1. Introduction
Transparent ceramic materials have been widely used in building high-performance solid-state lasers due to their significant advantages over single-crystal materials such as large-size fabrication, reduced production cycle, high doping concentration and stable chemical properties[Reference Sanghera, Kim, Villalobos, Shaw, Baker, Frantz, Sadowski and Aggarwal1–Reference Liu, Zeng, Li, Shen, Bo, Zong, Wang, Xu, Xu, Cui, Peng, Xu, Zhang and Pan4]. Early research into potential substitutions of high-power laser gain media was focused on neodymium-doped YAG ceramics. In 1995, Ikesue demonstrated a 1.1 at.% Nd:YAG ceramic laser for the first time and observed that the continuous-wave (CW) laser had a slope efficiency of 28% at 1060 nm[Reference Ikesue, Kinoshita, Kamata and Yoshida5]. In 2001, Lu demonstrated laser oscillation with 1.0 at.% Nd:YAG ceramic and the output power reached 72 W[Reference Lu, Lu, Murai, Takaichi, Uematsu, Ueda, Yagi, Yanagitani and Kaminskii6]. One year later, a group led by Ueda increased the output power of a Nd:YAG ceramic laser to 1.46 kW, corresponding to a slope efficiency of 42%[Reference Lu, Takaichi, Uematsu, Shirakawa, Musha, Ueda, Yagi, Yanagitani and Kaminskii7]. However, the gain bandwidth of the 
$\text{Nd}^{3+}$ ion is merely 1 nm, limiting the output pulse width to picosecond magnitude in mode-locked operation for the purpose of generating ultrafast lasers[Reference Taira, Mukai, Nozawa and Kobayashi8–Reference Yoshioka, Nakamura, Ogawa and Wada10].
Compared with 
$\text{Nd}^{3+}$, 
$\text{Yb}^{3+}$ has been recognized as a more attractive alternative for high-power or ultrashort-pulse laser operation due to its simple two-manifold energy levels and large emission cross-section, which is beneficial for the elimination of undesired effects such as up-conversion, cross relaxation and excited-state absorption[Reference Shirakawa, Takaichi, Yagi, Bisson, Lu, Musha, Ueda, Yanaqitani, Petrov and Kaminskii11–Reference Xu, Tang, Zhu and Zhang15]. In particular, the relatively broadband absorption cross-section of Yb ions enables high pump absorption efficiency from 941 to 970 nm, which is within the emission wavelength range of high-power, low-cost InGaAs laser diodes, thus making diode-pumped solid-state Yb-doped lasers attractive for practical high-power and high-energy applications[Reference Tsunekane and Taira16–Reference Kaminskii, Bagayev, Ueda, Takaichi, Yagi and Yanagitani18]. Moreover, the large emission cross-section of Yb material enables high-efficiency and widely tunable laser operation[Reference Hao, Li, Zeng, Yang, Dou, Zhou and Lu19–Reference Pirri, Vannini, Babin, Niki and Toci22].
However, with the substantial increase of output power in solid-state lasers, the heavy thermal load of the quasi-three-level structure has become a severe problem that restricts the power scaling of Yb-doped solid-state lasers. Therefore, the exploration of advanced materials with superior heat transfer performance has become a hot scientific focus. In 2006, Ikesue and Aung demonstrated the fabrication of a laser ceramic with a complicated structure without precise polishing and diffusion bonding; the advanced ceramic processing technique they presented paved the way to the design of laser gain media with flexibility and convenience[Reference Ikesue, Aung, Taira, Kamimura, Yoshida and Messing23]. Soon after, they carried out in-depth research into the advantages of composite-structure laser ceramic, which was reported in 2008. Through a comparison of the thermal distribution between a conventional ceramic with uniform doping concentration and a novel ceramic with smooth gradient doping concentration in an edge-pumped laser, the experimental results showed that thermal lensing effects were efficiently suppressed in the composite ceramic due to the excellent gradient distribution of doping ions[Reference Ikesue and Aung24]. Moreover, the composite ceramic structure also allows for effective beam mode and pattern control of Q-switched or mode-locked lasers, which will promote the integration and downsizing of solid-state laser oscillators[Reference Li, Pan, Zeng, Liu, Jiang and Guo25–Reference Ba, Li, Pan, Jing, Jiang, Liu, Kou and Guo27]. Recently, a diode corner-pumped high-power slab laser with an output power of more than 520 W and a slope efficiency of 32% was introduced due to the easy synthesis of ceramic active media and a simple pumping system[Reference Liu, Gong, Lu, Gong and Li28].
In this paper, we report the fabrication of a multilayer composite YAG/5 at.% Yb:YAG/YAG laser ceramic by a non-aqueous tape-casting and vacuum-sintering process. The transmittance characteristics and laser performance of this composite ceramic are demonstrated. The experimental results indicate that low-threshold and continually tunable properties are achieved under high-brightness diode laser pumping.
2. Experimental setup and results
The non-aqueous tape-casting technique was adopted in our study to fabricate multilayer ceramics. To begin with, we prepared two groups of high-purity commercial powders as starting materials. An 
$\text{Al}_{2}\text{O}_{3}$ and 
$\text{Y}_{2}\text{O}_{3}$ mixture was used to produce undoped YAG slurry, while an 
$\text{Yb}_{2}\text{O}_{3}$, 
$\text{Al}_{2}\text{O}_{3}$ and 
$\text{Y}_{2}\text{O}_{3}$ mixture was used to produce 5 at.% Yb:YAG slurry. The weight of each powder was precisely controlled with a stoichiometric ratio of YAG and 5 at.% Yb:YAG, respectively. We used polyvinyl butyral, PEG-400 and butyl benzyl phthalate as the binder and the plasticizers. The binder and the plasticizers were added into the powders and the mixture was then ball-milled for 10 h. A degassing process was subsequently carried out in a vacuum device for 30 min. After that, the tape-casting process was implemented with a height between the blade and the substrate of 350 
${\rm\mu}$m and a casting velocity of 
$100\ \text{mm min}^{-1}$. The eight stacked tapes obtained had a thickness of approximately 1 mm and were clipped into wafers with a diameter of 20 mm. We chose 15 undoped tapes as the bottom layer, stacked 10 doped tapes as the interlayer, and placed another 15 undoped tapes onto them as the top layer. These sandwich structure multi-tapes were laminated at 
$85\,^{\circ }\text{C}$ for 30 min under a pressure of 40 MPa to obtain the green samples. The samples were sintered at 
$1760\,^{\circ }\text{C}$ for 50 h under vacuum conditions at a pressure of less than 
$1\times 10^{-3}$ Pa and annealed at 
$1450\,^{\circ }\text{C}$ for 10 h in a stove to improve the optical quality of the ceramic samples.
An image of the YAG/Yb:YAG/YAG (top: unannealed; bottom: annealed).
Figure 1 shows photographs of YAG/Yb:YAG/YAG ceramic synthesized by the non-aqueous tape-casting method. The top image is the unannealed green body and the bottom image is the annealed laser ceramic. The color of the unannealed ceramic was green because of light absorption in the visible range; both of the two undoped layers and the doped interlayer in the unannealed ceramic are affected by lattice defects caused by oxygen vacancies. The doped interlayer exhibits a darker shade of green due to the effects of lattice defects as well as ‘F’ color centers caused by ion transformations from 
$\text{Yb}^{3+}$ to 
$\text{Yb}^{2+}$[Reference Yang, Deng and Yin29, Reference Xu, Zhao, Song, Zhou, Xu and Deng30]. The annealed ceramic was completely transparent, which can be seen from the image. The optical transmittance of the ceramic from 200 to 1100 nm is shown in Figure 2. It can be seen that the transmittance is as high as 83% from 400 to 800 nm and attains a higher value around 1100 nm. From 800 to 1100 nm, there are three strong absorption peaks around 915, 940 and 970 nm, which correspond to the conventional broadband absorption spectrum of 
$\text{Yb}^{3+}$ ions. The strongest absorption rate reaches 90% at 940 nm, which corresponds to the energy transition between two manifolds of 
$\text{Yb}^{3+}$ ions.
The transmittance curve of the YAG/Yb:YAG/YAG ceramic from 800 to 1100 nm. The inset is the transmittance curve from 200 to 1100 nm.
The experimental setup of the laser cavity.
(a) The typical output laser spectrum and (b) the output power of the YAG/Yb:YAG/YAG multilayered ceramic laser using OCs with 
$T=2\%$, 
$T=5\%$, and 
$T=10\%$.
In order to test the performance of the ceramic laser, a folded laser cavity was built, as schematically shown in Figure 3. The total cavity length was 450 mm, which was pumped by a high-brightness fiber-pigtailed diode laser with a maximum output power of 30 W at 974 nm. The core diameter of the fiber was 100 
${\rm\mu}$m, and the numerical aperture was 0.22. Two lenses were employed to build a 1:1 imaging system for efficient focusing of the pump beam into the surface of the ceramic sample with a spot size of about 100 
${\rm\mu}$m. The size of the ceramic used in our experiment was 
$5\ \text{mm}\times 5\ \text{mm}\times 4\ \text{mm}\ (X\times Y\times Z)$ and the doping concentration of 
$\text{Yb}^{3+}$ ions was 5 at.%. The ceramic was wrapped with indium foil and fixed tightly into a three-dimensional adjustment stage. The temperature of the gain medium was controlled at 
$14\,^{\circ }\text{C}$ by a water-cooled copper heat sink. We adopted a stable three-mirror folded resonator. The resonator consisted of one dichroic input coupler (M1), one folding mirror (M2) with a radius of curvature of 300 mm and one output coupler (OC). The dichroic mirror M1 and folding mirror M2 were anti-reflection coated at 976 nm and high-reflection coated in a wavelength range from 1020 to 1120 nm. Three OCs with different transmissions of 
$T=2\%$, 
$T=5\%$, and 
$T=10\%$ from 1020 to 1120 nm were used to measure the laser performance. The output CW laser was monitored by a fiber spectrometer and a power meter.
In the process of adjusting the laser, we found that the output power hardly changed when we moved the focus spot of the pumping laser diode on the ceramic by using the three-dimensional adjustment frame (
$X\times Y$), which implied uniform distribution of the ceramic. Figure 4(a) shows the CW laser spectrum of the YAG/5 at.% Yb:YAG/YAG ceramic. The output wavelength was about 1031 nm with a full width at half maximum (FWHM) of over 3 nm. The CW laser oscillation could be observed by the fiber spectrometer with a threshold incident pump power of 0.45 W when using the 
$T=2\%$ OC. In order to optimize the laser performance of the ceramic laser, we monitored the output power obtained using OCs with 
$T=2\%$, 
$T=5\%$, and 
$T=10\%$, as shown in Figure 4(b). The maximum output power of 3.2 W was obtained when using an OC with 
$T=10\%$ with an incident pump power of 26.6 W, corresponding to a slope efficiency and optical conversion efficiency of 20.4% and 12.0%, respectively. Once the incident pump power exceeded 20 W, the output power tended to be unchanged with increase of pump power. This was mainly caused by the wavelength drift of the incident pump laser away from the narrow absorption bandwidth of 976 nm for the gain media due to the temperature change of the diode laser. By employing a pump laser with a stable wavelength and improving the cavity structure, a higher output power could be expected.
To further characterize the wavelength tunability of this YAG/Yb:YAG/YAG ceramic laser, we tuned the laser wavelength by inserting an SF57 Brewster prism into the resonator. It was located between the folded mirror and the OC at the Brewster angle. Under 5 W incident pump power, the Yb:YAG laser could be tuned from 1021 to 1058 nm, with a maximum output power of 178 mW at 1031 nm, as shown in Figure 5. The tuning curve was continuous and smooth, and the broadband tunability indicated the possibility of ultrashort femtosecond pulse generation by such a composite ceramic. The coating of all cavity mirrors had a reflection range starting from 1020 nm; by employing a reflection coating with a lower cut-off wavelength, laser oscillation at wavelengths shorter than 1020 nm would be expected.
The tuning curve obtained with an intracavity dispersive prism for the YAG/5 at.% Yb:YAG/YAG ceramic laser.
3. Conclusion
In summary, a continuously tunable YAG/Yb:YAG/YAG composite ceramic laser was demonstrated in a three-mirror folded cavity. The multilayer composite ceramic with a sandwich structure was fabricated by the non-aqueous tape-casting method. At 26.6 W incident pump power, a 3.2 W maximum average output power of the CW laser was achieved at 1031 nm with a slope efficiency of 20.4%. The minimum threshold incident pump power for laser oscillation was 0.45 W. By inserting an SF57 prism in the laser cavity, a smooth tunable curve from 1021 to 1058 nm was obtained. Such multilayer composite ceramics support different doping concentrations, with high transmission efficiency from visible to infrared and continuous broadband wavelength tunability. Our results demonstrated that composite ceramics fabricated by the tape-casting technique could be tailored with flexible structures and improved heat dissipation, which are desirable qualities for the next generation of high-power ultrashort solid-state lasers.
Acknowledgements
This work was supported by the Major Program of the National Natural Science Foundation of China (Grant No. 50990301) and the Project for Young Scientists Fund of the National Natural Science Foundation of China (Grant Nos. 51002172 and 51302298).
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