splitting ratios are possible with active fiber amplification. Diode laser interconnections can have an efficiency of 56% with no increase in the laser threshold. Finally, techniques for large fanouts and high interconnectivity through holograms are considered.