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CW laser operations of thin-disk lasers with Lu-based oxide ceramics are reported. An output power of 166 W and a slope efficiency of 72.2% were obtained with an Yb:LuAG ceramic disk. We have also successfully demonstrated thin-disk lasers with Yb:Lu2O3 ceramics which were bonded by our soldering and gluing techniques. Slope efficiencies of 60.6% and 55.6% were obtained from a soldered disk and a glued disk, respectively.
© 2014 Optical Society of America
1. Introduction
Yb3+-doped active media have great potentials for compact, highly efficient, high power/energy laser applications due to their unique energy level scheme. Yb:Y3Al5O12 (YAG), one of the traditional gain media, have produced many applications and have of course been investigated. However, unavoidable thermo-optical distortions such as thermal lensing always limit the beam quality, power/energy scaling and efficiencies in case of the bulk-shaped medium. In terms of overcoming the thermal issues, employing the thin-disk concept which provides excellent thermal treatment is one of the most effective approaches [1,2]. Many excellent demonstrations have been reported with single crystalline Yb:YAG thin-disk laser/amplifier architectures e.g. SESAM mode-locking with 80 μJ pulse energy [3], Kerr-lens mode-locking in negative/positive dispersion regime [4,5], and regenerative amplification with an amplified average power of 300 W [6]. To enhance the performances more, selecting the gain media with appropriate properties for the system is the next step. While the short thickness of the gain medium performs an important role in the thin-disk concept, it leads to quite small pump absorption and gain. Thus multi-pass pumping system and high Yb-doping are required. However, realizing the high thermal conductivity and high Yb-doping simultaneously can be difficult due to rapid reduction of the thermal conductivity caused by the mismatch of the atomic mass between a host atom (Y3+, 173 g/mol) and a dopant atom (Yb3+, 88.9 g/mol) [7,8]. From this point of view, further improvement of the thermal exhaust can be difficult in case of the Yb:YAG with the thin-disk concept. Using Lu-based gain media is one of the solutions to avoid this problem. Since the atomic mass of Lu3+ (175 g/mol) is almost the same as that of Yb3+ [8], the thermal conductivities of Yb3+-doped Lu-based materials decrease only moderately and Yb3+-doped Lu-based materials exhibit high thermal conductivity even in high doping situation [9,10].
In this paper, we focus on the Yb3+-doped Lu-based oxides such as Yb:Lu3Al5O12 (LuAG) and Yb:Lu2O3. Yb:LuAG is one of very attractive materials with a great potential to enhance the performances of existing Yb:YAG-based laser sources due to its higher thermal conductivity than that of Yb:YAG in heavy doping regime [9]. First lasing of Yb:LuAG single crystal was demonstrated in 1976 [11]. Recently, Yb:LuAG single crystals have been indicating excellent laser performances e.g. 5.0 kW laser oscillation [9] and 742 W fundamental-mode oscillation with the thin-disk geometry [12]. More recently, highly transparent polycrystalline Yb:LuAG ceramics have been developed as a novel gain media [13,14]. We reported highly efficient continuous-wave (cw) laser oscillation and first passive mode-locking with bulk Yb:LuAG ceramic [14], and first demonstration of the cw lasing of the Yb:LuAG ceramic thin-disk [15]. Furthermore, multi-pass amplifier with bulk Yb:LuAG ceramic has been reported [16]. Yb:Lu2O3, one of sesquioxides (RE2O3, RE = Y, Sc, Lu), is also a promising gain medium especially for ultrashort short pulsed lasers [10]. Many fascinating results have been reported with Yb3+-doped sesquioxides: for example, SESAM mode-locking with an average power of 141 W [17] and high power cw operation with an output power of 670 W [18]. However, their high melting points (e.g. Y2O3: 2430 °C) can prevent the growth of the single crystals with good optical qualities and large aperture sizes. Recently, optimized heat exchanger method enables the growth of the single crystals with high optical quality, and actually, highly efficient Yb:Lu2O3 thin-disk laser have been demonstrated [19]. The ceramic technology is another approach to fabricate Yb:RE2O3 with high optical qualities and large aperture sizes due to their relatively low sintering temperatures compared with the melt growth. Recently, Yb:Lu2O3 ceramic fabricated using not only our vacuum sintering and nanocrystalline technology [20] but also the hot-press method have demonstrated the cw laser oscillation with high efficiency [21]. However, almost all demonstrations with the thin-disk concept, including other materials, have been achieved with single crystals. To the best of our knowledge, only Yb:YAG ceramic thin-disk lasers [22,23] and a mode-locked Yb:Y2O3 ceramic thin-disk laser [24] have been reported.
Here we report the cw laser operations of Yb:LuAG ceramic and Yb:Lu2O3 ceramic thin disks. A maximum output power of 166 W, a maximum optical efficiency of 60.3% and a slope efficiency of 72.2% were successfully obtained with an Yb:LuAG ceramic disk. We have also demonstrated the cw laser operation of the thin-disk lasers with Yb:Lu2O3 ceramics which were bonded by our bonding techniques. With the soldered disk, a maximum output power of 45.1 W, a maximum optical efficiency of 45.1% and a slope efficiency of 60.6% were obtained. In case of the glued disk, a maximum output power of 34.5 W, a maximum optical efficiency of 43.5% and a slope efficiency of 55.6% were successfully achieved.
2. Thermal and optical properties of Yb:LuAG and Yb:Lu2O3
Samples of un-doped LuAG, un-doped Lu2O3, 3 at.% Yb:Lu2O3 and 10 at.% Yb:LuAG ceramics were fabricated using our vacuum sintering and nanocrystalline technology [25,26]. Each ceramic has a similar average grain size of ≈3 μm with nearly uniform distributions of the nanocrystals.
2.1 Thermal conductivities
The thermal conductivities of the ceramics were measured by the flash method (LFA447, NanoFlash, NETZSCH Inc.) to be 8.80 W/mK and 8.04 W/mK for un-doped LuAG and 10 at.% Yb:LuAG [14], and 13.9 W/mK for 3 at.% Yb:Lu2O3. The thermal conductivity of un-doped Lu2O3 ceramic was reported to be 10.9 W/mK (measured by the temperature wave method) [27]. The main reason why the thermal conductivity of Yb:Lu2O3 ceramic is higher than that of un-doped ceramic will be the difference of the measurement methods. Yb:LuAG and Yb:Lu2O3 ceramics indicate high thermal conductivities in heavy doping situation like single crystals.
2.2 Optical properties
Yb:LuAG ceramic has its zero-phonon absorption peak at 968 nm with a narrow bandwidth and in addition exhibits a broad absorption band around 940 nm [14]. Since its narrow bandwidth and slight blue shifted wavelength of the zero-phonon line compared with that of Yb:YAG, the non-wavelength stabilized LD operating at 940 nm was employed as a pump source for laser operation. A main emission band of Yb:LuAG is centered at 1030 nm (corresponding to an emission cross section σem≈2.5 × 10−20 cm2) with a FWHM of 6.1 nm [14]. The emission cross section at 1030 nm is 25% higher than that of Yb:YAG. These two properties of Yb:LuAG ceramic, namely the high thermal conductivity at high doping levels and the large emission cross section, make it a very promising material for thin-disk lase/amplifier architectures.
The zero-phonon line of Yb:Lu2O3 ceramic is peaked at 976 nm with a FWHM of 3.3 nm [27]. We pumped into the zero-phonon line with the LD whose wavelength was stabilized by volume-Bragg-grating (VBG) to operate at 976 nm in the laser experiment. The peak wavelength of the emission band which was used for laser oscillation in this investigation is 1032 nm (corresponding to an emission cross section σem≈1.35 × 10−20 cm2) with a FWHM of 13 nm [27]. Its broader gain bandwidth and larger thermal conductivity than those of Yb:YAG are very suitable properties especially for ultrashort pulsed thin-disk laser.
3. Experiments and results
3.1 Setup for the Yb:LuAG ceramic thin-disk laser
A 10 at.% Yb:LuAG ceramic disk with a thickness of 150 μm was prepared. Figure 1 shows the schematic of the experimental setup. The disk was coated by a commercial supplier with anti-reflection (AR) coating on the front surface of the disk and high-reflection coating (HR) on the back surface of the disk. After that, the disk was glued onto a Cu heat sink. It was then mounted in a standard pumping module with 24 pump beam passes as commonly provided by the IFSW. The radii of curvature of the disk were measured to be 16 m in both x and y directions, respectively. A fiber-coupled LD operating at 940 nm with a maximum pump power of 280 W was used as a pump source. The pump spot diameter on the disk was 2.9 mm. This leads to a maximum pump power density of ≈4 kW/cm2 which is well below the usual damage threshold (>5 kW/cm2). A V-shaped resonator was designed and set up to operate the laser in a multi-mode regime. It consists of a concave HR mirror with a radius of curvature of 500 mm and a plane output coupler (OC) with different transmittances (3%, 5%, 6%). The overall resonator length was 390 mm.
3.2 Results of the Yb:LuAG ceramic thin-disk laser
The power performances of the multi-mode laser resonator are shown in Fig. 2. A maximum output power of 158W, a maximum optical efficiency of 57.1% and a slope efficiency of 63.7% were obtained with 3% of output coupling. Using 6% of output coupling, the output power reaches 164 W with an optical and a slope efficiency of 59.9% and 68.5%, respectively. The best performance was achieved with 5% of output coupling: a maximum power of 166W, a maximum optical efficiency of 60.3% and a slope efficiency of 72.2% were obtained. The emission wavelength was measured to be always around 1031 nm for all OCs. No signs of roll over were observed and hence further power scaling is expected to be easily possible with higher pumping powers and larger pump spot diameters. The beam quality factor (M2) at the maximum pump power was measured with a slit scanning beam profiler (BP 109-ir, Thorlabs Inc.) to be 21 for 3% and 5% of output couplings and 22 for 6% of output coupling.
3.3 Setup for the Yb:Lu2O3 ceramic thin-disk laser
Two 3 at.% Yb:Lu2O3 ceramic disks with thicknesses of 300 μm and 250 μm were prepared. The disks have the same coating as mentioned above. The disk with a thickness of 300 μm was soldered onto a Cu heat sink (Fig. 3(a) and the disk with the thickness of 250 μm was glued onto a Cu heat sink (Fig. 3(b)) by our bonding technique.Figure 4(a) indicates the schematic of the experimental setup. The disk with the heat sink was mounted in our hand-built pumping module with 16 pump beam passes (Fig. 4(b)). The pump source was a VBG-locked fiber coupled LD operating at 976 nm with a maximum pump power of 100 W. Therefore, with a pump spot diameter on the disk of 1.7 mm, the maximum pump densities were estimated to be 4.4 kW/cm2. The radii of curvatures of the center part of the disk were measured by using the focus retrocollimated interferometry [28] and estimated to be 10 m and 8.2 m concave for x and y directions, respectively in case of the soldered disk, and 18 m convex for both x and y directions in case of the glued disk. For the first test of the disks with our bonding technique, a simple I-shaped resonator was constructed by the HR coating of the disk and a concave output coupler with a radius of curvature of 500 mm and the transmittance of 3%. To evaluate the mode matching effect, the resonator length was changed from 250 mm to 450 mm and from 150 mm to 350 mm in the cases of soldered and glued disk, respectively.
Fig. 4 (a) Schematic of the setup of the Yb:Lu2O3 ceramic thin-disk laser. (b) Photograph of the pump module.
3.4 Results of the Yb:Lu2O3 ceramic thin-disk laser
Figures 5(a) and 5(b) indicate the power performances obtained from soldered disk and glued disk. Maximum output powers, maximum optical efficiencies, slope efficiencies and M2mean at maximum pump powers were summarized in Table 1.. In case of the soldered disk, with the maximum pump power of 100 W, the best performance was obtained with a 350 mm resonator length. A maximum output power of 45.1 W, a maximum optical efficiency of 45.1% and a slope efficiency of 60.6% were obtained with M2mean of 7.6. The optical efficiency did not show saturation above 350 mm of the resonator length, and no signs of roll over were obtained. Therefore, the laser performances should be improved with higher pump powers. In the case of glued disk, with the maximum pump power of 80 W (since the LD power degraded), the best performance was achieved with a 250 mm resonator length. A maximum output power of 34.5 W, a maximum optical efficiency of 43.5% and a slope efficiency of 55.6% were obtained with M2mean of 9.5. As is the case of soldered disk, further power scaling should be possible. In both case, the lasing wavelength was always around 1034 nm. We did not obtain any damage and instability during the experiment. Consequently, our bonding techniques were expected to be successfully working.
Fig. 5 Power performances of Yb:Lu2O3 ceramic thin-disk lasers with (a) soldered disk and (b) glued disk.
Table 1. The maximum values of output power, optical efficiency, slope efficiency and M2mean.
Conclusion
We have demonstrated highly efficient cw laser oscillation of Yb:LuAG ceramic thin disk laser. A maximum output power of 166 W, a maximum optical efficiency of 60.3% and a slope efficiency of 72.2% were successfully obtained. We have also demonstrated cw laser operation of a thin-disk laser with Yb:Lu2O3 ceramic. With the soldered disk, a maximum output power of 45.1 W, a maximum optical efficiency of 45.1% and a slope efficiency of 60.6% were obtained. In case of the glued disk, a maximum output power of 34.5 W, a maximum optical efficiency of 43.5% and a slope efficiency of 55.6% were successfully achieved.
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