Abstract

There is debate on the comparative usefulness of optical (or optoelectronic) and electronic devices in digital processors. Here I point out one feature of optics not so far stressed, optical detectors, sources, and modulators, because they can be quantum devices and can operate as efficient impedance transformers, which can reduce the energy required to communicate information between logic devices. Because of noise and threshold uniformity, room-temperature electronic logic levels are ~1 V. Because electromagnetic propagation is practically low impedance, large dissipations result in irreversible electrical communication of logic-level signals (e.g., 20 pJ for a 1-V, 1-ns pulse on a 50-Ω line). Optical quantum detectors, sources, and modulators can, however, effectively impedance match small (high-impedance) electronic devices to this propagation. For example, a photodiode giving one electron per absorbed photon and operating into high impedance can generate ~1 V with ~1 nW of light, although there is only ~600-μV classical voltage in such a beam. A similar argument works in reverse for ideal quantum emitters or modulators, generating or modulating one photon per electron of current. Properly integrated, semiconductor photodetectors and quantum well modulators (or possibly low-threshold lasers) could actually reduce energy for all except the shortest intrachip interconnects.

© 1988 Optical Society of America