Contents 1. INTRODUCTION
The development of novel active devices based on broadband amplified spontaneous emission (ASE), broadband signal amplification and multi-wavelength laser emission in the wavelength range λ = 1.5 – 2.2 µm constitutes a hot topic for the recent research. In fact, the availability of novel active devices at these wavelengths promise several applications including medicine diagnostic and therapy, microscopy, laser radar for remote sensing, earth and atmosphere monitoring and novel communications optical systems [1]–[13]. The ASE and/or lasing at these wavelengths have been obtained in a variety of glasses, among which the silicate, fluorophosphate, tellurite, chalcogenide, antimony and germanate glasses. Low phonon germanium-based glass allows high rare-earth-ion solubility, if compared with chalcogenide glass, and high physicochemical stability, if compared with fluoride glass. Another strength point is the spectral transmittance, extended up to 5 – 6 µm wavelength, and the quite low probability of non-radiative relaxation [1]–[3]. Unfortunately, the high number of spectroscopic parameters, pertaining to each rare earth ion transition and to the energy transfer phenomena among the different rare earth ions, makes the optimization of the rare earth doped optical glasses and devices not trivial. In particular, the multiple rare earth doped glass design is further complicated because the energy transfer phenomena nonlinearly depend on the dopant concentration levels and the ion population levels.
