Transparent ceramics for laser applications

Introduction: Transparent ceramics represent a state-of-the-art class of ceramic materials, with prerequisites of transparency being the absence of defects, secondary phases and pores. Since transparent ceramics are polycrystalline, they have good thermal and mechanical properties that make them valid substitutes to glasses and single crystals as solid state laser hosts. The crystalline structure and higher thermal conductivity of ceramics compared to glasses provide much better performance for high power pulsed lasers and more efficient cooling. On the other hand, the wide range of shaping methods represents a strong advantage of ceramic technology in comparison with that of single crystals, as it allows the production of near-net-shape components with a well-defined internal structure. Furthermore, in comparison with the ceramic production, common single crystal growth methods are time consuming and expensive processes. In the case of high-power lasers the performance may be negatively affected by the presence of temperature gradients inside the gain medium, and eventually may lead to damage and destruction of the component. The presented work shows that it is possible to exploit the ceramic process for a better thermal management by the introduction of a non-uniform distribution of the laser-active ions within the medium. The system studied in the presented work is Yb-doped YAG (yttrium aluminium garnet, Y3Al5O12) and structures with both uniform and variable dopant distribution were produced. Materials and Methods: Yb:YAG ceramics were prepared by solid state reaction sintering, where the cubic garnet phase is formed by reaction of oxide powders. The production process can be described as follows: mixing and homogenization of oxide powders and sintering additive - shaping - heat treatments in air and under vacuum (sintering) - annealing, polishing. Shaping was performed by two different methods: 1. pressing of granulated powders and 2. tape casting followed by thermal compression of stacked tapes. Structures with variable doping were obtained by combining of layers or tapes with different Yb concentration. Material quality was evaluated by SEM-EDX analysis of microstructure, measurements of optical transmittance, and characterization of laser efficiency. Results: Materials with good optical quality (transmittance higher than 80 %, i.e 95 % of the theoretical transmittance) and laser performance (laser slope efficiency exceeding 50%) were prepared by both approaches. Structures with layered dopant distribution were produced, and both the pressing of powders as well as tape casting provided good results. The use of tape casting allowed a better control of dopant distribution and is more promising for the production of larger parts. Discussion: Ceramic technology proved to be a convenient approach for the production of gain media for solid state lasers. A particular advantage lies in the possibility to produce in-situ not only a product of desired shape, but also with a controlled inner structure, the latter leading to improved laser efficiency under high thermal loading.