Compact, short-wavelength lasers are continuously considered as one of the most attractive among all coherent light sources. A very wide range of applications such as data storage, photolithography, medical diagnostics and sterilization motivates research and development on new active materials which can efficiently emit light in the UV spectral range. There are three general methods to achieve UV and visible light emission. The first are semiconductor-based LEDs and laser diodes - in the recent years a rapid progress in the field of semiconductor lasers has been observed resulting in a variety of commercially available short-wavelength laser diodes. These impressive devices, however, are not free from some disadvantages, like poor quality of the beam and limited output power. That’s why other sources continue to attract researchers’ attention. The second method deploys nonlinear crystals, which enable obtaining short wavelength emission by the frequency multiplication. The third way to achieve UV and visible light is to directly use the optical transitions within rare earth ions in solid state media [1–2]. This work is focused on this last method.
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