method of optical amplification was determined to have the potential to simultaneously boost the power of many closely spaced optical chan- nels. Thus, EDFA technology opened the door to the ability to design optical systems that could contain an increasing number of channels at different wavelengths, with the power of all the signals boosted by one optical amplifier. One practical result of EDFA was the ability to develop economical WDM systems, which, as a result of erbium doping, had an extended range prior to requiring conventional amplification. After the year 2000, WDM quickly became dense -WDM (DWDM). In fact, the ITU (Inter- national Telecommunication Union)-standardized DWDM by defining a grid of wavelengths that we discuss later in this chapter. Because of the demand for an increased level of transmission capacity, vendors introduced proprietary systems that use as few as 0.8 nm sections of fre- quency, enabling 80 channels to be created in the frequency spectrum Fiber in the WAN associated with the main erbium-doped fiber amplifier (EDFA) band. Shortly thereafter a second EDFA band was used, making it possible to derive an additional 80 channels in the 1570- to 1610-nm band, thus dou- bling the capacity to 160 channels. This band is referred to as the L band, and the use of erbium-doped amplifiers in this band is referred to as L-band EDFA. Although the idea behind L-band EDFA dates to the early 1990s, the fraction of erbium ions that can be excited in the C band, referred to as ion inver- sion, is between 70 and 80 percent, while its use in the L band provides an inversion of approximately 40 percent. This results in L-band amplification approximately 20 percent that of a C-band EDFA, requir- ing more amplifiers to be inserted at an additional cost when the L band is used. Not quite content with 160 channels, Lucent Technologies as well as other vendors, according to trade press reports, were on the verge of introducing systems that could support over 1000 distinct wavelengths for transmission over a 