Thermo-optical properties of uniaxial NaT(XO_4)_2 laser host crystals (where T = Y, La, Gd or Bi, and X = W or Mo)

摘要

Thermo-optic coefficients dn_o/dT and dn_e/dT were measured in tetragonal double tungstate and double molybdate crystals NaT(XO_4)_2 (where T = Y, La, Gd or Bi and X = W or Mo) by a laser beam deviation method in the spectral range 0.4-1.1 μm. Thermal expansion coefficients in the directions of a and c crystallographic axes were also measured. Analytical expressions for thermo-optic dispersion formulas were derived as series in 1/λ~2. All dn/dT values for NaT(XO_4)_2 crystals were found to be negative. Their absolute values satisfy the relation ∣dn_e/dT∣ >∣dn_o/dT∣ for crystals without Bi and ∣dn_o/ dT∣ >∣dn_e/dT∣ for crystals with Bi. A clear tendency for dn/dT values to decrease with the increase of the volumetric thermal expansion coefficient α_(vol) of the crystal was observed. This is related with dominant contribution of volumetric thermal expansion effect to the temperature dependence of the refractive index. Thermal coefficients of the optical path W = dn/dT + (n — 1 )α_T governing thermal lensing effect were calculated for different light propagation directions and polarizations as well as crystal athermal directions.