Fibre optic communication systems have traditionally carried data using binary (on-off) encoding of the light amplitude. However, next generation systems will exploit both the amplitude and phase of the optical carrier to achieve higher spectral efficiencies and thus higher overall data capacities 1,2. Although this approach requires highly complex transmitters and receivers, the increased capacity and many further practical benefits that accrue from a full knowledge of the amplitude and phase of the optical-field 3 , more than outweigh this additional hardware complexity and can greatly simplify optical network design. However, use of the complex optical-field gives rise to a new dominant limitation to system performance, namely nonlinear phase noise 4,5. A device for removal of this noise therefore becomes of great technical importance. Here we report the development of the first practical ('black-box') all-optical regenerator capable of removing both phase and amplitude noise from binary phase-encoded optical communication signals.

We discuss recent advances in phase-sensitive amplification technology and review its application to the regeneration of multi-level phase-encoded signals.

In this paper, we investigate the properties of transmission links amplified by phase-sensitive amplifiers (PSAs). Using an analytic description, we explain the principles enabling improved sensitivity compared to conventional links amplified by phase-insensitive amplifiers (PIAs) and mitigation of nonlinear transmission distortions. We demonstrate these features using numerical simulations, and in particular, we show the possibility of efficiently mitigating both self-phase modulation (SPM)-induced distortions and nonlinear phase noise (NLPN) if the link dispersion map is optimized. The properties of the noise on signal and idler are important and to enable NLPN mitigation, the noise must be correlated at the link input. We investigate the role of the dispersion map in detail and show that in a link with standard single mode fiber (SSMF) the optimum dispersion map for efficient nonlinearity mitigation corresponds to precompensation of an amount equal to the effective loss length. Furthermore, we experimentally demonstrate both improved sensitivity and mitigation of nonlinearities in a 105 km PSA-amplified link transmitting 10 GBd 16-ary quadrature amplitude modulation (16QAM) data. We measure a combined effect allowing for more than 12 dB larger span loss in a PSA-amplified link compared to a conventional PIA-amplified link to reach the same bit error ratio (BER) of 1 × 10 −3 .

We investigate a non-degenerate phase-sensitive amplifier (PSA) operating in gain saturation experimentally and numerically using the three-wave model. The phase-dependent gain and phase-to-phase transfer functions are obtained for different levels of saturation with good agreement between experimental and numerical data when higher-order FWM is small. Moreover, we identify an operating point where the PSA is found to be able to significantly reduce both phase- and amplitude noise simultaneously.