single fiber when this book was written. Howev- er, it was not stated what band would be used or the gain that could be expected through the use of EDFAs. While EDFA was found suitable for amplification of signals up to 10 Gbits/s, as vendors experimented in extending data transmission rates to 40 Gbits/s, it became clear that system noise was a major limitation associated with developing very-high-speed optical networks. At 40 Gbits/s a maxi- mum transmission distance of approximately 40 km was obtainable prior to requiring signal regeneration. Because the existing long-haul infrastruc- ture of most communications carriers includes huts where power supplies and electronics were previously installed to support optical amplification, from an economic perspective it is important to be able to use the existing infrastructure To accomplish this requires a new method of amplification at a rate of 40 Gbits/s. That method of amplification is referred to as Raman amplification. RAMAN AMPLIFICATION A Raman amplifier is based on a phe- nomenon referred to as the Raman effect in optical fibers. The Raman effect occurs when a large continuous-wave laser signal is colaunched at a lower wavelength than the signal to be amplified, resulting in a gain whose value is dependent primarily on the strength of the laser pump and the frequency offset between the pump and the signal. When the pump photon gives up its energy to create a new photon, Chapter Six some residual energy is also created. The residual energy is absorbed in an optical fiber as vibrational energy that results in gain or ampli- fication of the signal to be amplified. The key to Raman amplifica- tion capability is the launching of pump light into a fiber at ampli- fier sites opposite the signal direction. This action results in Raman amplification functioning as a low-noise preamplifier that maintains signal integrity. In addition, because the signal power is weaker at the output end of a fiber, no additional nonlinear fiber phenomena are introduced. Another advanta