After languishing for many years as an interesting technology without a cost-effective application, wavelength-division multiplexing started playing a major role in telecommunications networks in the early 1990s, This resulted from the surge in demand for high-capacity links and the limitation of the installed fiber plant in handling high-rate optical signals over any substantial distance.
This limitation led to a rapid capacity exhaustion of long-haul fiber networks.
While installing an optical fiber cable plant is both expensive and extremely time consuming, expanding the capacity of an installed network is economically attractive. Tradition carries upgraded their link capacity by increasing the transmission rate. This worked well initially, with speeds eventually reaching 2.5 Gb/s. However, when going to the next multiplexing level of 10Gb/s, people starts to encounter the effects that can seriously degrade WDM network performance such as the dispersion, reflections, scattering, etc.
New fiber designs, special dispersion-compensation techniques, and optical isolators can mitigate these limitations, and newly installed links are operating very well as 10Gb/s per wavelength.
However, a large portion of the older installed fiber base is limited to OC-48 rates (2.5Gb/s) at a given wavelength. Thus, a great interest has been established in using WDM, not only for older links but also to have a very high capacity new links.
For a typical WDM link. At the transmitting end, there are several independently modulated light sources, each emitting signals at a unique wavelength. Here a multiplexer is needed to combine these optical outputs into a continuous spectrum of signals and couple them onto a single fiber. At the receiving end, a demultiplexer is required to separate the optical signals into appropriate detection channels for signal processing. At the transmitter, the basic design challenge is to have the multiplexer provide a low-loss path from each optical source to the multiplexer output. Since the optical signals that are combined generally do not emit any significant amount of optical power outside of the designated channel spectral width, interchannel cross-talk factors are relatively unimportant at the transmitting end.
Wavelength multiplexers are specialized devices that combine a number of optical streams at distinct wavelengths and launch all their powers in parallel into a single fiber channel. This
combination need not be uniform for all wavelengths; that is. One may want to combine 50% of the power from on wavelength, 75% from another source, and 100% from other wavelengths. However, for WDM applications it is usually desirable that the multiplexers combine the optical powers from multiple wavelengths onto a single fiber with little loss. Wavelength demultiplexers divide a composite multichannel optical signal into different output fibers according to wavelength without splitting loss. This section describes a phased-array-based WDM multiplexer and a fiber-grating multiplexer as examples of such components.