In recent years, the development of rare-earth-doped sesquioxides for use as laser materials has made significant progress. The development of Yb-doped lutetium oxide (Yb:Lu₂O₃) has attracted particular attention due to its beneficial combination of spectral and thermal properties, making it a promising material for scaling the average power of a continuous-wave (cw) thin-disk laser (TDL) oscillator as well as for the average and peak power scaling of ultrafast TDL oscillators and amplifiers. As a first step, the scaling of the cw average power of a TDL oscillator with Yb:Lu₂O₃ is reported in the present work.

With a comparatively large emission bandwidth of 13 nm full width at half-maximum (FWHM) [1], Yb:Lu₂O₃ supports the amplification of pulses with durations of less than 100 fs in mode-locked TDL oscillators and TDL amplifiers. This is the basis for achieving high peak powers with subpicosecond lasers. With regard to the scaling of the average output power of a TDL, important parameters of the laser medium are the thermal conductivity and the quantum defect. In comparison to the established material Yb:YAG with a thermal conductivity of 7 W/mK, the thermal conductivity of Yb:Lu₂O₃ amounts to 12 W/mK, and is therefore almost twice as high (both values are for an Yb-ion density of 8 × 10²⁰ cm^-3) [2]. In addition, the quantum defect of Yb:Lu₂O₃ pumped at a wavelength of 976 nm is 5.6% and therefore slightly smaller than the quantum defect of Yb:YAG, 5.9%, when pumped at a wavelength of 969 nm [1]. Consequently, the fractional heat load of Yb:Lu₂O₃ can be lower and the generated heat can be removed more efficiently than with Yb:YAG.

Figure 1 shows the output powers of Yb:Lu₂O₃ TDL oscillators in cw multimode operation that have been published in recent years. For single crystals (black dots), one of the highest slope efficiencies, 85%, was published by Peters et al. in 2011 [2] for a 2 at. %-doped single-crystal Yb:Lu₂O₃ disk with an output power of 301 W. Further power scaling was limited by the small diameter of the disk: 5 mm. The highest average output power demonstrated with Yb:Lu₂O₃ single crystals was reported by Weichelt et al. in 2012 [5]; the output power was 670 W and the slope efficiency was 80% when using a 5 at. %-doped disk with a diameter of 7 mm. The comparatively small size of the disk in combination with edge defects led to permanent damage to the disk, preventing further power scaling. An overall limiting factor on the average output power achieved with Yb:Lu₂O₃ is the limited availability of high-quality and large-size single crystals. The main reason for this is the high melting temperature of Yb:Lu₂O₃, 2450 °C, which is a challenge in the conventional growth of single crystals as it requires special techniques such as the heat exchanger method [10]. Alternatively, Yb:Lu₂O₃ can be produced as a polycrystalline transparent ceramic, profiting from sintering temperatures of < 1700 °C [11], which are far below the melting temperature. The results achieved in terms of average output power in cw operation for Yb:Lu₂O₃ ceramics are shown in Fig. 1 (red triangles). The highest slope efficiency of a TDL with ceramic Yb:Lu₂O₃ was published by Sanghera et al. in 2011 [7], and amounted to 74% at an output power of 16 W using a 10 at. %-doped disk with a diameter of 3 mm. So far, the highest output power achieved with ceramic Yb:Lu₂O₃ was reported by Kitajima et al. in 2017 [9]. The authors obtained an average power of 174 W with a slope efficiency of 54% using a 3 at. %-doped Yb:Lu₂O₃ ceramic thin disk with a diameter of 6.3 mm.

 

Fig. 1. Progress in the cw multimode average TDL output power obtained with Yb:Lu₂O₃ single crystals (black dots) and polycrystalline ceramics (red triangles) in recent years (single crystals [2–5] and ceramics [6–9]).

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In the present letter, we report on the scaling of the average output power of a TDL oscillator up to the kilowatt level using a polycrystalline Yb:Lu₂O₃ ceramic.

A 4 at. %-doped Yb:Lu₂O₃ polycrystalline ceramic disk was used. The ceramic was fabricated using a combination of vacuum sintering and hot isostatic pressing. The powder synthesis and sintering process were similar to those described in [11]. The transparent ceramic raw material with a diameter of 11 mm and a thickness of 2 mm was polished down to a thickness of 120 µm and was coated to make it antireflective at its front face and highly reflective at its rear face at both the pump and laser wavelengths. The thin disk was glued onto a water-cooled diamond heat sink with a minimum radius of curvature (RoC), R_minor, of 3.7 m and a maximum RoC, R_major, of 3.9 m (in two planes of symmetry). The disk was mounted in a multi-pass pumping module allowing for 24 passes of the pump light through the crystal. The diameter of the pump spot was set to 5.5 mm. The pump was a fiber-coupled diode laser delivering up to 2 kW of optical output power. The emission spectrum of the pump laser was wavelength stabilized using volume Bragg gratings. The central wavelength of the pump radiation was measured to range from 976.3 nm at low powers to 977 nm at the maximum output power, and the spectral bandwidth was measured to be 0.32 nm FWHM over the whole power range. This ensures a sufficient overlap with the absorption line of Yb:Lu₂O₃, which is centered at 976 nm and has a linewidth of 2.9 nm FWHM [2]. The absorption of the pump light after 24 pump passes was estimated by inserting a thin uncoated glass plate at an angle of 45° into the beam path outside the pump module (see Fig. 2) in order to record the power launched into the pumping module as well as the power leaving the pumping module. For this analysis, the overall losses of the operation.

 

Fig. 2. Setup of the TDL in multimode operation: ceramic Yb:Lu₂O₃ disk (green) in a multi-pass pumping module and a V-shaped laser cavity formed by a plane output coupler (OC) and a concave end mirror. Water-cooled power meters (PMs) are depicted in blue.

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The experimental setup for the laser experiment in multimode operation is shown in Fig. 2. A V-shaped laser cavity was formed by a concave end mirror with a RoC of 3 m, the ceramic Yb:Lu₂O₃ disk acting as a folding mirror, and a plane output coupler (OC). The optimum output coupling was experimentally determined to be 4%. The resonator was designed to support multiple transverse modes, and the beam propagation factor of the output beam was measured to be M²= 15 in both the horizontal and the vertical planes.

Figure 3 shows the measured output power and the optical efficiency as a function of the pump power. The pumping of the ceramic disk was limited by the available pump power to a power of 1975 W. A maximum laser output power of 1190 W was reached with an optical efficiency of 60.3% with respect to the pump power. The slope efficiency was measured to be 65% and the laser threshold was reached at a pump power of 122 W. The emission spectrum at the maximum laser output power is shown in the inset in Fig. 3 and was found to be centered around a wavelength of 1034 nm.

 

Fig. 3. Output power (black dots), linear fit (black dashed line), and optical efficiency (red triangles) versus pump power (launched into the pumping module) in cw multimode operation. The inset shows the laser emission spectrum measured at the maximum output power.

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Compared with previously published results with Yb:Lu₂O₃ ceramics, the average output power was increased by a factor of approximately seven; compared with single crystals, the output power could almost be doubled (see Fig. 1). Yet, the slope efficiency of 65% reported herein is lower than the record slope efficiencies for ceramics (74%) [7] and single crystals (88%) [2], indicating the presence of scattering losses arising from the ceramic material itself.

The surface temperature of the thin ceramic disk of Yb:Lu₂O₃ was monitored during laser operation by means of a thermal camera. Figure 4 shows the peak surface temperature as a function of the pump power. At the maximum pump power of 1975 W, the peak temperature of the disk was measured to be 132 °C. The inset shows the thermal image recorded at the maximum pump power. It is worth noting that no hot spot or thermal degradation was observed during this high-power operation, in contrast to what has been observed in experiments conducted with single-crystalline Yb:Lu₂O₃ [12].

 

Fig. 4. Maximum temperature on the surface of the Yb:Lu₂O₃ ceramic disk versus the pump power (launched into the pumping module) measured during cw multimode operation. The inset shows the thermal image recorded at the maximum pump power. The edge of the pump spot is marked by a dashed circle for reference.

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The laser cavity used for the fundamental-mode experiment is shown in Fig. 5, and was formed by the ceramic Yb:Lu₂O₃ disk, a plane OC (4% transmission), two plane folding mirrors, and a concave end mirror with a RoC of 1 m. This cavity was optimized for fundamental-mode operation at pump power levels above 900 W. For this purpose, the thermal lens of the disk was measured in advance in fundamental-mode operation with pump powers of up to 700 W using a common-path UV interferometer [12]. With a pump-spot diameter of 5.5 mm, the change in the refractive power of the thin disk was measured to be −9.4 mdpt/kW/cm².

 

Fig. 5. Setup for fundamental-mode operation of the TDL: ceramic Yb:Lu₂O₃ disk (green) in a multi-pass pumping module and a W-shaped laser cavity formed by a plane output coupler (OC), two plane folding mirrors, and a concave end mirror.

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Figure 6(a) shows the measured fundamental-mode output power and the optical efficiency as functions of the pump power. The corresponding beam propagation factor M² is shown in Fig. 6(b).

 

Fig. 6. (a) Output power (black dots) and efficiency (red triangles) versus pump power (launched into the pumping module) in cw fundamental-mode operation. The inset shows the emission spectrum at maximum output power. (b) Values of M² versus pump power.

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A maximum fundamental-mode output power of 409 W with an optical efficiency of 35.6% was reached at a pump power of 1150 W. At this power level, the beam quality was measured according to ISO 11146 [13] to be M²_x = 1.11 in the horizontal and M²_y = 1.09 in the vertical planes of symmetry. The evolution of the beam quality as a function of the launched pump power is shown in Fig. 6(b). Up to a pump power of 800 W, the M² values were measured to be significantly larger than 1, whereas a pump power between 950 and 1150 W led to beam quality that was close to the diffraction limit (M² < 1.2). This behavior is due to the thermal lens of the disk in combination with this specific resonator design. The transition of the M² value is also reflected in both the laser output power and the efficiency, as can be seen in Fig. 6(a). The drop in efficiency from 37% at a launched pump power of 790 W to 35.3% at 950 W is most probably due to the transition from a slightly multimode beam to the fundamental mode, which has a smaller geometrical overlap with the pump spot than a mix of higher-order transverse modes. The caustic of the laser beam measured at an output power of 409 W is shown in Fig. 7, which reveals a stigmatic beam with only minor distortions of the far-field intensity distribution.

 

Fig. 7. Caustic of the near-fundamental-mode laser beam measured at an output power of 409 W. The insets show examples of the corresponding intensity distributions.

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In summary, the investigations presented in this letter show the potential of a high-quality and large-size polycrystalline Yb:Lu₂O₃ ceramic. Its high-power capability was investigated in a TDL oscillator in multimode operation, where an output power of 1190 W at an optical efficiency of 60.3% and a slope efficiency of 65% was achieved. The average output power was scaled by a factor of approximately seven compared with previously published results obtained with Yb:Lu₂O₃ ceramics, and was only limited by the available pump power. Compared with results achieved with single crystals, the output power could almost be doubled. On the other hand, a significantly lower slope efficiency was measured in our work compared with the record slope efficiencies for Yb:Lu₂O₃ ceramics and single crystals, indicating the presence of non-negligible scattering losses from this ceramic material.

In fundamental-mode operation, an output power of 409 W with an efficiency of 35.6% was demonstrated. This underlines the optical quality of the new ceramics with regard to wavefront distortions.

For further power scaling in multimode operation towards the 2 kW output power range, ceramics with larger apertures are required. Scaling of the output power in fundamental-mode operation may be obtained by using an improved resonator design to address the thermal lens of the laser active medium.

Further work will be dedicated to the manufacturing of larger ceramics with even higher optical quality and the systematic comparison of their laser performance with those of established materials such as single-crystalline Yb:YAG. Furthermore, the ceramics presented here will be tested in a mode-locked TDL experiment in order to explore their potential for scaling the average and the peak power of ultrafast TDLs.