Co2+ ions when located in tetrahedral (Td) sites attract attention due to their intense and wide absorption band related to the transition. The corresponding saturable absorption is used for the passive Q-switching of erbium (Er) eye-safe lasers at ~1.5 μm. As compared with classical Er phosphate glass with poor thermo-mechanical properties, Er-doped crystals, namely YAG, borates like YAB or YCOB, silicates like CAS, vanadate YVO4 etc., can provide much better power scaling capabilities . Laser wavelength in this case is shifted up to ~1.7 μm, so state-of-the art saturable absorber (SA), Co:MgAl2O4 single crystal cannot be used efficiently.
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