High-power and High Efficiency Yb:YAG Ceramic Laser at Room Temperature

Ceramic laser media fabricated by vacuum sintering (Ikesue et al., 1995), (Ikesue et al., 1996), and nanocrystalline (Lu et al., 2000) technology are very attractive materials because they have several remarkable advantages compared with single crystal laser materials. Ceramic samples with a large size can be easily fabricated, whereas this is extremely difficult for single crystals; multiplayer and multifunctional ceramic laser materials are possible because of the polycrystallinity of ceramics (Taira, 2007). Potentially, because of their short fabrication period and because they can be mass-produced, the cost of ceramic laser materials could be much lower than that of single crystals. Furthermore, no complex facilities and critical techniques are required for the growth of ceramics. Since 1995, Ikesue and coworkers have been developing several types of ceramic laser material (Ikesue et al., 1995), (Ikesue et al., 1996), and they found in 2000 that the output power from a 3.4 at.% Nd:YAG ceramic microchip laser is twice that from a Nd:YAG crystal microchip laser of the same size (Shoji et al., 2000). At a low doping concentration, it was found that the efficiency of a diode-end-pumped Nd:YAG ceramic laser is even higher than that of a Nd:YAG single crystal laser. Since 1998, Yanagitani and coworkers have been developing several types of ceramic lasers, and Lu et al. reported the Nd:YAG ceramic laser as one of them in 2001 (Lu et al., 2001). The mechanical properties of YAG ceramics were reported by Kaminskii et al. in ref. (Kaminskii et al., 2003). YAG ceramics had a 10% higher hardness than a YAG single crystal, and the fracture toughness of the YAG ceramics was more than threefold that of the YAG single crystal. Therefore, the ceramics had a higher resistance to thermal shock than the single crystal. Ytterbium (Yb3+)-doped materials are very attractive for diode-pumped solid-state lasers (DPSSLs) (Krupke, 2000). The Yb3+-doped materials have high quantum efficiency and exhibit no concentration quenching simply because the Yb3+ ion has only two manifolds, namely, the ground state 2F7/2 and the upper level 2F5/2. Thus far, many articles about Yb:YAG crystal lasers have been published (Lacovara et al., 1991), (Hönninger et al., 1995), (Sumida et al., 1998). Yb:YAG has broad absorption and emission bands. The broad absorption band in the near-IR region is suitable for laser-diode (LD) pumping, and the broad emission band enables the generation of ultrashort pulses (Hönninger et al., 1995). However, an Yb:YAG laser is known as a quasi-three-level laser or a quasi-four-level laser, and a finite population exists at the Stark level of the lower manifold 2F7/2, where laser transition terminates, which requires high-intensity pumping, a high-brightness pump