## 4 Discussion  
Final concentrations of the dopants in the fibers depend on the porosity of the silica frit as well as on the nanocrystal size, composition, and affinity toward the silica frit [6]. The silica frit porosity is proportional to the deposition flow rate of SiCl\({}_{4}\)[34]. The center of the Gaussian pore size distribution shifts from 100 to 500 nm for flow rates between 100 and 400 sccm. The affinity depends on the nanocrystal composition, solvent, or applied surfactant. The structural methods, XRD and DLS, provided different nanocrystal's sizes. The exact determination of nanocrystal sizes is a rather complex task limited by the fundamental principles, capabilities and resolutions of analytical methods available. XRD analysis provides information on the size of primary nanocrystals. The primary nanocrystals are usually aggregated into larger clusters. Sizes of these clusters dispersed in a solvent can be measured by dynamic light scattering which provides the information about the behavior of aggregated nanocrystals in the solvent. This measurement is the most important for the deposition because the size of the formed clusters limits their penetration into the silica frit. Comparing the mean nanocrystal's size of Er:YAG calculated from XRD analysis, i.e., 140 nm, with the effective diameter of the clusters calculated from DLS, i.e., 234 nm, we can estimate that several primary nanocrystals are aggregated together forming larger clusters. The presence of isolated primary nanocrystals or clusters larger than 320 nm were an exception as was reported in Figure 3 and supported by TEM observations in Figure 4. However, none of these method allows us to determine low concentrations of amorphous nanocrystals and therefore direct observation by electron microscopy is necessary. Contrary to expectations, the analysis of the synthetized Er:YAG nanopowder confirmed the presence of two kind of nanoparticles: nanocrystalline Er:YAG and side-formed amorphous aluminum oxide. The concentration of side-formed amorphous aluminum oxide nanoparticles in synthetized Er:YAG was very low and their presence cannot be effectively detected by conventional structural methods, except for TEM. The unexpected presence of aluminum oxide nanoparticles could be attributed to the crystallization kinetics of YAG. It has been found that several processes contribute to the crystallization kinetics of YAG prepared by bottom-up approaches [35] and Y\({}^{3+}\) ions are incorporated into YAG matrix prior to the Al\({}^{3+}\) ions [35; 36]. The crystallization kinetics, that strongly depends on the heating-rate [35], causes formation of a diffusion barrier on the nanocrystal surface which could lead to a formation of unreacted amorphous aluminum oxide nanoparticles in the prepared nanocrystalline powders. The small size and the isoelectric point of aluminum oxide nanoparticles supported their preferential adsorption into the porous silica frit resulting to an increase in the concentration of Al\({}^{3+}\) ions in the fibers in comparison with the stoichiometric ratio of YAG nanocrystals. Similar behavior during the crystallization and deposition into the silica frit can also be expected for other ternary nanocrystals. Optical properties of the fibers significantly depend on the nanocrystal size and frit's porosity: Attenuation of optical fibers can be divided into intrinsic and extrinsic components [13]. The intrinsic attenuation is given by optical properties of core material, i.e., silica glass. It covers material absorption, chromatic dispersion, scattering phenomena etc. It corresponds to the background losses of optical fibers. Recently prepared telecommunication optical fibers usually reach a theoretical value of 0.4 dB\(\cdot\)km\({}^{-1}\)[28]. Rare-earth doped optical fibers usually have background losses on the level of 0.05-0.5 dB\(\cdot\)m\({}^{-1}\)[6; 22; 37]. The reason can be found in the formation of local fluctuations inside the glass structure which enhance scattering effects.