While rare-earth-doped crystals exhibit most of the optical and thermomechanical properties required to get high power laser beams, their maximum size is limited. Also, their maximum doping level is limited by the activator ions segregation undergone during the growth processes from the melt.
Glasses can be synthesised in various shape and without the same size limitation. However, their mechanical properties are not sufficient and their thermal conductivity does not allow enough heat evacuation during laser operation. Also, the maximum doping level is limited due to rare-earth (RE) clustering tendency. Moreover, the optical properties of the RE ions suffer from inhomogeneous broadening.
In this study, we have studied fluoride materials. With respect to the usual oxide type materials, a fluoride material can offer a low phonon environment favourable to enhance the radiative rate and quantum efficiency. The excited states lifetime are increased.
A first alternative route to single crystals has been investigated. This consists in using oxyfluoride glass ceramics in which RE ions benefit of a crystalline and fluoride environment. These materials are obtained using glass synthesis technique and thermally activated crystallisation of nanocrystallites thank to RE nucleating effect. With thermomechanical properties higher than glasses and crystal like optical properties, the glass ceramics are competitive materials with an easy shaping and fabrication in the air. Two approaches developed in the lab will be discussed, either PbF2 phase inside germanate glass and CaF2 phase inside silica glass.
Finally, transparent ceramics prepared from nanopowders are a logical outcome for high power laser materials. Many results have been recently obtained with YAG and RE sesquioxides activated with Nd and Yb. We will present here preliminary results in which a fluoride route is investigated using CaF2 as a model material for ceramming processes.