area networks, especially in "metro-access" rings. In most cases, CWDM operation is unamplified. However, the channels at 1531 and 1551 nm lie within the gain bandwidth of the C-band Erbium- Doped Fiber Amplifier (EDFA), should it be needed. Semiconductor optical amplifiers can also easily provide gain for channels at 1311 nm and other wavelengths, as described by Thiele and Nebeling (2007). Laser wavelength 20-nm spaced Can use low-cost can drift channels: nonlinear or use optical filters that can low-cost, low- crosstalk (especially be broadband specification lasers four-wave mixing) and not well aligned less likely Wavelength (nm) Fig. 6.2 Channel plan for CWDM. Wavelength Division Multiplexing ~1310 nm ~1550 nm downstream Fiber upstream Unwanted reflection due to Simple, low-cost filter low-quality component, can separate the reflection splice, connector, etc. from the downstream signal Fig. 6.3 Bidirectional operation with two wavelengths. The wavelengths used are usually from the CWDM grid. In wavelength diplexing the objective is to propagate light in two directions in the same optical fiber, as shown in Figure 6.3. The main applications are in "Passive Optical Networks" (PONs) to satisfy the needs of access networking, but the strategy might also be of interest to optical fiber-based LANs or other forms of fiber-computer interconnects (see Volume 3). Access technologies are particularly cost sensitive, and SO the aim is to use only one fiber to guide the light in two directions. Unfortunately, bidirectional operation is poten- tially problematic if the transmitters at the opposite ends of the fiber use the same wave- length. Any phenomenon, such as unintended point reflections or Rayleigh backscattering, that provokes the guided waves to counter-propagate causes crosstalk at the receivers. However, the difficulty is easily overcome by sending the counter-propagating signals at different wavelengths because low-cost optical filters in front of the two receivers can easily separate the intended signal from 