8Q A M regeneration using a phase-sensitive amplifier with dual-conjugated pumps

Abstract

Summary form only given. For the growing demand on higher capacity in fiber-optic communication systems, advanced modulation formats, i.e. quadrature amplitude modulation (QAM), provide a promising solution for significant capacity gains. However, these multi-level amplitude- and phase-encoded signals are sensitive to amplitude and phase noise, induced by i.e. amplified spontaneous emission and nonlinear phase noise. Hence, also regeneration of signal phase is crucial for long-haul transmission systems. The most promising processing approaches are four-wave mixing (FWM) with higher order idler waves [1] and using phase-conjugated pumps [2].In this work, we investigate numerically the performance of a phase-sensitive amplifier (PSA) for a star8QAM, 40 Gbaud signal with two amplitude and four phase states. A FWM scheme with two phase-conjugated pumps, which are equally spaced around the 8QAM signal, is used [2]. The operation principle of the PSA is illustrated in Fig. 1(a). The two phase-conjugated pumps are produced in another highly nonlinear fiber (HNLF) by an FWM process with two CW pumps, which are equally spaced around the signal by 250 GHz. The idlers are additionally amplified before entering the second stage. In the second HNLF, the amplified idlers from the first stage serve as phase-conjugated pumps for the PSA and allow a regeneration of the 8QAM signal through a degenerate four-wave mixing process with the phase-matching condition ΨS + ΨI - ΨP1 - ΨP2 = 2ΨS-(ΨS) -(ΨS) = 4ΨS for four phase states. Simulations have shown that the regeneration process is most efficient if the following condition for the signal amplitude at the PSA output is fulfilled [1,3]: Aout exp(iψout) = exp(iψ ) + exp(-imΨi) , with m=3 for four phase states Typical results of the 8QAM-signal processing are presented in Fig. 1(b,c). The simulations are made with help of VPItransmissionMaker. Additional phase noise was imposed on the signal by a phase modulator driven by a Gaussian noise signal. The PSA parameters have been optimized for the higher amplitude level in that case, leading to less efficient phase noise suppression in the low-power states. The reason is the deviation of the PSA gain from the optimum for different power levels. The four phase states of the higher amplitude level are rotated with respect to the ones of the lower amplitude level which is due to cross-phase modulation in the second PSA stage. Possibility of PSA performance improvement using either operation of the first stage of conjugate generation in saturation and/or additional active modulation of conjugate amplitude and phase before the PSA stage will be also considered.