Chromium ion valence transformations in Cr,Ca:YAG used for passive Q-switching

Composite laser devices of passively Q-switched Nd:YAG were prepared by optical contacting between Nd:YAG and Cr,Ca:YAG crystal wafers, followed by prolonged heating at elevated temperatures. Thermal treatment of YAG crystals co-doped with Cr⁴⁺ and Ca²⁺ exhibits reversible oxidation-reduction reactions of the chromium ions, which depend on both temperature and ambient atmosphere. The dynamics of the oxidation-reduction reactions were studied by the spectral absorption characteristics of Cr,Ca:YAG crystals following different thermal treatment procedures. The absorption spectra were resolved into different peaks, of modified Lorentzian line-shape. Full reduction of Cr⁴⁺ into Cr³⁺ is obtained by prolonged heating in an ambient atmosphere of Ar:5vol.%H₂ mixture at 900-1200°C. Saturated oxidation is obtained by prolonged heating at elevated temperatures in free air. Knowing the process parameters now allows one to optimize oxidization procedures administered to Cr,Ca:YAG crystal devices that are used as laser gain media or as saturable absorbers. The device thus obtained was end-pumped by a fiber-optic-coupled diode laser, and exhibited high repetition-rate short (∼5ns) pulsing. Laser damage threshold and Findly-Clay analysis were also perform on the composite devices.