Читать книгу Processing of Ceramics - Группа авторов - Страница 27

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.

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.

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

Figure 2.27 Forward single pass experimental setup for evaluating EDFA performance.

Source: Ikesue and Aung [24].

Figure 2.28 Transmission Spectra of Sapphire Crystals with 3 mm thickness by Cz method and ACT process.

Source: Ikesue and Aung [24].

As shown in Figure 2.28, the transmittance is equal to or higher than that of the sapphire single crystal synthesized by the Czochralski method, the optical loss is at least <0.1%/cm, and the UV transmittance property is excellent. Generally, when synthesizing alumina from the gas phase, the deposition rate is low, and only a film thinner than several μm can be formed. According to this chemical transport method, the crystal growth rate is as high as 5 mm/hour and a low‐temperature synthesis is possible so that the material is definitely an ultra‐high‐quality material. It is believed that laser‐active elements such as Nd have too large an ionic radius to replace Al in Al₂O₃. But taking advantage of the benefits of the low‐temperature synthesis process, the replacement of Nd ions may become possible. Once this technology is established, above‐described techniques such as fine‐grain formation and orientation control by strong magnetic field will be no more required, and in addition, there is a high possibility that not only the synthesis of optically anisotropic laser gain medium but also the synthesis of new materials that are difficult to produce even by the conventional melt‐growth method or sintering technique will be beneficial.