Processing of Ceramics. Группа авторов
bonding interface by visual observation. At the surrounding area of the bonding interface, only the grain size of the YAG‐Nd:YAG ceramic is different, and there are no interstices at the interface. Further, scattering from the bonding interface cannot be detected by laser tomography measurement, and the scattering at the bonding interface can hardly be measured because the transmittance is the same as that of YAG and Nd:YAG samples having the same length. Since the same size monolithic Nd:YAG ceramic has a wavefront distortion of 0.12λ/5? inches and the same value was obtained for the composite material, there is no increase in the wavefront distortion accompanying the formation of the composite by bonding (see Figure 2.32c). With this material, an output of higher than 5 kW was obtained by side pumping with a high output 808 nm LD although it is a part of an unpublished work.
Figure 2.32 (a) Appearance of end‐cap structured YAG‐Nd:YAG‐YAG slab before and after bonding, (b) bonding inspection by laser tomography and (c) Schlieren and wavefront distortion image of composite slab.
Figure 2.33 is a large slab with a volume of 100 × 100 × 20 mm3. The core material is Nd:YAG, and the cladding material is Sm:YAG ceramics to suppress parasitic oscillation. The material was tested at the Lawrence Livermore National Laboratory (LLNL) in the United States as a laser gain medium for a heat capacity laser (SSHCL) and was found to achieve an output of 67 kW. Regarding high‐power ceramic lasers, Textron and Northrop Grumman of the United States have reported 100 kW‐class laser generation.
Figure 2.33 Nd:YAG (core)‐Sm:YAG (cladding = supersaturated absorber) composite used for heat capacity laser at Lawrence Livermore National Laboratory in the United States.
Source: Yanagitani and Yagi [27].
Various types of interesting composite laser elements are introduced below. Figure 2.34a shows a YAG‐Nd:YAG‐YAG composite in which 11 layers of materials with different compositions are bonded, and it is possible to control rapid heat generation during side excitation. Figure 2.34b shows a composite with five layers of materials having different compositions in the length direction, to control heat generation and beam quality during laser oscillation. Although the idea of composite design is correct, the problem is that it is difficult to minimize scattering because the refractive index of each layer is different. Generally, scattering occurs at the bonding interface of YAG and Nd:YAG due to the difference in their refractive index. But this new type of composite has a uniform refractive because gadolinium Gd is doped in YAG and Nd:YAG components as necessary in order to compensate for the refractive index fluctuation throughout the whole composite slab sample. Figure 2.34c shows a laser gain medium slab with 40 × 160 × t4 mm dimension. At first glance, it looks like a monolithic slab, but this slab is composed of seven components with different compositions, and its purpose is to generate a laser with high power and high beam quality. As of 2018, approximately 7 kW of laser has been successfully generated, but the ultimate goal is 20 kW per slab.
Figure 2.34 (a) YAG‐Nd:YAG‐YAG composite with 11 layers. (b) Five‐layer composite in which index mismatching controlled by doping with Gd. (c) Five‐layer composite with different Nd doping in length direction and pure YAG is attached to both sides.
References
1 1 Ikesue, A., Kinoshita, T., Kamata, K., and Yoshida, K. (1995). Fabrication and optical properties of high‐performance polycrystalline Nd:YAG ceramics for solid‐state lasers. J. Am. Ceram. Soc. 78 (4): 1033–1040.
2 2 Matsui, K. (2010). Phase‐transformation mechanism in yttria‐stabilized tetragonal zirconia polycrystal: discovery of grain boundary segregation‐induced phase transformation. TOSOH Res. Technol. Rev. 54: 3–15.
3 3 Haneda, H., Yanagitani, T., Watanabe, A., and Shirasaki, S. (1990). Preparation of ytterbium iron garnet powder by homogeneous preparation method and its sintering. J. Cerma. Soc. Jpn. 98 (3): 285–291.
4 4 Sarthou, J., Aball, P., Patriarche, G. et al. (2016). Wet‐route synthesis and characterization of Yb:CaF2 optical ceramics. J. Am. Ceram. Soc.: 1–9. https://doi.org/10.1111/jace.14216.
5 5 Pawlowski, E., Klugel, M., Menke, Y. et al. (2010). Proc. SPIE Photonic West 757815‐1: 7578.
6 6 Ikesue, A. and Aung, Y.L. (2016). Synthesis of Yb:YAG ceramics without sintering additives and their performance. Rapid Comm. J. Am. Ceram. Soc.: 1–4.
7 7 A. Ikesue, Y. L. Aung, and T. Kamimura. (2015). Optical properties of P‐type, neutral, N‐type and pure Nd:YAG ceramics. Fr‐O‐2‐I‐1, 11th Laser Ceramics Symposium International Symposium on Transparent Ceramics for Photonic Application, Xuzhou, China, December 4, 2015.
8 8 Schad, S.‐S., Gottwald, T., Kuhn, V. et al. (2016). Recent development of disk lasers at TRUMPF. In: Proc. SPIE 9726, 972615–972615‐972616.
9 9 Sato, Y. et al. (2004). Spectroscopic properties and laser operation pf Nd:Y3ScAl4O12 polycrystalline gain media, solid solutions of Nd:Y3Al5O12 and Nd:Y3Sc2Al3O12 ceramics. J. Ceram. Soc. Jpn. 112: S313–S316.
10 10 Saikawa, J., Sato, Y., Taira, T., and Ikesue, A. (2004). Passive mode locking of a mixed garnet Yb:Y3Sc1Al4O12 ceramic laser. Appl. Phys. Lett. 85: 5845–5847.
11 11 N. Ter‐Gabrielyan, L. D. Merkel, G. A. Newburgh, M. Dubinskii, and A. Ikesue. (2008). Proc. Adv. Solid State Photon., Nara, Japan, TuB4.
12 12 Kaminskii, A.A., Kurokawa, H., Shirakawa, A. et al. (2009). Ba(Mg,Zr,Ta)O3 fine‐grained ceramic: a novel laser gain material with disordered structure for high‐power laser systems. Laser Phys. Lett. 6: 304–310.
13 13 Sarthou, J., Aballea, P., Patriarche, G. et al. (2016). Wet‐route synthesis and characterization of Yb:CaF2 optical ceramics. J. Am. Ceram. Soc. https://doi.org/10.1111/jace.14216.
14 14 Aballea, P., Suganuma, A., Druon, F. et al. (2015). Laser performance of diode‐pumed Yb:CaF2 optical ceramics synthesized using an energy‐efficient process. Optica 2 (4): 288–291.
15 15 Kitajima, S., Shirakawa, A., Ueda, K., and Ishizawa, H. (2017). Femstosecond mode‐locked Yb3+‐doped CaF2‐LaF3 ceramic laser. IEEE: 1–1. https://doi.org/10.1109/CLEOE‐EQEC.2017.8086286.
16 16 Chen, H., Ikesue, A., Noto, H. et al. (2019). Nd3+‐activated CaF2 ceramic lasers. Opt. Lett. 44: 3378–3381.
17 17 Mirov, S., Fedorov, V., Moskalev, I., and Martyshkin, D. (2007). IEEE J.