Processing of Ceramics. Группа авторов

Figure 2.21 (a) X‐ray diffraction pattern of FAP powder as a raw material for Yb:FAP ceramics and Yb:FAP ceramics. Diffraction from Yb:FAP ceramics were from the surface of 3 mm × 3 mm. (b) Transmission and absorption spectra of Yb:FAP ceramics.

      Source: Sato et al. [20]© 2014, The Optical Society.

Figure 2.22 (a) Experimental configuration of the confirming of laser grade quality. This setup included an 880‐nm laser diode as a pump source, a delivering fiber, collimating and focusing lens, Nd:YVO₄ microchip, flat output coupler, and an Yb:FAP ceramic sample. (b) Slope efficiency of Nd:YVO₄ laser as a function of output coupling. 2 at.% Yb:FAP ceramics with the thickness of 0.6 mm was placed in the resonator.

      Source: Sato et al. [20]© 2014, The Optical Society.

Figure 2.23 Schematics of optical scattering in fine‐grained non‐cubic ceramics.

      Source: Furuse et al. [22]. Licensed under CC BY 4.0.

As shown in Figure 2.24, the transmittance is the theoretical transmittance (87%) in the laser oscillation region (1 μm band), but the transmittance becomes lower as the wavelength becomes shorter. It is considered that significant Rayleigh scattering occurs in the gain medium because it is a polycrystalline material in which hexagonal crystallites are randomly oriented, and theoretically birefringence always exists. Although the optical loss appears to be small when the sample thickness is 1 mm, the loss is still very large when the optical constant is converted to the unit of %/cm, which is commonly used for laser gain medium. It is necessary to show quantitative data on how much the birefringence component has been reduced by micro‐crystallization (i.e., downsizing the grain size of polycrystalline materials), and this will be clearer if the extinction ratio is measured in this case. This material succeeded in laser oscillation with a slope efficiency of 6.5% by improving the inline transmittance due to the decrease in birefringence due to the formation of fine grains of ceramics. To realize highly efficient and high beam quality laser generation with randomly oriented sintered compacts, it is important to study the effects of grain size, refractive index distribution in the grain and the entire sample, and birefringent light scattering on laser oscillation characteristics. If these correlations can be clarified, a breakthrough will occur in this material system.

Figure 2.24 Transmitted spectrum and loss coefficient of Nd:FAP ceramics. The dashed line is the theoretical transmittance, and the red dotted lines are calculated ones. The inset shows a 1‐mm thick Nd:FAP ceramic sample after polishing.

      Source: Furuse et al. [22] Licensed under CC BY 4.0.

E. H. Penilla et al. [23] reported a dense translucent Nd:Al₂O₃ composed of randomly oriented fine grains prepared by the SPS method. The doped concentration of Nd is 0.25–0.35%, and the sample size is thin as shown in Figure 2.25.

As shown in Figure 2.26a, when Nd with a large ionic radius is added to Al₂O₃, the Nd is subjected to strong stress and the absorption and emission spectra are broadened, resulting in tunability and ultrashort pulse oscillation. On the other hand, Nd is segregated at the grain boundary, and due to this segregation, Rayleigh scattering occurs as seen in the transmission curve shown in Figure 2.26b. The transmission characteristics have a strong wavelength dependence, and the shorter the wavelength, the stronger the Rayleigh scattering. Since the base material Al₂O₃ is a hexagonal material and the grain size is about 200 nm, the problem of birefringence has not been solved, and it seems that significant technological innovation is needed to be able to apply it as a laser gain medium.

Figure 2.25 Effect of CAPAD temperature on the relative density of undoped and samples doped with 0.25 and 0.35 at.% Nd. The inset is a picture demonstrating long‐range transparency.

      Source: Penilla et al. [23]. Licensed under CC BY 4.0.

      It is the most effective method to produce a laser gain medium having an anisotropic structure in a polycrystalline form. However, no effective idea for reducing the optical loss to the level of a single crystal has been proposed. As one solution to solve this problem, the synthesis of single crystal by the sintering method will be described in Chapter 7.

      The development of polycrystalline ceramics having optical anisotropy has not been shown theoretically or technically to have innovative results. Instead of challenging technical issues that cannot be theoretically overcome, the authors chose an unprecedented material synthesis with a new method. In recent years, the author has succeeded in synthesizing bulk single crystals by chemical transport, which sublimates polycrystals and synthesizes them at low temperature [24].

Forward single pass experimental setup for evaluating EDFA performance is shown in Figure 2.27. High‐purity alumina sintered body and carbon are prepared as starting materials, and when this material is reacted at a high temperature of higher than 1700 °C, AlO (gas) is generated. The generated AlO is deposited on a c‐axis oriented alumina substrate on the low‐temperature side using Ar–H₂ as a carrier gas, whereby a bulk crystal (single crystal) can be synthesized. Since the synthesis temperature is 1000–700 °C, the optical quality is extremely high, and the material is a high‐quality material with almost no dislocations.

Figure 2.26 (a) PL emission spectra for the 0.25 and 0.35 at.% Nd³⁺:Al₂O₃ samples along with 0.5 at.% Nd³⁺:Glass and 1.1 at.% Nd³⁺:YAG single crystal. The pump source is an 806 nm laser diode. The PL reveal broadened lines attributed to the ₄F^3/2 → ₄I^11/2 electronic transitions. (b) Transmission measurements of the Nd:Al₂O₃ and undoped Al₂O₃. All the ceramics show high transmission, and importantly, the Nd‐doped samples have absorption bands characteristic of Nd³⁺ transmission. The corresponding absorption cross sections in the area of interest are shown in the inset.

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