Raman amplifiers in WDM systems

Dense WDM has enabled an explosive increase in the capacity of optical transmission systems with announced commercial systems now reaching capacities beyond 1 Tb/s. At present, long distance systems utilize the C- and L-bands of erbium doped fiber amplifiers (EDFA) and carry up to 80 wavelength channels per direction. To obtain even higher capacity, it will be necessary to 1) increase the spectral efficiency of these C- and L-band systems or 2) utilize wavelength bands outside the erbium window. As we discuss in this paper, amplifiers based on Raman scattering enable the capacity of future high-capacity systems to be increased using both these approaches. Raman amplification, discrete Raman amplifiers and hybrid Raman/EDFA are considered