We demonstrated a high-power continuous-wave (CW) polycrystalline Yb:YAG ceramic laser pumped by fiber-pigtailed laser diode at 968 nm with 400 μm fiber core. The Yb:YAG ceramic laser performance was compared for different Yb3+ ion concentrations in the ceramics by using a conventional end-pump laser cavity consisting of two flat mirrors with output couplers of different transmissions. A CW laser output of 40 W average power with M2 factor of 5.8 was obtained with 5 mol% Yb concentration under 120 W incident pump power. This is to the best of our knowledge the highest output power in end-pumped bulk Yb:YAG ceramic laser.
©2009 Optical Society of America
Transparent laser ceramics are considered as the potential replacements for single-crystal counterparts with several remarkable advantages such as high concentration, easy fabrication of large-size and multilayer ceramic laser materials [1–4]. Fabricated by using the vacuum sintering and nanocrystalline techniques, ceramics can be easily manufactured to varieties of shape and size with appropriate processing containers. Besides easier fabrication, ceramics exhibit higher doping concentration than single crystals, higher thermal conductivity and better mechanical property than laser glasses. Typically, cubic crystals of low-symmetry sites with no distinction of refractive index along the optical axis are the optimum choice of preparation for transparent laser ceramics, such as garnet (YAG) and rare-earth sesquioxide (Y2O3). Efficient high-power lasers have been demonstrated in Nd- and Yb-doped ceramics, manifesting as promising candidates for high-power laser-diode-pumped lasers with rod, slab and microchip configurations [5–7]. And the superiority of only two manifolds 2F5/2 and 2F7/2 makes Yb3+ as one of the most preferable active ions for ceramics. When compared to the Nd-doped counterparts, Yb-doped materials avoid many undesired effects, such as excited-state absorption and concentration quenching, and even provide a broader absorption and emission bandwidth. A Yb:YAG ceramic microchip laser was realized with 2 W average output power and near-diffraction-limited laser beam quality , and as high as 6.8 W average power was realized with a Yb:YAG ceramic in the end-pump configuration with a slope efficiency of 72% . Heavy-doped Yb:YAG ceramics have been proved to exhibit better laser performance than their single-crystal counterparts, but the average output power does not exceed 3 W . As low as 400 mW pump threshold and 68-nm tunable range was achieved with Yb3+:Y1.8La0.2O3 ceramic . Up to date, the highest output power for Yb-doped ceramic laser was reported as 9.2 W of diode-end-pumped Yb:Y2O3 ceramic laser .
Power scaling of Yb:YAG ceramic laser for high intensity laser application is restricted by the thermal lens effect [13–16], which is enforced by the cumulative thermal loads under high power pump. The scattering losses caused by pores and grain boundaries produce additional hurdle to high-power scaling of ceramic laser. In this letter, we report a high-power CW Yb:YAG ceramic laser, which outputs as high as 39.6 W average power at 1050 nm under 120 W pump power at 968 nm. The M2 factor was measured as 5.6 at full output power. The corresponding slope efficiency and optical-to-optical efficiency were 39.3% and 33%, respectively.
2. Experimental setup and results
As shown in Fig. 1 , the Yb:YAG ceramic laser was operated with a simple laser cavity consisting of two flat mirrors, DM (AR@ 940-980 nm, HR@ 1020-1120 nm) and output coupler (PR@ 1020-1120 nm). Transparent Yb:YAG ceramics were fabricated by solid-state reaction at 1780 °C in vacuum. Commercial Al2O3, Y2O3, and Yb2O3 powders were mixed in ethanol and doped with MgO and tetraethoxysilane. All of the Yb:YAG ceramics were cut into the size of 3.5-mm long, 4 × 4 mm2 aperture and AR-coated from 940 to 980 nm on both surfaces. To remove the thermal loads, the ceramics were wrapped with indium foil and mounted in a copper holder water-cooled around 12 °C. A fiber-coupled high-power laser diode emitting around 968 nm with 3-nm bandwidth and 400-μm fiber core was used as the pump source. The incident pump was 2.5:1 relayed with 160-μm pump beam diameter in the ceramic.
Firstly, the CW laser performance was tested with 10 mol% Yb-doped YAG ceramic. In order to evaluate the laser properties, we measured the Yb:YAG ceramic laser output with different output coupler transmission T = 2%, T = 5%, and T = 9%. Figure 2(a) presents the corresponding experimental results. The maximum slope efficiency was obtained with T = 5% output coupler, which reached up to 43.4%. The incident pump power at laser threshold was 8 W, corresponding to 40 kW/cm2 pump power density. Under the incident pump power of 41 W, as high as 13.5 W average output power was achieved at 1050 nm, corresponding to an intracavity power of 257 W. The slope efficiency was reduced to 29.2% and 35.3% as T = 2% and T = 9% output coupler was used, respectively. However, the 10 mol% Yb-doped ceramic obviously suffered the thermal effect from intensive absorption and the unwanted impurities from raw materials. As a result of the cumulative thermal loads, the laser output power declined with constant pump power for a long time. In order to lessen the thermal effect and achieve a high-power output, we reduced the Yb concentration to 5 mol%, which would inevitably reduce the laser slope efficiency. As shown in Fig. 2(b), the best laser performance was achieved with T = 5% output coupler. The highest output power was 39.6 W at 1050 nm under 120 W incident pump power, with a slope efficiency of 39.3% and an intracavity power of 752 W. The laser threshold of incident pump power was measured as 7.5 W pump power, corresponding to 38 kW/cm2 pump power density. Although the fluorescence intensity at 1030 nm was much larger than that at 1050 nm, the highest output power was obtained at 1050 nm (shown in Fig. 3(b) ). This took place as the emission laser wavelength was determined by the emission spectrum and re-absorption loss in the quasi-three-level Yb3+ in ceramic.
Residual pores, secondary phase and grain boundary affect not only the available output power, but also the transverse beam profile . Thermal loads and mechanical stress would induce inhomogeneous change of the refractive index and thus distort the intracavity laser transverse mode, resulting in a deteriorated beam quality and reduced optical efficiency. In order to investigate the beam quality of the Yb:YAG ceramic laser, a CCD was employed to record the beam pattern and its change under different pump powers. Figure 3(a) shows the M2 factor as a function of the output power for the 5% Yb:YAG ceramic. There is an approximately linear relationship between the beam quality M2 factor and the laser output power. The insets display the transverse beam patterns of the Yb:YAG ceramic laser with 2, 10, 22, and 38 W output power, and the corresponding divergence angle of laser beam was measured as ~0, 3.6, 6.8, and 10 mrad, respectively. At 2 W average output power, the beam pattern have a symmetric profile. The beam quality for high-power laser operation was deteriorated, as can be seen from the calculated M2-numbers given in Fig. 3(a). The M2 factor was calculated as 5.8 when the laser output power was 38 W. The most likely reason for this was the beam distortion caused by the refractive index step of the Yb:YAG ceramic and diffraction impact under high power pump. In order to obtain high output power from the ceramic laser, further improvements of the sample with the porosity and grain boundary and a more efficient cooling on the gain media is necessary.
To further qualify our Yb:YAG ceramic laser, the tunability of 10% and 5% Yb concentration with 968 nm diode laser pump was investigated by a three-mirror laser cavity and an intracavity SF10 dispersion prism, which has been employed in the earlier experiments [9,18,19]. As shown in Fig. 4 , the 10% Yb:YAG exhibits a broader and smoother tunable spectrum than the 5% one. Under 30 W incident pump power, the 10% Yb:YAG could be tuned from 1017 to 1094 nm, with a maximum output power of 2.3 W at 1042 nm. However, there were only two narrow-tunable bands for the 5% Yb:YAG ceramic, the corresponding output power was 6.9 W for 1032 nm and 3.2 W for 1050 nm.
In conclusion, the high-power diode-pumped Yb-doped ceramic laser was boosted to 39.6 W average output power with a slope efficiency of 43.3%. The M2 factor was measure to be ~1.0, 2.1, 4.1, and 5.8 with the average output power of 2, 10, 22, and 38 W, respectively. The tunability of Yb:YAG ceramic laser was also investigated for different Yb-doping concentrations. Further improvements of the sample and a more efficient cooling on the gain media are necessary when scaling to higher powers.
This work was supported by National Natural Science Fund (10525416 & 10774045), National Key Project for Basic Research (2006CB806005), Shanghai leading Academic Discipline project (B408), High Technology R&D Program of China (2007AA03Z523), and Natural Science Foundation of Shanghai (09ZR1435600).
References and links
1. J. Lu, H. Yagi, K. Takaichi, T. Uematsu, J. Bison, Y. Feng, A. Shirakawa, K. Ueda, T. Yanagitani, and A. Kaminskii, “110 W ceramic Nd3+:Y3Al5O12 laser,” Appl. Phys. B 79(1), 25–28 (2004). [CrossRef]
3. Q. Yang, C. Dou, J. Ding, X. Hu, and J. Xu, “Spectral characterization of transparent (Nd0.01Y0.94La0.05)2O3 laser ceramics,” Appl. Phys. Lett. 91(11), 111918 (2007). [CrossRef]
4. S. Ye, B. Zhu, J. Luo, J. Chen, G. Lakshminarayana, and J. Qiu, “Enhanced cooperative quantum cutting in Tm3+- Yb3+ codoped glass ceramics containing LaF3 nanocrystals,” Opt. Express 16(12), 8989–8994 (2008). [CrossRef] [PubMed]
5. J. Lu, J. Lu, T. Murai, K. Takaichi, T. Uematsu, J. Xu, K. Ueda, H. Yagi, T. Yanagitani, and A. A. Kaminskii, “36-W diode-pumped continuous-wave 1319-nm Nd:YAG ceramic laser,” Opt. Lett. 27(13), 1120–1122 (2002). [CrossRef]
6. B. Chen, Y. Chen, and M. Bass, “Edge- and end-pumped slab lasers with both efficient and uniform pumping,” IEEE J. Quantum Electron. 42(5), 483–489 (2006). [CrossRef]
7. M. Tsunekanea and T. Taira, “High-power operation of diode edge-pumped, composite all-ceramic Yb: Y3Al5O12 microchip laser,” Appl. Phys. Lett. 90(12), 121101 (2007). [CrossRef]
8. J. Dong, A. Shirakawa, K. Ueda, H. Yagi, T. Yanagitani, and A. A. Kaminskii, “Efficient Yb3+:Y3Al5O12 ceramic microchip lasers,” Appl. Phys. Lett. 89(9), 091114 (2006). [CrossRef]
9. S. Nakamura, H. Yoshioka, Y. Matsubara, T. Ogawa, and S. Wada, “Efficient tunable Yb:YAG ceramic laser,” Opt. Commun. 281(17), 4411–4414 (2008). [CrossRef]
11. Q. Hao, W. Li, H. Zeng, Q. Yang, C. Dou, H. Zhou, and W. Lu, “Low-threshold and broadly tunable lasers of Yb3+-doped yttrium lanthanum oxide ceramic,” Appl. Phys. Lett. 92(21), 211106 (2008). [CrossRef]
12. J. Kong, D. Y. Tang, B. Zhao, J. Lu, K. Ueda, H. Yagi, and T. Yanagitani, “9.2-W diode–end–pumped Yb:Y2O3 ceramic laser,” Appl. Phys. Lett. 86(16), 161116 (2005). [CrossRef]
14. A. Sell, A. Leitenstorfer, and R. Huber, “Phase-locked generation and field-resolved detection of widely tunable terahertz pulses with amplitudes exceeding 100 MV/cm,” Opt. Lett. 33(23), 2767–2769 (2008). [CrossRef] [PubMed]
15. X. Yang, Z. Z. Xu, Y. X. Leng, H. H. Lu, L. H. Lin, Z. Q. Zhang, R. X. Li, W. Q. Zhang, D. J. Yin, and B. Tang, “Multiterawatt laser system based on optical parametric chirped pulse amplification,” Opt. Lett. 27(13), 1135–1137 (2002). [CrossRef]
16. K. Stelmaszczyk, P. Rohwetter, G. Méjean, J. Yu, E. Salmon, J. Kasparian, R. Ackermann, J.-P. Wolf, L. Wöste, P Rohwetter,, G Méjean, J. Yu, E Salmon, J Kasparian, R Ackermann, J Wolf, and L Wöste, “Long-distance remote laser-induced breakdown spectroscopy using filamentation in air,” Appl. Phys. Lett. 85(18), 3977–3979 (2004). [CrossRef]
17. A. Ikesue and Y. Aung, “Ceramic laser materials,” Nat. Photonics 2(12), 721–727 (2008). [CrossRef]
18. S. Nakamura, H. Yoshioka, T. Ogawa, and S. Wada, “Broadly tunable Yb3+-doped Y3Al5O12 ceramic laser at room temperature,” Jpn. J. Appl. Phys. 48(No. 6), 060205 (2009). [CrossRef]