Thermal properties of monoclinic KLu(WO_4)_2 as a promising solid state laser host

Abstract

Thermal analysis of the monoclinic solid state laser host KLu(WO4)2 is presented. The specific heat was measured by the relaxation method in the temperature range from 1.9 to 385 K: its value at room temperature is 0.324 J/gK. The Debye temperature and the sound velocity amount to 303+/-3 K and 3734 m/s. The linear thermal expansion tensor was measured by X-ray powder diffraction from room temperature up to 773 K. The eigenvalues of this tensor are alpha'(11)=8.98 x 10(-6) K(-1), alpha'(22)=3.35 x 10(-6) K(-1), and alpha'(33)=16.72 x 10(-6) K(-1), with the maximum value in the a-b crystallographic plane, at 31.94 degrees from the N(g) principal optical axis. The thermal diffusivity and its anisotropy in the temperature range between 300 and 500 K were measured by the pyroelectric method to determine the thermal conductivity tensor. The eigenvalues of the thermal conductivity are kappa'(11)=2.95 Wm(-1)K(-1), kappa'(22)=2.36 Wm-1K-1, and kappa'(33)=4.06 Wm(-1)K(-1), with the maximum value along a direction again in the a-b crystallographic plane, at 40.75 degrees from the N(g) principal optical axis. Simulation of the temperature distribution in a bulk sample of KLu(WO4)2 with dimensions 3 x 3 x 3 mm(3) shows that pump and laser beam directions along the N(p) principal optical axis in terms of thermal effects are preferable because the propagation is along a quasi-isothermal path.

Thermal-Lens-Driven Effects in $N_{\mathrm {g}}$ -Cut Yb-and Tm-Doped Monoclinic KLu(WO4)2 Crystals

“…In particular, the angle N m^X ' 1 amounts to 39.3º [20], while N m^X ' 1κ = 40.8º [15]. The temperature dependence of the thermal conductivity of KLuW was reported in [15] while the temperature-induced change of α and dn/dT coefficients seem to be vanishing (for temperatures that are of practical interest for laser crystals, below 200 ºC). The eigenvalues α' ii and κ' ii , as well as the temperature coefficients dκ' ii /dT, for KLuW are collected in Table 1.…”

“…Thermal lensing in solid-state laser crystals is determined by three principal effects, namely temperature-induced variation of refractive index (expressed by the TOC, dn/dT), [15] the photo-elastic effect (dependence of refractive index on thermally induced stress, expressed by the P PE term), and microscopic bulging of crystal end faces due to the nonuniform thermal expansion (expressed by Q dist ). The sum of these contributions is denoted as "generalized" TOC, Δ.…”

“…The main limitations of the monoclinic DTs are the modest thermal conductivity (~ 3W/mK) [15] and strongly astigmatic negative thermal lensing (that arises mainly from large negative thermo-optic coefficients dn/dT [16][17][18]) which restricts the power scaling capabilities. In this way, a thermal lens can result in abrupt drop of the laser output, distortion of the laser beam profile, and thermally-induced birefringence losses [19].…”

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

“…Tm-doped double tungstates were also utilized in laser systems with Q-switched [21][22][23] and mode-locked [24][25][26] operation modes. The power scaling capabilities for these materials are restricted by moderate value of thermal conductivity [27] and profound anisotropy of physical properties [28] which lead to complicated thermo-optic distortions pattern. KREW crystals are optically biaxial and their optical properties should be described within the frame of optical indicatrix axes (N p , N m and N g ) [29].…”

Potassium double tungstate waveguides with high ytterbium concentration for optical amplification

“…The orientation of the X'3 is ~8.5−13.5˚ counterclockwise from c. The respective angle between the c and X'3 axes, which is denoted as φc3, for different types of double tungstates is listed in Table 3.3. It should be noted that the orientation of the thermal ellipsoid deviates slightly with the change of temperature [115,116].…”