ceramic laser gain media (original) (raw)
Definition: laser gain media which have a ceramic (polycrystalline) microscopic structure
Categories: 
optical materials, 
laser devices and laser physics
Related: laser gain medialaser crystalslaser glassesneodymium-doped laser gain mediahigh-power lasersoptical materialsscattering
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DOI: 10.61835/hcs Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn
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Traditionally, solid-state laser gain media have been made either of crystals or glasses. In the case of crystals, these were typically single crystals (i.e., with a uniform crystal lattice throughout a large piece) 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 laser 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 [11].
- 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 optical 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 [8]. 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.
Frequently Asked Questions
What is a ceramic laser gain medium?
A ceramic laser gain medium is a polycrystalline material used for light amplification in a laser. It is composed of many very small crystal domains, and modern fabrication techniques allow it to have an optical quality similar to that of a single crystal.
What are the main advantages of ceramic gain media over single crystals?
Ceramic gain media can be fabricated in larger sizes and more complex shapes, can incorporate composite structures with varying doping, and often allow for higher doping concentrations. They can also be significantly cheaper to produce, particularly in large volumes.
How is it possible to make low-scattering ceramics for lasers?
Low-scattering ceramics are made with refined fabrication techniques like vacuum sintering. These processes use very fine starting powders to create a final material with extremely small crystallites and very low porosity, which minimizes optical scattering losses.
What laser materials can be made in ceramic form?
A prominent example is YAG (yttrium aluminum garnet), doped with neodymium or ytterbium. Other materials include sesquioxides like yttria (Y2O3) and scandia (Sc2O3), which are difficult to grow as single crystals, and vibronic media like Cr2+:ZnSe.
Are ceramics used for other parts in laser construction?
Yes, some ceramics like aluminum nitride have very high thermal conductivity while being excellent electrical insulators. This makes them useful for components like heat sinks for high-power laser diodes.
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Bibliography
| [1] | E. Carnall et al., “Optical studies on hot-pressed. polycrystalline CaF2 with clean grain boundaries”, Mater. Sci. Res. 3, 165 (1966) |
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| [14] | S. Esser et al., “Ceramic Yb:Lu2O3 thin-disk laser oscillator delivering an average power exceeding 1 kW in continuous-wave operation”, Opt. Lett. 46 (24), 6063 (2021); doi:10.1364/OL.445637 |
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