Transmission of multi-dimensional signals for next generation optical communication systems

Over the past several decades, the data-carrying capacity of a single optical fiber have been increased significantly by fully exploring and optimizing physical dimensions of the light. Generally there are five major candidates of dimensions, including time, wavelength, polarization, space and quadrature (phase and amplitude). Multi-dimension, utilizing more than two dimensions of the light simultaneously, is one of the essential characteristics of next generation optical communication systems. We review recent advances in transmission of multi-dimensional signals, and highlight innovative ways of exploring the polarization dimension to further increase the capacity or spectral efficiency of a single optical fiber, a so-called pseudo-polarization-division-multiplexing (PPDM) technique. Related demonstrations include non-orthogonal PDM, PPDM of three and four states (PPDM-3 and PPDM-4). Brief discussions on trends of multi-dimensional signal transmission technologies are also presented.