Equation (6) can be generalized to analyze multispan optically
amplified systems, where the total intensity fluctuation
at the receiver is the accumulation of XPM contributions
created by each fiber span. For a system having a total of
N amplified fiber spans, the XPM created in the mth span
produces an intensity modulation AiiT)(O, LN) at the end
of the system. Even though the phase modulation creation
depends only on the pump power and the walk-off within the
mth span, the phase-to-intensity conversion depends on the
accumulated dispersion of the fibers from the mth to the Nth
spans. Therefore, we have