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Up-conversion processes are studied in (Yb 3+ , Tb 3+ ) co-doped fluorophosphate glasses. These glasses contain a large concentration of terbium fluoride, up to 36 mol%, and still luminesc. Samples are excited at 980 nm, within the Yb 3+ absorption band. For `low' Yb 3+ concentrations, i.e., below 16 mol%, the typical green luminescence of terbium is observed. With increasing Yb 3+ concentration, the green luminescence vanishes and a blue emission appears. Both emissions are due to a cooperative effect between two Yb 3+ ions.

Novel fluorophosphate glasses containing a heavy concentration of Tb 3+ ions are studied in terms of optical absorption and emission, and decaytimes. From the 5 D 4 emitting level, an intense green luminescence is observed. The decay is single-exponential whatever the terbium concentration which can be as large as 36 mol%. At high concentrations, a fast diffusion process is responsible for the energy migration from that level. For 5 D 3 , on the other hand, emissions are strongly concentration-dependent, and the decays are well accounted for by the Inokuti-Hirayama model.

A resonant technique, based on the sinusoidal modulation of the pump beam intensity, is demonstrated in the case of the precise measurement of the lifetime of the 4 I 13 2 level of erbium ions in fluorophosphate hosts. This intensity modulation is obtained via a rotation of the polarization of the pump beam. The large signal-to-noise ratio enables the use of low pump power and apparent lifetime shortenings due to ion-ion energy transfer processes and to amplification of spontaneous emission are circumvented. A parallel ith decay times deduced from usual fluorescence decay measurements is performed. Extension of the method to the spectroscopy of other rare-earth ions is discussed.