Phase-Conjugated Twin Waves for Communication beyond the Kerr Nonlinearity Limit

Optical fiber communication has enabled the exponential growth in communication capacity of the information era. The theoretical upper bound of the capacity per unit bandwidth of a linear communication link is set by the Shannon limit1. However, as optical fiber transmission capacity continues to increase so is the needed optical signal power to ensure sufficient signal-to-noise ratio, which results in signal distortions due to fiber Kerr nonlinearity, leading to the so-called nonlinear Shannon limit. Here we show that the nonlinear distortions imposed on a pair of co-propagating phase-conjugated twin waves are anti-correlated, even in the presence of large chromatic dispersion during transmission, so complete cancellation of signal-to-signal nonlinear interaction can essentially be achieved by coherently superimposing the twin waves at the receiver. Modulating a pair of phase-conjugated twin signals on two orthogonal polarizations of a same optical carrier, we experimentally demonstrate the nonlinearity cancellation in both non-dispersive and dispersive transmissions, reducing nonlinear distortions by >8.5 dB. In the case of dispersive transmission, full nonlinearity cancellation additionally requires a symmetry condition, which can be satisfied by appropriately pre-dispersing the twin signals. Furthermore, we show that the phase-conjugated twin signals provides a much improved immunity to inter-channel nonlinear effects resulting from other channels in wavelength-division-multiplexed transmission, unattainable by conventional nonlinear compensation schemes. Compared to the well-known mid-link phase conjugation, this novel twin-wave based approach removes the need for additional components inside the transmission link. The concept of using phase-conjugated twin waves to cancel out signal-to-signal nonlinear interactions, especially those from other wavelength channels, may offer hope for high-performance communication beyond the limit imposed by the Kerr nonlinearity, and open new possibilities in other physical systems that are governed by the nonlinear Schrödinger equation in general.