Multilevel Phase-Preserving Amplitude Regeneration Using a Single Nonlinear Amplifying Loop Mirror

A possibility of multilevel phase-preserving amplitude regeneration using a nonlinear amplifying loop mirror (NALM) is presented for the optical star-8 quadrature amplitude modulation (QAM) transmission format as an example. Two significantly different state power ratios for the QAM signal, 1:3 and 1:7, were investigated. After the optimization of the coupler splitting ratio and the directional phase bias in the NALM, amplitude noise can be efficiently suppressed at both signal power levels simultaneously. Bit-error-ratio (BER) simulations have shown that in a system limited by nonlinear phase noise, the deployment of the NALM allows an increase of the fiber launch power by 1.9 and 2.2 dB at a BER of 10^-3 for a state power ratio of 1:3 and 1:7, respectively. The regeneration limits due to imperfections of the power transfer characteristic are also discussed.