Shaping Lightwaves in Time and Frequency for Optical Fiber Communication

In optical communications, sphere shaping is used to limit the energy of lightwaves to within a certain value over a period of time. This fundamentally minimizes the energy to contain information, allowing the rate of information transmission to approach the theoretical limit if the lightwaves are transmitted through a linear medium. However, when the lightwaves are transmitted through optical fiber, the sphere shaping affects the Kerr nonlinearity in a peculiar way that makes analysis of transmission performance difficult. In this work, we show that the impact of sphere shaping on the Kerr nonlinearity varies with the chromatic dispersion of fiber and the shaping block size in time and frequency, and give insights into why the block size matters. Then, by optimizing the block size, we experimentally demonstrate that the information rate can be increased by up to 25% in low-dispersion channels on a 2824-km dispersion-managed wavelength-division multiplexed optical fiber link.