We consider networks for interconnecting parallel computers that are parameterized by their degree (fan-out) of the switches that are used to build the network. Electronic networks tend towards the small fan-out/large diameter end of the spectrum due to limitations on the fan-out of electronic switches. The fan-out of systems of optical switches is anticipated to be significantly higher than electronic switches. Thus it has been suggested that optics can be used to build the complete network which is the most desirable of Clos's family of networks. Even optics, however, has limits on the fan-out of switches in any given configuration. Thus in this paper, we describe an implementation of a network with a significantly smaller degree and slightly larger network diameter than the complete network. The network that we implement is called the mesh-connected bus network. We remark, however, that it is essentially an implementation of the second most desirable network in the family of networks defined by Clos, which is widely referred to as the Clos network. We implement the network using novel free-space optical interconnection techniques along with a multiple-access scheme based on wavelength-division multiplexing.>

The authors discuss digital optics, a technology for processing, transport, and storage of optical digital information. Digital optics offers both the high temporal bandwidth of fiber communications and the high connectivity and information density of optical imaging. The energy dissipation per bit of communicated information, as well as the chip area dedicated to interconnections, can be significantly lower in optics than in high-speed electronics. This motivates the introduction of parallel optical interconnections through free space in communication-intensive areas of digital information processing such as switching in telecommunications and within multiprocessors. Digital optical circuits can be constructed by cascading two-dimensional planar arrays of optical logic gates interconnected in free space. The state of the art and the trends in digital optical information processing systems for optical logic, optoelectronic interfaces, and optical free-space interconnection systems are reviewed.>