**4. Effect of electron irradiation upon optical properties of Bi-doped silica fibers**

Bi-doped silica fibers with core-glass codoped with Al, Ge, or P are currently of increasing interest, being a promising active medium for amplifying and lasing in the spectral range 1.1– 1.6 μm (see e.g. [49–59]). In spite of remarkable success in the field, there remain certain obstacles for further improvements of Bi fiber lasers and amplifiers because the main problem is lack of clarity in the nature of Bi "active" centers (Further—BACs) in silica glass. Thus, any research aiming to recover the essences of BACs would have value.

Below we highlight the effect of irradiation of Bi-doped germano- and alumino-silicate fibers by a beam of free electrons of high energy. The main result of the treatment was found to be decrement ("bleaching")/increment (rise of resonant absorption) in the characteristic peaks, being ascribed to BACs in Bi-doped germano-silicate/alumino-silicate fibers. (Note that analogous trends were reported for similar fibers and glasses under the action of UV laser pulses and γ-quanta [60, 61]). Given that the other optical properties of the fibers under scope, such as BACs fluorescence spectra and lifetimes, were found to be weakly affected by electron irradiation, the changes in the absorption spectra should be associated with the changes in BACs concentration, as firmly justified in our study.

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

The Bi-doped silica fibers were drawn from Ge and Al codoped silicate-glass preforms, fabricated applying the MCVD/SD technique. Core radii of the fibers were measured to be in the 2…3 μm range. The representative attenuation spectra of pristine (as-received) Bi-doped germano- and alumino-silicate fibers, having comparable contents of BACs, are shown in **Figure 16**. Hereafter, the emission-active BACs are referred to as Bi(Ge,Si) and Bi(Al), respec‐ tively, in these two types of fiber. Impact of electron irradiation on the basic characteristics of the fibers, referred to further as *Bi-1, Bi-2*, and *Bi-3* (germano-silicate) and *Bi-4* (aluminosilicate), is addressed subsequently.

**Figure 16.** Attenuation spectra of typical Bi-doped germano-silicate (curve 1) and alumino-silicate (curve 2) silica fi‐ bers. Arrow shows the pump wavelength (977 nm) used in the experiments on fluorescence spectra and lifetimes measurements. Dashed lines show schematically a trend of the background loss to grow toward shorter wavelengths. (Reproduced with permission from Kir'yanov [67]. Copyright© 2011, Optical Society of America).

Electron irradiation of Bi-1…Bi-4 fibers was proceeding in the conditions, described in Introduction; the indices "1," "2," and "3" label to doses 2 × 1012, 1 × 1013, and 5 × 1013 cm–2, respectively. The technique applied to reveal the spectral transformations in attenuation of the Bi-doped fibers as the result of irradiation was completely the same as described in Paragraphs 2 and 3 and is not repeated here. When measuring Bi-related fluorescence, we utilized the same LD (pump wavelength, 977 nm) for excitation. As seen from **Figure 16**, the pump wavelength was on the Stokes tail of the 750–950 nm absorption band of BACs in Bi-doped germano-silicate fiber and, correspondingly, on the anti-Stokes slope of the absorption band (centered at 1050 nm) of BACs in Bi-doped alumino-silicate fiber. We applied in the BACs fluorescence measurements the lateral detecting geometry, when it was collected from the surface of a Bidoped fiber sample; the same OSA and a Ge PD were handled to proceed the fluorescence measurements.

#### **4.2. Experimental**

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

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 β-

**4. Effect of electron irradiation upon optical properties of Bi-doped silica**

Bi-doped silica fibers with core-glass codoped with Al, Ge, or P are currently of increasing interest, being a promising active medium for amplifying and lasing in the spectral range 1.1– 1.6 μm (see e.g. [49–59]). In spite of remarkable success in the field, there remain certain obstacles for further improvements of Bi fiber lasers and amplifiers because the main problem is lack of clarity in the nature of Bi "active" centers (Further—BACs) in silica glass. Thus, any

Below we highlight the effect of irradiation of Bi-doped germano- and alumino-silicate fibers by a beam of free electrons of high energy. The main result of the treatment was found to be decrement ("bleaching")/increment (rise of resonant absorption) in the characteristic peaks, being ascribed to BACs in Bi-doped germano-silicate/alumino-silicate fibers. (Note that analogous trends were reported for similar fibers and glasses under the action of UV laser pulses and γ-quanta [60, 61]). Given that the other optical properties of the fibers under scope, such as BACs fluorescence spectra and lifetimes, were found to be weakly affected by electron irradiation, the changes in the absorption spectra should be associated with the changes in

The Bi-doped silica fibers were drawn from Ge and Al codoped silicate-glass preforms, fabricated applying the MCVD/SD technique. Core radii of the fibers were measured to be in the 2…3 μm range. The representative attenuation spectra of pristine (as-received) Bi-doped

electrons) and at OP (creation of carriers and color centers by pump-light).

research aiming to recover the essences of BACs would have value.

BACs concentration, as firmly justified in our study.

**4.1. Fiber samples and experimental arrangement**

in **Figures 14** and **15**) but not—at e-irradiation.

24 Radiation Effects in Materials

**fibers**

The experimental results are presented by **Figures 17**–**21**. The attenuation spectra of Bi-doped germano-silicate fiber sample Bi-2 subjected to electron irradiation with doses "2" and "3" are shown in **Figure 17(a)** along with the attenuation spectrum of a pristine (dose "0") fiber of the same type. A strong irradiation-induced bleaching effect can be revealed from the figure, seen as drop of magnitude of the absorption peaks labeled "1" (the 750–950 nm band) and "2" (the

**Figure 17.** (a) Attenuation spectra of Bi-doped germano-silicate fiber sample Bi-2 obtained before (dose "0") and after (doses "2" and "3") irradiation. The spectral area, comprising the resonant-absorption peaks "1" and "2" which attrib‐ ute Bi(Ge,Si) centers in the host glass, is shown (b) Insight to the spectral area of peak "2" in a vaster scale. (Repro‐ duced with permission from Kir'yanov [67]. Copyright© 2011, Optical Society of America).

**Figure 18.** Attenuation spectra of Bi-doped germano-silicate fibers Bi-1 (curves 1 and 2) and Bi-3 (curves 3 and 4), ob‐ tained before (dose "0") and after (dose "3") electron irradiation. [The spectral area for the peak "1" is zoomed.] (Re‐ produced with permission from Kir'yanov [67]. Copyright© 2011, Optical Society of America).

1250–1450 nm band). It is accompanied by an increase of background loss at shorter wave‐ lengths (refer to the left side of **Figure 17(a)**), a well-known feature in experiments on influence of various type of irradiations on optical properties of Ge-doped silica fibers (see e.g. Refs. [62– 66]). Unfortunately, such a drastic growth of background loss did not allow us to make wellresolved measurements of the irradiation-induced transformations of BACs band peaked at ~500 nm (see **Figure 16**), so we inspected mostly the changes in peaks "1" and "2". Also notice that almost no changes arise in the attenuation peak at 1180 nm, which corresponds to the cutoff wavelength: this and other Bi-doped germano-silicate fiber samples were drawn to provide single-mode propagation for wavelengths >1200 nm.

Effects of Electron Irradiation Upon Absorptive and Fluorescent Properties of Some Doped Optical Fibers http://dx.doi.org/10.5772/63939 27

**Figure 19.** Dose dependences of attenuation of the resonant-absorption peaks "1" (~820 nm) (a) and "2" (~1400 nm) (b): The data for Bi-1 (curves 1), Bi-2 (curves 2), and Bi-3 (curve 3) are shown. (c) insights dose dependences of the peaks magnitudes' ratios (820…1400 nm – curve I and 500…1400 nm – curve II), for fibers Bi-1 (circles) and Bi-2 (squares). (Reproduced with permission from Kir'yanov [67]. Copyright© 2011, Optical Society of America).

**Figure 20.** Dose dependences of background loss measured at 700 nm for fibers Bi-1 (1), Bi-2 (2), and Bi-3 (3). (Repro‐ duced with permission from Kir'yanov [67]. Copyright© 2011, Optical Society of America).

1250–1450 nm band). It is accompanied by an increase of background loss at shorter wave‐ lengths (refer to the left side of **Figure 17(a)**), a well-known feature in experiments on influence of various type of irradiations on optical properties of Ge-doped silica fibers (see e.g. Refs. [62– 66]). Unfortunately, such a drastic growth of background loss did not allow us to make wellresolved measurements of the irradiation-induced transformations of BACs band peaked at ~500 nm (see **Figure 16**), so we inspected mostly the changes in peaks "1" and "2". Also notice that almost no changes arise in the attenuation peak at 1180 nm, which corresponds to the cutoff wavelength: this and other Bi-doped germano-silicate fiber samples were drawn to

**Figure 18.** Attenuation spectra of Bi-doped germano-silicate fibers Bi-1 (curves 1 and 2) and Bi-3 (curves 3 and 4), ob‐ tained before (dose "0") and after (dose "3") electron irradiation. [The spectral area for the peak "1" is zoomed.] (Re‐

produced with permission from Kir'yanov [67]. Copyright© 2011, Optical Society of America).

**Figure 17.** (a) Attenuation spectra of Bi-doped germano-silicate fiber sample Bi-2 obtained before (dose "0") and after (doses "2" and "3") irradiation. The spectral area, comprising the resonant-absorption peaks "1" and "2" which attrib‐ ute Bi(Ge,Si) centers in the host glass, is shown (b) Insight to the spectral area of peak "2" in a vaster scale. (Repro‐

duced with permission from Kir'yanov [67]. Copyright© 2011, Optical Society of America).

26 Radiation Effects in Materials

provide single-mode propagation for wavelengths >1200 nm.

**Figure 21.** Attenuation spectra of Bi-doped alumino-silicate fiber Bi-4, obtained before (curve 1, dose "0") and after (curve 2, dose "3") electron irradiation. A part of the spectra is shown where the main resonant-absorption peaks of Bi(Al) centers are observed. Inset highlights the behavior of one of the peaks (at ~700 nm) against the irradiation dose. (Reproduced with permission from Kir'yanov [67]. Copyright© 2011, Optical Society of America).

Of separate interest is the behavior of absorption peak "2." Since absorption of BACs in this spectral area is covered by an absorption peak of OH groups (1385 nm), we found reasonable to zoom the spectral transformations for this range (see **Figure 17(b)**). From the figure, it is seen that the contribution in attenuation which comes from contaminating by OH groups is unchanged after irradiation, while the one stemming from the presence of the Bi-dopants is substantially reduced.

One more example of the irradiation-induced bleaching effect is given in **Figure 18** where we make insight to the spectral transformations in the absorption peaks within the 750–950 nm band ("1") after electron irradiation of the rest of Bi-doped germano-silicate fibers, Bi-1 and Bi-3. These two have, in pristine state, a higher and lower than Bi-2 concentration of BACs, correspondingly (see **Figure 17**). The spectra shown in **Figure 18** have been obtained before (dose "0": black curves 1 and 3) and after (dose "3": blue curves 2 and 4) electron irradiation. Qualitatively, the same law, viz., bleaching of the resonant-absorption peaks through the interval 750–950 nm as the result of electron irradiation, is revealed, now for fiber samples Bi-1 and Bi-3. Hence, the bleaching effect is found to be a general essence of the Bi-doped germanosilicate fibers.

The next graphs plotted in **Figure 19** (a, b) demonstrate how absorption peaks "1" (namely, its main subpeak centered at 820 nm) and "2" (the one centered at ~1400 nm) are reduced via electron irradiation (these dose dependences are shown for all fibers: Bi-1, Bi-2, and Bi-3). The initial absorption values (in peaks; these are given near each curve in **Figure 19(a, b)**) were taken from the attenuation spectra of pristine (dose "0") samples. Curves 1–3 for resonantabsorption peaks "1" (Figure 19(a)) and "2" (Figure 19(b)) were obtained from the spectra shown in Figures 16 and 17 after subtracting the background loss, which grows at irradiation (refer to **Figure 16** and also to **Figure 20**). Note that, for fiber Bi-3 characterized by the lowest content of Bi centers, the data are provided for peak "1" only because the measurements for peak "2" were below the resolution limit. It is seen from **Figure 19(a, b)** that bleaching of the resonant-absorption bands after electron irradiation is a characteristic feature of the Bi-doped germano-silicate fibers. Furthermore, resonant absorption bleaching in peaks "1" and "2" has almost the same character, which is evident from **Figure 19(c)** where we plot the ratio of absorption coefficients in peaks "1" and "2" in function of irradiation dose for Bi-1 and Bi-2 samples; see curve I. As seen, this quantity is kept virtually unchanged via irradiation, being equal to its initial value measured in pristine state. The same conclusion can be made for the ratio of absorption coefficients in peaks at ~500 nm and ~1400 nm ("2"), see curve II in **Figure 19(c)**. This is a justification of that resonant-absorption bands peaked at ~500, ~820, and ~1400 nm (and accordingly emission-active BACs attributed by these peaks, see Figures 16– 18) are affected by the same or by a very similar manner by electron irradiation.

Then, as seen from **Figure 20**, the background loss (measured in the dip at 700 nm, between the absorption peaks ascribed to BACs in germano-silicate fiber; see **Figure 17**) monotonously increases with dose, a common effect for all kinds of Ge-doped silica materials. (Growth of the background loss is even more pronounceable in the UV.)

The results of electron irradiation of the Bi-doped alumino-silicate fibers (exemplified for fiber Bi-4) deserve a separate attention. **Figure 21** shows how the attenuation spectra of this fiber are changed after a maximal dose of electron irradiation. It is seen from a direct comparison of curves 1 and 2 (obtained before and after irradiation) that in the Bi-doped aluminate fiber an opposite (to the case of the Bi doped germanate fiber) trend exists, viz. instead of resonantabsorption bleaching (see **Figures 17–19**), weaker but detectable extra absorption arises in the peaks centered at ~520 and ~700 nm. Inset to **Figure 21** examples the dynamics of the absorption peak at ~700 nm upon dose; note that almost the same dose behavior is observed for the peaks at ~520 and ~1050 nm.

We do not present here the results of measuring fluorescence spectra and fluorescence lifetimes adherent to BACs, obtained before and after irradiation; the reader is advised to refer to [67] for details. The only thing to mention in this regard is that the fluorescence spectra of both types of the Bi-doped fibers (germano- and alumino-silicate) were not affected qualitatively by electron irradiation, with a sole result of the latter being a decrease/increase of integrated fluorescence power emitted by the germano-/alumino-silicate fibers. Also note that almost no change was detected in the fluorescence kinetics for pristine and irradiated fibers of these two types (0.38 ± 0.03/0.89 ± 0.04 ms). Thus, the changes in the resonant-absorption peaks, detected above, should be related to a decrement/increment of the BACs concentration in the germa‐ nate/aluminate Bi-doped fibers.

### **4.3. Discussion**

Of separate interest is the behavior of absorption peak "2." Since absorption of BACs in this spectral area is covered by an absorption peak of OH groups (1385 nm), we found reasonable to zoom the spectral transformations for this range (see **Figure 17(b)**). From the figure, it is seen that the contribution in attenuation which comes from contaminating by OH groups is unchanged after irradiation, while the one stemming from the presence of the Bi-dopants is

One more example of the irradiation-induced bleaching effect is given in **Figure 18** where we make insight to the spectral transformations in the absorption peaks within the 750–950 nm band ("1") after electron irradiation of the rest of Bi-doped germano-silicate fibers, Bi-1 and Bi-3. These two have, in pristine state, a higher and lower than Bi-2 concentration of BACs, correspondingly (see **Figure 17**). The spectra shown in **Figure 18** have been obtained before (dose "0": black curves 1 and 3) and after (dose "3": blue curves 2 and 4) electron irradiation. Qualitatively, the same law, viz., bleaching of the resonant-absorption peaks through the interval 750–950 nm as the result of electron irradiation, is revealed, now for fiber samples Bi-1 and Bi-3. Hence, the bleaching effect is found to be a general essence of the Bi-doped germano-

The next graphs plotted in **Figure 19** (a, b) demonstrate how absorption peaks "1" (namely, its main subpeak centered at 820 nm) and "2" (the one centered at ~1400 nm) are reduced via electron irradiation (these dose dependences are shown for all fibers: Bi-1, Bi-2, and Bi-3). The initial absorption values (in peaks; these are given near each curve in **Figure 19(a, b)**) were taken from the attenuation spectra of pristine (dose "0") samples. Curves 1–3 for resonantabsorption peaks "1" (Figure 19(a)) and "2" (Figure 19(b)) were obtained from the spectra shown in Figures 16 and 17 after subtracting the background loss, which grows at irradiation (refer to **Figure 16** and also to **Figure 20**). Note that, for fiber Bi-3 characterized by the lowest content of Bi centers, the data are provided for peak "1" only because the measurements for peak "2" were below the resolution limit. It is seen from **Figure 19(a, b)** that bleaching of the resonant-absorption bands after electron irradiation is a characteristic feature of the Bi-doped germano-silicate fibers. Furthermore, resonant absorption bleaching in peaks "1" and "2" has almost the same character, which is evident from **Figure 19(c)** where we plot the ratio of absorption coefficients in peaks "1" and "2" in function of irradiation dose for Bi-1 and Bi-2 samples; see curve I. As seen, this quantity is kept virtually unchanged via irradiation, being equal to its initial value measured in pristine state. The same conclusion can be made for the ratio of absorption coefficients in peaks at ~500 nm and ~1400 nm ("2"), see curve II in **Figure 19(c)**. This is a justification of that resonant-absorption bands peaked at ~500, ~820, and ~1400 nm (and accordingly emission-active BACs attributed by these peaks, see Figures 16–

18) are affected by the same or by a very similar manner by electron irradiation.

background loss is even more pronounceable in the UV.)

Then, as seen from **Figure 20**, the background loss (measured in the dip at 700 nm, between the absorption peaks ascribed to BACs in germano-silicate fiber; see **Figure 17**) monotonously increases with dose, a common effect for all kinds of Ge-doped silica materials. (Growth of the

The results of electron irradiation of the Bi-doped alumino-silicate fibers (exemplified for fiber Bi-4) deserve a separate attention. **Figure 21** shows how the attenuation spectra of this fiber

substantially reduced.

28 Radiation Effects in Materials

silicate fibers.

First of all, the attenuation spectra of typical pristine Bi-doped germano- and alumino-silicate fibers **(Figure 16)** need examination. From these spectra that cover an extended wavelengths interval (400–1600 nm), one can recognize the "fingerprints" of Bi dopants in the fibers, appearing through the correspondent resonant-absorption bands: these were referred to as Bi(Ge,Si) and Bi(Al) centers. Specifically, the main absorption peaks at 520, 700, and 1050 nm (the Bi-doped alumino-silicate fiber) seem to belong to the center Bi(Al), whereas the ones at 500, 820 (910), and 1400 nm (the Bi-doped germano-silicate fiber)—to physically similar Bi(Ge) and Bi(Si) BACs. Note that the peaks at 1400 nm look indistinguishable for both fiber types; so they can be related to Si forming host of both the glasses (see e.g. Refs. [68, 69]). (Other spectral features not linked to the presence in the fibers of Ge, Al, and Si originate either from contaminating by water (OH peaks at 1385 and 1240 nm) or from special design of the fibers (the cutoff peaks). Regarding the experimental results on electron irradiation, they are remarkable but not enough to make a definite conclusion on real processes involved. The only thing to propose is possible correlation of the rise and decrease of IR emission-active BACs concentrations after electron irradiation in alumino- and germano-silicate fibers, respectively, with known facts that substitutional four-coordinated Al in alumino-silicate glass is a hole trap, whereas substitutional Ge in germanate glass is an electron trap [64, 66, 70–72]. This difference can strongly affect the residuary charge state of the Bi specie after the electron irradiation. The process of radiation-induced charge trapping of both electrons and holes can be accompanied by the formation of different point defects (say, Ge(1), Ge(2), GeE', Al-E', and Al-ODCs [73, 74]), detectable in ESR and optical spectra' measurements.
