alt="Schematic illustration of dependence of the average output power on the absorbed pump power under quasi-cw pumping."/>
Figure 2.16 Dependence of the average output power on the absorbed pump power under quasi‐cw pumping.
Source: Chen et al. [16].
Figure 2.17 Appearance of Cr‐doped ZnSe, and Fe‐doped ZnSe ceramics by hot press and Fe:ZnSe single crystal as reference.
Source: Mirov et al. [17].
Figure 2.18 Absorption and fluorescence spectra of (a) Cr‐doped ZnSe, ZnS, and CdSe and (b) Fe‐doped ZnSe and CdMnTe.
Source: Modified from [17].
2.3.5 Fiber Ceramics as Laser Gain Media
In general, glass materials have large σ (stimulated emission cross section) and τ (fluorescence lifetime) and are materials that have a large energy storage capacity and large output of laser power. However, due to their low thermal conductivity and low repetition rate of continuous‐wave and pulse oscillation, crystalline materials are industrially being applied. The above problem can be solved by adding a laser‐active element to a glass fiber having a diameter of about several tens of μm or smaller, and the laser emitted from the fiber can simplify the optical system or it can be mounted on a robot arm and so on. Hence, the technical range is different from the lasers that use bulky solid gain media. However, regarding the fiber laser, if a crystalline fiber (ceramic) can be made instead of glass fiber, the laser gain length (fiber length) of the fiber can be shortened and the heat dissipation characteristics will be excellent, so further improvements in laser performance are highly expected.
H. Kim et al. made a slurry with a planetary mill using YAG, Ho2O3 powder, an organic binder, and ion‐exchanged water as a solvent and extruded the slurry at a pressure of 20–35 MPa using a nozzle with a diameter of 50 μm to form a fiber‐like green body [18, 19]. After drying at room temperature, it was calcined at 600 °C to remove organic components and then sintered in a vacuum furnace at 1700–1800 °C, followed by annealing at 1400–1500 °C to produce a fiber‐like sintered body. Since the surface of the obtained fiber is uneven, the surface is polished. The outer periphery of the polished fiber is coated with glass powder and then heat‐treated for cladding.
In Figure 2.19a–c, SEM micrographs of the surfaces of the polycrystalline YAG fibers with different degrees of surface polishing are shown, and it is noted that the surface roughness is finished with up to RMS = 0.03 μm. Figure 2.19d is a cross section of the as‐sintered sample, and the fiber diameter is about 20 μm, and it is a dense microstructure composed of grains of several μm.
Figure 2.19 SEM micrographs of the surfaces of the polycrystalline YAG fibers with different degrees of surface polishing: (a) shows the surface of the as‐sintered fiber and (c) is further polished than (b). Surface roughness values of (a) and (c) are given in root mean square (RMS). (d) SEM micrograph of the cross section of the polycrystalline Ho:YAG fiber before surface polishing.
Source: Kim et al. [19].
Figure 2.20 Output power as a function of input power for the HR + Fresnel configuration during power scaling efforts.
Source: Kim et al. [19].© 2017, The Optical Society.
Figure 2.20 shows the input and output characteristics when a laser test was performed using a Tm:glass fiber laser with a wavelength of 1908 nm. The output level was several tens of mW, and the slope efficiency was low at 1–4%. However, the glass cladding fiber successfully oscillates a laser beam with a wavelength of 2091 nm with a slope efficiency of 7%. Although the scattering of the ceramic fiber material has not been sufficiently controlled, this is the first successful example using fiber‐shaped ceramics.
2.3.6 Optically Anisotropic Ceramics
YAG laser ceramics developed by Ikesue in 1995, and all subsequent laser ceramics are a cubic crystal system. When a polycrystalline ceramic having a cubic crystal structure is synthesized, it basically becomes an optically isotropic body, so that it is possible to oscillate a laser with this material if the optical scattering is low. Cubic materials are not always optically isotropic, and many of the synthesized cubic ceramics contain optical anisotropy such as birefringence. Even now, only a few researchers can synthesize high‐quality ceramic laser materials worldwide.
From the viewpoint of material science, there are many fascinating laser gain materials other than the cubic system. If anisotropic ceramics can oscillate laser with high efficiency and high quality, the applications of ceramics will be further expanded. For instance, Sato et al. synthesized Yb:FAP (Yb‐doped Ca5[PO4]3F) ceramics with apatite structure and hexagonal crystal system [20].
Firstly, a slurry is prepared by mixing the raw materials with a solvent. Then, the slurry is molded by slip casting under a strong magnetic field (1.4 T) to give the preformed granules a certain orientation. After sintering the molded body at 1600 °C, it was finally treated in HIP (hot isostatic pressing) furnace at 1600 °C for one hour with a pressure of 190 MPa to obtain a transparent body. The fabricated Yb:FAP ceramics is a highly dense sintered body, and it is oriented in the c‐axis direction as shown in Figure 2.21a. In general, when a hexagonal material is synthesized with random orientation, even if the sintered body has a high density, birefringence is significant, so that the sintered body has poor linear (inline?) transmittance. The sample thickness is 0.6 mm but the transmittance in the laser oscillation region (1 μm) reaches c. 84% by orienting each grain constituting the sintered body in the c‐axis direction (see Figure 2.21b). However, since the theoretical transmittance of this material is 87%, the internal loss can be estimated to be higher than 50%/cm, so that the optical loss is more than a few hundred times larger
