Abstract
Having in mind the capacity optimization of power-constrained submarine
links, by following the work in [1] we first compare the achievable
information rate (AIR) of gain-flattened and un-flattened blocks of N b
≤ 12 spans with span loss 16.5dB and with end-span single-stage
co-pumped erbium-doped fiber amplifiers (EDFA) when the transmitted
wavelength division multiplexed (WDM) channels all have the same
transmitted power. All EDFAs have the same pump power and the same
physical parameters. In the flattened case, each EDFA is followed by an
ideal gain-flattening filter (GFF) that chops off the EDFA gain
exceeding the span loss. No GFFs are used in the un-flattended case. We
show that, for block length Nb > 7, at large-enough input
power the AIR of the GFF block exceeds that of the no-GFF block, while
for Nb ≤ 7 at large input power the AIR is about the same. We next build
a long submarine link by concatenating the Nb-span no-GFF blocks, and
placing a GFF at the last EDFA of each block in order to flatten the
block gain down to the Nb-span loss, and calculate the AIR of the
resulting sparse-GFF submarine link, accounting also for nonlinear
interference. For the 287-span case-study link with span loss 9.5dB used
in [5], [9], we show that the best power efficiency is achieved
by blocks of size Nb = 6 (i.e., one GFF every 6 spans) when the pump is
around 12 mW. When the GFF excess loss is 0.3dB the top-AIR gain over
the standard all GFF system is 9.5%, a value that decreases to 4% when
the excess loss is zero. Considering that modern submarine-grade GFFs
have almost zero excess loss, and that the most efficient pump power is
likely too low to operate with, we conclude that sparse-GFF links offer
little advantage in practice over the current design.