Mutual information expression for the nonlinear Schrödinger channel in the low-noise limit using scattering data

Transmission through optical fibers offers ultrafast and long-haul communications. However, the search for its ultimate capacity limits in the presence of distributed amplifier noise is complicated by the competition between wave dispersion and nonlinearity. In this paper, we exploit the integrability of the nonlinear Schrödinger equation, which accurately models optical fiber communications, to derive an expression for the mutual information of an optical fiber communications channel, expressed fully in the scattering data domain of the nonlinear Fourier transform and valid in the limit of low amplifier noise, which when optimized with respect to the input provides the corresponding capacity. We utilize the relationship between the derived noise-covariance operator and the Jacobian of the mapping between the signal and the scattering data to obtain the properties of the former. Emerging from the structure of the covariance operator is the significance of the Gordon-Haus effect in moderating and finally reversing the increase of the mutual information with power. This effect is showcased in numerical simulations for Gaussian input in the high-bandwidth regime.