Processing of Ceramics. Группа авторов
are highly expected.
H. Kim et al. made a slurry with a planetary mill using YAG, Ho2O3 powder, an organic binder, and ion‐exchanged water as a solvent and extruded the slurry at a pressure of 20–35 MPa using a nozzle with a diameter of 50 μm to form a fiber‐like green body [18, 19]. After drying at room temperature, it was calcined at 600 °C to remove organic components and then sintered in a vacuum furnace at 1700–1800 °C, followed by annealing at 1400–1500 °C to produce a fiber‐like sintered body. Since the surface of the obtained fiber is uneven, the surface is polished. The outer periphery of the polished fiber is coated with glass powder and then heat‐treated for cladding.
In Figure 2.19a–c, SEM micrographs of the surfaces of the polycrystalline YAG fibers with different degrees of surface polishing are shown, and it is noted that the surface roughness is finished with up to RMS = 0.03 μm. Figure 2.19d is a cross section of the as‐sintered sample, and the fiber diameter is about 20 μm, and it is a dense microstructure composed of grains of several μm.
Figure 2.19 SEM micrographs of the surfaces of the polycrystalline YAG fibers with different degrees of surface polishing: (a) shows the surface of the as‐sintered fiber and (c) is further polished than (b). Surface roughness values of (a) and (c) are given in root mean square (RMS). (d) SEM micrograph of the cross section of the polycrystalline Ho:YAG fiber before surface polishing.
Source: Kim et al. [19].
Figure 2.20 Output power as a function of input power for the HR + Fresnel configuration during power scaling efforts.
Source: Kim et al. [19].© 2017, The Optical Society.
Figure 2.20 shows the input and output characteristics when a laser test was performed using a Tm:glass fiber laser with a wavelength of 1908 nm. The output level was several tens of mW, and the slope efficiency was low at 1–4%. However, the glass cladding fiber successfully oscillates a laser beam with a wavelength of 2091 nm with a slope efficiency of 7%. Although the scattering of the ceramic fiber material has not been sufficiently controlled, this is the first successful example using fiber‐shaped ceramics.
2.3.6 Optically Anisotropic Ceramics
YAG laser ceramics developed by Ikesue in 1995, and all subsequent laser ceramics are a cubic crystal system. When a polycrystalline ceramic having a cubic crystal structure is synthesized, it basically becomes an optically isotropic body, so that it is possible to oscillate a laser with this material if the optical scattering is low. Cubic materials are not always optically isotropic, and many of the synthesized cubic ceramics contain optical anisotropy such as birefringence. Even now, only a few researchers can synthesize high‐quality ceramic laser materials worldwide.
From the viewpoint of material science, there are many fascinating laser gain materials other than the cubic system. If anisotropic ceramics can oscillate laser with high efficiency and high quality, the applications of ceramics will be further expanded. For instance, Sato et al. synthesized Yb:FAP (Yb‐doped Ca5[PO4]3F) ceramics with apatite structure and hexagonal crystal system [20].
Firstly, a slurry is prepared by mixing the raw materials with a solvent. Then, the slurry is molded by slip casting under a strong magnetic field (1.4 T) to give the preformed granules a certain orientation. After sintering the molded body at 1600 °C, it was finally treated in HIP (hot isostatic pressing) furnace at 1600 °C for one hour with a pressure of 190 MPa to obtain a transparent body. The fabricated Yb:FAP ceramics is a highly dense sintered body, and it is oriented in the c‐axis direction as shown in Figure 2.21a. In general, when a hexagonal material is synthesized with random orientation, even if the sintered body has a high density, birefringence is significant, so that the sintered body has poor linear (inline?) transmittance. The sample thickness is 0.6 mm but the transmittance in the laser oscillation region (1 μm) reaches c. 84% by orienting each grain constituting the sintered body in the c‐axis direction (see Figure 2.21b). However, since the theoretical transmittance of this material is 87%, the internal loss can be estimated to be higher than 50%/cm, so that the optical loss is more than a few hundred times larger than that of the cubic ceramic laser gain medium.
Figure 2.22a shows the experimental setup reported by Sato et al. [20] using a 10%Yb:YVO4 single crystal as the laser oscillation medium and inserting the fabricated Yb:FAP anisotropic ceramics in the optical resonator to confirm whether laser oscillation occurs or not. (That is, if the optical quality of Yb:FAP ceramics is too poor, laser oscillation may stop.) Generally, the lasing slope efficiency of the Yb:YVO4 single crystal is approximately higher than 85%. It was successful with laser oscillation in this configuration by making the thickness of the ceramic material (0.6 mm) as a microchip design, however, as shown in Figure 2.22b, the lasing slope efficiency was saturated at around 14%. This means that the internal loss of the Yb:FAP ceramics is not satisfactory as a laser gain medium, and it will be an important technical issue on how to significantly reduce the internal loss of this type of material in the future.
Akiyama et al. succeeded in synthesizing Nd:FAP and reported a 15% scattering loss (which is five times larger than the loss in this case) in a microchip‐shaped laser element [21]. This means that it is necessary to significantly improve the optical quality of this material to be applied as a laser gain medium.
Furuse et al. reported that laser oscillation is performed using a random‐oriented Nd:FAP ceramics composed of fine grains synthesized without orienting in strong magnetic field [22]. They synthesized transparent Nd:FAP ceramics using the spark plasma sintering (SPS) method. The XRD pattern of the sintered body was similar to powder (random orientation), and the average grain size of the sintered body was as small as 140 nm. It is a dense body and does not include any secondary phases, so that it shows translucency. Figure 2.23 is an illustration of the effect of the grain size of the sintered body on the inline transmittance in hexagonal materials. This idea has already been reported for alumina ceramics with the excellent linear transmission, and it has been proven that the actual linear transmittance is also increased. When the grain size is comparable to the wavelength of light, Mie scattering occurs at grain boundaries due to a discrepancy between refractive indices of different crystal orientations. However, when the grain size is sufficiently small compared to the wavelength, Mie scattering at grain boundaries is suppressed to permit light passing through the sample.