New view on optical quality of Cr4+:Mg2SiO4 laser crystals: analysis of absorption spectra

Nowadays chromium-doped forsterite single crystals (Cr⁴⁺:Mg₂SiO₄) are one of the key elements of high power femtosecond lasers. Optical quality of crystal is commonly determined by the parameter FOM = alpha_p/alpha_par, where alpha_p is absorption of the crystal at the pump wavelength and alpha_par is parasitic losses at the laser emission wavelength. In the present work we analyzed the spectra of more than twenty crystals with various chrome concentration. All the crystals were grown by the Czochralski technique in the atmosphere with various content of oxygen in laboratories of Kharkov and Moscow. The parasitic absorption under consideration was observed in the majority of the examined crystals and probably belongs to the chromium absorption bands with charge transfer. The appearance of parasitic absorption correlates with the appearance of absorption in the visible part of spectrum. Evidently this explains good correlation between output power of the CW Cr⁴⁺:Mg₂SiO₄ laser and the FOM, calculated with the usage of absorption in the visible part of crystal spectrum.