High-power Yb : GGG thin-disk laser oscillator : first demonstration and power-scaling prospects

We present the first demonstration of a thin-disk laser based on the gain material Yb:GGG. This material has many desirable properties for the thin-disk geometry: a high thermal conductivity, which is nearly independent of the doping concentration, a low quantum defect, low-temperature growth, and a broadband absorption spectrum, making it a promising contender to the well-established Yb:YAG for high-power applications. In continuous wave laser operation, we demonstrate output powers above 50 W, which is an order of magnitude higher than previously achieved with this material in the bulk geometry. We compare this performance with an Yb:YAG disk under identical pumping conditions and find comparable output characteristics (with typical optical-to-optical slope efficiencies >66%). Additionally, with the help of finite-element-method simulations, we show the advantageous heat-removal capabilities of Yb:GGG compared to Yb:YAG, resulting in >50% lower thermal lensing for thin Yb:GGG disks compared to Yb:YAG disks. The equivalent optical performance of the two crystals in combination with the easy growth and the significant thermal benefits of Yb:GGG show the large potential of future high-power thindisk amplifiers and lasers based on this material, both for industrial and scientific applications. © 2017 Optical Society of America OCIS codes: (140.0140) Lasers and laser optics; (140.3380) Laser materials; (140.3480) Lasers, diode-pumped; (140.3580) Lasers, solid-state; (140.3615) Lasers, ytterbium; (140.6810) Thermal effects. References and links 1. P. Lacovara, H. K. Choi, C. A. Wang, R. L. Aggarwal, and T. Y. Fan, “Room-temperature diode-pumped Yb:YAG laser,” Opt. Lett. 16(14), 1089–1091 (1991). 2. C. Kränkel, “Rare-earth doped sesquioxides for diode-pumped high power lasers in the 1-, 2-, and 3-μm spectral range,” IEEE J. Sel. Top. Quant. Electron. 21, 1602013 (2015). 3. A. Ellens, H. Andres, M. L. H. ter Heerdt, R. T. Wegh, A. Meijerink, and G. Blasse, “Spectral-line-broadening study of the trivalent lanthanide-ion series. II. The variation of the electron-phonon coupling strength through the series,” Phys. Rev. B 55(1), 180–186 (1997). 4. M. N. Zervas and D. A. Codemard, “High power fiber lasers: a review,” IEEE J. Sel. Top. Quantum Electron. 20(5), 0904123 (2014). 5. P. Russbueldt, D. Hoffmann, M. Hoefer, J. Loehring, J. Luttmann, A. Meissner, J. Weitenberg, M. Traub, T. Sartorius, D. Esser, R. Wester, P. Loosen, and R. Poprawe, “Innoslab Amplifiers,” IEEE J. Sel. Top. Quantum Electron. 21(1), 3100117 (2015). 6. A. Giesen, H. Hügel, A. Voss, K. Wittig, U. Brauch, and H. Opower, “Scalable concept for diode-pumped highpower solid-state lasers,” Appl. Phys. B 58(5), 365–372 (1994). 7. V. Kuhn, T. Gottwald, C. Stolzenburg, S.-S. Schad, A. Killi, and T. Ryba, “Latest advances in high brightness disk lasers,” Proc. SPIE 9342, 93420Y (2015). 8. Boeing, “30 kW multi thin disk laser,” http://boeing.mediaroom.com/Boeing-Thin-Disk-Laser-ExceedsPerformance-Requirements-During-Testing, retrieved on 2016/11/24 (2013). 9. J.-P. Negel, A. Loescher, A. Voss, D. Bauer, D. Sutter, A. Killi, M. A. Ahmed, and T. Graf, “Ultrafast thin-disk multipass laser amplifier delivering 1.4 kW (4.7 mJ, 1030 nm) average power converted to 820 W at 515 nm and 234 W at 343 nm,” Opt. Express 23(16), 21064–21077 (2015). Vol. 25, No. 2 | 23 Jan 2017 | OPTICS EXPRESS 1452


Introduction
Increasing the power available from diode-pumped solid-state lasers continues to be a hot topic for both industrial and scientific applications.Reaching high power requires gain materials with excellent thermal properties as well as optimized heat-removal schemes.Most important progress in this area was triggered by the pioneering experiments on laser operation of Yb:YAG [1].Yb 3+ (Yb-)doped gain materials have many spectroscopic advantages for high-power operation [2].They exhibit a small quantum defect, which results in inherently excellent thermal management.Their simple two-manifold energy level structure avoids detrimental loss processes like up conversion, cross relaxation, and excited-state absorption.Combined with the host material YAG, the relatively strong coupling of the electronic 4f-4f transitions to the phonons of the host lattice leads to broad absorption lines between 920 nm and 980 nm [3].This enables optical pumping with cost-efficient high-power laser-diode arrays and requires no additional external wavelength stabilization.With all these favorable properties Yb-doped materials have been at the forefront of most high-power continuous wave (cw) laser developments achieved in the last decades.Fiber [4], slab [5], and thin-disk [6] geometries have been used to push the available frontiers to previously unimaginable levels.
Among these technologies, thin-disk lasers [6] are particularly well suited for applications requiring excellent spatial and spectral beam quality.In this geometry, the gain medium is shaped as a disk with typical apertures of >1 cm and thickness in the order of 100 μm and used in reflection inside the cavity.The short gain length and therefore low single-pass pump absorption is compensated with a multi-pass pumping scheme.The large spot size versus thickness results in a quasi-1-D heat flow and extremely reduced thermal aberrations, which enables power scaling by increasing the mode area and the pump power accordingly.In cw operation, more than 8 kW of power were achieved from a multimode (M 2 >8) thin-disk laser (TDL) with one single disk as gain medium [7].Furthermore, up to 4 kW near single fundamental mode operation (M 2 >1.38) have recently been demonstrated [7].In a more complex layout, 30 kW of nearly diffraction-limited output power has been achieved [8] using ten disks in one resonator.
One important advantage of the thin-disk laser concept is the low cavity round-trip nonlinearity which is even more important for ultrafast pulse generation with very high peak power.The most powerful ultrafast laser demonstrated to date is based on a multipass thindisk amplifier generating an average output power of 1.4 kW at a pulse repetition rate of 300 kHz with 8 ps pulses [9], which has been even improved to 1.9 kW of average power most recently [10].Regenerative amplifiers based on TDLs have also made large progress in the last few years [11,12].For even higher megahertz pulse repetition rates modelocked ultrafast thin-disk laser oscillators are very attractive, as they generate hundreds of watts of average power directly from a compact oscillator with excellent beam quality (M 2 = 1) and low-noise properties [13][14][15].
All of these record-performance high-power thin-disk sources are based on Yb:YAG.Apart from the advantages highlighted above, this gain material exhibits an attractive combination of spectroscopic and thermo-mechanical properties ideally suited for the thindisk geometry.The high hardness enables high-quality thin-disk preparation.Yb:YAG has good heat-removal capabilities at low doping concentrations and a relatively high absorption cross section, which is beneficial for efficient laser operation with thin gain media.Yb:YAG is easily grown in large sizes using the standard Czochralski method.Furthermore, for ultrafast applications, Yb:YAG offers an attractive combination of high emission cross section and a sufficiently broad bandwidth to support ultrashort high-power pulse durations of down to several 100s of fs with passive modelocking [13,15].Further power scaling would benefit from some additional properties.In particular, the disk thicknesses will need to be further decreased for efficient heat removal and reduction of thermal aberrations.This requires high doping concentrations for sufficient absorption [16][17][18].The thermal conductivity κ of Yb:YAG, however, drops significantly with increased disk doping concentration (9 W/m/K at 2 at.% and 6 W/m/K at 10 at.% [19]).In recent years, remarkable research efforts were carried out to find new host materials for the Yb ion specifically for high-power operation in the thin-disk geometry [20][21][22][23].A particularly promising alternative is the mixed sesquioxide Yb:Lu 2 O 3 , which exhibits a thermal conductivity that stays almost constant up to highest doping concentrations (12 W/m/K) [2].Additionally, its emission spectrum is broader than Yb:YAG's, thus supporting shorter pulse durations [13].Unfortunately, because Yb:Lu 2 O 3 's melting temperature is 500°C higher than Yb:YAG's (2450°C versus 1940°C), it can only be grown with the complex and costly heatexchanger method [2].Furthermore, its narrow absorption spectrum requires wavelengthstabilized diodes.Therefore, Yb:YAG stays the preferred material in commercial and lowcost applications due to the possibility to grow large crystals in reproducible and excellent optical quality by the well-established Czochralski technique.
In this paper, we investigate the potential of another promising gain crystal for the highpower thin-disk geometry, Yb:Gd 3 Ga 5 O 12 (Yb:GGG).Table 1 summarizes the key material properties in comparison to Yb:YAG.The most striking feature of the Yb:GGG gain material is its thermal conductivity κ of 7.8 W/m/K, which remains nearly independent of doping concentration up to >20 at.%, and is higher than Yb:YAG at doping concentrations >4 at.% [19,24,29].Additionally, the quantum defect of Yb:GGG can be 20% lower than the one of Yb:YAG.The other spectroscopic characteristics of Yb:GGG are similar to Yb:YAG, such as a broadband absorption spectrum, a sufficiently wide emission spectrum, and favorable quenching behavior.The material hardness of Yb:GGG is 7.5 Mohs, which is still sufficiently high for thin-disk preparation and very close to Yb:YAG (8.5 Mohs, Table 1).Furthermore, the low melting temperature of 1750°C and large segregation coefficient of rare earth ions allows for the large-scale Czochralski fabrication method.Diameters as large as 190 mm have already been demonstrated.Therefore, the new gain material Yb:GGG is particularly attractive for further power scaling of thin-disk laser sources, potentially replacing Yb:YAG in the future.
So far, this promising gain material had not been tested in the thin-disk configuration.Results of diode-pumped Yb:GGG were only achieved in the bulk geometry and at moderate power levels <6 W [24,30], both in cw and Q-switched configuration.Here, we present the first demonstration of an Yb:GGG thin-disk laser.With the help of finite-element-method (FEM) simulations we show the advantageous heat-removal capabilities of Yb:GGG over Yb:YAG, resulting in >50% lower temperature rise and thermal lensing for thin Yb:GGG disks compared to Yb:YAG disks.Furthermore, we demonstrate the mechanical suitability of this crystal for all the steps required for thin-disk mounting.In transverse multimode cw operation, we scale the previously achieved output powers up by an order of magnitude to >50 W. Additionally, we compare this performance with an Yb:YAG disk under identical pumping conditions and find similar output characteristics.The equivalent optical performance of the two crystals in combination with the easy growth and the significant thermal benefits of Yb:GGG shows the large potential of high-power thin-disk amplifiers and lasers based on this material, both for industrial and scientific use.
This paper is organized as follows.In section 2 we give an overview of the key parameters of Yb:GGG in comparison to Yb:YAG.We then describe the growth and contacting process of our crystal.Furthermore, we add simulations based on the rate equations for quasi-three-level systems and FEM to demonstrate the thermal-lensing benefits of Yb:GGG over Yb:YAG in future power-scaled thin-disk lasers.Section 3 highlights the results of our high-power lasing experiments.We then summarize our results in the final section 4 and provide an outlook.

Yb:GGG crystal properties, growth, and thin-disk fabrication
In this chapter, we provide an overview of the most important spectroscopic and thermomechanical properties of Yb:YAG and Yb:GGG, in particular for high-power thin-disk laser operation.These parameters are summarized in Table 1.
Gadolinium Gallium Garnet (GGG) crystallizes in the cubic structure in the Ia3d space group, just as YAG.During doping with Yb 3+ ions, the dopant and the replaced Gd 3+ ions have the same charge state and comparable ionic radii, therefore the dopants do not disturb the crystal lattice and do not alter the crystal properties [31].Additionally, thanks to the larger lattice parameter of GGG compared to YAG (12.38 Å versus 12.01 Å), the average distance between Yb ions in GGG crystals is larger than in YAG crystals for the same molar concentration.This reduces non-radiative transition effects such as cooperative luminescence caused by ion-ion interaction and decreases the degree of lattice distortion defects after Yb doping.In particular, this makes the GGG crystal matrix more favorable for high Yb doping levels compared to YAG.Namely, Yb:GGG's high thermal conductivity κ remains nearly independent of doping concentration up to >20 at.%, in contrary to Yb:YAG [19,24].
Thanks to their similar host-crystal structures, the spectroscopic characteristics of Yb:GGG are comparable to Yb:YAG [24,25], see Fig. 1.In particular, both Yb:YAG and Yb:GGG have broadband absorption plateaus around 940 nm, which allow for cost-effective pumping with non-wavelength-stabilized diode arrays.From the zero-phonon line and the maximum emission wavelength it follows that the quantum defect of Yb:GGG can be up to 20% lower than the one of Yb:YAG, resulting in an overall significantly reduced thermal load during lasing.The Yb:GGG crystal used for our experiments was grown by the Czochralski method.This allows for straightforward and large-scale growth with Iridium crucibles, which could enable large-volume and commercial growth in the future.The polycrystalline Yb:GGG material was synthesized by a conventional solid-state reaction at 1280°C for 48 h.The starting material had an Yb 3+ concentration of 10 at.% and was melt completely in an Iridium crucible.The single crystal was grown in a RF-induction-heating Czochralski furnace.A pure GGG crystal rod with <111> orientation was used as seed.The pulling and the rotation rates were set to 0.8 mm/h and 15 rpm, respectively.After growth, the obtained crystal was annealed in air at 1400°C for 40 h to release thermal stress.In conclus becomes serio TDLs can po thermally-ind advantage for

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In order to c thickness) wi diameter, 100 Trumpf.
We set up pumped the d wavelength r absorption cr absorbed pum disks were te curved output due to increas ncrease compa distance of 10 cm from the disk, with OC rates ranging from 0.4% to 1.8% as summarized in Table 2.We measured the beam quality factor M 2 of the output beam to be ~60.We achieved the highest optical-to-optical efficiencies of 53% (Yb:GGG) and 55% (Yb:YAG), relative to an absorbed power of 100 W, with an output coupler of 1.3% [Fig.5(a)].The corresponding slope efficiencies, relative to the absorbed power, are 67% (Yb:GGG) and 66% (Yb:YAG), see Table 2.These values stand in good agreement with the simulations for quasi-three-level TDL systems [32] as introduced earlier.The resulting maximum output power was 51 W for the Yb:GGG crystal, and 53 W for the Yb:YAG crystal.For these results we calculated [32] the loss of the optical disk for a single pass and obtained <0.2% for both Yb:YAG and Yb:GGG, which are typical values for state-of-the-art coated thin disks with high optical quality.Additionally, we experimentally verified the lower emission wavelength for Yb:GGG compared to Yb:YAG (see Table 1), ultimately decreasing the quantum defect by up to 20%.The comparable cw multimode performance obtained with these two crystals shows the huge potential of Yb:GGG for high-power operation in the thindisk geometry.
To investigate the thermal behavior, we used a thermal camera (FLIR SC640) to measure the temperature increase of the surface of the disks under pumping in fluorescence mode (i.e.without cavity feedback and lasing conditions).The transverse spatial resolution of our thermal camera measurements was around 250 μm.In general, we expect to find a higher temperature in fluorescence mode of operation than in laser operation due to non-radiative effects such as concentration quenching or growth defects and impurities [33,34].These nonradiative effects are quantified with the radiative quantum efficiency η r [33,34], i.e. a high η r is an indicator for small non-radiative effects.For high-quality crystals, η r is in the same range for both Yb:YAG and Yb:GGG (~70%-80% for 10-at.%-dopedYb:YAG and Yb:GGG [37]).Using this knowledge and combining it with our COMSOL simulations, we found excellent agreement between measurement and simulation for our Yb:YAG disk [Fig.5(b)].
In our experiment, we measured an unexpectedly high temperature rise for our Yb:GGG disk, which ultimately led to damage of the disk.The experimental temperature increase corresponds to an increased heat fraction caused by a low η r of about 50%.It should be noted that in laser operation far above threshold, the non-radiative channels are suppressed in favor of the radiative ones [37], which explains the good optical behavior of our Yb:GGG disk in lasing conditions [Fig.5(a)].which is material's this disk compared to a similar Yb:YAG disk at the same pumping intensity.This can be explained by growth-induced impurity defects, which can be greatly improved in future growth runs.
Our goal for this paper was to compare this material with a state-of-the-art Yb:YAG disk, therefore we used a doping concentration of 10 at.%.In future experiments, we will increase the doping concentration to >15 at.%, which should support efficient laser operation using thinner disks of around 50 µm with outstanding thermal properties.Additionally, we will investigate the behavior of Yb:GGG in single-fundamental-mode thin-disk operation.We expect this material to to reach several 100s of W cw output powers in the thin-disk configuration in the near future using thinner disks with high doping from improved growth runs.Furthermore, it is also a potentially interesting candidate for high-power picosecond lasers for precision micromachining applications.

Fig. 1
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