with 1 μm light source, (b) sample with grain boundary phase, and (c) sample with no including anisotropic phase measured by tomography.
Figure 1.15a shows the setup of the laser tomography used for detecting the light scattering inside the material (the light source has a wavelength of 1 μm). Here, the material is irradiated with a laser beam, and scattered light inside the material is captured with a CCD camera installed in a perpendicular direction. Figure 1.15b shows a material with a grain boundary phase, Figure 1.15c shows a sample with no grain boundary phase and a scattering coefficient equivalent to that of a single crystal. In the sample shown in Figure 1.15b, scattering from the grain boundary area can be easily detected, but in the sample of Figure 1.15c it is difficult to detect scattered light. Recently, we fabricated Yb:YAG ceramics (size 80 × 190 × t6 mm) for high power laser oscillation. After optical polishing and AR coating, then its optical loss at 1 μm was measured. Its scattering loss was as low as 0.02%/cm. We confirmed extremely low scattering. Generally, the optical loss of a single crystal is 0.1%/cm, but nowadays, the optical loss of polycrystalline ceramics is significantly lower than that of the single crystal. Although the obtained results cannot be understood from the conventional material science, actually how to consider these real facts is to open the “further science doors.” As stated at the beginning, “pursuing the truth of natural science and fully understanding the theory” leads to a conclusion. In other words, there seems to be a possibility that the pursuit of natural science has been insufficient, and that the future of ceramic materials could be easily predicted with insufficient understanding of the theory.
In Figure 1.16a, the outside landscape can be clearly observed through the fabricated Nd:YAG ceramic rod with a length of 300 mm (size is 10 × 10 × L300 mm), because scattering of this material is equal to or higher than that of high‐quality single crystal. The Schlieren image and the transmitted wavefront image of this material are also shown in Figure 1.16b,c, respectively, but the inhomogeneous parts cannot be detected inside the material and very low scattering and high uniformity guarantee the optical quality. Figure 1.16d shows the result of irradiating the ceramic sample with a He–Ne laser. It is understood that the quality of this material is so high that internal scattering cannot be visually observed.
Figure 1.16 (a) Appearance of Nd:YAG ceramics with 300 mm length, (b) Schlieren image and (c) transmitted wavefront image of this ceramics, (d) condition of internal scattering line when He–Ne laser irradiated into Nd:YAG ceramics (we cannot detect scattering line in this ceramic)
Figure 1.17 shows an appearance of a high‐quality Nd:YAG single crystal (manufactured by the Cz method, 10 × 10 × L 25 mm) having the same composition as the 1% Nd:YAG ceramics (Φ10 × L25 mm) produced in this study and the transmitted wave front distortion measured by the interferometer.
The laser beam pattern when irradiated with He–Ne laser (wavelength 633 nm) is also shown. Although it is difficult to distinguish the appearance of ceramics and single crystal, the transmitted wavefront distortion per inch length of ceramics is λ/19.5, that is, 0.051λ/in. (λ = 633 nm, the smaller the wavefront distortion is, the more uniform the gain material). It is generally about 0.10λ/in. for single crystals. In fact, this result is the most advantageous condition since the wavefront distortion measurement was taken with respect to the crystal growth direction after cutting off the high‐quality part of the produced single crystal ingot. There are countless facets (nonuniform parts) in the direction perpendicular to crystal growth, which makes it difficult to measure wavefront distortion. When these materials are irradiated with a Gaussian mode, He–Ne laser, the beam patterns of the transmitted lasers of both samples were similar to the initials (this meaning is considered to be some reasons that (i) the optical quality of both materials happens to be similar accidentally at the irradiated part and (ii) the difference cannot be detected by using only one pass of detection light for both materials with different wavefront distortion). However, the same result is obtained even when the laser is irradiated in the direction perpendicular to the main measurement direction in ceramics (that is, there is no optical anisotropy of the optical quality), whereas in the case of the single crystal material, the laser is irradiated in the vertical direction, distinctive beam distortion is confirmed. By excluding the anisotropy of the optical quality, the single crystal material is utilized so that there is no big problem in laser application. Without limiting to YAG materials, single crystals remain a major problem as optical materials, and even as long as 50 years have passed since the report of laser oscillation by Nd:YAG single crystal in 1964, even an idea for solving those conventional problems has not been proposed.
Figure 1.17 Comparative data of (a) Polycrystalline and (b) single crystal by Czochralski growth of 1%Nd:YAG materials by appearance, wave front distortion, beam patter of He–Ne laser which passed through test sample.
The author has advanced further research even in materials systems other than YAG. As shown in Figure 1.18, the transmission behavior of single crystal and ceramics of spinel ceramics was investigated. Both of the transmission characteristics in the infrared to visible wavelength range are equivalent, but the transmission characteristics of ultraviolet wavelength range of ceramics composed of fine grains exceeded single crystal.
Furthermore, as seen in the above inset figure, ceramics could transmit through to shorter than 200 nm, where the single crystal was hard to transmit. The optical band gap of spinel ceramics is 6.8 eV (absorption edge is 180 nm), far superior to 4.9 and 5.5 eV of the same single crystal fabricated by Bernoulli method or CZ method. It has been reported that the lasing characteristics of ceramics have also approached that of single crystals as the transmission properties of ceramics have approached single crystals, and some scientific understandings have been achieved. In conventional perception; however, it is theoretically impossible for the optical characteristics of polycrystalline ceramics to exceed that of single crystals in the short wavelength region, in particular, in the ultraviolet region where Rayleigh scattering is significant due to the presence of grain boundaries (dislocations). Nevertheless, the achievements in this invention fundamentally reverse the conventional perception.
Figure 1.18 Transmittance spectra between UV (vacuum) ~ infrared wavelength of most excellent polycrystalline Spinel ceramics, single crystal Spinel by Verneuil and Czochralski method.
