Effects of co-dopant concentrations and excitation conditions on the 2 ?m fluorescence dynamics in Tm, Ho:YLF crystals

Abstract

Abstract.Results of spectroscopic studies carried out on Tm, H o : Y L F crystals are reported. In particular, we have investigated the effects of the co-dopant concentrations on the time evolution of the fluorescence at 2 gm emitted in response to short-pulse laser excitation at 792 nm. The fluorescence intensity profiles are analyzed in terms of their temporal and amplitude characteristics over a wide range of excitation conditions. PACS: 42.70Laser action at approximately 2 p.m involving 517--,5I s transitions of Ho 3÷ is receiving considerable attention because efficient optical pumping with currently available laser diodes is possible by using Tm 3 + as co-dopant sensitizer ions. Besides Yttrium Aluminum Garnet (YAG) [1,2], Yttrium Lithium Fluoride (YLF) is considered as one of the most interesting among the crystals that can be activated with Tm 3÷ and Ho 3+ ions [3, 41. To optimize laser operation from Tm,Ho:YLF crystals several factors have to be considered. An essential one is the choice of the concentrations of the sensitizer and activator ions, which is dependent on the operating conditions and the mode of laser operation (cw or Qswitched). In order to operate the most appropriate choice one has to understand the effects of the concentrations on the relevant spectroscopic and lasing properties, and how these effects vary with the operating conditions. At present, this understanding is not complete, even though the Tm,Ho:YLF system has been extensively studied in recent years [5][6][7][8], and a significant amount of work has been done on measuring the fundamental system properties [9][10][11][12]. A better understanding may be gained by monitoring the concentration dependence of selected processes taking place in the excited crystal. One of the most important among these processes is the time evolution of the Ho 3 + ion lowest excited level population, i.e., of the upper laser level population for lasers operating at 2 p.m.Measurements of the transient behavior of the fluorescence from this level have been reported in the literature for a large variety of co-dopant concentrations [9,11,12]. These data, together with those relative to fluorescence from the other excited levels in the Tm,Ho:YLF system, have permitted to draw a qualitative picture of the energy distribution mechanisms and to put forth hypotheses on those which predominate. Nevertheless, there is still a number of open questions that, if answered, may provide a better understanding. One of them is concerned with the dependence of the fluorescence time evolution on the excited-state density. The presence of loss mechanisms which reduce the effective upper laser level lifetime under intense pumping is well recognized in Tm and Tm,Ho lasers [7,12]. However, these effects and their dependence on co-dopant concentrations have not yet been clearly quantified. In fact the upper laser level fluorescence measurements which are reported in the literature are usually performed at low excitation densities. Just few data are available under intens...

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