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

Abstract

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 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).

Microchip laser operation of Tm,Ho:KLu(WO_4)_2 crystal

“…The resulting population of the lower-lying 3 F 4 excited state is decreased due to the highly efficient two-for-one cross-relaxation scheme that reduces the heat release. Indeed, for Tm:KLuW, η h was measured to be only ~0.25 ± 0.05 [25]. For a Tm,Ho-codoped crystal, a fraction of the energy stored in the 3 F 4 manifold of Tm is transferred to the 5 I 7 state of Ho.…”

Ho:KLuW microchip laser intracavity pumped by a diode-pumped Tm:KLuW laser

“…The contribution of the lens in the Ho:KLuW crystal (which is also positive) is weak. Indeed, in the latter case, the fractional heat loading is determined as quantum defect, η h = 1 − λ Tm /λ Ho = 0.1 ± 0.02 in our case, compared with η h = 0.25 ± 0.05 [21] for Tm:KLuW. The latter value is related to the efficient two-for-one crossrelaxation scheme for Tm 3+ ions that reduces the heat loading.…”

Thermal lensing in Yb:KLu(WO4)2crystals cut along the optical indicatrix axes

“…Let us discuss their contributions. The dn/dT coefficient depends only on the polarization state (E || N m or N p , in our case); it is taken from the literature [24]. The term Q dist = (n − 1)α depends on the crystal orientation (via the thermal expansion coefficient α [10]) and on the laser polarization (via the refractive index n [1]); it can be calculated.…”