We report on a ceramic Yb:LuAG thin-disk laser in continuous wave operation. The Yb:LuAG ceramic was fabricated using solid-state reactive sintering method. In multi-mode operation in open-air, an output power of 1.74 kW with an optical-to-optical efficiency of 65.0% and slope efficiency of 71.2% was obtained. In near-fundamental mode operation we obtained an output power of 1.29 kW and an average beam quality factor of M2 = 1.44 with an optical-to-optical efficiency of 48.2%. The near-fundamental mode result was realized with a simple evacuated, stable resonator cavity with just the thin-disk gain medium and output coupler. To the best of the authors’ knowledge, this is not only the first time more than 1 kW has been demonstrated from a ceramic Yb:LuAG medium, but this is also currently the brightest continuous wave Yb-doped ceramic laser.
© 2015 Optical Society of America
Since the conceptualization of the thin-disk laser by Giesen et al , the subsequent decades have seen the development of Yb-doped thin-disk laser into various high-power, high-brightness systems suitable for materials processing. In recent years, Trumpf GmbH has also achieved more than 10 kW of laser output from a single disk in multi-mode operation and also 4 kW in single-mode operation .
The recent approaches to the power scaling of thin-disk lasers generally involve the reduction of thermally induced aberrations. A relatively straight forward way is to pump the disk at the zero-phonon line with volume Bragg grating stabilized diodes at 969 nm [2, 3]. This reduces the heat generation in the gain medium by almost a third and has also been shown to suppress nonlinear phonon relaxation and the heating associated with it .
The alternative approach is to explore the use of other host materials with better thermal conductivity. One possibility that is compatible with current 940 nm pump sources for Yb:YAG is Yb:LuAG. The thermal conductivity of single crystal Yb:LuAG can be up to 20% better than Yb:YAG at 10% Yb3+-concentration and more than 5 kW output has been demonstrated from a single disk .
However, the commercial availability of large sized Yb:LuAG single crystals remains limited when compared to Yb:YAG. This motivates the development of Yb:LuAG polycrystalline ceramics as a gain medium.
Ikesue et al first demonstrated the efficient lasing of a Nd:YAG ceramic polycrystalline laser  and the development of high power Nd:YAG ceramic lasers followed on in the 2000s. For Yb-doped ceramic polycrystalline lasers, Latham et al achieved 6.5 kW from a 200 μm thick ceramic Yb:YAG thin-disk laser with a 1 mm undoped YAG cap . Continuous wave (CW) lasing for ceramic polycrystalline Yb:LuAG was first demonstrated in 2012  followed by a mode-locked laser in 2014 . A maximum of 101 W in multi-mode operation and 49 W in fundamental-mode was subsequently demonstrated in thin-disk laser architecture .
In this paper, we report on a high brightness (i.e. high power and good beam quality) ceramic Yb:LuAG thin-disk laser in CW operation. We will begin with the fabrication of the thin-disk, followed by the lasing results in multi-mode and near fundamental-mode operation.
2. Fabrication of Yb:LuAG ceramic thin-disk
A 16 mm diameter, 8.5% at. Yb:LuAG ceramic was produced in DSO using the solid state reaction method first described by Ikesue . High purity (99.99 wt%) powders of Lu2O3, Al2O3 and Yb2O3 were mixed via wet milling and then granulated to tens of microns using spray drying. The powder compact was then sintered at 1750°C for 20 hours under vacuum and further annealed in air at 1400°C for 10 hours to form the 16 mm diameter ceramic disk. These disks were then ground to required thickness, lapped and polished to the final specified thickness of 150 μm by a supplier. AR and HR coatings suitable for the thin-disk laser architecture were then applied on the polished disks.
Figure 1 shows a sample of a coated ceramic Yb:LuAG after fabrication and before bonding. The disk was then glued in-house onto a diamond heatsink for support and for thermal management. The effective radius of curvature of the glued disk was estimated to be about 3.5 m from lasing experiments performed with a single plane output coupler at the edge of cavity stability.
Figure 2 shows the calculated optical-to-optical efficiencies for the simulation parameters used in the inset for 24 and 48 pump passes with a 150 μm thick disk. The thickness of this disk was limited by the capabilities of the commercial vendor we used. To realize the 48 pump passes we used a commercial TDM30 module from Dausinger and Giesen (D + G GmbH) with a 2.5 kW fiber coupled laser source (1 mm diameter, 0.2 NA) from Laserline GmbH. With 48 pump passes, the optimal doping level was found to be at a lower doping level compared to 24 pump passes. The roll-over in efficiency for the 48 pump passes can be explained by the increase in lasing threshold outweighing the effects of the increase in absorption efficiency due to the increase in concentration of dopants.
3. Experiments and results
3.1 Multi-mode continuous wave lasing
The disk was first lased in a multi-mode cavity with a plane HR and plane output coupler in open-air. A schematic of the setup is shown in Fig. 3. The output coupling of the cavity was then varied between 2% and 10% to determine the optimal output coupling for this setup. The pump chamber provided 48 pump passes on the disk with a pump spot of 8 mm diameter. The fundamental-mode diameter on the disk is expected to be about 1.5 mm and this resulted in a multimode output for the laser.
Figure 4(a) shows the results obtained for an output coupling of 4%. A maximum output power of 1740 W was obtained with an optical-to-optical efficiency of 65.0%. This was also the highest power obtained from the experiments. The slope efficiency, determined by linear regression, was 71.2%. The roll-off for the output power seen in the graph can be attributed to the wavelength of the pump - at full pump power the wavelength of the pump source is centered at 940 nm with a FWHM of 3nm which is beyond the optimal wavelength for pumping Yb:LuAG at 938 nm .
The output coupler was then varied between 2% and 10% and the data taken again. The slope efficiencies were then obtained and plotted against the output coupling transmittance used. The results for these are shown in Fig. 4(b). The peak slope efficiency obtained is 71.7% at 6% output coupling transmittance.
Using Contag’s zero-dimensional model  and temperature dependent cross-sections from  at an estimated average disk temperature of 393 K, we estimated the total round trip cavity loss to be less than 0.5% in order for the peak slope efficiency to be between 4% to 6% output coupling transmittance. However, we are unable to ascertain the actual transmission losses through the disk from this value because it is also affected by the scattering off the optical coatings and surfaces of the intra-cavity optics and as well as any surface irregularity of the disk after bonding.
3.2 Near fundamental-mode continuous wave lasing
To further ascertain the quality of the mounted thin-disk, a near fundamental-mode cavity was designed. The design principles for this cavity are similar to that of our previous work : a large fundamental mode diameter was used on the disk to filter out the higher order modes, with an evacuated cavity to mitigate the thermal effects due to the heating of the air in front of the disk.
The setup for this experiment is shown in Fig. 5. It was ascertained in the previous section that an output coupling in the range of 4% to 6% would be ideal for a V-shaped cavity with two reflections off the disk per cavity round trip. Hence for this I-shaped cavity with one reflection off the disk per cavity round trip, 2% to 3% would be ideal. We chose a 2% output coupler as this was the only piece readily available in our laboratory with a suitable curvature of 1 m RCX. The curvature of the output coupler and distance to the disk was chosen to give a fundamental mode diameter of about 5.6 mm on the disk (~0.7 times the pump spot size).
At the laser output, an additional 0.25 m convex mirror was used to expand the beam going into the power meter (Ophir-Spiricon 10kW power meter) to prevent damage to it. The leak-through of the laser from the first 45 degree folding mirror (AR coated on the rear side) was then folded into a Spiricon M2-200s for beam quality measurement. The additional uncoated wedge was used to further attenuate the power of the laser entering the diagnostic device.
The results for this experiment are shown in Fig. 6(a). A maximum output power of 1290 W was obtained with 48.2% optical-to-optical efficiency. The decrease in efficiency as the power was increased can be attributed to the change in modal content (towards more single mode) as the power was increased. Figure 6(b) shows the results from the Spiricon M2-200s, with the insets showing the mode profile. The beam quality factors at the maximum output power were measured to be 1.22 and 1.71 in the X and Y axes respectively resulting in an average beam quality factor of M2 = 1.44. We believe the difference in the beam quality in the two directions can be attributed to a misaligned pump spot that was larger in the Y-axis resulting in more gain for higher order modes in this direction that was not effectively filtered out with this large fundamental mode diameter on the disk.
In conclusion, we have successfully fabricated a Yb:LuAG ceramic gain medium using a solid-state reactive sintering method and lased it in a thin-disk laser architecture. In multi-mode lasing, we achieved a maximum output power of 1740 W with an optical-to-optical efficiency of 65.0%. This is an order of magnitude greater than previous results . In near-fundamental mode lasing, we achieved a maximum output power of 1290 W with an optical-to-optical efficiency of 48.2% and an average M2 of 1.44. To the best of our knowledge, this is not only the brightest CW Yb:LuAG ceramic laser reported so far, but also the brightest CW Yb-doped ceramic laser reported.
References and links
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