  An upper bound to the information capacity of a wavelength-division multi-
plexed optical fiber communication system is derived in a model incorporating
the nonlinear propagation effects of cross-phase modulation (XPM). This work is
based on the paper by Mitra et al., finding lower bounds to the channel
capacity, in which physical models for propagation are used to calculate
statistical properties of the conditional probability distribution relating
input and output in a single WDM channel. In this paper we present a tractable
channel model incorporating the effects of cross phase modulation. Using this
model we find an upper bound to the information capacity of the fiber optical
communication channel at high SNR. The results provide physical insight into
the manner in which nonlinearities degrade the information capacity.
