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Four-wave mixing experiments in a low-birefringence optical fiber reveal an unexpected sudden increase of the conversion efficiency as the signal power crosses a threshold value. In this regime, peak frequency conversion is achieved outside the small-signal parametric gain spectrum. A nonlinear model of wave mixing fits the measured data well.

We derive an exact solution including pump depletion of collinear four‐wave mixing in a birefringent Kerr medium. We find mirrorless optical multistability for a linearly polarized backscattered signal with orthogonal pumps at cw power levels using a few meters of polarization‐maintaining silica fiber. An expression for the nonlinear phase distortion of ideal phase conjugation is obtained, showing a fundamental limitation to generating time‐reversed wave fronts using Kerr nonlinearity.

The fundamental limitations for all‐optical switching in a nonlinear directional coupler due to the saturation of the nonlinearity have been obtained by solving analytically the nonlinear coupled‐mode equations. We found that there is a minimum value required for the saturation index change to obtain efficient switching, and that the number of powers at which switching occurs at one beat length can double.

We report ultrafast all‐optical switching between two orthogonal linear polarizations in a periodically twisted, birefringent fiber filter.