Three-photon phenomena in the upconversion luminescence of erbium–ytterbium-codoped phosphate glass

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Abstract
Frequency upconversion into green and red luminescence in erbium–ytterbium-codoped phosphate glass pumped with a 970 nm diode laser is presented. The red upconversion emission results from a two-photon excitation process. The green emission results from a two- or three-photon excitation process with the two-photon process dominating at low pumping power. Based on measurement of the excitation and emission spectra, a model of the three-photon excitation process has been developed. This model provides a good basis for explaining the effect of the three-photon process on the saturation of 1.54 μm laser power of Er, Yb-codoped phosphate glass laser.