  The asymptotic probability density function of nonlinear phase noise, often
called the Gordon-Mollenauer effect, is derived analytically when the number of
fiber spans is very large. The nonlinear phase noise is the summation of
infinitely many independently distributed noncentral chi-square random
variables with two degrees of freedom. The mean and standard deviation of those
random variables are both proportional to the square of the reciprocal of all
odd natural numbers. The nonlinear phase noise can also be accurately modeled
as the summation of a noncentral chi-square random variable with two degrees of
freedom and a Gaussian random variable.
