nge, yellow, green, blue, indigo, and violet. We also noted earlier in this book that while colors are transmitted through the air together, they can be easily separat- ed by the use of a prism. Perhaps remembering Roy G. Biv and a high school physics class using a prism to separate light, early WDM systems injected light at 850 and 1300 nm into a multimode fiber using a simple fused coupler. At the distant end of the fiber, another coupler was employed to split the received light onto two short fibers. One fiber was connected to a silicon detector that was very sensitive to an 850-nm wave- length, while the second fiber employed a germanium or an InGaAs detector that was more sensitive to a 1300-nm wavelength. Through the use of optical filters unwanted wavelengths were removed, enabling each detec- tor to receive the applicable wavelength that it was designed to receive. Developments in optical technology, including the manufacture of economical single-mode fiber, resulted in most communication carriers installing this type of optical cable for use in their backbone infrastruc- ture. At first, the primary method employed to multiplex multiple light signals onto a fiber was based on the use of couplers, allowing utiliza- tion of 1300- and 1550-nm optical windows where attenuation in terms of dB / km is minimized. Although light sources in the 1300- and 1550- nm optical windows were adequate to enable WDM, one problem that arose was that this technique was incompatible with the emerging design of fiber. Specifically, because of different dispersion characteris- tics, glass fiber was designed and manufactured differently for use in 1300- and 1550-nm systems. In fact, fiber that was optimized for opera- tion in the 1300-nm optical window was used primarily for local metro- politan connections while the backbone infrastructure of communica- tions carriers, including long-haul and submarine cables, were based on the use of dispersion-shifted optical fiber that was optimized for perfor- mance in the 1550-nm optical wi