Coming: Faster, Cheaper Optical Data Links

Chris B. Chinnock

A new breed of optical interconnectors for linking video servers, computer clusters, and other applications will deliver the high-speed performance of fiber at less expensive prices. These new cables feature multiple channels of optical fiber with advanced lasers and optoelectronic receivers. Such devices should become commercially available this year. They will allow 400-Mbps point-to-point data transmission on each channel.

Today, fiber-optic systems are for the most part used for long-distance telecommunications l inks, communications backbones, and data WANs.

Copper cable will continue to be the best choice for many applications. For example, 50-ohm twin-axial copper cable ca n handle 150 Mbps over 100 meters, or up to 500 Mbps for less than 10 meters. But if you need to operate at over 500 Mbps at distances of 50 meters or more, optical fiber provides a better solution. The strength of a signal declines more rapidly over increased distances when you are using copper cable. You can extend the distance supported by copper using repeaters that amplify a signal, but that increases the cost of the network.

Companies such as Motorola's Logic IC Division (Chandler, AZ), IBM (Yorktown Heights, NY), Hewlett-Packard (Palo Alto, CA), and others are working to reduce the cost of parallel-optical bus architectures. Much of the cost of fiber-optic links is due to their connectors and components. One way to lower that cost is by assembling multiple channels, from four to 32, into a single connector to lower the overall cost per channel. Motorola's goal, for example, is to bring the price down to the area of $150 per link, which includes two transceivers and 10 meters of ribbon.

At the heart of these development efforts is an array of lasers called Vertical Cavity Surface Emitting Lasers (VCSELs). Anis Husain, a program manager at ARPA, which has funded research efforts in this area, says VCSELs will have a similar effect upon the optoelectronics field as CMOS technology had on silicon-based electronics (CMOS allows inexpensive, low-power-consumption chips).

Unlike edge-emitting lasers, VCSELs don't need to be packaged before testing. "You can probe-test VCSELs in wafer form," says Jerry Grula, a senior applications engineer at Motorola. "This means that from an overall system standpoint, you have known good laser arrays going into the final assembly." VCSEL technology developers include Motorola, Hewlett-Packard, Honeywell Technology Center (Bloomington, MN), and Vixel (Broomfield, CO).

Current serial standards such as Fibre Channel provide data connections of 266 Mbps and 1 Gbps now, with 4 Gbps expected in two years. Fibre Channel is a good solution for server-to-stora ge connections. But new parallel-optical interconnectors , which proponents say won't suffer from the overhead that comes with the serialization process, should enable emerging parallel-processing and/or shared-memory applications. Says John Crow, IBM's manager of optical communications, "You could tie together the data-processing capability of several workstations scattered around a department to perform sophisticated computer simulations and modeling that today require a multimillion-dollar supercomputer."

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Parallel-Optical Interconnectors At A Glance

Program         Companies          Channels           Data Rate      Length
                               (Transmit/Receive)    Per Channel


Optobus         Motorola           10 and 10          400 Mbps       1 to 300 
                                                      or higher      meters

POLO            HP, AMP, Dupont,   10 and 10          622 Mbps,
      1 to 300
(Parallel       SDL, University                       1 Gbps         meters
Optical Link    of Southern
Organization)   California

Jitney          IBM, LexMark, 3M   20 and 20          500 Mbps       1 to 50 
                                                                     meters