**3.3. Discussion**

Summarizing all the data, we notice that either at e-irradiation or at resonant OP substantial and complex but different in appearance changes arise within the resonant absorption band of Yb3+ ions ("reversible bleaching"), while monotonous growth of nonresonant background loss is observed in VIS ("darkening"). Furthermore, these trends are revealed to stem from the changes in concentrations of Yb3+ ions and, seemingly, of other centers, closely related to them and spectrally matching them near 977 nm. This is the main news of this study.

A general consequence of the experiments on e-irradiation, rise of background nonresonant loss in YFs in VIS (see **Figure 10 (c, d))**, is not surprising. This loss correlates spectrally with the excess loss arising in optical fibers at other types of irradiation (X-rays, *γ*-quanta, UV [33– 35]). Some other aspects are as follows:


Thus, the presence of Yb3+ dopants in the fibers results in a more pronounceable degra‐ dation at e-irradiation, with a probable reason being that Yb3+ ions are powerful sources of secondary carriers (electrons and holes) born at e-irradiation. That is, the changes within the resonant-absorption band of Yb3+ may stem from excitation of inner-shell (*f*) electrons of Yb3+ and their valence transformation through the charge-transfer (CT) processes (direct and reversed), sketched by the following reactions [36]: *e- + Yb3+ → Yb2+; e+ + Yb2+ → Yb3+*, where e– and e+ stand for secondary (irradiation induced) electrons and holes, and Yb2+ is the notation for Yb ions in 2+ valence state. In turn, the presence in the fibers of secondary carriers as the result of e-irradiation can produce such defects as oxygen-deficit center (ODC) and NBOH centers [47]. These centers are known to be responsible for the wide excess-loss spectral bands similar to the ones produced in the darkened fibers **(Fig‐ ures 9** and **14)**.

*3.2.3. What's about fluorescence?*

22 Radiation Effects in Materials

conditions and at the same pumps.)

35]). Some other aspects are as follows:

**3.3. Discussion**

changes occurred in the YFs in the sense of Yb3+ 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

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,

Summarizing all the data, we notice that either at e-irradiation or at resonant OP substantial and complex but different in appearance changes arise within the resonant absorption band of Yb3+ ions ("reversible bleaching"), while monotonous growth of nonresonant background loss is observed in VIS ("darkening"). Furthermore, these trends are revealed to stem from the changes in concentrations of Yb3+ ions and, seemingly, of other centers, closely related to them

A general consequence of the experiments on e-irradiation, rise of background nonresonant loss in YFs in VIS (see **Figure 10 (c, d))**, is not surprising. This loss correlates spectrally with the excess loss arising in optical fibers at other types of irradiation (X-rays, *γ*-quanta, UV [33–

**1.** A monotonic increase of the background loss in VIS (darkening) with increasing Yb3+ content in the YFs, which demonstrates that the presence of Yb3+ dopants leads to a higher

**2.** A notable decrease followed by equally notable increase arising in the resonant-absorption peaks of Yb3+ (at 920 and 977 nm) with increasing e-irradiation dose **(Figure 10 (a, b))**, the

Thus, the presence of Yb3+ dopants in the fibers results in a more pronounceable degra‐ dation at e-irradiation, with a probable reason being that Yb3+ ions are powerful sources of secondary carriers (electrons and holes) born at e-irradiation. That is, the changes within the resonant-absorption band of Yb3+ may stem from excitation of inner-shell (*f*) electrons of Yb3+ and their valence transformation through the charge-transfer (CT) processes (direct and reversed), sketched by the following reactions [36]: *e- + Yb3+ → Yb2+; e+ + Yb2+ → Yb3+*,

and spectrally matching them near 977 nm. This is the main news of this study.

degree of the fibers' degradation at e-irradiation **(Figure 11(a))**.

effect also dependent on Yb3+ concentration **(Figure 11(b))**.

Qualitatively similar observations can be made regarding the spectral transformations in the YFs as the result of OP at 977 nm (refer to **Figure 11(c, d)** and **Figures 12–15**). Analo‐ gously, the following trends are drawn:


The observations (3–5), when gathered together, tell us that PD in the YFs at high-power, longterm OP at 977 nm arises among the centers concentration of which is a nonlinear (almost quadratic) function of Yb3+ ions concentration. These are most probably the centers composed of couples of Yb3+ ions (*pairs*), or agglomerates of the latter. Furthermore, similar reactions: *e- + Ybp 3+ → Yb<sup>p</sup> 2+*; *e+ + Ybp 2+ → Yb<sup>p</sup> 3+* (see above) can be proposed to address these transformations at OP, where index *p* stands to show that a pair of Yb3+ ions is involved in the processes and notations *e–* and *e+* are used for an electron and a hole, free or trapped by the nearest ligand, say oxygen. Such reactions can go at the assistance of CT-processes between ion pairs where both constituents are in the excited state. Hence, the spectrally wide background loss (PD) in the fibers (see **Figures 12** and **14**) can be produced Ybp 2+ and of *e–* /*e+* -related centers (say, ODCs and NBOHCs) at OP, like this takes place at e-irradiation.

It is currently accepted that PD occurs among clusters of Yb3+ ions (obviously, pairs are their kind). However, a novelty found here is the spectral feature, occurring at OP (see dotted rings in **Figures 14** and **15**) but not—at e-irradiation.

There are evidences for that PD can be itself associated with nonbinding oxygen near surfaces of Yb/Al clusters that can be formed in alumino-silicate glass (our case). The nonbinding oxygen originates from Yb3+ substituting Si4+ sites. When subjecting a YF to 977 nm OP, the excess energy is radiated as phonons, causing a lone electron of a nonbinding oxygen atom to shift to a nearest neighbor nonbinding oxygen atom with creation of a hole and a pair of lone electrons, which results in a Coulomb field between the oxygen atoms to form an unstable "color" center. Conversion of such an unstable center to a semistable center requires shifting of one electron of the lone electron pair to a nearest neighbor site. As a result of this, the formation of Yb-related ODC can happen. On the other hand, PD in alumino-silicate YFs may take place via breaking of ODC, which gives rise to release of free electrons. The released electrons may be trapped at Al or Yb sites to form a color center resulting in PD. These hypotheses can serve as the arguments, bringing more clarity in understanding the similarity of the spectral transformations in YFs at e-irradiation (creation of "secondary" carriers by βelectrons) and at OP (creation of carriers and color centers by pump-light).
