1.5 ENABLING TECHNOLOGIES FOR WDM OPTICAL NETWORKS
WDM optical networking is enabled by a range of technologies. At the foundation is the extremely high-bandwidth (25-THz), low-attenuation-loss (0.2-dB/km in the 1.55-micron low-attenuation band) single-mode optical fiber allowing long-distance transmission. A new type of fiber, called AllWave fiber, does not have the "waterpeak window" at 1.39 microns, which previously prevented the use of that part of the spectral region of the fiber, and hence it provides a more usable optical spectrum. EDFAs provide optical amplification of all the wavelengths at the same time to compensate for power loss in optical signal transmission. Conventional or C-band EDFAs amplify signals in the range 1.5301.562 microns. Long-wavelength-or L-band EDFAs (which use longer lengths of erbium-doped fiber) allow amplifi-cation of signals in the wavelength range 1.5701.610 microns and will be the next generation of EDFAs . A combined (C+L)-band EDFA can provide 10 THz of bandwidth. While EDFAs work over 1.5301.610 microns wavelength range, Ra-man amplifiers can amplify signals from 1.270 to 1.670 microns to further increase the capacity of optical fibers. Most optical transmission systems use semiconductor lasers as their light sources. The most commonly used light sources are distributed Bragg reflector and distributed feedback lasers. The transmitters used in WDM networks often require the capability to tune to different wavelengths. The well-established approaches for realizing tunable optical sources include mechanically tuned lasers, acousto-optically and electro-optically tuned lasers, and injection current tuned lasers. Two recent developments include array sources and switched sources. These tunable lasers differ mainly in two characteristics: tuning range (this refers to the range of wavelengths over which the laser may be operated) and tuning time (this specifies the time required for the laser to tune from one wavelength to another). Semiconductor lasers with a tunable range of 0.04 microns are becoming commercially available. Single-channel transmission speeds greater than 10 Gb/s are enabled by Mach-Zehnder laser modulators with modulation bandwidths in excess of 10 GHz. Tunable filters, which allow splitting and combining of the available wavelength band into several individual wavelength channels, are another key technology. A variety of tunable filters such as fiber Fabry-Perot and fiber Bragg grating filters are commercially available. The switching times of electromechanical optical switches are now typically between a few and tens of milliseconds. In the very near future it will be possible to realize high-performance, low-cost optical components (such as optical switches, tunable lasers, and variable optical attenuatorsvariable optical attenuators are used inside optical amplifier, add/drop multiplexers, and crossconnects to control the optical power) using MEMS (Micro-ElectroMechanical Systems) technologies . MEMS devices are miniature structures fabricated on silicon substrates in a similar manner to silicon integrated circuits. However, unlike electronic circuits, these are mechanical devices. Optical MEMS switches built using several micromirrors on silicon have already been demonstrated. Commercial PIN (p-type, intrinsic, n-type) photodetectors and avalanche photodetectors provide bit rates of 10 Gb/s and higher at the receiver. EDFA-based optical preamplifers, which raise the power level at the input of the receiver, enable high sensitivity of these optical receivers.
Today's widely installed WDM optical networks are opaque, that is, a signal path (connection) between any two end nodes or users in these networks is not totally optical. This means the path involves opticalelectronicoptical conversion operations at intermediate nodes and these conversion operations affect the network speeds or bit rates. WDM optical networks are migrating from just point-to-point WDM links to all-optical networks, where more and more switching and routing functions are carried out in optical domain. In all-optical networks each connection (lightpath) is totally optical except at the end nodes.
Note that the designers of WDM networks must be aware of the properties and limitations of optical fiber and components (without making unrealistic assumptions about these) in order to realize a practical network and its associated protocols/algorithms which exploit the full potential of WDM. For network operators, who like to deploy equipment from multiple vendors that operate together in a single network, interoperability among the equipment is very important. WDM network standards are being developed to achieve WDM multivendor inter-operability. Standards (which focus on interfaces that specify how equipment is physically interconnected and what procedures are used to operate across differ-ent equipment) allow network operators to have choice of buying equipment from different, competing vendors, rather than being committed to buying equipment from a single vendor. Achieving optical interoperability is not an easy task because it requires standardizing parameters such as wavelength, optical power, signal-to-noise ratio, pilot tone (for keeping track of the origin of each optical signal that is monitored), and supervisory channel (for carrying control information). There are several standards organizations and industry consortia working in this area (standards), including the International Telecommunications UnionTelecommunication Standardization Sector (ITU-T), Internet Engineering Task Force (IETF), Optical Domain Service Initiative (ODSI), and Optical Internetworking Forum (OIF). Appendix A gives a Web resources list, linking to home pages of several WDM optical component/system vendors.