Line Verification in Luminescence Spectra of Fluorophosphate Glass Doped with Ytterbium and Thulium Ions by the Degree of Nonlinearity of Up-Conversion Processes

Up-conversion luminescence (UCL) spectra of fluorophosphate glasses doped with the rare-earth ions Yb3+ with concentrations of 4-10% and Tm3+ with concentrations of 10-5-3% were investigated using continuous-wave excitation at 975 nm. Each of the observed UCL bands was the result of several processes corresponding to various Tm3+ excited states and characterized by a different degree of nonlinearity regarding the number of absorbed photons required to excite these states. The influence of the pumping power and concentrations of doping ions on the contributions to UCL spectra from excitation processes with various nonlinearities was analyzed. The simultaneous appearance of several lines in each UCL spectral region was shown to be typical of low Tm3+ concentrations (fractions of a percent and less) and rather high Yb3+ concentrations (4-10%). UCL appeared primarily in the 755-840-nm band with a maximum near 793 nm if the Tm3+ concentration was increased up to 3% with an Yb3+ concentration of 5%.