Multidimensional Modulation Formats for Coherent Single- and Multi-Core Fiber-Optical Communication Systems

Doctoral thesis, 2015

This thesis covers multidimensional modulation formats for coherent optical communication systems including spatial division multiplexed systems using multicore fibers. The single-mode optical signal has four dimensions which are spanned by the two orthogonal polarizations and the in-phase and quadrature components. By optimizing modulation formats in the four-dimensional (4D) signal space, formats with increased asymptotic power efficiency and/or spectral efficiency can be found. In this thesis, a range of 4D modulation formats are studied and several experimental realizations of different 4D formats are presented. This thesis also includes modulation formats with higher dimensionality. Two different experimental realizations of biorthogonal modulation in eight dimensions are presented where either two consecutive timeslots or two wavelength channels are used to span the eight dimensions. In the experiments, the multidimensional formats are compared to conventional two-dimensional formats in terms of achievable transmission reach. Multicore fibers systems are also investigated in this thesis and the impact of inter-core crosstalk on quadrature phase shift keying signals is studied. Further, multidimensional modulation formats over spatial superchannels consisting of signals in several cores are explored. Experimental demonstrations of low-complexity formats capable of increased transmission reach at a small reduction in spectral efficiency are presented. This thesis also studies the impact on the achievable information rate using mutual information for different assumptions of the channel distribution in fiber-optical transmission experiments. It is shown that decoders operating in four-dimensions can achieve significant higher achievable information rates for highly nonlinear fiber channels.