any unwanted reflected light. The wavelengths used in diplexed operation are commonly on the CWDM grid. For example, the access network categories known as B-PON, G-PON, and E-PON ("Broad- band," "Gigabit," and "Ethernet" PONs, as the case may be) use wavelength combinations of ~1490 nm and ~1550 nm downstream and ~1310 nm upstream (Keiser, 2006; ITU-T G.983.1-G.983.5; ITU-T G.984.1-G.984.6; ITU-T G.985). Dense Wavelength Division Multiplexing Dense Wavelength Division Multiplexing (DWDM) is used in high-capacity and long-span optical transmission systems. The channel plan, which is illustrated in Figure 6.4, is specified in terms of (absolute) frequencies rather than wavelengths. ITU-T standard G.694.1 (ITU-T, 2006) specifies a central reference frequency of 193.1 THz and an equally spaced grid to higher and lower values. The standards documentation advises the use of an accurate value for the speed of light in a vacuum when converting between frequency and wavelengths: C = 2.99792458 X 108 m/s. With this value, the reference frequency is the equivalent of ~1552.52 nm, which is in the lowest loss region of silica fiber. The grid lines shown at the top of Figure 6.4 are at 100-GHz intervals, but they do not all have to be occupied. For example, we could use every second line or some irregularly spaced combination. An operator might want to use only 4-8 lines approximately equally spaced across the gain profile of an Erbium-Doped Fiber Amplifier (EDFA) in intermediate- Chapter 6 Optical Fiber Transmission 193.1 THz 100 GHz (corresponds to (corresponds to about 1552.52 nm) about 0.8 nm) 192.6 192.7 193.8 192.9 193.0 193.2 193.3 193.4 193.5 193.6 Frequency (THz) Grid with Greater Channel Density 50-GHz spacing Continued Reduction in Spacing: 25 GHz and 12.5 GHz Fig. 6.4 DWDM: the ITU-T frequency grids. capacity, but acceptable-cost, metropolitan area networks. The DWDM grid also permits very high channel densities. For example, by using 50-GHz spacing, it is possible to fill the C-band gain profile of an EDFA with a