Processing of Ceramics. Группа авторов
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.
2.3.7 Laser Oscillation by Composite Laser Elements
A composite is a “laser gain medium” with a complex structure in which elements with the same crystal structure and different compositions are basically bonded and aims to create functions that cannot be realized with a monolithic structure. (In exceptional cases, bonding of different materials such as YAG and sapphire has been reported, but the optical quality, etc. are unknown.) Figure 2.29 summarizes the image of composites that can be made using ceramic technology and the expected advantages from these composites. For example, an end‐cap type in which pure YAG are bonded on both ends of laser medium, a clad‐core type in which the outer periphery of the laser gain material is covered with a material with a low refractive index similar to the optical fiber, and so on. In addition, it is possible to create a high‐performance laser element that combines a self‐Q switch (laser generation and pulse generation by switching) function by bonding a laser gain medium such as Nd:YAG and Cr4+:YAG, and by cladding the periphery of the Nd:YAG disk with Sm:YAG (or cladding the periphery of the Yb:YAG disk with Cr4+:YAG). This cladding design also can prevent parasitic oscillation when a high‐power laser is generated. Therefore, advanced ceramic bonding technology can realize functions that have not been realized until now. For these composites, basically bonding technology must provide a seamless state of bonding in ceramics, unless it may cause optical problems such as scattering or distortion caused by bonding. Details of these results are explained in Chapter 7.2.
Figure 2.29 Various configurations of producible composite element and their technological functions.
Source: Ikesue and Aung [25].
Composite technology enables laser elements with complex design, resulting in higher laser beam quality, higher power, and new functionality, etc. Here are some examples. Figure 2.30a shows a waveguide laser element having a three‐layer structure of YAG‐Nd:YAG‐YAG. The dimension of the sample is 12 × 32 × t1.2 mm, and it has a 400 μm core (0.6% Nd:YAG), cladding with YAG (thickness, 400 μm) on both sides of 12 × 32 mm. Figure 2.30b shows the setup of the laser oscillator. Cooling was performed using a 250 W chiller and side‐excitation scheme pumping with a LD (808 nm, max. Output 500 W). Figure 2.30c shows the output characteristics of the waveguide laser. The slope efficiency was 46% and the maximum output was 120 W. By cladding YAG which has a lower refractive index than the core Nd:YAG, the laser light is amplified while zigzag propagating inside the core (and does not propagate in the same place), so that a laser with high beam quality can be obtained.
Figure 2.30 (a) Appearance of YAG‐Nd:YAG‐YAG waveguide structured composite ceramics, (b) setup of laser oscillator, (c) laser performance of waveguide laser ceramics.
Figure 2.31 (a) Appearance of cylindrical clad‐micro‐core structured composite, (b) doping profile of Nd ions in composite, (c) laser gain of transverse mode, (d) longitudinal mode, and (e) oscillation performance by composite ceramics.
Source: Zheng et al. [26].
Figure 2.31a shows a composite laser element having a microcore‐clad structure. Core material is 0.6%Nd:YAG and its diameter is approximately 400 μm. Figure 2.31b shows the Nd concentration distribution in the core. Although the core is very small, the Nd concentration distribution is formed from the center of the core to the outer periphery with a Gaussian distribution. The transverse mode of the laser light obtained by exciting this material with a LD light having a wavelength of 940 nm has the function shown in TEM00 mode. (TEM is the abbreviation of transverse electro magnetic wave, and 00 means transverse mode with single mode.) Figure 2.31c shows the oscillation spectrum of the obtained laser. The longitudinal mode (longitudinal) is also single (i.e., a single wavelength). Figure 2.31d,e show the laser oscillation characteristics using this laser element. The laser has high efficiency with a single mode in the vertical and horizontal directions.
Figure 2.32a shows the appearance of a composite element of a slab on an end cap (YAG‐Nd:YAG‐YAG), and Figure 2.32b shows the scattering state near the bonding interface measured by a laser