**2.1. Fiber samples and experimental arrangement**

accelerator's chamber for various time intervals, which provided growing irradiation doses. The irradiated fibers were then left for 2 weeks prior to the main-course spectral measure‐ ments to avoid the role of short-living components in the decay of induced absorption (IA). The measurements were done during a limited time (viz., the following 2…3 weeks) for diminishing the effect of spontaneous IA recovering. Note that ionization, that is, the production of β-induced carriers by an electron beam (i.e., of secondary free holes and electrons), is the main cause of the spectral transformations in the fibers. This happens because high-energy primary β-electrons are virtually nondissipating at the propagation through a fiber sample; on the other hand, certain contribution in ionization of the fibers' core-glasses arising from *γ*-quanta born at inelastic scattering of the high-energy electrons

We demonstrate below first a study of the resistance of a couple of cerium (Ce)-doped alumino-phospho-silicate fibers (one of them being codoped with gold (Au)), to β-electrons. The experimental data reveal a severe effect of β-irradiation upon the fibers' absorptive properties, given by noticeable susceptibility of Ce ions being in Ce3+/Ce4+ states to the treatment, arising as growth followed by saturation of IA. We also report the essentials of posterior bleaching of β-darkened fibers, also in terms of attenuation spectra' transforma‐ tions, at exposing them to low-power green (a He-Ne laser) and ultra violet (UV, a mercury lamp) light. It is shown that both phenomena are less expressed in Ce fiber codoped with Au than in Au-free one and that the spectral changes in the former are more regular versus dose

Then, we provide a comparative experimental analysis of IA, induced by β-electrons, for a series of ytterbium (Yb)-doped alumino-germano-silicate fibers with different concentrations of Yb3+ ions and compare this effect with the photodarkening (PD) phenomenon in the same fibers, arising at resonant (into 977 nm absorption peak of Yb3+ ions) optical pumping. The experimental data obtained reveals that, in these two circumstances, substantial and complex but different in appearance changes affecting the resonant absorption band of Yb3+ ions and

Finally, we report a study of attenuation spectra' transformations in a set of bismuth (Bi)-doped silica fibers with various contents of emission-active Bi centers, which occur as the result of β-irradiation. Among the data obtained, notice a substantial decrease of concentration of Bi centers, associated with the presence of Germanium (Ge) in core-glass, with increasing irradiation dose (the "bleaching" effect), while, on the contrary, an opposite trend, that is, dosedependent growth of resonant-absorption ascribed to Bi active centers, associated with the presence in core-glass of Aluminium (Al). These results are worth noticing for understanding

**2. The effects of electron irradiation and posterior optical bleaching in**

Development of suitable host glasses and fibers for dosimetry, which are based on formation of radiation-induced defects leading to glass coloration [1–6] or filling pre-existing traps,

**Ce-doped and Ce/Au-codoped alumino-phospho-silicate fibers**

the off-resonance background loss are produced in the fibers.

the nature of Bi-related centers in silica fibers, yet uncovered.

cannot be disregarded.

4 Radiation Effects in Materials

and bleaching time.

The sourcing Ce-doped and Ce/Au-codoped fiber preforms based on alumino-phosphosilicate glass have been made by means of modified chemical vapor deposition (MCVD) process employed in conjunction with solution doping (SD) technique; the final fibers have been drawn from the preforms using a drawing tower. Core diameters/numerical apertures of the two fibers were measured to be ~25 μm/0.15…0.16, respectively.

Estimated from EDX, average doping levels were found to be 5.0 wt.% Al2O3, 0.15 wt.% P2O5, 0.3 wt.% CeO2 (in the Ce-doped fiber) and 5.1 wt.% Al2O3, 0.15 wt.% P2O5, 0.27 wt.% CeO2, and 0.2 wt.% Au2O3 (in the Ce/Au-codoped fiber). Both fibers had multimode wave-guiding, which make them useful for sensor applications. A sample of standard multimode Al-doped (~6 wt. % Al2O3) fiber was used in experiments for comparison. The β-irradiation dosage below corresponds to 1 × 1012 ("dose 1"), 5 × 1012 ("dose 2"), 1 × 1013 ("dose 3"), 5 × 1013 ("dose 4"), 1 × 1014 ("dose 5"), and 2.5 × 1015 ("dose 6") cm–2.

Optical transmission spectra of fiber samples were measured (employing the cutback method), using a white light source and optical spectrum analyzer (OSA), turned to a 5 nm resolution. Such spectra were recorded before and after each stage of β-irradiation and at posterior exposure to light of a He-Ne laser (543 nm) or UV lamp (λ <450 nm). The attenuation spectra presented below were obtained after recalculating the measured transmissions into loss [dB/m]. In some of the figures below the difference spectra in terms of IA are provided, which were obtained after subtraction of the attenuation spectra of pristine samples from the ones taken after a certain dose of β-irradiation; this allows one straightforward view on the "net" spectral loss changes in the darkened fibers. The transmission dynamics at optical bleaching of β-darkened fibers by 543 nm light was inspected applying "frontal" detecting geometry where a beam of the He-Ne laser was coupled into a fiber sample, while the transmitted light was detected using a Si photodetector; this permitted detection of the changes in transmission *in situ*. The results of the measurements are given below in terms of absorption difference (AD) at bleaching with respect to the initial (β-darkened) state of the fiber. The experiments on optical bleaching of β-irradiated fibers by UV light were as well proceeding *in situ*, where transmission change at long-term exposure to UV light was analyzed. All experiments were made at room temperature.

#### **2.2. Experimental**

#### *2.2.1. IA as a result of β-irradiation*

In **Figure 1**, we demonstrate (a) attenuation spectra of the Ce-doped (black solid curve 1) and Ce/Au-codoped (grey dashed curve 2) fibers before irradiation, that is, in their "pristine" state, and (b and c) the fibers' cross-sections, obtained at white light illumination. Long (meters)

**Figure 1.** (a) Attenuation spectra of pristine Ce-doped (1), Ce/Au-codoped (2), and Al-doped Ce-free (3) fibers in a VISto-near-IR spectral range and micro-photographs of pristine Ce-doped (b) and Ce/Au-codoped (c) fibers. (Reproduced with permission from Kir'yanov et al. [75]. Copyright© 2014, Optical Society of America).

fibers were used in the measurements applying the cutback method, whereas short (10 centimeters) pieces of fibers—at microscopy. For comparison, spectral loss of "standard" Aldoped Ce-free fiber is presented in **Figure 1(a)**—see red dash-dotted curve 3.

% Al2O3) fiber was used in experiments for comparison. The β-irradiation dosage below corresponds to 1 × 1012 ("dose 1"), 5 × 1012 ("dose 2"), 1 × 1013 ("dose 3"), 5 × 1013 ("dose 4"), 1

Optical transmission spectra of fiber samples were measured (employing the cutback method), using a white light source and optical spectrum analyzer (OSA), turned to a 5 nm resolution. Such spectra were recorded before and after each stage of β-irradiation and at posterior exposure to light of a He-Ne laser (543 nm) or UV lamp (λ <450 nm). The attenuation spectra presented below were obtained after recalculating the measured transmissions into loss [dB/m]. In some of the figures below the difference spectra in terms of IA are provided, which were obtained after subtraction of the attenuation spectra of pristine samples from the ones taken after a certain dose of β-irradiation; this allows one straightforward view on the "net" spectral loss changes in the darkened fibers. The transmission dynamics at optical bleaching of β-darkened fibers by 543 nm light was inspected applying "frontal" detecting geometry where a beam of the He-Ne laser was coupled into a fiber sample, while the transmitted light was detected using a Si photodetector; this permitted detection of the changes in transmission *in situ*. The results of the measurements are given below in terms of absorption difference (AD) at bleaching with respect to the initial (β-darkened) state of the fiber. The experiments on optical bleaching of β-irradiated fibers by UV light were as well proceeding *in situ*, where transmission change at long-term exposure to UV light was analyzed. All experiments were

In **Figure 1**, we demonstrate (a) attenuation spectra of the Ce-doped (black solid curve 1) and Ce/Au-codoped (grey dashed curve 2) fibers before irradiation, that is, in their "pristine" state, and (b and c) the fibers' cross-sections, obtained at white light illumination. Long (meters)

**Figure 1.** (a) Attenuation spectra of pristine Ce-doped (1), Ce/Au-codoped (2), and Al-doped Ce-free (3) fibers in a VISto-near-IR spectral range and micro-photographs of pristine Ce-doped (b) and Ce/Au-codoped (c) fibers. (Reproduced

with permission from Kir'yanov et al. [75]. Copyright© 2014, Optical Society of America).

× 1014 ("dose 5"), and 2.5 × 1015 ("dose 6") cm–2.

made at room temperature.

*2.2.1. IA as a result of β-irradiation*

**2.2. Experimental**

6 Radiation Effects in Materials

We reveal from (a) that, in both Ce-doped and Ce/Au-codoped fibers, dramatic growth of absorption occurs toward UV, below ~550 nm, which is known to be a shoulder of the strong absorption bands adherent to Ce3+/Ce4+ ions (mostly located in UV [23, 24]), and that no such feature is observed in the reference Ce-free fiber. Also notice steep loss rise in Ce-doped and Ce/Au-codoped fibers toward IR and a small peak at ~520 nm (asterisked), the features not observed in case of the Ce-free fiber.

**Figure 2** shows the trends occurring in the fibers' attenuation spectra as the result of βirradiation at moderate dose 4. Note that in this case, the measurements were proceeding with shorter fiber samples (~a few cm) in virtue of strong IA, established after β-irradiation.

**Figure 2.** (a) Attenuation spectra of Ce-doped (1), Ce/Au-codoped (2), and Al-doped cerium-free (3) fibers, all meas‐ ured after β-irradiation with dose 4 (5 × 1013 cm–2) and micro-photographs of Ce-doped (b) and Ce/Au-codoped (c) fi‐ bers recorded after irradiation with this dose. (Reproduced with permission from Kir'yanov et al. [75], Copyright© 2014, Optical Society of America).

It is seen that IA in the Ce-free fiber is ~two times bigger than in the Ce-doped and Ce/Aucodoped ones. The other fact is that IA maxima are located near 400 and 500 nm in these two fibers, whereas the ones in the Ce-free one—at ~400 and ~600 nm, that is, in the range most probably attributing to well-known nonbridging oxygen-holes (NBOHCs) [26] (while the presence of other defect states in it—such as Si-/Al-defect centers cannot be excluded). Furthermore, it is seen from photos (b) and (c) that, in the Ce-doped and Ce/Au-codoped fibers, the core and adjacent core-cladding areas suffer darkening after β-irradiation, in the former, the effect being more pronounced.

**Figure 3** demonstrates that IA in the Ce-doped (a) and Ce/Au-codoped (b) fibers increases monotonously with dose; this trend is noticeable for the 400–700 nm range, while for bigger wavelengths it fades. The other detail seen is that for moderate doses (1–4), IA is stronger in the Ce-doped fiber.

**Figure 3.** Main frames: IA spectra of Ce-doped (a) and Ce/Au-codoped (b) fibers; curves 1–6 correspond to doses of irradiation (in both figures) being: 1 × 1012 (1), 5 × 1012 (2), 1 × 1013 (3), 5 × 1013 (4), 1 × 1014 (5), and 2.5 × 1015 (6) cm–2. Insets: average IA-losses measured within the 1300–1550 nm range *vs*. irradiation dose. (Reproduced with permission from Kir'yanov et al. [75]. Copyright© 2014, Optical Society of America).

The two-peaks structure of the IA spectra is apparent at higher irradiation doses for both fibers, with the first peak (bigger in magnitude) locating at ~415 ± 10 nm and the second one (lower in magnitude)—at ~520 ± 10 nm (compared to the ~520 nm peak asterisked in the attenuation spectra of pristine fibers in **Figure 1(a)**). To evaluate IA strength in the fibers in function of βirradiation dose, let us compare the IA spectra with the attenuation spectra of the same fibers being in pristine state (refer to **Figure 1**). It is known that attenuation growth toward UV is common for Ce-doped glass, as stemming from the transitions inherent to Ce3+/Ce4+ ions. (Unfortunately, IA arising in the UV-region, below 400 nm, was undetectable using our experimental equipment.) Regarding IA in the near-IR, note that the spectral transformations in this region are more complex (see insets to **Figure 3**) whose nature is unclear at the moment.

**Figure 4.** Main frames: dose dependences of IA for Ce-doped (a) and Ce/Au-codoped (b) fibers; blue and red symbols and lines show IA magnitudes of bands 1 and 2, obtained after deconvolution of the spectra shown in **Figure 3**. Insets: examples of deconvolution of the data obtained for the fibers, irradiated with dose 5 (spectra are plotted in eV-do‐ main). (Reproduced with permission from Kir'yanov et al. [75]. Copyright© 2014, Optical Society of America).

Deconvolution of IA spectra (**Figure 3**) allows a closer view on their two-band structure (see insets in **Figure 4(a) and (b)**). Spectral locations of the bands (1 and 2) were found to be almost independent of irradiation dose, for both fibers: they are centered at ~3.0 and ~2.4 (±0.1) eV and are measured in half-widths at a 3 dB level by ~0.3 and ~0.5 (±0.05) eV, respectively. In main frames of **Figure 4**, IA—in terms of these two peaks' magnitudes—is plotted versus irradiation dose; these dependences are shown, respectively, by blue (band 1) and red (band 2) symbols. Fitting them within domain of smaller doses (up to ~2 × 1013 cm–2), linear growth of IA in both bands versus dose is revealed (see the blue and red lines in the figure).

The slopes' values estimated as the result of fitting were found to be ~1.7 (~1.2 dB/m/cm2 ) (Cedoped fiber) and ~1.3 (~0.9 dB/m/cm2 ) (Ce/Au-codoped fiber), correspondingly, for bands 2 and 1. It deserves mentioning that these ratios, on one hand, are almost equal for both fibers (~1.5) and, on the other hand, the slopes' ratios, when compared for bands 1 and 2, are vastly equal as well (~1.3). Furthermore, at bigger irradiation doses IA, in both bands and for either fiber, steadily approaches the "plateaus", marked by black dotted lines in the figures. It is interesting that IA in maxima of bands 1 and 2 at the plateaus (i.e. at doses exceeding 2 × 1014 cm-2) has virtually the same magnitude, for both fibers.

#### *2.2.2. Bleaching of IA as a result of posterior exposure to 543 nm/UV light*

**Figure 3.** Main frames: IA spectra of Ce-doped (a) and Ce/Au-codoped (b) fibers; curves 1–6 correspond to doses of irradiation (in both figures) being: 1 × 1012 (1), 5 × 1012 (2), 1 × 1013 (3), 5 × 1013 (4), 1 × 1014 (5), and 2.5 × 1015 (6) cm–2. Insets: average IA-losses measured within the 1300–1550 nm range *vs*. irradiation dose. (Reproduced with permission

The two-peaks structure of the IA spectra is apparent at higher irradiation doses for both fibers, with the first peak (bigger in magnitude) locating at ~415 ± 10 nm and the second one (lower in magnitude)—at ~520 ± 10 nm (compared to the ~520 nm peak asterisked in the attenuation spectra of pristine fibers in **Figure 1(a)**). To evaluate IA strength in the fibers in function of βirradiation dose, let us compare the IA spectra with the attenuation spectra of the same fibers being in pristine state (refer to **Figure 1**). It is known that attenuation growth toward UV is common for Ce-doped glass, as stemming from the transitions inherent to Ce3+/Ce4+ ions. (Unfortunately, IA arising in the UV-region, below 400 nm, was undetectable using our experimental equipment.) Regarding IA in the near-IR, note that the spectral transformations in this region are more complex (see insets to **Figure 3**) whose nature is unclear at the moment.

**Figure 4.** Main frames: dose dependences of IA for Ce-doped (a) and Ce/Au-codoped (b) fibers; blue and red symbols and lines show IA magnitudes of bands 1 and 2, obtained after deconvolution of the spectra shown in **Figure 3**. Insets: examples of deconvolution of the data obtained for the fibers, irradiated with dose 5 (spectra are plotted in eV-do‐ main). (Reproduced with permission from Kir'yanov et al. [75]. Copyright© 2014, Optical Society of America).

Deconvolution of IA spectra (**Figure 3**) allows a closer view on their two-band structure (see insets in **Figure 4(a) and (b)**). Spectral locations of the bands (1 and 2) were found to be almost independent of irradiation dose, for both fibers: they are centered at ~3.0 and ~2.4 (±0.1) eV and are measured in half-widths at a 3 dB level by ~0.3 and ~0.5 (±0.05) eV, respectively. In main frames of **Figure 4**, IA—in terms of these two peaks' magnitudes—is plotted versus irradiation dose; these dependences are shown, respectively, by blue (band 1) and red (band

from Kir'yanov et al. [75]. Copyright© 2014, Optical Society of America).

8 Radiation Effects in Materials

Hereafter, the featuring data on optical bleaching of β-irradiated Ce-doped and Ce/Aucodoped fibers by a low-power He-Ne 543 nm laser and UV mercury lamp are reported.

**Figure 5.** Dynamics of attenuation decay in terms of AD in Ce-doped (a) and Ce/Au-codoped (b) fibers under the ac‐ tion of 543 nm light (~0.5 mW); bleaching (negative AD) was realized after β-irradiation with doses 2 (curves 1), 4 (curves 2), and 6 (curves 3), for which AD is taken to be zero. (c) Micro-photographs of darkened (dose 5 of β-irradia‐ tion) Ce-doped fiber prior to (top) and after bleaching during 7.5 h (bottom). (d) Examples of the initial 543 nm bleach‐ ing stage, zooming the dependences shown by curves 2 in (a) and (b), respectively. (Reproduced with permission from Kir'yanov et al. [75]. Copyright© 2014, Optical Society of America).

Keeping in mind that, IA, a signature of color centers or defects in glass matrix produced at different kinds of irradiation, can be "bleached" by light (see e.g. [59–61]), we found reasonable to check whether such treatment has effect in our case.

First, we inspected the effect of weak 543 nm light delivered from a 1.5 mW He-Ne laser. In the experiments, power launched into both fibers was fixed (~0.5 mW; coupling efficiency ~30%). In this case, very short pieces (1…2 cm) of β-irradiated fibers were handled, given big IA, measured by hundreds of dB/m (refer to **Figures 3** and **4**), being established at β-darkening. The results are shown in **Figure 5**.

In the left part of **Figure 5**, we show the temporal dynamics of changes in attenuation of the Ce-doped (a) and Ce/Au-codoped (b) fibers under the action of 543 nm light, measured at the same wavelength. The effect of partial bleaching of β-induced loss (the negative AD) is apparent. Note that optical bleaching of both fibers demonstrates a saturating behavior and that the decay rate is bigger for the fiber codoped with Au than for Au-free one (compare curves 1–3 in (a) and (b)); also notice an almost exponential character of bleaching when the process gets starting (see (d) on the right side of **Figure 5**). The bleaching effect is clearly demonstrated by the photographs in **Figure 5(c)**, exemplifying the case of Ce-doped fiber. It is seen that its initial state (before β-irradiation) was almost restored under the action of 543 nm light: compare the photos in **Figure 1(b)** and **Figure 5(c)**.

One would speculate on whether bleaching of the Ce-doped and Ce/Au-codoped fibers arises solely due to laser-light-induced recombination or due to thermally assisted recombination, too, but as for us, the former appears to play a vital role.

**Figure 6(a)** and **(b)** shows how the bleached (main frames) and unbleached (insets) loss in the Ce-doped and Ce/Au-codoped fibers behave at 543 nm bleaching. Note that unbleached (remnant) loss is bigger in Ce/Au- than in Ce-doped fiber, that is, codoping of a Ce-doped fiber with Au results in a similar property of lesser susceptibility to exterior influence (compare with the results on β-irradiation); however, in the case of bleaching this feature appears to be a disadvantage.

The results of illuminating darkened Ce-doped and Ce/Au-codoped fibers with UV light are shown in **Figure 7**. In **Figure 7(a)** we exemplify the spectral dynamics of transmission of βirradiated (at dose 5) Ce/Au-codoped fiber. The photographs in **Figure 7(b)** visualize the result of treatment, being almost a full fading of IA loss. This effect can be quantified by a shift of wavelength's transmission, measured at a 3 dB level (see gray line in **Figure 7(a)**), from near-

**Figure 6.** Bleached (main frames) and unbleached (insets) spectral loss in Ce-doped (a) and Ce/Au-codoped (b) fibers after ~0.5 mW 543 nm treatment, posterior to β-irradiation with doses 2 (curves 1), 4 (curves 2), and 6 (curves 3). For comparison, curves 0 demonstrate the attenuation spectra of pristine fibers. (Reproduced with permission from Kir'ya‐ nov et al. [75]. Copyright© 2014, Optical Society of America).

IR to VIS. It is seen from **Figure 7(c)**, where we demonstrate the results of experiments with Ce/Au-codoped (black open dots) and Ce-doped (gray open squares) fibers, that it has a similar character for both fibers.

**Figure 7.** (a) Dynamics of attenuation decay (in terms of transmission of Ce/Au-codoped fiber under UV-lamp illumi‐ nation with maximal spectral power @350 nm). Bleaching was realized in the darkened fiber, posterior to β-irradiation with dose 4. (b) Micro-photographs of darkened (dose 5 of β-irradiation) Ce/Au-codoped prior to optical bleaching with UV-lamp (top) and after continuous bleaching during 10,000 h (bottom). (c) Examples of the spectral transforma‐ tions during UV-bleaching in terms of shifting of the fiber's transmission edge wavelength measured at -3 dB level; the data were obtained for Ce/Au-codoped (open black dots) and Ce-doped (open grey squares) fibers, preliminary β-irra‐ diated with dose 4; both the data are fitted to the eye by dotted red lines. (Reproduced with permission from Kir'yanov et al. [75]. Copyright© 2014, Optical Society of America).

#### **2.3. Discussion**

~30%). In this case, very short pieces (1…2 cm) of β-irradiated fibers were handled, given big IA, measured by hundreds of dB/m (refer to **Figures 3** and **4**), being established at β-darkening.

In the left part of **Figure 5**, we show the temporal dynamics of changes in attenuation of the Ce-doped (a) and Ce/Au-codoped (b) fibers under the action of 543 nm light, measured at the same wavelength. The effect of partial bleaching of β-induced loss (the negative AD) is apparent. Note that optical bleaching of both fibers demonstrates a saturating behavior and that the decay rate is bigger for the fiber codoped with Au than for Au-free one (compare curves 1–3 in (a) and (b)); also notice an almost exponential character of bleaching when the process gets starting (see (d) on the right side of **Figure 5**). The bleaching effect is clearly demonstrated by the photographs in **Figure 5(c)**, exemplifying the case of Ce-doped fiber. It is seen that its initial state (before β-irradiation) was almost restored under the action of 543

One would speculate on whether bleaching of the Ce-doped and Ce/Au-codoped fibers arises solely due to laser-light-induced recombination or due to thermally assisted recombination,

**Figure 6(a)** and **(b)** shows how the bleached (main frames) and unbleached (insets) loss in the Ce-doped and Ce/Au-codoped fibers behave at 543 nm bleaching. Note that unbleached (remnant) loss is bigger in Ce/Au- than in Ce-doped fiber, that is, codoping of a Ce-doped fiber with Au results in a similar property of lesser susceptibility to exterior influence (compare with the results on β-irradiation); however, in the case of bleaching this feature appears to be a

The results of illuminating darkened Ce-doped and Ce/Au-codoped fibers with UV light are shown in **Figure 7**. In **Figure 7(a)** we exemplify the spectral dynamics of transmission of βirradiated (at dose 5) Ce/Au-codoped fiber. The photographs in **Figure 7(b)** visualize the result of treatment, being almost a full fading of IA loss. This effect can be quantified by a shift of wavelength's transmission, measured at a 3 dB level (see gray line in **Figure 7(a)**), from near-

**Figure 6.** Bleached (main frames) and unbleached (insets) spectral loss in Ce-doped (a) and Ce/Au-codoped (b) fibers after ~0.5 mW 543 nm treatment, posterior to β-irradiation with doses 2 (curves 1), 4 (curves 2), and 6 (curves 3). For comparison, curves 0 demonstrate the attenuation spectra of pristine fibers. (Reproduced with permission from Kir'ya‐

The results are shown in **Figure 5**.

10 Radiation Effects in Materials

disadvantage.

nm light: compare the photos in **Figure 1(b)** and **Figure 5(c)**.

too, but as for us, the former appears to play a vital role.

nov et al. [75]. Copyright© 2014, Optical Society of America).

### *2.3.1. Pristine fibers*

Regarding pristine Ce-doped and Ce/Au-codoped fibers (**Figure 1**), apart from a strong growth of absorption seen at shorter (VIS to UV) wavelengths (apparently connected with the presence of Ce in valences Ce3+/Ce4+ [23, 24]), the other two points deserve mentioning, being (*i*) monotonous growth of loss toward IR in both fibers (of not clear origin but inherent to Ce doping since such trend is absent in the reference Ce free fiber) and (*ii*) a distinct peak at ~520 nm (~2.4 eV) in the absorption spectra of both fibers (but absent in the Ce free one). We suppose that this peak has the same origin as band 2 risen at β-irradiation and located at ~2.4 eV (see **Figure 4**). This feature has not been reported for bulk Ce-doped silica but is frequently observed in Ce-doped fibers subjected to ionizing radiations [17, 24]. It can be related to quite stable Ce3+h+ centers, or alternatively while hypothetically, to Ce4+e<sup>−</sup> centers (existence of which was not documented), but apparently not to sole Ce ions being in either trivalent or tetravalent state. As for us, a more realistic cause for the existence of Ce3+h+ or/and Ce4+e<sup>−</sup> defect centers in pristine fibers, attributable by the 520 nm peak, can be ionization, that is, generation of electrons e− and holes h+ , at the fiber preform's collapse stage [24] or during the fiber's drawing with posterior covering by acrylic outer cladding when—in both situations—strong UV light is produced, with a result being a trapping of free carriers by Ce3+/Ce4+ species.
