Adiabatic femtosecond soliton active nonlinear directional coupler.

The optimal all-optical switching, amplification, and temporal compression of femtosecond solitons by means of a nonlinear erbium-doped twin-core fiber may be achieved by suitably tapering the linear coupling strength. We analytically show that, irrespective of the soliton self-frequency shift, tapering may in principle reduce the switching power threshold to arbitrarily low values. A substantial reduction is also demonstrated in a practical case in which the full complex susceptibility of the erbium doping and the Raman response function are considered.