Thermal lensing study and athermal directions in flashlamp-pumped Nd:KGd(WO4)2 laser crystal

Abstract

Thermal lensing has been studied by a probe beam technique in flashlamp-pumped Nd:KGdW laser rods with rod axes positioned in the N p -N g plane and making different angles with the N g optical indicatrix axis. It was found that the orientation of principal meridional planes of the astigmatic thermal lens in Nd:KGdW corresponds to the directions with maximum and minimum values of thermalexpansion coefficient in the plane perpendicular to the rod axis. The athermal Nd:KGdW laser crystal orientation defined as orientation with weak positive near spherical thermal lens has been determined to make an angle of 30 • -40 • with the N g axis for light polarization E N m .

Thermal lensing and microchip laser performance of N g-cut Tm3+:KY(WO4)2 crystal

“…For N pcut Tm:KYW crystal at the wavelength of 1.94 µm (light polarization is E||N m ) the Δ value is −7.2 × 10 −6 K −1 (α p = 2.0 × 10 −6 K −1 [38]). This is consistent with negative sign of thermal lens for isostructural N p -cut Nd:KGdW crystals [31,32]. Moreover, particularly this fact was crucial for poor power scaling capabilities of Yb,Tm:KYW microchip laser in [33].…”

“…It cannot be used for microchip operation, as plano-plano laser cavity with negative internal lens is unstable. Indeed, thermal lens in the isostructural N p -cut Nd:KGdW crystal was found to be negative [31,32]. Thus, the attempt to create the Tm,Yb:KYW microchip laser with N p -cut crystal was not successful [33].…”

Anisotropy of the photo-elastic effect in Nd : KGd(WO4)2laser crystals

“…However, the photo-elastic effect in general case should be different for different polari-sations of light [16,18]; therefore the values previously calculated for E┴N m will not be valid for E||N m and, strictly speaking, not only the value, but even the sign of the lens cannot be predicted from the data available in this study. However, our measurements of thermal lens in the O-cut Nd:KGW crystal in E||N m configuration under the flash-lamp pumping shows that it is positive along direction A and negative along B direction [15]. This allows us to estimate the maximum and minimum values of the photo-elastic contribution to thermal lens (see Table 3) under the current pump conditions.…”

“…N g +43° in the N p -N g plane [15]. Strong astigmatism of the thermo-optical properties of the material results in significant variation in the sign and value of the thermal lens in crystals cut for light propagation along the N p and N g axes under diode-or flash-lamp pumping [9,12,15,16]. Thermal lens measurements were reported previously for the N p and N g -cut Nd:KGW crystals at the wavelength of 1.06 μm under the flash-lamp-pumping [9,15,16] and at the wavelength of 1.35 μm under the diode -pumping [12].…”

“…KGW is a biaxial crystal, with three crystal orientations having been employed so far for laser engineering: along the b (N p ) axis [1][2][3][5][6][7][8], along the N g axis [9][10][11][12] and along the optical axis (O) [13,14], i.e. N g +43° in the N p -N g plane [15]. Strong astigmatism of the thermo-optical properties of the material results in significant variation in the sign and value of the thermal lens in crystals cut for light propagation along the N p and N g axes under diode-or flash-lamp pumping [9,12,15,16].…”