Processing of Ceramics. Группа авторов
pumped by 940 nm LD.
Source: Pawlowski et al. [5].
(b) High‐power laser generation of 10%Yb:YAG thin‐disk ceramics without sintering additive.
Source: Ikesue and Aung [6].
By the way, a trace amount of SiO2 sintering aid is added to almost all garnet type laser ceramics developed in the past. The sintering aid is very effective for making the material transparent, lowering the sintering temperature, and shortening the sintering time, but it also has disadvantages. The author pointed out the disadvantages of the sintering aid, especially the problem of heat generation (especially for high‐power laser operation) in the wavelength region of 1 μm or longer. Laser ceramics that do not use sintering aids are ideal, but other than this result [6, 7] has not been reported. Due to the development of high‐purity ceramics, recent research has confirmed that the output exceeds 5.5 kW from a single Ф10 × 0.15 mm thin disk (excitation area is Ф8 × 0.15 mm) although it is a part of an unpublished work. In addition, the power density (output per 1 cm3 of gain medium) reaches about 0.36 MW/cm3 (nearly 1 MW/cm3 in the latest case), it was just the era of super high output power by ceramic laser technology. Although there are reports (literature) that laser oscillation of 10 kW was successful using an Yb:YAG single crystal from a thin‐disk shape [8], it is necessary to investigate with the same resonator when comparing laser characteristics between the laser materials. In our experiments, it was possible to generate 1 kW laser even with a single crystal material, but we confirmed that cracks occasionally occurred during oscillation. Although the grain boundaries of ceramics have been considered disadvantageous for optical properties, this is also simple speculation. On the contrary, we must also consider the development of a laser gain medium with strong laser damage property using the grain boundaries inherent in ceramics.
Figure 2.6a is a photograph of the appearance of 0.6%Nd:YAG (slab) ceramics before polishing. This slab was made by bonding three samples of 55 × 55 × 6 mm size. The slab after polishing is 40 × 155 × t2.5 mm, and both surfaces are polished at the Brewster angle (the angle where there is no surface scattering when the laser passes through the gain medium). Figure 2.6b shows the input/output characteristics of the laser when this material is side pumped by a high‐power LD of 808 nm. The maximum output was 4.3 kW, and the slope efficiency was 37%. Although this slab is a monolithic structure fabricated by diffusion bonding, the bonding interface after this 4.3 kW laser generation test was no damage at all. From this result, it is shown that the bonding cross‐sectional area with respect to the maximum output of the ceramic laser gain medium can sufficiently withstand a laser power density of 4.3 kW/cm2.
Figure 2.6 (a) Appearance of 0.6%Nd:YAG slab ceramics (165 × 55 × 6 mm) which bonded three pieces of Nd:YAG ceramics with 55 × 55 × t6 mm. (b) Input of 808 nm diode laser vs. output powder of 1.06 μm laser from ceramics.
In garnet‐based materials, disordered solid solution gain media are interesting materials. Figure 2.7 shows the emission spectrum of a laser medium with Nd:Y3ScxAl5−xO12 composition in which the x value is serially changed from 0 to 2.0. That is, if Al sites in the YAG material are substituted with Sc, spectral shift, and broadening occur. This spectral shift is very effective for making the laser tunable and short pulse oscillation.
Figure 2.8a shows a setup of the laser oscillator for 15%Yb:Y3Sc1Al4O12 ceramic (Ф10 mm, t = 1 mm) which was used as the laser oscillation medium, and a commercially available Ti:Sapphire laser was used as the excitation source. Figure 2.8b shows the CW laser oscillation characteristics obtained from the laser oscillator. The efficiency depends on the transmittance of the output coupler, and a maximum slope efficiency of 72% was obtained at T = 20%. Figure 2.8c shows an oscillation wave type of a short pulse laser obtained by mode‐locked oscillation. The oscillation wavelength width was 4.1 nm, and the pulse width was 280 fs. The spectral linewidth of Yb:YSAG ceramics is wider than Yb:YAG (similar to Nd:YSAG), making it easier to generate short‐pulse lasers.
Figure 2.7 Emission cross sections of Nd:Y3ScxAl5−xO12 ceramics (x = 0.3–2.0) and Nd:YAG single crystal (x = 0) at room temperature.
Source: Sato et al. [9], Image courtesy of The Ceramic Society of Japan.
2.3.3 Synthesis and Laser Performance of Sesquioxide Ceramics
Sesquioxide has been attracting attention as a promising laser host. For example, in the case of Y2O3, since the melting point is above 2400 °C, the Bridgman method requires a crucible such as Re (rhenium). In addition, at around 2200 °C after solidification, distortion, and cracks occur inside the material due to a volume change accompanying the transition from hexagonal crystal system to the cubic crystal system. It is impossible to produce a high‐quality single crystal by the conventional melt solidification method. The author himself has ever tried to produce a Y2O3‐Tb2O3‐based single crystal material by the FZ method. As shown in Figure 2.9, a solid substance (crystal) is formed but when it is analyzed with EBD (electron‐beam diffraction), the second phase is also formed in the produced crystal. It was not possible to synthesize an optically isotropic and homogeneous substance.
If the sesquioxide ceramic materials can be synthesized at a temperature below the phase transition point and a low temperature, problems unique to this material can be solved. In the 1980s, Tsukuda et al. attempted to synthesize transparent Y2O3 ceramics, but the grain size was very large, and it only showed a slight translucency. In 1974, Greskovich succeeded in synthesizing Nd:10%ThO2‐Y2O3 ceramics and also pulsed laser oscillation at room temperature but lasing efficiency was about 0.1%. After this achievement, quality improvement was performed but there has been no significant progress at all. Since a large amount (10%) of radioactive ThO2 was added as a sintering aid in this material, a large amount of secondary phase (precipitates = scattering source) named as “orange peel” generated. For this reason, optical distortion in the material caused by the “orange peels” becomes significant, and the broadening of absorption and emission spectrum occurs. Also, a large amount of point defects is appeared as Th4+ replacing Y3+ in the host material (Y2O3), and it adversely affected the laser characteristics. Therefore, (i) low‐temperature synthesis, and (ii) lower amount of sintering aid are the keyword for the synthesis of sesquioxide ceramics.