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Timestamp: 2019-04-19 13:28:05+00:00

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We unveil a gas-lens effect in kW-class thin-disk lasers, which accounts in our experiments for 33% of the overall disk thermal lensing. By operating the laser in vacuum, the gas lens vanishes. This leads to a lower overall thermal lensing and hence to a significantly extended power range of optimal beam quality. In our high-power continuous-wave (cw) thin-disk laser, we obtain single-transverse-mode operation, i.e. M2 < 1.1, in a helium or vacuum environment over an output-power range from 300 W to 800 W, which is 70% broader than in an air environment. In order to predict the magnitude of the gas-lens effect in different thin-disk laser systems and gain a deeper understanding of the effect of the heated gas in front of the disk, we develop a new numerical model. It takes into account the heat transfer between the thin disk and the surrounding gas and calculates the lensing effect of the heated gas. Using this model, we accurately reproduce our experimental results and additionally predict, for the first time by means of a theoretical tool, the existence of the known gas-wedge effect due to gas convection. The gas-lens and gas-wedge effects are relevant to all high-power thin-disk systems, both oscillators and amplifiers, operating in cw as well as pulsed mode. Specifically, canceling the gas-lens effect becomes crucial for kW power scaling of thin-disk oscillators because of the larger mode area on the disk and the resulting higher sensitivity to the disk thermal lens.
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Fig. 1 (a) Schematic of the single-mode cw thin disk laser (TDL) cavity setup, including an output coupler (OC), a 2-m convex (CX) mirror, and the highly-reflective (HR) end mirror. The three cavity arms are labeled as a, b, and c, respectively. (b) Output-power slope for high-power single-mode operation. The power range of good beam quality (i.e. M2 < 1.1) is ~70% broader in vacuum as compared to 1-bar air.
Fig. 2 (a) Disk thermal-lensing measurements in fluorescence mode, total disk thermal lens Δ F total , disk peak temperature change Δ T . Both methods, interferometer (Int.) and laser focus (Focus), stand in good agreement with each other. Measurements in vacuum yielded similar results as 1-bar He, and measurements in 1-bar N2 (not shown here) yielded similar results as 1-bar air. The disk-temperature increase is independent of the gas environment. (b) Disk thermal-lensing measurements in multimode (MM) operation, reaching 1.4 kW of output power [see Fig. 2(c)]. Due to the low sensitivity of the highly multimode test cavity to the cavity element curvatures, the output power was independent of the gas environment within the uncertainty of the used power meter (~5%). Note also that a slightly wider range of pump intensities (up to 5.5 kW/cm2) is used for the multimode data.
Fig. 3 (a) Sample output beam profile for different values of M2. (b) Calculated stability zone of the single-mode cavity (black solid line) in comparison to the measured mode quality (M2) versus the total disk diopter change ( Δ F total ) for both operation in 1-bar air and vacuum. The overlaid numbers indicate the corresponding output power, see also Fig. 1(b). In this particular cavity setup, an ideal overlap of pump spot and laser spot, leading to an M2 < 1.1, is achieved for disk thermal lensing between −5*10−3 1/m and −11*10−3 1/m.
Fig. 4 (a) Schematic of the model showing, left to right, the diamond heatsink, the disk with the super-Gaussian temperature profile on it, and the gas in front of it. (b) Simulated and measured gas-lens and vertical-gas-wedge effect for a disk’s peak temperature increase ΔT = 57 °C. We fitted the gas-lens-simulation data with the function ΔFgas,sim = α (dspot)β finding β = −1.4. The gas-lens effect increases for smaller pump-spot diameter while the gas wedge (black squares) only slightly depends on it.
Fig. 5 Simulation of the gas refractive index (n(x,y,z)-1) profiles for (a) 1-bar air and (b) 1-bar He. The disk is situated at z = 0 mm with a peak temperature of 81 °C (ΔT = 57 °C). Both the gas-lens and gas-wedge effects are clearly visible. Due to the higher thermal conductivity of He, see Table 2, the heat from the disk extends further into the gas. Nonetheless, thanks to the order of magnitude lower n-1, the thermo-optic effects for He are significantly less pronounced as compared to air, as in our experimental data summarized in Table 1.
Table 1 Slope of disk thermal lensing for different gas environments. Total diopter change ( Δ F t o t a l , see also Fig. 2) in both fluorescence and multimode operation, diopter change due to the gas-lens effect ( Δ F g a s , e x p = Δ F t o t a l − Δ F v a c u u m ) inferred from the fluorescence measurement, and simulated diopter change due to the gas-lens effect ( Δ F g a s , s i m , see section 4), all per disk peak temperature increase ( Δ T ). The fit uncertainty of ( Δ F t o t a l / Δ T ) is ~3%. The resulting uncertainty of Δ F g a s , e x p / Δ T is presented in the table.
Table 2 Thermal and optical properties of vacuum, helium, nitrogen, and air, at 25°C and 1030 nm [33–35].
Slope of disk thermal lensing for different gas environments. Total diopter change ( Δ F t o t a l , see also Fig. 2) in both fluorescence and multimode operation, diopter change due to the gas-lens effect ( Δ F g a s , e x p = Δ F t o t a l − Δ F v a c u u m ) inferred from the fluorescence measurement, and simulated diopter change due to the gas-lens effect ( Δ F g a s , s i m , see section 4), all per disk peak temperature increase ( Δ T ). The fit uncertainty of ( Δ F t o t a l / Δ T ) is ~3%. The resulting uncertainty of Δ F g a s , e x p / Δ T is presented in the table.
Thermal and optical properties of vacuum, helium, nitrogen, and air, at 25°C and 1030 nm [33–35].

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