Abstract
We investigate experimentally and theoretically the shape of the optimum spectral
pre-emphasis for ultra-long-haul networks and systems in the presence of optical
nonlinearities. In a 40×10 Gbits/s×2240 km system, we experimentally show the advantages
of a pre-emphasis scheme that ensures minimum variations of path-averaged power among
the wavelength-division-multiplexing channels between the cleanups performed by dynamic
gain-flattening filters. This is achieved by enforcing a straight-line spectrum near the
middle of the link between two consecutive gain-flattening nodes. We explain these
results by a simple model, in which we derive the conditions for optimum pre-emphasis in
the presence of either distortionlike (e.g., self-phase modulation) or noiselike (e.g.,
four-wave mixing) nonlinearities. We show that, for relatively small gain ripple, both
of these optimum pre-emphasis techniques are equivalent to equalizing the optical
signal-to-noise ratio for all channels and producing a flat power spectrum near the
middle of the link. Hence, enforcing the straight-line spectrum in the middle of the
link is the closest practical approximation to the optimum pre-emphasis.
© 2006 Optical Society of America
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