Amplitude noise reduction in multilevel QAM formats using a nonlinear amplifying loop mirror with a saturable amplifier

Quadrature amplitude modulation (QAM) allows increase of transmission capacity while keeping the symbol rate, But QAM signals show a high sensitivity to amplitude and phase noise. In particular, amplitude noise does not only enlarge the overlap between the signal states, but due to the Gordon-Mollenauer effect in the transmission line, it is also converted into nonlinear phase noise which is the major limiting factor for phase-shift-keying (PSK) transmission. Phase-preserving amplitude regeneration was considered to reduce amplitude fluctuations and, therefore, the origin of nonlinear phase noise for binary and quadrature PSK. It has already been demonstrated that the periodic behaviour of the power transfer characteristic of a nonlinear amplifying loop mirror (NALM) can create multiple flat, so-called plateau regions, which can be used also for QAM signal regeneration. Instead of using two different amplifiers, linear and nonlinear one, as it was proposed in, the authors have considered a simpler scheme with only one saturable amplifier.