he exit route. An isolator at each end protects the system from unwanted reflections. Just as EDFAs were critical to the development of WDM, future advances in WDM capacity also require new developments in amplifiers. One approach to increasing the capacity of WDM systems is to expand the usual frequencies to make room for more channels. However, EDFAs present a problem: They boost light only in the C-band (1,530 to 1,565 nm) and L-band (1,565 to 1,625 nm), and a different EDFA is required in each band. To use both bands, a 5-nm separation is required between bands, and the unused bandwidth represents about 120Gbps of lost capacity. One solution to the problem posed by EDFAs is to use Raman ampli- fiers. A Raman amplifier uses a powerful laser source to boost the signal power in standard optical fiber. A single Raman amplifier can boost both C- and L-bands, eliminating the need for the 5-nm separation, thereby increasing capacity. Another development in amplifier technology is the emergence of the SOA, which is built on a single chip. SOAs can be integrated into multifunction optical chips and can be less expensive than EDFAs. The disadvantage of SOAs is a higher signal-to-noise ratio. However, SOAs are attractive for optical signal processing (e.g., all-optical switching, wavelength conversion). Much research is therefore being done on SOAs as optical computing components. WDM Equipment WDM works by spatially separating, or multiplexing, different wavelengths of light down a single optical fiber. Current fiber-optic systems use only a fraction of the available bandwidth. They carry just one wavelength, when, in fact, thousands of wavelengths can be derived. The data rate supported by each wavelength depends on the type of light source. Today, each wavelength can carry from 2.5Gbps (OC-48) to roughly 10Gbps (OC-192). Recently we have seen some deployment of 40Gbps (OC-768) systems, but we are in the early stages of deploying that level. Nonethe- End-to-End Optical Networking less, in the very near future, we're e