We report on the generation of continuous-wave, intra-cavity frequency-doubled, multi-mode laser radiation in an Yb:LuAG thin-disk laser. Output powers of up to 1 kW at a wavelength of 515 nm were achieved at an unprecedented optical efficiency of 51.6% with respect to the pumping power of the thin-disk laser. The wavelength stabilization and spectral narrowing as well as the polarization selection, which is necessary for a stable and efficient second-harmonic generation, was achieved by the integration of a diffraction grating into the dielectric end mirror of the cavity, which exhibits a diffraction efficiency of 99.8%. At a frequency-doubled output power of 820 W the peak-to-valley power fluctuations measured during 100 minutes of laser operation amounted to only 8.2 W (1.0%). The beam parameter product of the frequency-doubled output was 3.4 mm·mrad (M2 ≈ 20), which is suitable for standard beam delivery using fibers with a core diameter of 100 µm and a NA of 0.2.
© 2017 Optical Society of America
Efficient sources of high-power green laser beams have been of increasing interest in the last few years. Driven by the emergence of the e-mobility sector and renewable energy sources, material processing of copper has been playing an important role for high-power electric circuitry and battery technology. The higher absorptivity of copper at room temperature (ca. 35%) in the green spectral range in comparison of its absorptivity of infra-red radiation (ca. 5%) makes laser processing of copper less susceptible to variations in the surface conditions and less challenging to control [1,2]. Although there are many more applications e.g. pumping of Ti:Sa-oscillators and amplifiers  or the generation of UV radiation , there are currently no efficient sources for the direct generation of green light with sufficient output power to address all these applications. Hence, high-power green laser beams are commonly generated by frequency doubling the infrared radiation provided by solid-state lasers.
Frequency doubling by second-harmonic generation (SHG) can either be performed outside (extra-cavity) or inside (intra-cavity) a laser cavity. For the extra-cavity approach, a high frequency conversion efficiency of about 70% has recently been demonstrated in the ps regime with up to 820 W of average output power . In this case pulsed operation guarantees for the high intensities, necessary for a sufficiently high SHG conversion efficiency. For continuous-wave operation, the highest output power achieved by extra-cavity SHG reported so far was 350 W of green radiation by frequency doubling a 1 kW single-mode fiber laser with a wavelength of 1030 nm .
For intra-cavity SHG the achieved output is limited by losses and thermo-optical issues caused by the additional intra-cavity elements, which are necessary to obtain stable phase-matching conditions. In addition, the conversion process directly influences the gain-loss-balance of the infrared beam inside the cavity, which makes the design of intra-cavity frequency-doubled laser systems more demanding. On the other hand, the radiation intensity of the fundamental beam is much higher inside the laser cavity which means that several hundred watts of frequency-doubled radiation can easily be achieved with conversion efficiencies as low as 2% to 6% which again conveniently corresponds to the typical output coupling of efficient thin-disk lasers.
So far, the intra-cavity SHG approach led to the generation of up to 1.8 kW of green average output power in a Q-switched thin-disk laser emitting pulses with duration in the order of 100 ns . Just recently, S. Pricking et al. demonstrated SHG with 8 kW of pulse peak power for a pulse duration of 1 ms and a repetition rate of 100 Hz as well as 6 kW of pulse peak power at 10 ms pulse duration and a repetition rate of 10 Hz . The same publication reports on 1.7 kW of multimode green radiation in continuous-wave operation but without information about cavity design and the used approach for frequency and polarization control. The optical efficiency with respect to the diode pump was stated to be 45% at 1 kW of green output power, decreasing towards around 40% at the maximum output power of 1.7 kW.
In the present paper we report on the detailed requirements for efficient SHG inside a thin-disk laser cavity, including the cavity design and the used optical elements for spectral and polarization control, and discuss the resulting laser performance. Compared to the previous report  which was devoted to diffraction-limited output the main focus of the present work was to investigate the suitability of the approach and the involved key components for power scaling into the kW power level. In order to scale the output and the efficiency of intra-cavity frequency-doubled continuous-wave thin-disk lasers to the kW level we used grating-waveguide mirrors for the frequency and polarization selection, as these elements are very efficient and withstand high power densities. At a frequency-doubled green output power of 1016 W the optical efficiency was 51.5% with respect to the diode pumping, which to the best of our knowledge is the highest value reported to date.
2. Experimental set-up
For stable and efficient frequency conversion additional elements have to be implemented into a laser resonator to reduce the spectral bandwidth and to generate a linear polarization state . Compared to the use of conventional intra-cavity elements (e.g. thin-film polarizers, etalons and brewster plates) a grating waveguide mirror (GWM) , as used in our experiments, has the advantage of combining the wavelength and polarization selection in one single reflective device and therefore reducing both round-trip losses as well as thermally induced effects.
The GWM was implemented as one of the cavity end mirrors and operated in Littrow configuration, where the −1st diffraction order is fed back into the resonator. Due to the dispersion of the grating, the Littrow angle exhibits a strong wavelength dependence and is therefore suitable for the intra-cavity wavelength selection. Furthermore, being polarization selective by design, the use of the GWM leads to a pure linear polarization of the intra-cavity laser beam. Further details about the principle of GWMs can be found in .
For an efficient laser operation the diffraction efficiency of the GWM has to be comparable to the reflectivity of standard HR-mirrors. The diffraction efficiency of the GWM was therefore measured and compared to the values calculated during the grating design process based on the method described in , as shown in Fig. 1. The angle of incidence to fulfill Littrow condition at the corresponding wavelength was adapted for each data point. At a wavelength of 1030 nm and a grating period of 619 nm the Littrow angle θL is 56.3°.
The diffraction efficiency for TE-polarized (electrical field parallel to the grating lines) radiation in the −1st diffraction order was measured to be 99.8 ± 0.1% for wavelengths ranging between 1010 nm and 1040 nm, whereas TM-polarized (electrical field perpendicular to the grating lines) radiation suffers lower reflectivity of only about 16% at 1030 nm. This leads to the suppression of TM-polarized modes in the resonator and consequently to the generation of a highly linearly TE-polarized beam. A small discrepancy between the simulated and measured reflectivity curves can be observed in Fig. 1. Imperfections in the manufacturing process are assumed to be the reason. The benefits of such GWM’s have already been reported in  for the generation of near-diffraction limited green laser beams at 400 W of output power.
The thin-disk laser cavity with a non-linear crystal and the implemented GWM is schematically shown in Fig. 2(a). An Yb:LuAG disk with a thickness of approximately 130 µm, a doping concentration of about 10 at. % and a diameter of 15 mm was used throughout all experimental investigations. Yb:LuAG was used since it exhibits enhanced thermal properties as compared to Yb:YAG . The disk provided by TRUMPF Laser GmbH was mounted on a diamond heat sink for efficient heat extraction. The radius of curvature of the thin-disk was measured to be 3.6 m and the pump spot diameter on the disk was set to 7.2 mm. In order to reduce the thermal load in the laser crystal, a spectrally stabilized laser diode provided by DILAS Diodenlaser GmbH with a maximum pump power of 2 kW was used to pump the laser crystal at the so-called zero-phonon-line of Yb:LuAG i.e. at a wavelength of 969 nm .
The resonator comprises two plane cavity end mirrors (one being the GWM) and three curved mirrors (convex 5 m, HR 1030 nm; concave 2 m, HR 1030 nm; and concave 0.5 m, dichroic HR 1030 nm / AR 515 nm) in order to reduce the beam radius by a factor of about 7 from the end of the resonator comprising the disk and the GWM to the end where the nonlinear crystal is placed. The calculated radius of the oscillating beam throughout the resonator is shown in Fig. 2(b) assuming a beam parameter product of 3.4 mm·mrad (M2 ≈ 10) by design. The total resonator length is 4.14 m. The beam radius at the conventional plane end mirror is about 500 µm. At the other end of the resonator, the generated beam is collimated between the disk and the GWM (with a beam radius of about 3.6 mm).
3. Infra-red lasing performance
In a first step, the performance of the resonator was evaluated both without SHG crystal and without GWM. For this purpose the plane end mirror (HR 1030 nm and HR 515 nm) was replaced by a plane output coupler with a transmission of 4.3%. The GWM was replaced by a plane HR end mirror. As shown by Fig. 3(a) the laser emitted up to 1295 W of (IR) power with an optical efficiency of 66.6% at a maximum incident pump power of 1.95 kW. Two main spectral ranges of emission at around 1030.1 nm and 1031.4 nm were observed with a total spectral width of approximately 1.2 nm < λ < 1.6 nm, Fig. 3(b).
When the GWM was implemented as cavity end mirror, the laser emitted up to 1200 W of (IR) output power with an optical efficiency of 61.7% at a maximum pump power of 1.95 kW. The comparison shows that the optical efficiency is only moderately reduced by about 4.9% points when using the GWM. This slight drop can essentially be attributed to the additional intra-cavity losses introduced by the GWM since its diffraction efficiency in the −1st diffraction order was measured to be 99.8 ± 0.1%, which is slightly lower than the reflectivity of commercially available high-power cw HR mirrors (99.98%). Furthermore depolarization losses in the range of 0.1% to 0.2% per round-trip contribute to lowering of the laser performances. This sums up to about 0.4% of additional losses which, assuming 0.75% of further intra-cavity losses in both resonator configurations, is consistent with the observed reduction of the output power by 95 W.
This minor reduction in power is fully acceptable in view of the significant advantage that with the GWM the laser output is linearly polarized and the spectral emission is narrowed to a single peak with a bandwidth of Δλ = 185 pm (FWHM) centered at a wavelength of 1030.18 nm, as can be seen from Fig. 3(b). The degree of linear polarization of the emitted beam was measured to be > 99.9% which was measured with a commercially available 1D-polarimeter. The beam quality factors in the two planes of symmetry were measured to be (h = horizontal, drawing plane in Fig. 2) and (v = vertical) at 1.24 kW of (IR) output power and is consistent with the value expected from the resonator design (see Fig. 2). The slight astigmatism is introduced by the tilted spherical folding mirrors (angle of incidence = 5°) of the cavity.
From the measured output power one can derive the circulating intra-cavity power Pω byFig. 4 for both resonator configurations either with a conventional HR end mirror or the GWM. At the maximum pump power of 1.95 kW the intra-cavity powers were 29.5 kW and 27.3 kW for the resonator with the HR end mirror and the GWM, respectively.
4. Intra-cavity SHG performance
After the confirmation of the stable spectral narrowing and polarization selection obtained by means of the GWM, a lithium-triborate (LiB3O5) crystal was introduced into the resonator (as shown in Fig. 2) for intra-cavity SHG. The LBO crystal provided by Crystal Laser S. A. was cut for type I critical phase-matching at a temperature of 110°C (ϕ = 10.1°, θ = 90°). The temperature of the LBO crystal was actively controlled. The size of the crystal was (6 x 6) mm2 and 12 mm in length. The surfaces of the LBO crystal were AR coated for both wavelengths of 1030 nm and 515 nm. The high power densities in the LBO crystal which are necessary in order to obtain the desired conversion efficiency were achieved by designing the cavity in such a way that the beam radius at the position of the LBO crystal was wLBO = 525 µm.
A plane mirror, HR coated for both the wavelengths of 1030 nm and 515 nm, was placed at a distance of 40 mm from the non-linear crystal. This allows for a second pass of both the fundamental and the frequency-doubled wave through the nonlinear crystal increasing the conversion provided that the distance between non-linear crystal and the mirror fulfills the phase-matching condition which is influenced by the dispersion in air as well as the coating of the end mirror [15,16].
To calculate the output power of the frequency-doubled beam P2ω we used the equations given by 
Here, l is the length of the double-pass in the non-linear crystal (twice the geometrical length), w is the beam radius, Δk is the thermal, angular or spectral phase-mismatch, deff is the effective non-linear coefficient (deff ≈ 8.31 · 10−13 m/V for LBO), c is the speed of light in vacuum, λ0 is the wavelength of the fundamental beam, n0 is the refractive index of the fundamental beam and ϵ0 is the permittivity of free space. The conversion efficiency is given by
A first estimation of the conversion efficiency in the case of perfect phase matching (Δk = 0) in the LBO crystal can be performed by considering the value of Pω presented in the previous section. For the maximum of Pω = 27.3 kW measured with the GWM implemented in the laser cavity, Eqs. (2) and (4) lead to a maximum conversion efficiency of ηSHG ≈ 4.1%. This corresponds well with the optimum output coupling (≈ 4.7%) for the fundamental laser oscillation of the considered thin-disk laser resonator. The optimum output coupling ratio was evaluated based on the rate equations for quasi-three-level thin-disk laser systems .
The performance of the frequency doubled laser is shown in Fig. 5. Up to 1016 W of green laser radiation was generated at an unprecedented overall optical efficiency of 51.6% with respect to the pump power of the thin-disk laser. Already at 500 W of output power (corresponding to 990 W of pump power) the frequency-doubled laser operated at 50% optical efficiency. At the limit of the available pump power, the slight onset of thermal roll-over could be observed. Therefore, further power scaling is limited for this particular setup. The beam parameter product of the SHG output was measured to be 3.4 mm·mrad (M2 ≈ 20 at 515 nm) at maximum output power. A non-negligible residual absorption by the bulk crystal material was observed. This lead to a thermally induced phase-mismatch which had to be compensated for by adjusting the temperature of the crystal oven. At maximum pump power the temperature of the oven had to be reduced by ΔT = 10.8°C. The absorption of the bulk crystal material (LBO) in the IR and green spectral range was estimated by the supplier to be ≈ 1.5 · 10−5 1/cm.
The measurement of the output power stability was performed at a (green) output power of 820 W (1.57 kW of pump power), which corresponds to the operation point with the maximum achieved overall optical conversion efficiency of 52.5%. The result is shown in Fig. 6.
An average output power of 820.6 W with a peak-to-valley deviation of only 8.2 W was observed over a time of 100 minutes. This corresponds to a total power fluctuation of ± 0.5%. The standard deviation of the recorded data was calculated to be ± 1.2 W (corresponding to ± 0.2% with respect to the average output power). The noticeable oscillation of the output power, which was noticed both in the fundamental and the second harmonic beam, is assumed to be caused by the temperature controller of the non-linear crystal. Figure 7(a) shows the evaluated power Pω available for the SHG process, as measured through one of the folding mirrors and using Eq. (1).
A maximum value of 31.9 kW was measured. The parabolic rise of the green output power expected from Eq. (2) is qualitatively seen in Fig. 7(b) where the (green) output power and the overall optical efficiency with respect to the circulating intra-cavity power Pω are shown. Figure 7(c) shows the fraction of power which is the reflected into the 0th diffraction order of the GWM and which therefore is not fed back into the resonator. At the maximum output power this reflection amounted to 59.4 W of power. Related to Pω this corresponds to a diffraction efficiency of the grating of ≈ 99.8% into the −1st diffraction order. We believe that the decrease of the grating efficiency might be attributed to increased depolarization with increasing oscillating power. Figure 7(d) shows the SHG conversion efficiency evaluated with respect to Pω. The maximum conversion efficiency was about 3.2% and therefore somewhat lower than the theoretical value used for the design of the set-up and explains the slightly higher maximum oscillating power Pω as compared to the experiments with the fixed output coupling of 4.3% and without SHG presented in the previous section.
In principle, an increase of the conversion efficiency and output power, respectively, could be achieved by decreasing the beam diameter on the LBO surface. However, this would lead to an increased risk of thermally induced damage. Another possibility to further scale the output characteristic would be the reduction of the residual absorption of the bulk crystal material for the fundamental and frequency-doubled wave leading to a lower thermally induced phase-mismatch. This will be the suspect of our further investigation.
We have demonstrated an intra-cavity frequency-doubled continuous-wave thin-disk laser providing green laser radiation with an output power of up to 1 kW with an unprecedented overall optical efficiency of 51.6% (with respect to the power provided by the pumping diodes). The wavelength stabilization and spectral narrowing as well as the polarization selection were achieved by the use of a grating waveguide mirror (GWM) implemented as one of the cavity end mirrors. The GWM was operated in Littrow configuration and exhibited diffraction efficiencies exceeding 99.8%. At a (green) output power of 821 W the overall optical efficiency was 52.5%. To the best of our knowledge, this is the highest efficiency for high-power intra-cavity frequency-doubled continuous-wave solid-state laser system presented to date. With a peak-to-valley deviation of 8.2 W within a duration of 100 minutes of continuous operation, the laser proved to be very stable. The beam parameter product of the frequency-doubled output was 3.4 mm·mrad (M2 ≈ 20), which is suitable for standard beam delivery using fibers with a core diameter of 100 µm and a NA = 0.2. Further optimization and investigation on power scaling will be subject of future experiments.
European Union Seventh Framework Programme [ICT-2013.3.2- Photonics] (619177 - Project TiSa-TD, www.tisa-td.eu). In addition, this work was supported by the German Research Foundation (DFG) within the funding program Open Access Publishing.
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