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

We investigate theoretically and experimentally the strong conversion regim e of parametric four-photon amplification or induced modulational instabili ty in the normal dispersion regime of propagation in a highly birefringent fiber. Such optical mixing is observed by injecting a tunable linearly pola rized (along the fast axis) anti-Stokes signal wave copropagating with a pu mp equisplitted between the fiber axes (i.e., linearly polarized at 45 degr ees) which entail that the two pump modes experience cross-phase modulation . In agreement with a four-wave model developed to study the depleted regim e of the mixing process, we observe that the strongest conversion occurs ou tside the parametric gain bandwidth, or in other words, under conditions of modulational stability of the pump beam. This proves that the optimum sign al frequency deviates significantly from the prediction of the Linear stabi lity analysis or the usual phase-matching argument.