Strong four-photon conversion regime of cross-phase-modulation-induced modulational instability

Abstract

We investigate theoretically and experimentally the strong conversion regime of parametric four-photon amplification or induced modulational instability in the normal dispersion regime of propagation in a highly birefringent fiber. Such optical mixing is observed by injecting a tunable linearly polarized (along the fast axis) anti-Stokes signal wave copropagating with a pump equisplitted between the fiber axes (i.e., linearly polarized at 45\ifmmode^\circ\else\textdegree\fi{}) which entail that the two pump modes experience cross-phase modulation. In agreement with a four-wave model developed to study the depleted regime of the mixing process, we observe that the strongest conversion occurs outside the parametric gain bandwidth, or in other words, under conditions of modulational stability of the pump beam. This proves that the optimum signal frequency deviates significantly from the prediction of the linear stability analysis or the usual phase-matching argument.