An apparatus and method are described for combining optical amplification and
dispersion compensation in a Raman amplifier. A Dispersion-Managing Raman Amplifier
(DMRA) combines Raman amplification with dispersion compensation by selecting the
length and dispersion of the gain fiber to balance the dispersion of the link.
This gain fiber is also single-mode at the signal and pump wavelengths. The pumping
level is adjusted to balance the losses from the gain fiber and transmission link,
while the pumping configuration is selected to remain within the 3 dB loss length
for the pumping light. When the amplifier is split into two segments, the two segments
may be joined by an isolator, a gain equalization element, and/or an optical add/drop
multiplexer. For WDM transmission systems based on dispersion-shifted fiber (DSF),
operation in the "violet band" between 1430-1530 nm is based on Raman amplification.
By using a DMRA, a dispersion and nonlinearity managed system can be implemented.
In particular, 4WM does not phase match in such a system, and modulation instability
is absent in the transmission link. Furthermore, gain equalization can be added
to the DMRA by cascading one or two Mach-Zehnder frequency filters. The invention
also includes a method for symmetrically adding channels below and above the C-band,
the gain tilt within the C-band can be minimized. Therefore, a roughly equal number
of channels should be placed in the short-wavelength S-band and the long-wavelength
L-band to minimize the Raman energy exchange in the C-band. Also, whereas C- and
L-bands can be amplified using erbium-doped fiber amplifiers, the S-band can use
either discrete or distributed Raman amplifiers. To minimize the interaction between
pumps for different bands, alternate band pumps can be spatially dispersed and/or
cross-polarized. The distributed Raman amplification can be achieved by pumping
the transmission line with discrete laser diodes or by a Raman oscillator.