Large signal-to-noise-ratio enhancement of ultrashort pulsed optical signals using a power-symmetric Nonlinear Optical Loop Mirror with output polarisation selection

A numerical study of a fibre-based scheme for the regeneration of ultrashort-pulsed optical signals is presented. The setup is made of a power-symmetric Nonlinear Optical Loop Mirror (NOLM) followed by a polariser. The NOLM operates through nonlinear polarisation rotation, and includes twisted, anomalous-dispersion fibre and a quarter-wave retarder. When the orientations of the linear input polarisation and of the output polariser are properly adjusted, the output energy characteristic flattens at high power, a property that can be used to eliminate large amplitude fluctuations on the logical ones of an optical signal. When the input pulse parameters closely match those of fundamental solitons or of stable elliptically polarised solitary waves, a wide and flat plateau is obtained, allowing the reduction of ∼30% amplitude fluctuations to less than 1%. Very large amplitude fluctuations beyond 50% can also be reduced down to a few %. Although the output pulses are slightly chirped, they are free of pedestal, thanks to the zero low-power transmission of the NOLM, which also allows the simultaneous regeneration of logical zeros. We believe that this setup will be useful for the regeneration of highly degraded signals in future ultrafast transmission networks.