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The main goal of such a study is to understand the fluorescence processes occurring in these systems. Many different mechanisms are generally involved and it is important to really identify the main ones and to try to determine the exact transfer route. Models valuable for a given system are too often imprudently generalized to other systems without considering that the efficiencies of the energy transfers strongly depend not only upon the nature of the ions, but also on the active ion concentrations and on the intensity and range of excitation. A good knowledge of the excited state dynamics should be helpful for the determination of the best doping densities for a given laser emission and for given excitation domain and mode. Optimized materials present lower laser threshold energy and higher power conversion slope efficiency. The solid-state laser materials containing Er3+, Tm3+ and Ho3+ ions are particularly interesting for infrared emissions and many infrared laser actions have been demonstrated in the past/1/. Recently these systems have been proposed as mid infrared diode-pumped solid state lasers/2,3/ because recent progress make an attractive alternative to lamp-pumped operation/4/ and also because of very good spectral match exists between the GaAlAs diode emission and the Er3+ and Tm3+ absorption near 795 nm. Ho3+ ions do not absorb this radiation but its 2μm laser emission in which we will be greatly interested in the following can be sensitized by Er3+ and Tm3+ ions. The systems investigated in the present study are Er3+, Tm3+ and He+ doped LiYF4 single crystals and fluoride glasses. The so-called ZBLA, BATY and BIZYT glasses which are investigated are described in detail in the LUCAS's paper of the present book.
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