Criteria of optical quality of Cr4+:Mg2SiO4 laser crystals

Optical quality of forsterite crystals (Cr⁴⁺:Mg₂SiO₄), which are widely used as active media of femtoseconds lasers, is commonly determined by the parameter FOM=α₁₀₆₄/α₁₂₅₀, where α₁₀₆₄ is absorption of the crystal at the pump wavelength (1064 nm) and α₁₂₅₀ is absorption at the laser emission wavelength (1250 nm). FOM is not usually high enough (12-40). However, even for such values of FOM it is hardly measurable by standard techniques. We propose other, more convenient techniques for evaluation of FOM. Parasitic absorption at the laser emission wavelength, which limiting FOM, consists of the superposition of absorption edge of Cr⁴⁺ and absorption of Cr²⁺ ions. Wide absorption bands of Cr²⁺ ions which correspond to ⁵E-³T₁ transition, have the maxima at 1400 nm (E||b) and 1700 nm (E||c). The latter band (α₁₇₀₀) is essentially stronger and suitable for exact detection. Moreover, in the case of this band there do not overlaps with other absorption bands. So, we can modify the parameter FOM to the parameter FOM´=α₁₀₆₄/α₁₇₀₀.