There has been an increased interest in enhancing the security of optical communications systems and networks. All-optical cryptography methods have been considered as an alternative to electronic data encryption. In this paper we propose and verify the use of a novel all-optical scheme based on cryptographic keys applied on the spectral signal for encryption of the M-QAM modulated data with bit rates of up to 200 gigabits per second.

A theoretical and experimental investigation of catastrophic amplification of broadband noise in optical fibers near the zero-dispersion frequency (0) pumped by two lasers with frequencies that are symmetrically tuned relative to 0 is presented. The effect is due to a four-wave mixing (FWM) process, phase matched up to thirdorder dispersion, between the spectral components of noise and the lasers. We observed a FWM gain of 16 dB (10-dB net gain) over a 22-nm bandwidth (limited by fourth-order dispersion) for 18-dBm power lasers at Ϯ1 THz (8 nm) from 0 in a 25-km-long dispersion-shifted fiber. A simple analytical model is proposed that will permit us to investigate this effect numerically in amplified links with concatenated amplifiers and consider random fluctuations of 0 along the fiber.

The impact of multipath interference (MPI) in all-Raman multispan dispersion-compensated links operating at 42.67 Gb/s was studied. D+/D− and D+/D−/D+ compensation schemes were considered using different fiber sets. Optical signalto-noise ratio (OSNR) penalties due to MPI were calculated for several backward-pumped system configurations, imposing system target OSNR levels suitable for intensity modulation with direct detection (IMDD) and differential phase-shift keying (DPSK). The analysis in this paper confirmed that the D+/D−/D+ OSNR penalty is typically much less than that of the D+/D− scheme. The authors then estimated the increase due to MPI in the number of spans required to satisfy a target OSNR for a given total link length, taking into account Kerr nonlinearities. It turned out that such an increase can be very significant (up to 15%-20%) with the D+/D− scheme and lower but nonnegligible (5%-10%) with the D+/D−/D+ schemes. The analysis confirms that, to substantially curtail the span increase, both forward and backward pumping should be adopted, as recent experimental results have shown. Finally, at the lower OSNR levels required by DPSK with respect to IMDD, the impact of MPI was shown to be smaller across all configurations. Index Terms-Differential phase-shift keying (DPSK), double Rayleigh backscatter (DRBS), forward pumping, multipath interference (MPI), Raman amplification, ultra-long-haul systems.

The ever-growing demand for optical network security can be addressed by data encryption at different network layers. In this work, we consider all-optical cryptography technique that applies a spectral phase change and delay encoding on spectrum slices of a specified WDM channel. In this case we have investigated a novel approach where signals are double encrypted to achieve an enhanced degree of security. Simulation results indicate that such double cyphering scheme can be applied to high data-rate M-QAM signals propagating in metro/regional networks.