Worldwide, higher-order modulation formats are intensively investigated to further increase the spectral efficiency for
building the next generation of high-speed metro systems. IQ-modulators, coherent receivers and electronic equalizers
are hereby discussed as key devices. We report on system design issues as well as on HHI's latest achievements in
developing InP based high-speed modulators and coherent receiver frontends.
The concept of a nonlinear transfer function of a fibre-optic communication link is reviewed. Also an approximation of the nonlinear transfer function is introduced, which allows to define an equivalent single-span model of a dispersion-managed multi-span system. In this paper we will show its limits of validity and try to extent these limits by enhancing the theoretical model. In this respect we will discuss the impact of dispersion precompensation and show the influence of residual dispersion per span, number of spans and local dispersion on transmission systems with on-off keying and differential phase-shift keying modulation formats. This approach allows fast assessment of the performance of a given modulation format over various dispersion maps by reducing the need for extensive numerical simulations.
In this paper, the concept of the nonlinear diffusion bandwidth of a fiber is introduced. This simple criterion enables the characterization of nonlinear impairments in single-span transmission systems. Furthermore it is shown how to extend this criterion to multi-span dispersion-managed transmission systems. This enables to easily predict the performance of a given modulation format over various transmission lines with different fiber chromatic dispersion and dispersion maps.