*3.2.3. What's about fluorescence?*

The fluorescence spectra obtained using pristine YF-1, YF-2, and YF-3 fibers at 977 nm pumping are shown in **Figure 8(b)**. All these are similar in appearance and their intensities are propor‐ tional to Yb3+ ions concentrations in the fibers (The measurements were made at the same conditions and at the same pumps.)

We also measured the fluorescence spectra of the YFs after irradiation by an electron beam and after long-term OP at 977 nm, but almost no qualitative spectral changes were captured in the Yb3+ fluorescence band; so we don't provide them here. We could only notice a small decrease in the fluorescence power as the result of the treatment, but this trend could not be quantified. Furthermore, it was found that the characteristic Yb3+ fluorescence decay time slightly decreases in the set of pristine YFs. This is a result of the presence of two exponents in the fluorescence kinetics, measured by ~0.7 and ~0.2…0.3 ms. Note that insignificant growth of the latter contribution was detected for the fiber with the highest Yb3+ content (YF-3); see also Ref. [46–48]. However, the time constants obtained at fitting were nonaffected neither by e-irradiation nor by long-term OP. Concluding, we can reveal that none, or very insignificant, changes occurred in the YFs in the sense of Yb3+ fluorescence.
