Figure 2.25 Effect of CAPAD temperature on the relative density of undoped and samples doped with 0.25 and 0.35 at.% Nd. The inset is a picture demonstrating long‐range transparency.
Source: Penilla et al. [23]. Licensed under CC BY 4.0.
It is the most effective method to produce a laser gain medium having an anisotropic structure in a polycrystalline form. However, no effective idea for reducing the optical loss to the level of a single crystal has been proposed. As one solution to solve this problem, the synthesis of single crystal by the sintering method will be described in Chapter 7.
The development of polycrystalline ceramics having optical anisotropy has not been shown theoretically or technically to have innovative results. Instead of challenging technical issues that cannot be theoretically overcome, the authors chose an unprecedented material synthesis with a new method. In recent years, the author has succeeded in synthesizing bulk single crystals by chemical transport, which sublimates polycrystals and synthesizes them at low temperature [24].
Forward single pass experimental setup for evaluating EDFA performance is shown in Figure 2.27. High‐purity alumina sintered body and carbon are prepared as starting materials, and when this material is reacted at a high temperature of higher than 1700 °C, AlO (gas) is generated. The generated AlO is deposited on a c‐axis oriented alumina substrate on the low‐temperature side using Ar–H2 as a carrier gas, whereby a bulk crystal (single crystal) can be synthesized. Since the synthesis temperature is 1000–700 °C, the optical quality is extremely high, and the material is a high‐quality material with almost no dislocations.
Figure 2.26 (a) PL emission spectra for the 0.25 and 0.35 at.% Nd3+:Al2O3 samples along with 0.5 at.% Nd3+:Glass and 1.1 at.% Nd3+:YAG single crystal. The pump source is an 806 nm laser diode. The PL reveal broadened lines attributed to the 4F3/2 → 4I11/2 electronic transitions. (b) Transmission measurements of the Nd:Al2O3 and undoped Al2O3. All the ceramics show high transmission, and importantly, the Nd‐doped samples have absorption bands characteristic of Nd3+ transmission. The corresponding absorption cross sections in the area of interest are shown in the inset.
Source: Penilla et al. [23]. Licensed under CC BY 4.0.
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
