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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 tomography with a measurement wavelength of 633 nm. Only, the difference in coloring of YAG‐Nd:YAG can be detected at the bonding interface by visual observation. At the surrounding area of the bonding interface, only the grain size of the YAG‐Nd:YAG ceramic is different, and there are no interstices at the interface. Further, scattering from the bonding interface cannot be detected by laser tomography measurement, and the scattering at the bonding interface can hardly be measured because the transmittance is the same as that of YAG and Nd:YAG samples having the same length. Since the same size monolithic Nd:YAG ceramic has a wavefront distortion of 0.12λ/5? inches and the same value was obtained for the composite material, there is no increase in the wavefront distortion accompanying the formation of the composite by bonding (see Figure 2.32c). With this material, an output of higher than 5 kW was obtained by side pumping with a high output 808 nm LD although it is a part of an unpublished work.

Figure 2.32 (a) Appearance of end‐cap structured YAG‐Nd:YAG‐YAG slab before and after bonding, (b) bonding inspection by laser tomography and (c) Schlieren and wavefront distortion image of composite slab.

Figure 2.33 is a large slab with a volume of 100 × 100 × 20 mm³. The core material is Nd:YAG, and the cladding material is Sm:YAG ceramics to suppress parasitic oscillation. The material was tested at the Lawrence Livermore National Laboratory (LLNL) in the United States as a laser gain medium for a heat capacity laser (SSHCL) and was found to achieve an output of 67 kW. Regarding high‐power ceramic lasers, Textron and Northrop Grumman of the United States have reported 100 kW‐class laser generation.

Figure 2.33 Nd:YAG (core)‐Sm:YAG (cladding = supersaturated absorber) composite used for heat capacity laser at Lawrence Livermore National Laboratory in the United States.

Source: Yanagitani and Yagi [27].

Various types of interesting composite laser elements are introduced below. Figure 2.34a shows a YAG‐Nd:YAG‐YAG composite in which 11 layers of materials with different compositions are bonded, and it is possible to control rapid heat generation during side excitation. Figure 2.34b shows a composite with five layers of materials having different compositions in the length direction, to control heat generation and beam quality during laser oscillation. Although the idea of composite design is correct, the problem is that it is difficult to minimize scattering because the refractive index of each layer is different. Generally, scattering occurs at the bonding interface of YAG and Nd:YAG due to the difference in their refractive index. But this new type of composite has a uniform refractive because gadolinium Gd is doped in YAG and Nd:YAG components as necessary in order to compensate for the refractive index fluctuation throughout the whole composite slab sample. Figure 2.34c shows a laser gain medium slab with 40 × 160 × t4 mm dimension. At first glance, it looks like a monolithic slab, but this slab is composed of seven components with different compositions, and its purpose is to generate a laser with high power and high beam quality. As of 2018, approximately 7 kW of laser has been successfully generated, but the ultimate goal is 20 kW per slab.

Figure 2.34 (a) YAG‐Nd:YAG‐YAG composite with 11 layers. (b) Five‐layer composite in which index mismatching controlled by doping with Gd. (c) Five‐layer composite with different Nd doping in length direction and pure YAG is attached to both sides.