ceramic laser gain medium.
Figure 2.22a shows the experimental setup reported by Sato et al. [20] using a 10%Yb:YVO4 single crystal as the laser oscillation medium and inserting the fabricated Yb:FAP anisotropic ceramics in the optical resonator to confirm whether laser oscillation occurs or not. (That is, if the optical quality of Yb:FAP ceramics is too poor, laser oscillation may stop.) Generally, the lasing slope efficiency of the Yb:YVO4 single crystal is approximately higher than 85%. It was successful with laser oscillation in this configuration by making the thickness of the ceramic material (0.6 mm) as a microchip design, however, as shown in Figure 2.22b, the lasing slope efficiency was saturated at around 14%. This means that the internal loss of the Yb:FAP ceramics is not satisfactory as a laser gain medium, and it will be an important technical issue on how to significantly reduce the internal loss of this type of material in the future.
Akiyama et al. succeeded in synthesizing Nd:FAP and reported a 15% scattering loss (which is five times larger than the loss in this case) in a microchip‐shaped laser element [21]. This means that it is necessary to significantly improve the optical quality of this material to be applied as a laser gain medium.
Furuse et al. reported that laser oscillation is performed using a random‐oriented Nd:FAP ceramics composed of fine grains synthesized without orienting in strong magnetic field [22]. They synthesized transparent Nd:FAP ceramics using the spark plasma sintering (SPS) method. The XRD pattern of the sintered body was similar to powder (random orientation), and the average grain size of the sintered body was as small as 140 nm. It is a dense body and does not include any secondary phases, so that it shows translucency. Figure 2.23 is an illustration of the effect of the grain size of the sintered body on the inline transmittance in hexagonal materials. This idea has already been reported for alumina ceramics with the excellent linear transmission, and it has been proven that the actual linear transmittance is also increased. When the grain size is comparable to the wavelength of light, Mie scattering occurs at grain boundaries due to a discrepancy between refractive indices of different crystal orientations. However, when the grain size is sufficiently small compared to the wavelength, Mie scattering at grain boundaries is suppressed to permit light passing through the sample.
Figure 2.21 (a) X‐ray diffraction pattern of FAP powder as a raw material for Yb:FAP ceramics and Yb:FAP ceramics. Diffraction from Yb:FAP ceramics were from the surface of 3 mm × 3 mm. (b) Transmission and absorption spectra of Yb:FAP ceramics.
Source: Sato et al. [20]© 2014, The Optical Society.
Figure 2.22 (a) Experimental configuration of the confirming of laser grade quality. This setup included an 880‐nm laser diode as a pump source, a delivering fiber, collimating and focusing lens, Nd:YVO4 microchip, flat output coupler, and an Yb:FAP ceramic sample. (b) Slope efficiency of Nd:YVO4 laser as a function of output coupling. 2 at.% Yb:FAP ceramics with the thickness of 0.6 mm was placed in the resonator.
Source: Sato et al. [20]© 2014, The Optical Society.
Figure 2.23 Schematics of optical scattering in fine‐grained non‐cubic ceramics.
Source: Furuse et al. [22]. Licensed under CC BY 4.0.
As shown in Figure 2.24, the transmittance is the theoretical transmittance (87%) in the laser oscillation region (1 μm band), but the transmittance becomes lower as the wavelength becomes shorter. It is considered that significant Rayleigh scattering occurs in the gain medium because it is a polycrystalline material in which hexagonal crystallites are randomly oriented, and theoretically birefringence always exists. Although the optical loss appears to be small when the sample thickness is 1 mm, the loss is still very large when the optical constant is converted to the unit of %/cm, which is commonly used for laser gain medium. It is necessary to show quantitative data on how much the birefringence component has been reduced by micro‐crystallization (i.e., downsizing the grain size of polycrystalline materials), and this will be clearer if the extinction ratio is measured in this case. This material succeeded in laser oscillation with a slope efficiency of 6.5% by improving the inline transmittance due to the decrease in birefringence due to the formation of fine grains of ceramics. To realize highly efficient and high beam quality laser generation with randomly oriented sintered compacts, it is important to study the effects of grain size, refractive index distribution in the grain and the entire sample, and birefringent light scattering on laser oscillation characteristics. If these correlations can be clarified, a breakthrough will occur in this material system.
Figure 2.24 Transmitted spectrum and loss coefficient of Nd:FAP ceramics. The dashed line is the theoretical transmittance, and the red dotted lines are calculated ones. The inset shows a 1‐mm thick Nd:FAP ceramic sample after polishing.
Source: Furuse et al. [22] Licensed under CC BY 4.0.
E. H. Penilla et al. [23] reported a dense translucent Nd:Al2O3 composed of randomly oriented fine grains prepared by the SPS method. The doped concentration of Nd is 0.25–0.35%, and the sample size is thin as shown in Figure 2.25.
As shown in Figure 2.26a, when Nd with a large ionic radius is added to Al2O3, the Nd is subjected to strong stress and the absorption and emission spectra are broadened, resulting in tunability and ultrashort pulse oscillation. On the other hand, Nd is segregated at the grain boundary, and due to this segregation, Rayleigh scattering occurs as seen in the transmission curve shown in Figure 2.26b. The transmission characteristics have a strong wavelength dependence, and the shorter the wavelength, the stronger the Rayleigh scattering. Since the base material Al2O3 is a hexagonal material and the grain size is about 200 nm, the problem of birefringence has not been solved, and it seems that significant technological innovation is needed to be able to apply it as a laser gain medium.
