In this chapter we describe the advances in optical networking that made possible the explosive growth of communication networks of the 1990s. We also point out the directions in which optical networking is likely to evolve.

The bandwidth of copper cables declines rapidly to 100 MHz over a 1-km distance, before signal regeneration is required. By contrast, an optical fiber has a bandwidth of 25,000 GHz over a distance of several tens of kilometers. That capacity is already used in several ways.

Cable TV fiber distribution networks utilize a bandwidth of about 1 GHz. Optical links have increased LAN speeds to 1 Gbps, and 100 Mbps Ethernet links to the desktop are no longer uncommon.

The capacity of telephone and data backbone networks increased by several orders of magnitude from DS-3 (45 Mbps) links in 1990 to OC-48 (2.5 Gbps) SONET fiber links in 1997. This is still a tiny fraction of the 25-THz fiber bandwidth. The reason for the limit is that electronic modulators today have a maximum speed of 2.5 Gbps. That speed will soon increase to 10 Gbps.

By 1997 commercial dense wave-division multiplexers (WDM) overcame the electronic speed limit by transporting forty 2.5-Gbps channels on the same fiber, increasing total link capacity to 100 Gbps. Companies have announced WDM products capable of carrying 160 10-Gbps channels. Thus very high-speed WDM links are being deployed.

The next wave of products will include optical cross-connects (OXCs) that allow combining links into "lightpaths" entirely in the optical domain.