Characterization of the thermal lens in 3 at.%Tm:KLu(WO₄)₂and microchip laser operation

Abstract

A comparative study of the thermal lensing effect is performed for monoclinic 3 at.%Tm:KLu(WO 4 ) 2 crystals cut along the three principal axes of the optical indicatrix, N p , N m and N g . Optical power, sensitivity factors and astigmatism of the thermal lens are determined. It is found that the thermal lens is differently signed for beam propagation along N p and N m (for m-and p-polarizations, respectively). In contrast, beam propagation along N g possesses a positive lens with nearly two-times reduced astigmatism (m-polarization). The results are utilized in the first realization of a continuous-wave quasi-microchip 3 at.%Tm:KLu(WO 4 ) 2 laser delivering 880 mW output power at 1.95 μm with a slope efficiency of 44%, and an M 2 factor below 1.05.

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

“…Indeed, if expressed as S/M mg , it equals only 4%. The thermo-optic properties of Tm,Ho:KLuW are summarized in Table 1, together with recent data for singly Tm-doped KLuW [21]. The crystal cut is along the N g -axis (light polarization E || N m ).…”

“…DTs are typically cut along the growth direction, or b-axis || N p -axis of the optical indicatrix, generating a negative thermal lens that leads to the cavity instability [19]. Only recently it was shown that another cut, along N g -axis, can provide the desired mode stabilization and Tm:DTs microchip lasers were developed [20,21]. Later on, the potential for power scaling to multi-watt levels was confirmed [22].…”

Tm:KLu(WO_4)_2 microchip laser Q-switched by a graphene-based saturable absorber

“…For the considered I the modulation depth will be only 1.2% that is . This is related to the low astigmatism of the thermal lens for N g -cut Tm:KLuW crystals [28]. The peak power is shown in Fig.…”

“…It was shown that Tm:DTs are suitable for the monolithic laser concept [26,27], provided a special crystal cut for light propagation along the N g dielectric axis is chosen. For this cut, the desired mode stabilization in the plano-plano cavity is ensured by a positive and nearly spherical thermal lens [28]; these properties are not available for the crystal cut along the growth direction, the b-axis. The specific features of the microchip concept allowed us to achieve ~10-fold improvement in terms of output power and pulse duration as compared with a recent report on Tm:KLuW laser with a hemispherical cavity Q-switched with a graphene SA [24].…”

Passive Q-switching of a Tm,Ho:KLu(WO_4)_2 microchip laser by a Cr:ZnS saturable absorber

“…It was oriented for light propagation along the N g -axis. This crystal orientation was selected because it provides a positive thermal lens [15,27] required for stabilization of the laser mode in a plano-plano microchip cavity. Both m × p crystal faces were polished to laser quality and remained uncoated.…”