Telecom routers and switches have been pushing the limits of conventional interconnect technology. As routers and switches have become larger and more complex the connection from rack-to-rack or system-to-system has become a bottleneck. Copper interconnects have reached the limit in terms of speed and distance. This need led to the development of parallel optical interconnects. These interconnects transmit data in parallel over a fiber ribbon, enabling large throughputs that are now reaching up to 40 Gb/s with one link.

There are two competing technologies for the parallel interconnect market. The first uses conventional 1310 nm Fabry Perot edge emitting lasers. These lasers provide the benefit of proven reliability and the ability to transmit over Single Mode (SM) or Multi-mode (MM) fiber. The second technology is based on 850 nm Vertical Cavity Surface Emitting Lasers (VCSEL's). This technology is attractive because of lower cost; however, they can only transmit over MM fiber. Both technologies can transmit over MM fiber with distances of 300 meters or longer. Interconnects that use SM fiber can transmit distances of several kilometers.

Potential Advantages
Although the market for large switches and routers has slowed in the current economy, thus the demand for parallel optical interconnects has decreased, there is a new emerging application for this technology: SONET/SDH (Synchronous Optical Network/ Synchronous Digital Hierarchy) compatible multi-channel modules. SONET/SDH is still the predominant technology for optical transmission to the ports for telecom switching and routing equipment. System vendors are trying to put more and more optical transceivers in one board. Because of this, the size of single channel transceivers has been reduced. Packaging for SONET/SDH transceivers has gone from bulky 1x9 transceivers to Small Form Factor (SFF) transceivers. Currently system vendors are placing 16, 32, or more SFF transceivers on one line card. This takes up a large amount of board real estate. The next logical step is to place multiple transmitters and receivers in one package. This is where the technology used to develop parallel interconnects comes into play. Companies are now introducing multi-channel links that are compatible with existing SONET/SDH standards. By using multi-channel links vendors can cut their real estate for optics in half, or more. By offering compatibility with existing standards and leveraging the price advantage of multi-channel packaging, there is a significant advantage to using multi-channel modules to replace SFF parts.

In addition to the density savings, there are potential cost savings as well. Currently a SFF OC-12 IR TRx costs up to $200. With multi-channel optics, this cost can be cut in half to $100/channel. The cost reduction coupled with the 50% density improvement provides a tremendous advantage to the system designer.

Compatibility Requirements
The requirements for compatibility between a multi-channel optical link and existing SONET/SDH standards are different than those for conventional parallel interconnects. One fundamental difference is that for interconnects the transmission is from one transmitter to a receiver over a fiber ribbon. For multi-channel SONET optics, the transmission can be from many different single channel transmitters to a single multi-channel receiver (or vice-versa) by using an optical fan-out (see diagram).

The specifications are also different. In general, the Extinction Ratio (ER) must be larger for SONET links. In addition, the channel-to-channel crosstalk is much more important. The large majority of the SONET market is for lower data rate parts, either OC-3 (155 Mb/s) or OC-12 (622 Mb/s). These links must meet the Intermediate reach (IR) specifications. In order to meet the distance requirements of more than 15 km, the receiver sensitivity is critical. The OC-12 IR specification requires sensitivity of better than -28 dBm. In order to meet this requirement, the channel-to-channel crosstalk is critical.

Today there are many companies developing parallel interconnect modules. However, as the economy continues to struggle along, the ability to find new applications and new markets will be key to the survival of vendors of parallel interconnect technology.

This article was written by Chris Keller, general manager of the Technology Department, for InterIntelligence, Inc. The author may be contacted at ckeller@interintelligence.com or (310) 768-2900, ext. 210. Visit InterIntelligence, Inc. online at www.interintelligence.com.

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