Spectroscopy and modeling of solid state lanthanide lasers: Application to trivalent Tm3+ and Ho3+ in YLiF4 and LuLiF4

Abstract

Lanthanide series ions are considered in the context of acquiring spectroscopic parameters and their application to modelling of quasifour-level lasers. Tm:Ho codoped crystals of ${\mathrm{YLiF}}_4$ (YLF) and the isomorphs ${\mathrm{LuLiF}}_4$ (LuLF) and ${\mathrm{GdLiF}}_4$ (GdLF) as 2.0 μm lasers are used for illustration of the experimental and theoretical techniques presented here. While these materials have similar physical properties, they differ in the strength of the crystal field at the site of optically active lanthanide dopant ions such as ${\mathrm{Tm}}^{\text{3+}}$ and ${\mathrm{Ho}}^{\text{3+}}.$ This is due in part to the size of the ${\mathrm{Lu}}^{\text{3+}},$ ${\mathrm{Y}}^{\text{3+}},$ and ${\mathrm{Gd}}^{\text{3+}}$ ions, which comprise part of the host lattice, but ionicity plays a role as well. This selection of lanthanide: host materials provides a useful basis on which to assess laser materials with regards to changes in the strength of the crystal field at the dopant ion site. It is demonstrated that Tm:Ho:LuLF has a larger crystal field splitting than Tm:Ho:YLF and Tm:Ho:GdLF, leading to smaller thermal populations in the Ho lower laser level. To assess this effect quantitatively, the energy levels of the first ten manifolds in Ho:LuLF have been determined. Measurement of ${\mathrm{Ho:XLiF}}_4$ (X=Y,Lu,Gd) emission cross sections at 2.0 μm, ${\mathrm{Tm:XLiF}}_4$ pump absorption cross sections around 0.78 μm, manifold to manifold decay times and energy transfer parameters in ${\mathrm{Tm:Ho:XLiF}}_4$ systems are also determined to provide a consistent set of parameters to use in laser modeling. The techniques presented here are applicable to any lanthanide series ion in a crystalline host. A theoretical laser model has been developed that is easily adapted to any lanthanide ion in a crystal host. The model is used to predict diode side-pumped laser performance of Tm:Ho:LuLF and Tm:Ho:YLF using input parameters determined from the spectroscopy presented here. An explanation is presented for the improved performance of Tm:Ho:LuLF over Tm:Ho:YLF by modeling the laser. A demonstration that small changes in lower laser thermal population can substantially alter laser performance is noted, an effect that has not been fully appreciated previously.