*2.3.3. Optically bleached fibers*

was not documented), but apparently not to sole Ce ions being in either trivalent or tetravalent

pristine fibers, attributable by the 520 nm peak, can be ionization, that is, generation of electrons

posterior covering by acrylic outer cladding when—in both situations—strong UV light is

Consider in more details the results of β-irradiation of Ce-doped and Ce/Au-codoped fibers (**Figures 2**–**4**). The processes, involved at irradiating the fibers with the result being rise followed by saturation of IA, described by the "stretched-exponent" law [14, 24, 26]), comprise:

(primary) electrons and their trapping on such glass imperfections as Ce ions (Ce3+/Ce4+), nonbridging oxygen centers, other centers associated with Al and P, and oxygen vacancies; (*ii*)

bination between the centers or defects that have arisen during and after β-irradiation. Concerning the role of Ce-doping, we assume that IA is produced via irradiation-induced

valences 3+/4+ in pristine fibers or/and being generated via irradiation. Note that determina‐ tion of relative contents of Ce3+/Ce4+ ions in the pristine state is hard and that at low Ce doping, mainly fluorescing Ce3+ are formed in the core-glass, while at higher overall Ce concentration both Ce3+ and Ce4+ (nonfluorescing) ions can be present. Unfortunately, the absorption spectra of glasses containing both Ce3+/Ce4+ ions have the featuring bands within a 200–400 nm (UV) range (not detectable by our spectral equipment); so any arguing about Ce3+↔Ce4+ transfor‐

 defect centers are expectedly located in VIS (see above), on one hand, and, on the other hand, the detected spectral changes at β-irradiation occur in VIS, too (band 2); thus, formation of

Furthermore, IA bands 1 (~3.0 eV) and 2 (~2.4 eV) (see **Figure 4**) have been undoubtedly separated; see above. The first of them, in Ce-doped and Ce/Au-codoped fibers, has seemingly the same origin as the one in the Ce-free fiber (see **Figure 2**), that is, it most probably belongs to one, most simply organized, type of the two NBOHCs centers, inherent to silica. The other would stem from Ce doping: it is seen from **Figure 2** that such band does not exist in the reference Ce-free fiber. However, the irradiated Ce-free fiber demonstrates ~600 nm band, probably attributing the other type of NBOHCs [26], absent in both Ce-doped fibers subjected to irradiation: compare spectra 1–3 shown in **Figure 2**. The fact that the dose dependences of IA (Figure 4) have different characters for the fibers points on different nature of the centers represented by bands 1 and 2. Therefore, our hypothesis that ~3.0 eV band stems from NBOHCs

In our case (alumino-silicate core glass of Ce-doped and Ce/Au-codoped fibers), such "point" defects as Al-E' and Al-oxygen-deficient centers can be also created at trapping secondary

as its result is a worthy proposal.

mations for this range is impossible. In the meantime, the absorption bands of Ce3+h+

produced, with a result being a trapping of free carriers by Ce3+/Ce4+ species.

(→?Ce4+) and Ce4++e<sup>−</sup>

, at the fiber preform's collapse stage [24] or during the fiber's drawing with

and electrons e<sup>−</sup>

recombination (annihilation); (*iii*) thermally or/and radiatively activated recom‐

→(?Ce4+e<sup>−</sup>

or/and Ce4+e<sup>−</sup> defect centers in

) in the core-glass matrix by β

)→Ce3+ [26–28], implying Ce was in

/Ce3+e<sup>−</sup>

) seems to be relevant.

and Ce4+e

state. As for us, a more realistic cause for the existence of Ce3+h+

e−

direct h+

−

and holes h+

12 Radiation Effects in Materials

*2.3.2. β-irradiated fibers*

…e<sup>−</sup>

reactions Ce3++h+

metastable centers Ce3+h+

(*i*) creating of secondary carriers (holes h+

→Ce3+h+

/Ce4+e<sup>−</sup>

and that ~2.4 eV one is associated with a Ce-related center (Ce3+h+

Let us discuss now the effect of partial bleaching of β-irradiated Ce-doped and Ce/Au-codoped fibers under the action of low-power VIS/UV light (see Figures 5–7). Whereas a doubtless conclusion on its nature is hard, some discourse about the matters involved can be made. The processes responsible for recombination of radiation-induced defects or color centers, seen as IA fading (bleaching) of darkened fibers, can be of thermally and/or optically induced origin. Bleaching, with its result being decreasing IA versus time, seems to be an example of mainly optically induced recombination of both types of centers, NBOHCs and Ce-related Ce3+h+ (assumed to be represented by bands 1 and 2, respectively) ones.

As seen from **Figure 5(d)** and **Figure 7(c)**, IA decreases almost exponentially at the beginning of bleaching. However, within the whole interval of optical bleaching, IA in bands 1 and 2 is seen to fade (in terms of negative AD at 543 nm illumination, see **Figure 5(a, b)**, as well as in terms of shifting the transmission edge to shorter wavelength, at exposure the fibers to UV light, see **Figure 7(a, c)**), which obeys a "stretched exponent" law [26]. An explanation for this behavior can be not only complexity of the mechanisms involved at optical bleaching but also a fact of limited "penetration" of bleaching light into a fiber sample (especially in the case of 543 nm bleaching).

Concerning the essence of IA at optical bleaching, we can, at the current stage of our knowl‐ edge, propose them only tentatively. If our attribution of IA bands 1 and 2 as "signatures" of NBOHCs and Ce-related Ce3+h+ centers is correct, then these centers, formed at trapping free holes, should be breaking via the holes' detrapping and annihilating with free electrons born at interaction with VIS/UV light. Weak intensity of bleaching light is guessed to produce mainly extra electrons rather than holes in the core-glass, leading to dominance of the processes relating to the hole-trapped centers, such as NBOHC (~3.0 eV band) and Ce3+h+ ones (~2.4 eV band). Note that a strong candidate to be "in-charge" of production of e− at the UV/VIS excitation may be Ce3+ ions themselves [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 ordered glass network, already noticed).
