Negative nonlinear absorption in Er³⁺-doped fluoroindate glass

We report the observation of negative nonlinear absorption in fluoroindate glasses doped with erbium ions. The pumping wavelength is 800 nm which is doubly resonant with Er³⁺ ions transitions. A large nonlinear intensity dependence of the optical transmittance and strong upconverted fluorescence are obtained. The dependence of the upconverted fluorescence intensity with the laser power is described by a system of coupled-rate equations for the energy levels' populations.