Since its early commercial deployment in the late 1980s, optical fiber has evolved to become the predominant carrier of the globe’s communications. Yet, after accommodating the world’s exponentially growing appetite for transmitted data for more than three decades, its ability to continue doing so is being challenged by fundamental factors. In this article, we review these factors and examine their consequences in terms of information capacity. In particular, we review the difficulties that are imposed by the nonlinear nature of fiber-optic transmission on the assessment of the capacity and on the definition of fundamental concepts, such as bandwidth and spectral efficiency. We discuss relevant approximations and regimes of operation in which bounds for the capacity can be effectively assessed while covering a broad range of applications ranging from interdatacenter communications to links spanning transoceanic distances. We relate to a broad variety of transmission schemes and discuss the potential benefits of spatial multiplexing with multimode and multicore fibers. State-of-the-art transmission experiments are also reviewed and compared with theoretical capacity bounds.

Fiber's nonlinearity fundamentally bounds the achievable information rates in fiber-optic communication systems. In a wavelength-division multiplexed system it induces a nonlinear interference between adjacent channels, an interference that was recently shown to have a strong dependance on the input distribution. In this work we show that a ball shaped input constellation may significantly reduce the nonlinear effects. We study the shaping gains in the fiber-optic channel and show that in certain scenarios the maximum gains may be higher than the 1.53dB ultimate shaping gain in linear additive white Gaussian noise channels. Furthermore, the maximum gain is achieved with a finite-dimensional ball shaping region.

We give a sufficient condition that insures that the impulse response of a channel is of minimum phase. The condition is that the energy of the first path be larger than the power spectral density of all subsequent paths. Establishing a lower bound to the first path energy, this condition is more likely to be met if transmitter and receiver are in line-of-sight. Our results show that the Hilbert transform may give unreliable results, if used to extract the channel phase response from frequency-resolved amplitude measurements when the first path is strongly attenuated.