The first demonstration of an Yb:YAl3(BO3)4 thin-disk laser is reported. An output power of 109 W with an optical efficiency of 50.2% was achieved in multimode CW-operation which is to the best of our knowledge a significant record performance compared to previous reports on CW-lasers with this material. At a lower power level of 19.3 W the material proved its suitability for efficient operation with an optical efficiency of 60.4%. In fundamental-mode operation the extracted output power was 10.4 W with an optical efficiency of 44.5% and a beam propagation factor M2 = 1.39. The broad emission bandwidth of the material was confirmed by measuring a continuous wavelength tuning range from 1001 to 1053 nm with a maximum output power of 36 W at 1040 nm.
© 2013 Optical Society of America
The laser active material Yb3+:YAl3(BO3)4 (Yb:YAB) features two interesting properties. On the one hand the self-frequency doubling capability of the YAB host [1,2] which led to intense research for the utilization of this material in microchip lasers emitting in the green wavelength region. On the other hand its broad gain bandwidth of more than 40 nm (σ-polarization), which demonstrated its potential for the use in ultrafast laser systems  with sub-100-fs pulse durations . Additionally, the good thermal conductivity of 4.7 W/ (m·K) at 5.6 at.% doping concentration  is promising for power scaling. Output powers of several hundred Watts should be feasible especially in thin-disk laser configuration. Despite this potential, the use of Yb:YAB was not demonstrated in thin-disk lasers until now, due to issues regarding the crystal quality, size and the manufacturing of thin Yb:YAB disks with thicknesses in the hundred micrometer range. Reports so far were restricted to end-pumped configurations with transverse cooling geometry where the highest output power published to date was 14 W .
In addition to the thermo-optical and spectroscopic properties the gain medium has to meet further specific requirements regarding the mechanical and chemical stability  to be usable in a thin-disk laser. For instance, the sensitivity of the material to moisture and potential chemical reactions with the polishing liquids have to be considered in the polishing process. Further critical parameters are the hardness, the tendency to cleave or break and the melting temperature of the crystal which exhibits a correlation to the generation of internal stress. These parameters are rarely discussed in thin-disk laser publications, but are of importance for the ability of polishing, coating and mounting the disk onto the heat sink.
At present, laser active materials  suitable for the thin-disk concept can be divided into two groups. One with gain media especially qualified for high average output powers in the multi-kW range and the other with laser active materials featuring large spectral emission bandwidths suitable to generate ultra-short pulses . Numerical values of the relevant spectroscopic, mechanical, thermal, thermo-mechanical, and thermo-optical parameters of Yb-doped laser active materials have been comprehensively published in the references [7–10]. For the time being only Yb:YAG and Yb:LuAG  can be used for kW average output powers, whereas the second group includes a broad range of materials: for example, sesquioxides like Yb:Lu2O3 [12,13] as well as several other oxide crystals such as Yb:CALGO  and Yb:SSO . The thermo-optical properties  of these latter materials are very promising; yet, the key for scaling the average power of mode-locked lasers with these crystals towards the kW-level would be the availability of crystals with substantially improved quality (e.g. regarding defects, impurities, internal stress etc.). This in turn requires further optimization of the crystal growth processes.
A relevant factor is thereby the melting temperature of the laser active material. Yb:CALGO and Yb:SSO offer the advantage of melting temperatures in the range of Yb:YAG (1940 °C) and the possibility to be grown with the well-known Czochralski method, whereas for all sesquioxides including Yb:Lu2O3 the melting point is much higher (around 2430 °C) which results in a more demanding growth process. To avoid the high temperatures required for drawing crystals from their melt, rare earth doped transparent ceramics  appear to be an interesting alternative, since vacuum sintering and hot pressing can be performed at temperatures well below the melting point. Further potential approaches for growing single crystal Yb-doped sesquioxides are the growth from a hydrothermal solution  or by high-temperature solution growth methods . These growth processes are performed at temperatures well below the melting temperatures resulting in substantially lower thermal stress of the laser active crystals, which is advantageous in high-power laser operation. For sesquoxides, however, all three approaches do not yet lead to a crystal or ceramic quality which is sufficient for high-power operation.
Due to the aforementioned facts, we decided to add the crystal quality and the mechanical robustness to the selection criteria for novel laser active materials suitable for the operation at the kW-level with the final goal of high brightness. Yb:YAB is therefore an interesting candidate which offers good spectroscopic properties including a large emission bandwidth with still sufficient thermal properties and a growth out of a flux. In this paper we present the first experimental investigations of Yb:YAB utilized in a thin-disk laser.
2. Crystal properties of Yb:YAB for the use in thin-disk lasers
For the fabrication of the Yb:YAB material employed in the experiments presented here the top-seeded solution growth (TSSG) was applied (solvent: Li2WO4; crystallization temperature ≈970 °C). Therefore, the dislocation densities are expected to be relatively small due to the inherently small temperature gradients typical of the TSSG growth process. The internal stress of the samples is likely to be also small. A sufficient crystal size could be obtained by the TSSG, which allowed the manufacturing of thin-disk crystals with a diameter of 6.3 mm. The Yb ion concentration of the grown crystal was 12at.%, corresponding to an Yb3+ volume concentration of 6.6·1020 cm−3. In order to be able to investigate the performance for both, IR and self-frequency-doubled green emission, the crystal was cut for type I SHG of 1030 nm under normal incidence having an angle of 31.3° to the c-axis. In view of the potential use of the sample for self-frequency doubling (not reported here) the thickness was chosen to be 250 µm, which is approximately 100 µm thicker than needed for efficient pump light absorption at this doping level but still offers reasonable heat removal. The calculation of the optimum disk thickness was done according to  with the material parameters of Yb:YAB. Nevertheless, in the following we only discuss experimental results obtained in the IR wavelength range.
The polishing quality achieved for the Yb:YAB thin-disk surfaces was comparable to that of commonly used Yb:YAG thin-disk crystals [see Fig. 1(a)]. This was facilitated by the good mechanical properties of the YAB crystals, including a comparatively high Mohs hardness of 7.5 . Furthermore no scattering defects or crack formation inside the thin-disk crystal were observed by microscopic examination. However, an inspection using a Nomarski interference contrast microscope revealed depolarising areas in the disk [see Fig. 1(b) and 1(c)]. These areas can be identified as the previously reported stacking faults in the lattice of the Yb:YAB crystal by Dekker and Dawes , that would mainly influence the frequency conversion but not the achievable infrared output power. A detailed analysis regarding the influence of the stacking faults on the polarization state of the laser emission as well as on possible depolarization losses, especially in the (polarized) fundamental-mode and mode-locked operation, is under preparation. Furthermore, the on-going progress in crystal growth technology may allow avoiding these defects soon.
Spectrometric measurements (unpolarized incident beam) of the manufactured thin-disk crystals with a thickness of 0.25 mm confirmed the broad absorption peak with > 30 nm width around the central wavelength of 975 nm as described in . This broad absorption spectrum considerably reduces the demands for diode pumping. In combination with the low quantum defect of 6.25% (assuming a pump wavelength of 975 nm and laser emission at 1040 nm), this material altogether offers favorable properties for high power operation.
3. Experimental investigations in high power CW-operation
The thin-disk resonator setup used for the experimental investigations is schematically shown in Fig. 2(a). The resonators comprises of a concave output coupler (T = 2%) with a radius of curvature (ROC2) of 0.5 m and a concave end-mirror with a ROC1 of 0.35 m or 0.5 m depending on the desired beam quality factor. The thin-disk crystal, which serves as a folding mirror in the resonator, had without incident pump radiation a ROCdisk in the order of 4 m (concave). All Yb:YAB thin-disk laser resonators reported here were optically stable. A summary of the resonator cavity parameters is given in Table 1.
Since the Yb:YAB thin-disk crystal, which has a refractive index of naverage ≈1.75, was coated within a coating run optimized for Yb:YAG with n = 1.82, the AR-coating on the disk front-side was not optimal and should be taken into consideration for the results presented in the following. The Yb:YAB crystal was glued with its HR-coated rear face onto copper heat sink. The thin-disk crystal was efficiently cooled by an impingement cooling operated at a water temperature of 15 °C at the back-side of the heat sink. A standard thin-disk pump module available at the IFSW and designed for 24 pump light passes through the disk  was employed to enable efficient pump light absorption.
To examine the power handling capability of Yb:YAB, the temperature of the front-side of the thin-disk crystal was measured as a function of the incident pump power density in both fluorescence and laser operation using a thermal imaging camera. For a pump power density of 2.7 kW/cm2, a maximum surface temperature of 71 °C during fluorescence emission and 56 °C in laser operation was measured [see Fig. 2(b)]. These values, which can be even further reduced by using thinner disks and a CVD diamond heat-sink, confirm the good suitability of Yb:YAB as laser active material for the thin-disk concept.
To prove this consideration further, high-power tests (with an unpolarized, fiber coupled pump diode: Pmax = 1 kW, λ = 975 nm) of the Yb:YAB thin-disk laser with larger pump spot diameters of 2.3 and 2.6 mm were performed. For these experiments an HR end-mirror with a ROC1 of 0.35 m was used in the V-shaped resonator described above. The length of the arm with the output coupler was 0.47 m. With this resonator, a maximum output power of 93.1 W, corresponding to an optical efficiency of 50.7%, at a pump spot diameter of 2.3 mm was reached. By increasing the pump spot size to 2.6 mm the output power was scaled further to 109 W with an optical efficiency of 50.2% [see Fig. 3(a)]. The beam quality factor M2 of these thin-disk lasers was in the range of 50 to 60.
Since the Yb:YAB crystal was damaged during the mounting process, the usable optical aperture of the disk was limited to < 3 mm. The decrease of the optical efficiencies in these experiments can be therefore mainly attributed to the fact that these pump spot diameters converge to the edge of the usable area which is limited by the damaged disk, introducing higher losses for pump and laser radiation. To demonstrate the good quality of the Yb:YAB material itself, we carried out experiments at moderate pump powers with a pump spot diameter of 1 mm. By varying the radius of curvature of the end mirror between 0.35 m and 0.5 m and adapting the length of the resonator arm with the output coupler from 0.33 up to 0.475 m, multimode operation with different M2 values was obtained [see Fig. 3(b)]. At M2 = 10 a maximum output power of 19.3 W was achieved with an optical efficiency of 60.4% corresponding to a slope efficiency of 76.4%. As expected from the oblique cut of the crystal, first measurements showed a high degree of linear polarization for the Yb:YAB thin-disk laser.
For the investigation on fundamental-mode operation the pump spot diameter was further reduced to 0.87 mm. The length of the output coupling arm was now reduced to 0.24 m. The ROC1 of the HR end-mirror was 0.35 m. By doing so the fundamental mode on the thin disk exhibited a diameter of approximately 0.66 mm, which corresponds to about 77% of the pump spot size and prevents the oscillation of higher order modes. The beam quality was measured by a dual-knife-edge beam measurement using a Coherent ModeMaster. The output performance of the Yb:YAB laser together with the M2 values is shown in Fig. 4. At the maximum output power of 10.4 W and an optical efficiency of 44.5%, the laser operated with a beam quality factor M2R of 1.39.
As fundamental-mode operation is the prerequisite for mode-locking, the successful realization of a diffraction-limited Yb:YAB thin-disk laser is the first step towards ultra-short pulse operation in future investigations. Based on the broad fluorescence bandwidth we expect that this should provide pulse durations down to about 200 fs or less in a passively mode-locked thin-disk oscillator.
4. Continuously tunable laser emission of Yb:YAB
The tuning range of the IR laser emission of Yb:YAB was investigated by inserting a mirror based on a grating waveguide structure (GWS) under Littrow configuration  in a multimode (M2 ≈5) V-shaped resonator. The GWS mirror with a diffraction efficiency of about 99.8% in the −1st order served as the output coupler (0.2% of output coupling). The 0.6 m long resonator was completed with a concave HR mirror with a ROC of 1.5 m. The measurements of the emission spectrum were performed with a HR4000 spectrometer from Ocean Optics.
As can be seen in Fig. 5, a wavelength tuning range of 52 nm from 1001 nm to 1053 nm was obtained. For wavelengths above 1053 nm the gain of Yb:YAB strongly decreases and the resonator round-trip losses were too high to maintain laser operation at longer wavelengths. In a free running Yb:YAB thin-disk laser resonator the emitted laser wavelength is at approx. 1040 nm with a FWHM spectral bandwidth of 4 nm (see dashed line in Fig. 5). This spectral bandwidth is reduced to a maximum FWHM of 1 nm when the laser oscillates using a GWS mirror. Output powers higher than 10 W are achieved for spectral range between 1009 nm and 1048 nm.
5. Conclusions and outlook
In conclusion we have demonstrated the suitability of Yb:YAB in the thin-disk laser configuration for the first time. At an emission wavelength of 1040.7 nm, a maximum optical efficiency of 60.3% at an output power of 19.3 W was obtained with a pump spot diameter of 1 mm. In high-power experiments with a pump spot diameter of 2.6 mm output powers exceeding 100 W together with 50.2% of optical efficiency were demonstrated. To the best of our knowledge, this is the highest output power reported to date for a CW Yb:YAB laser. In a near-diffraction-limited operation the output power was 10.4 W with an optical efficiency of 44.5%. Moreover, the large emission bandwidth of Yb:YAB was verified by continuously tuning the emission wavelength from 1001 to 1053 nm using a GWS mirror.
Taking into account the obvious room for further improvements e.g. by applying an optimum AR-coating or using a CVD diamond heat sink, power scaling of the Yb:YAB thin-disk laser to the multi-hundred watt level with high optical efficiencies seems to be within reach in the very near future. As FEM-simulations show, the maximum temperature of the Yb:YAB disk can be reduced by ≈20% when using a CVD diamond heat sink instead of a copper one. Further investigations will concentrate on the test of Yb:YAB thin-disk crystals configured for its use in mode-locked operation.
This work was supported by the German Research Foundation (DFG) within the funding programme Open Access Publishing.
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