Fluorescence mechanisms in ${\mathrm{Tm}}^{3+}$ singly doped and ${\mathrm{Tm}}^{3+}$, ${\mathrm{Ho}}^{3+}$ doubly doped indium-based fluoride glasses

The ${\mathrm{Tm}}^{3+}$→${\mathrm{Tm}}^{3+}$ and ${\mathrm{Tm}}^{3+}$⇄${\mathrm{Ho}}^{3+}$ energy transfers have been studied in both Tm-doped and (Tm,Ho)-doped indium-based fluoride glasses over a wide temperature range for several dopant concentrations. The regime of diffusion among the ${\mathrm{Tm}}^{3+}$ ions is clearly established and the cross-relaxation ${}_{}{}^3{}_{}{}^{}$ ${\mathit{H}}_4$, ${}_{}{}^3{}_{}{}^{}$ ${\mathit{H}}_6$ ${\to }^3$ ${\mathrm{F}}_4$ ${,}^3$ ${\mathit{F}}_4$ has been proved and its efficiency calculated. The temperature dependence of the thermal equilibrium observed between the lowest excited states ${}_{}{}^3{}_{}{}^{}$ ${\mathit{F}}_4$ of ${\mathrm{Tm}}^{3+}$ and ${}_{}{}^5{}_{}{}^{}$ ${\mathit{I}}_7$ of ${\mathrm{Ho}}^{3+}$ ions in codoped glasses is explained in terms of the Boltzmann distribution of the state populations governed by efficient forward and backward energy transfers. When ${\mathrm{Tm}}^{3+}$ ions are excited in the ${}_{}{}^3{}_{}{}^{}$ ${\mathit{H}}_4$ level near 0.8 μm, it is shown that two cross-relaxation processes compete: one between ${\mathrm{Tm}}^{3+}$ ions and the other between ${\mathrm{Tm}}^{3+}$ and ${\mathrm{Ho}}^{3+}$ ions. Their efficiencies are shown to strongly depend on the relative Tm and Ho concentrations.