Optical Technologies
Optical Transport Networks

On present days, optical technologies utilizing optical transmission media dominate in the transport level of telecommunication networks. Except an evolution of low-loss optical fibers, various optical components needed to realize optical-fiber transmission systems have been developed. The penetration of optical technologies into the various network levels is schematically illustrated in Figure below.

Penetration of photonic technologies into network levels

Optical technologies were first applied to transport networks, because these technologies are best suited to this application, which requires long-distance and wide-bandwidth transmission. Moreover, costs of the system are shared by many users (connections). Thus, the transport transmission cost per bit is much lower with optical media than with metallic media. Primary applications of optical technologies realize only point-to-point connection [50], [51]. However, to create large-bandwidth end-to-end connections cost-effectively, it is essential to reduce not only a transmission cost, but also a transport node cost. A key role in reducing both costs is playing by optical technologies that can be developed also in local area networks, in optical networks of the type Ethernet, FDDI or Fiber Channel. From first introduced optical-fiber transmission systems in 1981, a transmission capacity has been increased more than one order of magnitude per decade. This has resulted in a more than 90% reduction on a cost in this decade. The continued enhancement of electronic devices has also driven research into a high-speed transmission, so a rapid progress to multi-Gbps transmission systems is allowed. Another technology that must be emphasized is a technology of optical amplifiers. Their advantages including a wide bandwidth, a low noise, a high gain/power ratio and an easy connection to single-mode fibers greatly increase an application range of optical transmissions by overcoming the optical loss of signal transmission, devices and optical components. Furthermore, new optical technologies – a soliton transmission, a dense wavelength-division multiplexing DWDM, an optical frequency-division multiplexing OFDM and a high-speed optical signal processing – have the potential to enable further expansion of the transmission capacity and the network flexibility.

1G and 2G optical networks

In first-generation networks, the electronics at a node must not only handle all the data intended for that node, but also all the data that is being passed through that node on to other nodes in the network. If the latter data could be routed through in the optical domain, the burden on the underlying electronics at the node would be significantly reduced. This is one of key drivers for second-generation optical networks. Examples of these networks are the OTDM and WDM networks. WDM networks are expected to be deployed in the next few years, not only in interexchange networks and undersea networks, but also in local-exchange and access networks. OTDM networks constitute a longer-term approach.