## 4 Discussion
These fluctuations strongly depend on properties of the silica frit including its porosity [34; 37]. The Rayleigh scattering on the incorporated nanocrystal contributes to background losses [6]. Optical absorption of dopants and impurities presented in the fibers cause the extrinsic attenuation. Because the MCVD technology, including its nanocrystal-doping modification, provides extremely pure materials, the extrinsic attenuation practically corresponds to the absorption of introduced dopants and free -OH groups remaining in the silica glass matrix. The observed -OH concentration was higher than in the case of conventional solution doping with chlorides salts, has been already reported by doping the solution with LaF3 nanoparticles [38]. From the general point of view Er\({}^{3+}\) ions can be distributed inside the original YAG matrix, inside the amorphous or semi-amorphous Al\({}_{2}\)O\({}_{3}\) matrix or inside host silica matrix. The crystallinityand the chemical composition of the individual matrixes can significantly change the splitting of the energy levels of Er\({}^{3+}\) ions. Such a splitting should change the pronouncement of the absorption and emission bands and results to the existence of several luminescence decays [39].The observed luminescence lifetimes of the Er:YAG doped fibers exhibited single-exponential character and they were slightly shorter than 10.98 ms, which value was predicted for polycrystalline Er:YAG [40]. Besides an alternative reaction of the nanoparticles with the host silica matrix and its presence in side-formed Al\({}_{2}\)O\({}_{3}\), such a difference can be explained by the nanocrystal size. Smaller nanocrystals have larger specific surface area than the larger ones. The specific surface area is proportional to the concentration of surface defects contributing to the non-radiative energy transitions [12]. Considering a homogenous distribution of Er\({}^{3+}\) ions in nanocrystals, the smaller nanocrystals have higher concentration of Er\({}^{3+}\) ions at the nanocrystal's surface than the larger ones. Consequently, the nanocrystal size is inversely proportional to the quantum yield of the luminescence. It should be pointed out that the requirements imposed on nanocrystals in terms of fiber attenuation and luminescence properties are mutually contradictory. Smaller nanocrystals can reduce the Rayleigh scattering and thus the fiber attenuation. However, smaller nanocrystals can exhibit shorter luminescence lifetime. Considering the optimal performance of the fiber ring laser, the requirements are predominantly placed on high slope efficiency, low threshold power and short lengths of the employed fibers. These complex requirements were fulfilled for the fiber length of 7 m, which can be regarded as optimal. Despite the novel composition of the incorporated nanocrystals the results do not significantly differ from those achieved with fibers doped with now widely used aluminum oxide nanocrystals. More than a decade of intense research in the world-leading laboratories has led to the preparation of aluminum oxide nanocrystals doped fibers exhibiting lifetimes ranging from 10.3 to 11.8 ms [6; 20] and laser slope efficiency ranging from 8% to 75%. It should be pointed out that the reported results were achieved for a monolithic Fabry-Perot fiber laser setup, the slope efficiency of which is usually higher. However, the application of YAG nanocrystals opens new horizons for further tailoring laser parameters. Lower background losses and higher concentrations of dopants can be achieved by the incorporation of smaller and more uniform Er:YAG nanocrystals which could penetrate even in very small pores of the frit. Moreover, the Er\({}^{3+}\) ions incorporated inside the YAG matrix can be during the synthesis simply replaced by another rare-earth element allowing us to shift or extend the operating wavelength range of the laser.