Thermal lensing in Nm-cut monoclinic Tm:KLu(WO_4)_2 laser crystal

摘要

The thermal lensing effect is characterized in slab-shaped monoclinic Tm:KLu(WO_4)_2 crystal cut for light propagation along the N_m optical indicatrix axis (the light polarization is E ‖ N_p). The optical power of the thermal lens was measured directly by a modified probe beam technique. Alternatively, it was calculated on the basis of measured material parameters, including the thermal expansion coefficients along the optical indicatrix axes (α_p = 3.1, α_m = 11.1 and α_g = 13.1(10~(-6) K~(-1))) and the principal thermo-optic coefficients (dn_p/dT = -13.3, dn_m/dT = -5.9, dn_g/dT = -9.0(10~(-6) K~(-1)) at 1.95 μm). The two methods are in good agreement and confirm the defocusing action of the thermal lens; the 'effective' thermo-optic coefficient A for N_m-cut crystal is -2.1 ± 0.5 × 10~(-6) K~(-1). The fractional head load η_h for slab-shaped Tm:KLu(WO_4)_2 crystal is 0.33 ± 0.03 (as determined from ISO-standard laser calorimetry).