**3. Electron irradiation versus PD of Yb-doped germano-alumino-silicate fibers: The effects comparison**

Yb3+-doped silica fibers (YFs) with different core-glass hosts codoped with Al, Ge, or P have been of considerable interest during the past decades as extremely effective media for fiber lasers for the spectral region 1.0–1.1 μm, when pumped at 0.9–1.0 μm wavelengths. A variety of diode-pumping configurations (core and cladding) and pump wavelengths were examined so far, resulting in recognition of optimal arrangements for multi-watt release from YF-based lasers with high optical efficiency ~70–75% and perfect beam quality [29, 30]. However, in spite of a remarkable progress in the field, there remain obstacles that limit the performance of YFbased lasers, one of them being PD [31], that is, long-term (minutes to hours) degradation of laser power, measured by units to tens %. This hardly mitigated disadvantage becomes notable when dealing with a laser based on heavily doped YF where a high Yb3+ population inversion is created, either at high-power continuous-wave or moderate-power pulsed lasing. A number of studies were aimed to understand the PD phenomenon which however remained unclear, although a few hypotheses have been proposed for its explanation [32–42].

On the other hand, a few studies aiming the characterization of susceptibility of YFs under such irradiations as X-rays, *γ*-quanta, and UV have been reported [43–45]. The main motivation was inspection of YF-resistance to harmful environments. In many cases, the excess-loss spectra induced in YFs resemble the ones, characteristic for PD at resonant pumping into Yb3+ resonant-absorption band, the fact undoubtedly deserving attention.

Here, the results of two sets of experiments, where susceptibility of YFs with similar germanoalumino-silicate glass-cores, doped with Yb in different concentrations to irradiation by a beam of β-electrons and to resonant (into Yb3+ resonant band) optical pumping, are presented. In both circumstances, qualitatively similar trends are revealed, being strong and monotonous change in attenuation in VIS (darkening), accompanied by more complex transformations within the resonant absorption band of Yb3+ ions, either upon dose (the case of β-electron irradiation), or exposing time (the case of optical pumping at 977 nm). Below, we compare and discuss the experimental results and attempt to explain them.

### **3.1. Fiber samples and experimental arrangement**

The YFs inspected in these experiments were drawn from germano-alumino-silicate glass preforms fabricated using the "conventional" MCVD/SD route. The attenuation spectra of the fibers being in pristine (as-received) state are demonstrated in **Figure 8(a)**.

Effects of Electron Irradiation Upon Absorptive and Fluorescent Properties of Some Doped Optical Fibers http://dx.doi.org/10.5772/63939 15

Comparison of the bleaching effect in Ce-doped (without Au codoping) and Ce/Au-codoped fibers show that it is less expressed in the latter than in the former, which is probably related to lower susceptibility to exterior influence of Ce/Au-codoped fiber (a consequence of its more

**3. Electron irradiation versus PD of Yb-doped germano-alumino-silicate**

Yb3+-doped silica fibers (YFs) with different core-glass hosts codoped with Al, Ge, or P have been of considerable interest during the past decades as extremely effective media for fiber lasers for the spectral region 1.0–1.1 μm, when pumped at 0.9–1.0 μm wavelengths. A variety of diode-pumping configurations (core and cladding) and pump wavelengths were examined so far, resulting in recognition of optimal arrangements for multi-watt release from YF-based lasers with high optical efficiency ~70–75% and perfect beam quality [29, 30]. However, in spite of a remarkable progress in the field, there remain obstacles that limit the performance of YFbased lasers, one of them being PD [31], that is, long-term (minutes to hours) degradation of laser power, measured by units to tens %. This hardly mitigated disadvantage becomes notable when dealing with a laser based on heavily doped YF where a high Yb3+ population inversion is created, either at high-power continuous-wave or moderate-power pulsed lasing. A number of studies were aimed to understand the PD phenomenon which however remained unclear,

On the other hand, a few studies aiming the characterization of susceptibility of YFs under such irradiations as X-rays, *γ*-quanta, and UV have been reported [43–45]. The main motivation was inspection of YF-resistance to harmful environments. In many cases, the excess-loss spectra induced in YFs resemble the ones, characteristic for PD at resonant pumping into

Here, the results of two sets of experiments, where susceptibility of YFs with similar germanoalumino-silicate glass-cores, doped with Yb in different concentrations to irradiation by a beam of β-electrons and to resonant (into Yb3+ resonant band) optical pumping, are presented. In both circumstances, qualitatively similar trends are revealed, being strong and monotonous change in attenuation in VIS (darkening), accompanied by more complex transformations within the resonant absorption band of Yb3+ ions, either upon dose (the case of β-electron irradiation), or exposing time (the case of optical pumping at 977 nm). Below, we compare and

The YFs inspected in these experiments were drawn from germano-alumino-silicate glass preforms fabricated using the "conventional" MCVD/SD route. The attenuation spectra of the

although a few hypotheses have been proposed for its explanation [32–42].

Yb3+ resonant-absorption band, the fact undoubtedly deserving attention.

fibers being in pristine (as-received) state are demonstrated in **Figure 8(a)**.

discuss the experimental results and attempt to explain them.

**3.1. Fiber samples and experimental arrangement**

ordered glass network, already noticed).

14 Radiation Effects in Materials

**fibers: The effects comparison**

**Figure 8.** (a) Attenuation (small-signal absorption) spectra of fibers with low (YF-1), intermediate (YF-2), and high (YF-3) Yb3+ contents: curves 1, 2, and 3, respectively. (b) Fluorescence spectra of the fibers at resonant 977 nm excitation (pump power—300 mW). Labeling of curves 1, 2, and 3 is the same as in (a) and (b). Inset in (b) shows "cooperative" fluorescence in VIS. (Reproduced with permission from Kir'yanov [76]. Copyright© 2011, Scientific Research Publish‐ ing Inc).

The concentrations of Yb3+ ions in the fibers differed by more than an order of magnitude, so certain differences were expected after their exposure to β-electron irradiation (hereafter in this paragraph—*e-irradiation*) and to optical pumping (hereafter—*OP*) at 977 nm wavelength . The fibers, having the lowest, the intermediate, and the highest Yb3+ doping level, are referred further to as *YF-1*, *YF-2*, and *YF-3*, respectively.

The essences of experiments on e-irradiation of the YFs were completely the same as at irradiating the Ce-doped fibers (Paragraph 2). The indices "1," "2," and "3" label below the doses 2 × 1012, 1 × 1013, and 5 × 1013 cm–2, respectively.

Experiments on OP at 977 nm were made in a similar way as described in Ref. [36]. YF samples were pumped using a standard 300 mW 977 nm laser diode (LD). The pump light was launched from LD to an YF sample under study through a splice. The end of the latter was spliced to a piece of SMF-28 fiber that was, in turn, connected to an OSA for the transmission spectra' measurements. In these experiments, we handled short (a few cm) pieces of YFs to ensure nolasing conditions and negligible contribution of amplified spontaneous emission of Yb3+.

The optical transmission spectra of the YF samples were obtained using a white light source with a fiber output and the OSA, turned to a 1 nm resolution. These spectra were recorded over the spectral range 400–1200 nm, where the most interesting spectral transformations occur as the result of e-irradiation/OP. The output of the white light source was connected to a fiber set containing an YF sample (pristine or subjected to e-irradiation/OP), while its attenuation was measured using the OSA. The attenuation spectra were recorded before and after each stage of e-irradiation (*doses*) or OP at 977 nm pumping (*times*). Lengths of the YFs were chosen to be short enough, from <1 cm (YF-3) to tens cm (YF-1), to avoid spectral noise artifacts. In some of the figures below, the difference (IA) spectra are demonstrated which were obtained after subtracting the attenuation spectra of pristine samples from the ones taken after certain dose/time of e-irradiation/OP. This allows insight to "net" spectral loss, established after darkening of either type. All the spectra presented beneath have been obtained after recalcu‐ lating transmission coefficients in loss [dB/cm]. We also measured the fibers' fluorescence spectra and fluorescence kinetics of Yb3+ ions before and after e-irradiation/OP, applying the "lateral" geometry [46]. We used the same OSA for the fluorescence spectra measurements and a Ge photodetector (PD) and oscilloscope for the fluorescence-decay measurements. In the last case, LD power was modulated by a driver controlled by a function generator to achieve square-shaped pulses with sharp rise and fall edges. The time resolution of the setup was 8 μs. All the experiments were made at room temperature.

#### **3.2. Experimental**

#### *3.2.1. E-irradiation*

The attenuation spectra of samples YF-3 and YF-1, having correspondingly the highest and lowest Yb3+ concentrations, obtained after different doses of e-irradiation, along with the attenuation spectra of the samples in a pristine (dose "0") state are shown in **Figure 9(a, b)**.

**Figure 9.** Attenuation spectra of samples YF-3 (a) and YF-1 (b). The data are for e-irradiation doses increased from "0" (pristine samples) through "1" and "2"–"3". Insets show the difference spectra obtained after subtraction of the spectra of pristine samples from the ones after e-irradiation of the samples. Dashed lines show the positions of wavelengths for which the data in **Figure 10** are built. (Reproduced with permission from Kir'yanov [76]. Copyright© 2011, Scientific Research Publishing Inc).

First, a notable increase of background loss in VIS with increasing e-irradiation dose is revealed (see main frames of **Figure 9**). Also notice a specific spectral character of this loss for both fibers, viz. a drastic rise of loss-magnitude toward shorter wavelengths. This is a well-known for Yb3+-free silica fibers' trend in experiments on various kinds of irradiations. At the same time, apparent differences are seen in magnitude of e-irradiation-induced loss in these two fibers, that is, a higher degree of darkening in YF-3 than in YF-1. (For YF-2, intermediate in Yb3+ doping level, the effect of e-irradiation is intermediate, as compared with YF-3 and YF-1.]

Second, detectable but less pronounced spectral transformations are revealed for the resonantabsorption band of Yb3+ (850–1100 nm) (see insets to **Figure 9**), where the difference spectra are shown, obtained as explained above. Very weak in YF-1 (Figure 9(b)), the spectral trans‐ formations are noticeable in YF-3 (Figure 2(a)). These changes seem to be a result of some process, associated with e-irradiation of the fibers, which affects concentration of Yb3+ ions.

spectra and fluorescence kinetics of Yb3+ ions before and after e-irradiation/OP, applying the "lateral" geometry [46]. We used the same OSA for the fluorescence spectra measurements and a Ge photodetector (PD) and oscilloscope for the fluorescence-decay measurements. In the last case, LD power was modulated by a driver controlled by a function generator to achieve square-shaped pulses with sharp rise and fall edges. The time resolution of the setup was 8 μs.

The attenuation spectra of samples YF-3 and YF-1, having correspondingly the highest and lowest Yb3+ concentrations, obtained after different doses of e-irradiation, along with the attenuation spectra of the samples in a pristine (dose "0") state are shown in **Figure 9(a, b)**.

**Figure 9.** Attenuation spectra of samples YF-3 (a) and YF-1 (b). The data are for e-irradiation doses increased from "0" (pristine samples) through "1" and "2"–"3". Insets show the difference spectra obtained after subtraction of the spectra of pristine samples from the ones after e-irradiation of the samples. Dashed lines show the positions of wavelengths for which the data in **Figure 10** are built. (Reproduced with permission from Kir'yanov [76]. Copyright© 2011, Scientific

First, a notable increase of background loss in VIS with increasing e-irradiation dose is revealed (see main frames of **Figure 9**). Also notice a specific spectral character of this loss for both fibers, viz. a drastic rise of loss-magnitude toward shorter wavelengths. This is a well-known for Yb3+-free silica fibers' trend in experiments on various kinds of irradiations. At the same time, apparent differences are seen in magnitude of e-irradiation-induced loss in these two fibers, that is, a higher degree of darkening in YF-3 than in YF-1. (For YF-2, intermediate in Yb3+ doping

Second, detectable but less pronounced spectral transformations are revealed for the resonantabsorption band of Yb3+ (850–1100 nm) (see insets to **Figure 9**), where the difference spectra

level, the effect of e-irradiation is intermediate, as compared with YF-3 and YF-1.]

All the experiments were made at room temperature.

**3.2. Experimental**

16 Radiation Effects in Materials

*3.2.1. E-irradiation*

Research Publishing Inc).

More details are seen in **Figure 10** where we plot the results for samples YF-1 (a) and YF-3 (b), taken for all doses. **Figure 10(a, b)** demonstrates how attenuation within the resonantabsorption of Yb3+ ions (peaks at 920 and 977 nm, see also **Figure 8(a)**) changes throughout eirradiation: see curve 1 (for the 977 nm peak) and curve 2 (for the 920 nm peak), respectively. A decrease followed by an increase in the magnitude of small-signal absorption arises in both peaks with dose increasing in YF-3 (heavier doped with Yb3+); this trend is, in contrast, less expressed in YF-1 (lower doped with Yb3+).

For comparison, we plot in **Figure 10** the changes in attenuation of YF-3 (c) and YF-1 (d) fibers in VIS, where background (nonresonant) losses arise as the result of e-irradiation. Here we limit ourselves by the data, counted for 500 (curve 3) and 633 (curve 4) nm. It is seen that background loss steadily grows with dose, a common effect for silica fibers. Note that the rate of growth is higher in YF-3 than in YF-1. Furthermore, an initial level of background loss in pristine YF correlates with initial content of Yb3+ ions.

**Figure 10.** Dose dependences of attenuation in resonant-absorption Yb3+ peaks centered at 977 (curves 1) and 920 (curves 2) nm (top panels) and in VIS, for wavelengths 500 (curves 3) and 633 (curves 4) nm (bottom panels). The data are for samples YF-3 (a, c) and YF-1 (b, d). (Reproduced with permission from Kir'yanov [76]. Copyright© 2011, Scien‐ tific Research Publishing Inc).

**Figure 11(a)** and **(b)** gathers the experimental data obtained using all fibers, YF-1, YF2, and YF-3. From **Figure 11(a)**, it is seen that a monotonous increase of nonresonant loss in VIS (darkening), exampled by wavelengths 500 and 633 nm, with increasing Yb3+ concentration; the latter is proportional to YF small-signal absorption at 977 nm. This demonstrates that the presence of Yb3+ dopants gain their degradation at e-irradiation. (Here we show the results obtained at dose "3" only, because for other doses the dependences are similar, given by a smooth dependence of induced loss in VIS versus e-irradiation dose (see **Figure 10(c, d)**). From **Figure 11(b)**, it is seen that the lowest levels to which the values of absorption in the 977 nm peak approach throughout e-irradiation (minima of curves 1 in **Figure 9**) decrease with increasing Yb3+ content (a similar trend is observed for the other peak of Yb3+, at 920 nm). This fact seems to be in favor of that initial concentration of Yb3+ ions in pristine samples substan‐ tially decreases as a result of e-irradiation, at the primary stage. However, at the following stages, Yb3+ concentrations are re-established on levels comparable with those in pristine YFs (refer to **Figure 10(a)**). [The remainder of **Figure 11(c, d)** provides the data, obtained in the experiments on OP of the YFs, reported below.]

**Figure 11.** The results of experiments with fibers YF-1, YF-2, and YF-3, which were obtained for different e-irradiation doses (a, b) and OP times (c, d). The data are for the resonant-absorption peaks at 977 and 920 nm (filled and empty asterisks) (b, d)) and for the VIS region, exampled by wavelengths 500 nm (crossed squares) and 633 nm (crossed cir‐ cles) (a, c). Dotted lines are for visual purposes only. (Reproduced with permission from Kir'yanov [76]. Copyright© 2011, Scientific Research Publishing Inc).

#### *3.2.2. PD at OP*

We report here the results of OP experiments for sample YF-3 mainly (see **Figures 12**–**14**), having the biggest content of Yb3+ ions. Then, we summarize all the results, obtained for YF-1, YF-2, and YF-3 fibers, in **Figure 11(c, d)**.

the latter is proportional to YF small-signal absorption at 977 nm. This demonstrates that the presence of Yb3+ dopants gain their degradation at e-irradiation. (Here we show the results obtained at dose "3" only, because for other doses the dependences are similar, given by a smooth dependence of induced loss in VIS versus e-irradiation dose (see **Figure 10(c, d)**). From **Figure 11(b)**, it is seen that the lowest levels to which the values of absorption in the 977 nm peak approach throughout e-irradiation (minima of curves 1 in **Figure 9**) decrease with increasing Yb3+ content (a similar trend is observed for the other peak of Yb3+, at 920 nm). This fact seems to be in favor of that initial concentration of Yb3+ ions in pristine samples substan‐ tially decreases as a result of e-irradiation, at the primary stage. However, at the following stages, Yb3+ concentrations are re-established on levels comparable with those in pristine YFs (refer to **Figure 10(a)**). [The remainder of **Figure 11(c, d)** provides the data, obtained in the

**Figure 11.** The results of experiments with fibers YF-1, YF-2, and YF-3, which were obtained for different e-irradiation doses (a, b) and OP times (c, d). The data are for the resonant-absorption peaks at 977 and 920 nm (filled and empty asterisks) (b, d)) and for the VIS region, exampled by wavelengths 500 nm (crossed squares) and 633 nm (crossed cir‐ cles) (a, c). Dotted lines are for visual purposes only. (Reproduced with permission from Kir'yanov [76]. Copyright©

We report here the results of OP experiments for sample YF-3 mainly (see **Figures 12**–**14**), having the biggest content of Yb3+ ions. Then, we summarize all the results, obtained for YF-1,

experiments on OP of the YFs, reported below.]

18 Radiation Effects in Materials

2011, Scientific Research Publishing Inc).

YF-2, and YF-3 fibers, in **Figure 11(c, d)**.

*3.2.2. PD at OP*

**Figure 12.** Attenuation (small-signal absorption) spectra of fiber sample YF-3 after OP @ 977 nm. The data are for a pristine sample (curve 1: "0 min") and for photo darkened samples (curves 2 and 3, obtained after 40 and 150 min of OP, respectively). Dashed lines show the positions of wavelengths for which the data in **Figure 13** are built. (Repro‐ duced with permission from Kir'yanov [76]. Copyright© 2011, Scientific Research Publishing Inc).

**Figure 13.** Dose dependences of attenuation in resonant-absorption (Yb3+) peaks centered at 977 (curve 1) and 920 (curve 2) nm (a) and in VIS, for wavelengths 500 (curve 3) and 633 (curve 4) nm (b). The data are for sample YF-3. (Reproduced with permission from Kir'yanov [76]. Copyright© 2011, Scientific Research Publishing Inc).

**Figure 14.** Difference attenuation spectra after dose "3" of e-irradiation (curve 1) and after 2 h of OP at 977 nm (pump power is 300 mW) (curve 2); curve 3 is the difference of spectra 1 and 2. The data are for sample YF-3. (Reproduced with permission from Kir'yanov [76]. Copyright© 2011, Scientific Research Publishing Inc).

Figure 12 shows the attenuation spectra of sample YF-3 (length, 0.8 cm) after 40 and 120 min. of OP. The LD power was fixed in these experiments at 300 mW, the highest in our circum‐ stances level of Yb3+ ions inversion. For comparison, the attenuation spectrum of pristine (0 min) sample YF-3 is shown in **Figure 12**, too. Once compared with the attenuation spectra after e-irradiation (refer to **Figure 9(a)**), these spectra are seen to be similar. That is, a substantial increase of background loss is observed in VIS with increasing OP-time (the PD effect). Note that the spectral "signature" of PD resembles the one after e-darkening (see **Figure 9**).

In **Figure 13(a)**, we demonstrate the results of the experiments with sample YF-3, obtained at increasing OP time. Their representation is similar to the one used at the description of experiments on e-irradiation (see **Figure 10(a)**). From **Figure 13(a)**, it is seen how attenuations in the two absorption peaks of Yb3+ ions (at 977 and 920 nm) change throughout OP; see curves 1 and 2, respectively. The time dependence of OP-induced changes at 977 nm resembles the dose dependence at e-irradiation of sample YF-3. However, curve 1 in **Figure 13(a)** has "asymmetric" shape versus OP time, differing from "symmetric" shape of the dose depend‐ ence at e-irradiation given by curve 1 in **Figure 10(a)**. Furthermore, the time dependence of OP-induced changes at 920 nm, see curve 2 in **Figure 13(a)**, is very weak, being completely different from curve 2 in **Figure 10(a)** (e-irradiation). Therefore, we can propose that different mechanisms, responsible for the induced changes in the resonant-absorption band of Yb3+ at 977 and 920 nm, stand behind these two (e-irradiation and OP) treatments of the fibers.

In **Figure 13(b)**, we demonstrate the results of spectral transformations arising in YF-3 in VIS, at OP. Again, we provide in **Figure 13(b)**, the data for a couple of wavelengths, 500 (curves 3) and 633 (curves 4) nm, as most representative. In contrast to the dose dependences at eirradiation, long-term OP at 977 nm results in completely different dynamics of background loss in time. Indeed, it is essentially nonlinear versus time: there is a short timing interval in the beginning (few minutes) where PD increases dramatically, while afterward (tens of minutes) it slows down and tends to saturate.

**Figure 14** allows one to compare the attenuation spectra for YF-3 suffered dose "3" of eirradiation (curve 1) and 2 h of OP (curve 2). The spectra look qualitatively similar, which may tell that the mechanisms involved are similar in these two circumstances. At the same time, if one spectrum is subtracted from another, the result (curve 3 in **Figure 14**) brings some news. That is, apart from the difference presented in VIS (in background loss), there is a feature in the Yb3+ resonant band: though no deviation from "plain" behavior of curve 3 is seen near 920 nm peak of Yb3+, there is a well-defined (negative) 977 nm peak (it is marked by a dotted ring). This detail seems to be important as it lightens nonhomogeneity within the Yb3+ resonantabsorption band near 977 nm, present at OP but not—at e-irradiation.

This detail becomes expressed more when one analyzes the data obtained at PD of the other fibers, YF-1 and YF-2 (see **Figure 15**). In this figure, where we plot the difference spectra obtained for these fibers, analogous but clearer seen detail appears exactly within the 977 nm peak of Yb3+ ions (it is marked by a dotted ring in (a) and (b)).

**Figure 14.** Difference attenuation spectra after dose "3" of e-irradiation (curve 1) and after 2 h of OP at 977 nm (pump power is 300 mW) (curve 2); curve 3 is the difference of spectra 1 and 2. The data are for sample YF-3. (Reproduced

Figure 12 shows the attenuation spectra of sample YF-3 (length, 0.8 cm) after 40 and 120 min. of OP. The LD power was fixed in these experiments at 300 mW, the highest in our circum‐ stances level of Yb3+ ions inversion. For comparison, the attenuation spectrum of pristine (0 min) sample YF-3 is shown in **Figure 12**, too. Once compared with the attenuation spectra after e-irradiation (refer to **Figure 9(a)**), these spectra are seen to be similar. That is, a substantial increase of background loss is observed in VIS with increasing OP-time (the PD effect). Note

that the spectral "signature" of PD resembles the one after e-darkening (see **Figure 9**).

In **Figure 13(a)**, we demonstrate the results of the experiments with sample YF-3, obtained at increasing OP time. Their representation is similar to the one used at the description of experiments on e-irradiation (see **Figure 10(a)**). From **Figure 13(a)**, it is seen how attenuations in the two absorption peaks of Yb3+ ions (at 977 and 920 nm) change throughout OP; see curves 1 and 2, respectively. The time dependence of OP-induced changes at 977 nm resembles the dose dependence at e-irradiation of sample YF-3. However, curve 1 in **Figure 13(a)** has "asymmetric" shape versus OP time, differing from "symmetric" shape of the dose depend‐ ence at e-irradiation given by curve 1 in **Figure 10(a)**. Furthermore, the time dependence of OP-induced changes at 920 nm, see curve 2 in **Figure 13(a)**, is very weak, being completely different from curve 2 in **Figure 10(a)** (e-irradiation). Therefore, we can propose that different mechanisms, responsible for the induced changes in the resonant-absorption band of Yb3+ at 977 and 920 nm, stand behind these two (e-irradiation and OP) treatments of the fibers.

In **Figure 13(b)**, we demonstrate the results of spectral transformations arising in YF-3 in VIS, at OP. Again, we provide in **Figure 13(b)**, the data for a couple of wavelengths, 500 (curves 3) and 633 (curves 4) nm, as most representative. In contrast to the dose dependences at eirradiation, long-term OP at 977 nm results in completely different dynamics of background loss in time. Indeed, it is essentially nonlinear versus time: there is a short timing interval in the beginning (few minutes) where PD increases dramatically, while afterward (tens of

minutes) it slows down and tends to saturate.

with permission from Kir'yanov [76]. Copyright© 2011, Scientific Research Publishing Inc).

20 Radiation Effects in Materials

**Figure 15.** Difference loss spectra of YF-1 (a) and YF-2 (b), obtained after 1 h of OP at 977 nm. (Reproduced with per‐ mission from Kir'yanov [76]. Copyright© 2011, Scientific Research Publishing Inc).

Let us return to **Figure 11(c, d)**, where we gather the results on OP for all YF samples.

In contrast to the results on e-irradiation **(Figure 11(a, b))**, one can reveal first nonlinear growth of background loss at 500 and 633 nm with increasing Yb3+ concentration **(Figure 11(c))**. Apparently, this behavior is different from linear growth of background loss at e-irradiation **(Figure 11(a))**. Second, it is seen that, instead of a linear decrease of the resonant peaks at 977 and 920 nm with dose (occurring at primary stages of e-irradiation—see **Figure 11(b)**), a nonlinear law is obeyed by a decrease of the resonant peak at 977 nm while almost no change happens with the peak at 920 nm **(Figure 11(d))**.

Thus, the situation with OP-induced spectral transformations in the YFs is complex and curious at first glance. The 977 nm peak is strongly affected by OP, not the 920 nm one. This can be explained by the presence in the fibers of some other centers than Yb3+ dopants, but closely related to them and spectrally matching them near 977 nm. Moreover, partial weight of such centers in YF-core is expected to increase with increasing Yb3+ ions concentration. The nonlinear behavior of the nonresonant background loss versus OP time, discussed earlier (see **Figure 13(b)**), seems to be a related phenomenon.
