with the transmittance curve of Sapphire single crystal as a reference. In the case of alumina whose crystal structure belongs to hexagonal system, even the thickness of samples is as thin as 1 mm, their transmittances are very different to each other. This means that there will be a significant difference in transmittance when the thickness becomes larger. Optical loss is an important factor for knowing the transparency of the material. Even for materials with cubic crystal system such as MgO and Spinel (MgAl2O4), the optical loss of these transparent ceramics is remarkably larger than that of their single crystal counterparts. The optical performance of transparent ceramics was very poor in the material synthesis technology before laser ceramics was reported, and therefore, industrial utilization of translucent ceramics was extremely limited.
Figure 1.4 Relationship between wavelength and in‐line transmittance for commercial sapphire single crystal, Al2O3 ceramics by hot pressing, and Al2O3 ceramics by pressureless sintering.
Let us consider the relationship between microstructure of polycrystalline ceramics and its transmission characteristics. An image of light scattering caused by various types of microstructure defects in a common ceramic is illustrated in Figure 1.5. As shown in the figure, ceramics is composed of fine grains (single crystals) with random crystal orientation. Light scattering factors in ceramics are (i) existence of grain boundaries, (ii) residual pores localized at grain boundaries or inside grains, (iii) inclusions, (iv) reflection or double refraction due to grain boundaries, and (v) surface roughness. However, the fundamental differences on the microstructure affecting the light scattering between the single crystal material and the ceramic material are as follows: (i) the presence of grain boundaries and (ii) volume of residual pores. It is believed that these are the main sources of scattering and cause a large difference in optical performance.
On the other hand, Dr. Greskovich of GE developed a transparent Nd‐doped 10%ThO2‐Y2O3 ceramics, and in 1974, he succeeded in laser oscillation at room temperature with this polycrystalline ceramic for the first time in the world. 10%ThO2 was added as sintering aid to Y2O3, and after sintering it at high temperature of 2200 °C for about 100 hours, he succeeded to obtain transparent Nd:ThO2‐Y2O3 ceramics (about 10 years before his success in room temperature laser oscillation, there was a report on laser oscillation in cryostat system using Dy:CaF2 ceramics, but details of the physical properties of these ceramics are unknown).
Reflection micrograph and transmission micrograph of 1%Nd:ThO2‐Y2O3 ceramics are shown in Figure 1.6 [6, 7]. In the reflection micrograph, residual pores are observed on the surface, and microstructure defects (voids and grain boundary phases) are observed in the transmission micrograph. Also, in visual observation, it is a heterogeneous structure naming “orange peel.” Optical quality of this transparent ceramic material is remarkably higher than the conventional translucent ceramics. Although the level of optical loss of this ceramic reached about several %/cm, it is poor optical characteristics as laser material. It was succeeded at pulsed laser oscillation at room temperature by lamp excitation, but the slope efficiency was 0.1%, which is a very low result. His technology of successful laser oscillation at room temperature was the world's highest level in the 1970s, and laser oscillation by ceramics is a great dream for material scientists, but the next challenger did not appear for nearly 20 years afterward. This fact is also evidence that no one could exceed his technology and ideas in 20 years.
Figure 1.5 Illustration of optical scattering caused by various microstructure defects in common polycrystalline ceramics.
Let us discuss about the very strict requirement of optical properties for laser material in the following. Figure 1.7 shows the optical loss of material and the relative value of laser amplification caused by the optical loss. By continuously adding external energy (incoherent light) into the laser material, it is converted into “coherent light” in the laser gain medium by light amplification.
For this reason, the generated laser light repeats light amplification while reciprocating in the medium many times. At this time, if the optical loss in the medium is zero, it is 1n (n is the number of reciprocations in the medium). If the optical loss of the medium is 0.1 and 1%/pass and light amplification is carried out 10 times, then the output is computationally (0.999)10 = 0.99 and (0.990)10 = 0.90 (i.e., reduction of 1 and 10% output power, respectively). In the case that the light amplification is carried out 100 times, then the output is (0.999)100 = 0.90 and (0.990)100 = 0.36 (i.e., reduction of 10 and 64% output power, respectively), which leads to a significant reduction in lasing efficiency; therefore, laser amplification using a laser material with an extremely low optical loss is essential (this calculation simplifies the laser power subtracted by optical loss inside the gain medium).
Figure 1.6 (a) Reflection and (b) transmission microscopic photograph of 1%Nd:ThO2‐Y2O3 ceramics by Greskovich.
Source: Akio Ikesue, Yan Lin Aung, Voicu Lupei (2013), Ceramic Lasers, Cambridge University Press. https://doi.org/10.1017/CBO9780511978043.
Figure 1.7 Relationship between optical scattering loss and amplifying number of optical resonators depend on laser power.
1.4 Objective of Optical Grade Ceramics
The translucent ceramics that began to be developed in the late 1950s are used in high‐pressure sodium vapor lamp, and PLZT ((PbLa)ZrO3) ceramics are applied to flash‐protecting shutters utilizing the electro‐optic effect, after that development on various types of materials and application research were conducted, but research and development stagnated because fundamental problems related to optical technology cannot be solved. The purpose of writing this book is to describe; (i) how the author challenged with what kind of idea to develop the “optical grade polycrystalline ceramics” which is far superior to the conventional translucent ceramics and comparable to the optical grade single crystalline materials, (ii) how to evaluate the optical quality of fabricated optical ceramic materials, and (iii) how to feedback the obtained information from the evaluation into the fabrication process and so on.
Conventionally, optical grade single crystalline materials are produced by various methods such as Verneuil (VN), floating zone (FZ), Bridgman (BM), and Czochralski (Cz), and basically, the crystals are
