## 3 Results
### Characterization of Fibers  
Prepared preforms were drawn into optical fibers with outer diameters of 125 \(\pm\) 2 \(\upmu\)m and core diameters of 11 \(\upmu\)m. Selected properties of the optical fibers are summarized in Table 1. The fiber numerical apertures were calculated according to Equation (1):  
\[\text{NA}=\sqrt{n_{\text{core}}^{2}-n_{\text{cladding}}^{2}} \tag{1}\]  
where \(n_{\text{core}}\) is the refractive index of the core and \(n_{\text{cladding}}\) is the refractive index of the silica cladding, which is equal to 1.457. The calculated numerical apertures were proportional to the concentrations of dopants and reached the highest value of 0.162. Cutoff wavelengths of LP\({}_{11}\) mode ranged between 1080 and 1175 nm. These results lead to a conclusion that the drawn fibers were single-mode at lasing wavelength. Spectral attenuation of the fiber prepared at the flow rate of SiCI\({}_{4}\) equals to 400 sccm is shown in Figure 7. The most important parameters are listed in Table 1. The absorption spectrum exhibited a set of the absorption peaks typical of Er\({}^{3+}\) ions. The recorded background losses increased with increasing flow rate of SiCI\({}_{4}\) and achieved the maximal values of 0.022 and 0.138 dB-m\({}^{-1}\) at 850 and 1310 nm, respectively. The attenuations of fibers at 1310 nm were slightly higher because this spectral region overlaps the absorption peak of free -OH groups. The concentration of OH group was estimated to be -5 at. ppm. Although all prepared fibers exhibited wave-guiding properties, the final concentration of Er\({}^{3+}\) ions inside the fibers was not sufficient for the luminescence at 1550 nm to be observed, except the fiber prepared at the flow rate of SiCI\({}_{4}\) equal to 400 sccm. Therefore, only that fiber was used for the luminescence measurements and in the fiber ring laser. Although low erbium concentration limits the use of the fiber samples to fiber lasers with relatively long-cavity (several meters or more), the low concentration of this sample of about 110 at. ppm Er\({}^{3+}\) is beneficial in terms of mitigating energy up-conversion processes that often leads to decrease of the fiber laser efficiency [31]. To eliminate the cross-relaxation and amplified spontaneous emission effects, a technique of extrapolation from various pump powers to the \"zero\" value of the pump power was used. The extrapolations of the fluorescence lifetime values of the transition \({}^{4}\)I\({}_{13/2}\)\(\rightarrow\)\({}^{4}\)I\({}_{15/2}\) (at the 1/e level) evaluated for various fiber lengths and pump powers are shown in Figure 8. The values of measured lifetimes regularly decreased for longer fiber length and for higher pump powers. The lifetime value, extrapolated to the \"zero\" fiber length, was 10.6 \(\pm\) 0.1 ms. The extrapolation to the \"zero\" input power resulted to the lifetime 10.586 \(\pm\) 0.05 ms. The prepared fiber was tested as a gain medium in a fiber ring laser. This configuration was used in order to eliminate the risk of longitudinal mode instability and self-pulsing [32; 33]. The fiber was spliced into the ring laser setup and the dependence of the laser output power-Poutput-on the input pump power-Ppump-was measured. Laser thresholds for particular fiber lengths were calculated as the intercepts of linear fits from the dependence of the laser output power on the input pump power. Laser  
Figure 7: Absorption spectrum of the fiber prepared at flow rate of SiCI\({}_{4}\) equal to 400 sccm. output characteristics are shown in Figure 9. The laser threshold regularly increased with increasing length of the active fiber.