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Thermal nonlinear refractive index of graphene based saturable absorber in mode-locked fiber lasers

Graphene based optical modulators have been successfully implemented as a saturable absorber (SA) in ultrafast mode-locked fiber lasers (MLFLs). Interaction of evanescent field with graphene clad microfiber (GCM) causes heat generation inside the graphene. In this paper we numerically demonstrated that the temperature rise can severely change the effective nonlinear refractive index of GCM. We showed that temperature rise altered the nonlinear refractive index of microfiber from 3.67 × 10−7 cm2 GW−1 to almost 620 cm2 GW−1, when the waist of microfiber had the diameter of 4 μm. We believe this can affect on pulse duration by inducing the considerable change in self phase modulation (SPM) parameter.