ersion optical fiber that has the capability to operate at a higher data rate. If not, the fiber cannot support a higher transmission rate. If the exist- ing optical fiber is capable of supporting a higher data rate, the fiber can be considered as a potential candidate for upgrading. The term "potential" is used because, even though a fiber can support a higher data rate, it may not be economically viable to replace existing equip- ment to include the laser and amplifiers in each hut, along with the route of the fiber and the photodetector at its termination location. Thus, a communications carrier will more than likely sharpen its pencil and consider other possibilities. 3. Another possibility is to install a completely new system to include the latest type of fiber that provides the potential to upgrade to OC- 768's 40-Gbit/s transmission rate. However, as you might expect, this could be a very expensive solution to a capacity constraint. 4. Thus, another option, which has become the favorite of many com- munications carriers, is to transmit more lambdas (A) or lightwaves per fiber through the use of WDM, DWDM, and, as the number of wavelengths exceeds 80, ultradense WDM, referred to as UDWDM. Wavelength division multiplexing dates to the 1980s, when AT&T introduced its FT3C system. A wavelength division multiplexer operates by launching two or more optical signals at different wavelengths. The Chapter Six different wavelengths are coupled onto a common fiber, typically in the 1300- or 1550-nm region. When two wavelengths are coupled, this results in a doubling of the transmission capacity of a given optical fiber. Dur- ing the early to mid-1990s several WDM systems were introduced that doubled to quadrupled the capacity of existing optical fibers. OPERATION We can obtain an understanding of WDM operation by remembering our good friend who was introduced earlier in this book, Roy G. Biv. As we remember, Roy G. Biv represented a popular acronym for the different colors of light visible to the human eye: red, ora