Optical coherent transmission with spectral efficient modulation and detection based on the non-linear Fourier transform

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This project aims at improving optical communications using solitons based on the non-linear Fourier transform (NFT). It is a fundamentally new approach that implicitly takes into account the Kerr-nonlinearity when modulating signals onto the optical fiber. In contrast, conventional methods consider nonlinearities as a source of interference that needs to be mitigated. Thus, the performance of conventional linear communication schemes with respect to reach and data rates is limited by the the fiber nonlinearities; increasing launch powers results in reduced spectral efficiency although the signal-to-noise ratio may still improve. It is, thus, promising and intuitive to account for the Kerr-nonlinearity already in the system design to achieve an overall improved communication quality, even when further increasing the launch power.This project focuses on the achievable rate (mutual information) of NFT-based communication links including modulation, channel and detection; the theoretical capacity of the nonlinear optical fiber channel is not further considered. The improved transmission quality shall be achieved by spectral efficient modulation of the discrete nonlinear spectrum consisting of eigenvalues and respective spectral amplitudes. The continuous spectrum is not further considered and is set to zero, so that "pure" soliton impulses remain to be studied. Although solitons have been investigated extensively in the past, their application to data communication with high spectral efficiency is still in its beginnings; further research on their fundamental properties and potential advantages over classic "Nyquist"-based signaling is required.To achieve spectral efficient modulation, different eigenvalue constellations as well as constellations of the spectral amplitude are to be considered; in particular, the influence of the various degrees of freedom on the time-bandwidth-product along the fiber needs to be studied. Such degrees of freedom are the real and imaginary part of the eigenvalues of a constellation, the multiplicity and arrangement of the eigenvalues per constellation, and the number of concurrently transmitted eigenvalues. Moreover, the modulation of signal points of the respective spectral amplitude (constellation size and arrangement of points) needs to be investigated. Also, the influence of noise, nonlinear soliton interaction as well as other real-world imperfections on the optimized constellations and pulse sequences needs to be studied. Finally, the properties of NFT-based transmission optimized for spectral efficiency shall be considered in the context of an optical communication system using wavelength division multiplex.