Format-blind crossbar performs multiplication

A new semiconductor optical-amplifier-based optical crossbar can switch between eight channels with data rates of more than 1 GHz each. Because there are no electro-optical conversion stages within the system, it is format blind: the crossbar can route almost any analog or digital signal the connecting fibers can carry. The actual switching is done with semiconductor optical amplifiers (SOAs). Fibers link the SOAs to input and output channels forming a matrix-vector multiplier architecture. Developed by Optivision (Palo Alto, CA), the crossbar is currently being tested at MITRE Corp. (Bedford, MA) and TASC Inc. (Reading, MA).

The semiconductor optical amplifiers are buried double heterostructures fabricated in bulk InGaAsP that work in a similar way to semiconductor lasers. In each case, electrical energy is used to amplify an optical signal. With the amplifier, however, there is no laser cavity. Electricity is converted to light by raising the atoms within the SOA to higher energy levels. The signal stimulates photon emission as it travels through the de vice, and is thus amplified. The SOA current determines the level of amplification, so the signal can be controlled electronically.

Transparent architecture

The advantage of the Optivision architecture is that it uses the SOAs to connect fibers rather than operating in free space, which means that alignment ceases to be a problem. The data stream is fed in on standard 1300-nm single-mode fibers. The first stage of the crossbar amplifies each stream to compensate for the fan-out. The output of this stage is effectively a four-way pigtail (for a four-channel crossbar, see figure), and each channel speaks to a row of inputs at the crosspoint. The second stage of SOAs performs the switching. Signals leaving the crosspoint are then summed in columns, using fiberoptic combiners, to produce the system outputs.

In the current version, Optivision has been using two different types of controllers. In the first, the switching information is electronically transmitted from other computers to the crossbar, via either SCSI or ethernet ports. In the second, the addressing information is included as message headers in the data passing through the fibers. The channels are electronically polled in series, and switching takes place when a new addressing code is detected. The crossbar can also support wavelength-division multiplexing within the channels, and switching information could be sent via in-fiber, out-of-band transmissions at 1550 nm. Optivision researchers say that this feature is incorporated in a new controller, which will be unveiled in the next few months.

All-optical amplification

For transmission over distances greater than a few kilometers, the signal has to be amplified en route: the high bit rate and variable format reduce the length over which the signal is usable. In Massachusetts, for instance, the testbed network between MITRE Corp. in Bedford and TASC Inc. in Reading—a distance of 20 km—needs two optical repeater stages. To perform this function, Optivision developed amplifiers that use the same SOAs as the crossbar switch. In the MITRE/TASC testbed, the resulting all-optical network links analog sources, workstations, and an image server, allowing data to be accessed from many locations as if all were connected directly.

In addition to its all-optical products, Optivision has been commercializing hardware for electronic networks. In particular, it licensed high-performance parallel-interface (HIPPI) technology from Los Alamos National Laboratory in New Mexico. The HIPPI-SONET (synchronous optical network) gateway allows HIPPI data to be transmitted through standard fiber networks by cutting a signal up into six data streams. Each travels down its own fiber, and then the six data streams are recombined at the end.

For the December SuperCom '95 supercomputer show (San Diego, CA), Optivision has been asked to supply the link between the supercomputer center at the University of California at San Diego and the San Diego Convention Center. The latest version of its optically transparent switching system will be demonstrated at the Optical Fiber Communication conference in San Jose, CA, in February 1996.