Ceramic Gain Media
Traditionally, solid-state gain media have been made either of crystals or glasses. In the case of crystals, these were typically single crystals, because polycrystalline media usually exhibit strong scattering at domain boundaries. However, beginning in the 1990s, scattering losses of polycrystalline media with very small domains – called ceramics – have been greatly reduced with refined fabrication techniques, in particular with vacuum sintering. When the procedure is started with very small particles in the powders used and a refined treatment generates nanoparticles with a well-controlled size distribution, very small crystallites and very low porosity result, leading to scattering losses which are not significantly larger than for single crystals. This has been achieved in particular for YAG (yttrium aluminum garnet). Neodymium-doped YAG ceramics now allow for essentially the same laser efficiency as Nd:YAG single crystals. The same holds for some ytterbium-doped gain media. Ceramics are also suitable for vibronic laser gain media such as Cr2+:ZnSe.
Ceramic laser gain media offer a number of important advantages over single crystals:
- Their fabrication can be significantly cheaper, particularly for large pieces.
- Ceramic gain media can be fabricated with arbitrary shapes and size, whereas single-crystal growth techniques (e.g. the Czochralski method) set limits on the possible size.
- Ceramics are well suited to produce composite gain media, consisting e.g. of parts with different doping levels, or even different dopants. It is also possible to include a saturable absorber section for passive Q switching .
- Spatially varying doping profiles are relatively easily possible. These aspects give additional freedom in laser design.
- For neodymium-doped and ytterbium-doped YAG ceramics, a significantly higher doping concentration can be achieved without quenching effects degrading the laser efficiency.
- Some materials, e.g. yttria (Y2O3), scandia (Sc2O3) and other sesquioxides with their high melting temperatures, are very difficult to grow into single crystals, and much easier to obtain in ceramic form, because the sintering temperature can be much lower than the melting temperature . The high thermal conductivity of Y2O3 and Sc2O3 could make these materials preferable to YAG.
For these reasons, it is conceivable that ceramic gain media will in many cases replace single crystals, particularly in high-volume applications and those which need large gain media.
Note that ceramics are interesting for laser construction not only when used as gain media. Some ceramic media, such as aluminum nitride ceramic, have a very high thermal conductivity while being excellent electrical insulators. This makes them interesting for heat sinks of high-power laser diodes.
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