Mitigation of nonlinear fiber distortion using phase conjugation

Licentiatavhandling, 2016

This thesis is devoted to the study of fiber-optic communication systems that mitigate nonlinear distortion due to the Kerr effect by transmitting a phase-conjugated copy alongside the signal followed by coherent superposition. This kind of communication systems are referred to as phase-conjugated twin waves systems when the phase-conjugated copy is transmitted on the orthogonal polarization. The nonlinear distortion due to the Kerr effect is one of the main limiting factors in modern coherent fiber-optic communication systems and it is of high interest to design communication systems that have a higher tolerance for nonlinear distortion at high optical launch powers. With a perturbation analysis and numerical modelling, we investigate a time domain implementation, transmitting the signal and the phase-conjugated copy in different time slots. We show that even though this scheme is predicted by the perturbation analysis to perform worse compared to regular phase-conjugated twin waves, the numerical results show that it performs comparably. Also it is argued that the time domain scheme could be easier to implement in practice. We then make an experimental comparison between a link performing coherent superposition in digital signal processing and a link performing coherent superposition all-optically in a phase-sensitive amplifier. The results from that comparison show that the performance in the nonlinear regime is comparable with both approaches. Last, we investigate numerically the impact of the span power map in single- and multi-span links that perform the coherent superposition inline with the use of phase-sensitive amplifiers. It is shown that by employing distributed Raman amplification in a multispan phase-sensitive amplifier link, it is possible to achieve significant increases in tolerance against nonlinear distortion leading to increased transmission reach, in some cases by as much as a factor of 8.