Necessary conditions for a nonlinear few-mode excitation of modes in single-mode optical fiber

This paper presents the investigation of the necessary conditions for the non-linear excitation of higher order modes in the single mode optical fiber with step index profile. The required peak optical power for the non-linear excitation of higher modes was calculated. The dependency of this optical power from the normalized frequency of a step index optical fiber was studied. Calculations were performed for optical fiber with core of doped silica glass by germanium and cladding of pure silica glass. It was shown that for such a single mode optical fiber under pulse duration of less than 500 fs it is possible to create the necessary conditions for the nonlinear excitation of high order mode before the laser damage.