Abstract
An alternative application of distributed Raman amplification (DRA) for ultralong-haul optical fiber
transmission is proposed. In our study, the DRA is employed in a transmission system using midway optical phase
conjugation (OPC) for amplifying an optical signal and, at the same time, for constructing signal power evolution,
which is symmetrical with respect to the midpoint of the system where the OPC is performed. Then, the nonlinear
signal waveform distortions that are caused by the Kerr effect, as well as fiber dispersion, are almost completely
compensated by the OPC, whereas the fiber loss is compensated by the DRA. Three possible symmetrical signal power
maps—a power map that has a reverse sign of the power map that is caused by lump amplification, a flat signal
power map, and an arbitrary symmetrical signal power map—are numerically designed by using appropriate Raman
pump powers. We show that the flat power map exhibits smaller difference from the target and a higher optical
signal-to-noise ratio and requires lower pump power than the other two power maps. Numerical simulation results
demonstrate that, by employing the flat power maps with a span of 40 km, a single-wavelength signal whose data rate
is 160 Gb/s can be successfully transmitted over 5000 km, and the Kerr effect is sufficiently suppressed near
limitation due to the nonlinear accumulation of noise. Finally, we study the feasibility of expanding our method to
wavelength-division-multiplexed signal transmission by designing a DRA gain with multiple-wavelength pumping to
simultaneously obtain a flat power map and a wide-and-flat gain bandwidth. By using four-wavelength Raman pumps
while carefully choosing pump wavelengths and their powers, we achieve the DRA gain that simultaneously gives a
fluctuation of the signal power of only 3.5%, a gain ripple of only 5.3%, and, at the same time, a gain bandwidth of
as wide as 46 nm.
© 2007 IEEE
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