Thermal lensing inNm-cut monoclinic Tm:KLu(WO4)2laser crystal

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 (d n p /d T =− 13.3, d n m /d T =− 5.9, d n g /d T =− 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 Δ 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).