We report a study of the attenuation spectra transformations for a series of Bismuth (Bi) doped silica fibers with various contents of emission-active Bi centers, which arise as the result of irradiation by a beam of high-energy electrons. The experimental data reveal a substantial decrease of concentration of the Bi centers, associated with the presence of Germanium in silica glass, at increasing the irradiation dose (the resonant-absorption bleaching effect in germano-silicate fiber). In contrast, the spectral changes that appear in Bi doped alumino-silicate fiber have through irradiation a completely different character, viz., weak growth of the resonant-absorption peaks ascribed to the Bi centers, associated with the presence of Aluminum in silica glass. These results demonstrating high susceptibility of Bi centers to electron irradiation while opposite routes of the irradiation-induced spectral changes in Bi doped germanate and aluminate fibers seem to be of worth notice for understanding the nature of these centers.
©2011 Optical Society of America
Bismuth (Bi) doped silica fibers with host glass co-doped with Aluminum (Al), Germanium (Ge), or Phosphorous (P) are currently of increasing interest, being a promising active medium for amplifying and lasing in the spectral range 1.1–1.6 µm [1–9]. In spite of remarkable success in the field, there remain certain obstacles for further enhancement of Bi fiber lasers and amplifiers efficiency because of a lack of clarity in the nature of Bi centers in silica glass [10–20].
So far, various models were opted for emission-active Bi centers (e.g. Bi+, Bi2+, Bi5+, Bi0, Bi2, Bi2 -, BiO4, etc.); however each of them suffers from inability to explain at least some of the available experimental facts. Recently, one more hypothesis was proposed [21–23] that Bi-related centers are formed at the assistance of near-by located defects of surrounding network.
Our present research highlights the novelties that stem from experiments on irradiating Bi doped germano- and alumino-silicate fibers by a beam of free electrons of high energy (6 MeV). The main of them is a strong decrement (“bleaching”) of the resonant-absorption peaks ascribed to Bi centers in the Bi doped germano-silicate fiber. In the meantime, an opposite but weaker effect (a rise of the resonant-absorption peaks) takes place in the Bi doped alumino-silicate fiber (notice here that analogous trends were recently reported for similar fibers and bulk glasses after irradiation by UV laser pulses or γ quanta [24,25]). Since other optical properties of the fibers under scope, such as Bi centers’ fluorescence spectra and lifetimes, were found to be not subjected to qualitative changes after electron irradiation, the mentioned transformations in the absorption spectra are to be attributed to the irradiation-induced change in Bi centers concentration.
These experimental results, revealing high susceptibility of emission-active Bi centers to electron irradiation while qualitatively different trends in the absorption spectra transformations in correspondingly Bi doped germano- and alumino-silicate fibers, are briefly discussed in attempt of a reasonable explanation.
2. Experimental samples and methods
The Bi doped silica fibers we dealt with in experiments were drawn from Ge (or Al) co-doped silicate glass host preforms fabricated by the MCVD and solution-doping techniques; the details can be found in Refs. [8,26,27]. Core radii of the drawn fibers were measured to be in the range of 2.0 to 3.0 µm. The representative attenuation spectra of pristine (as-received) Bi doped germano- and alumino-silicate fibers, having comparable contents of active Bi centers, are shown in Fig. 1 ; these spectra will be examined and discussed in more detail in Section 4. Notice that in this and further figures the emission-active Bi centers are referred to as Bi(Ge,Si) and Bi(Al), respectively in the germano- and alumino-silicate fibers. The main characteristics of the Bi doped fiber samples that we subjected to electron irradiation – #46, #48, and #52 (germano-silicate) and #33 (alumino-silicate) – will be specified in Section 3.
A controllable linear accelerator (of the LU type), the source of mono-energetic (~6 MeV) electrons, was used in the main-course experiments where a pulsed (~5-µs) electron irradiation mode has been realized. The experiments were conducted at room temperature and the fiber samples were irradiated, being kept in the accelerator chamber during various time intervals. By such a way, growing irradiation doses, up to 5x1013 cm−2, were provided for the samples under study (indices “1”, “2”, and “3” below label doses 2x1012, 1x1013, and 5x1013 cm−2, accordingly). The irradiated fibers were aged during 10 days in order to exclude possible short-living instabilities in the host glasses, especially pronounceable in the fibers containing Ge . Notice that ionization, i.e. the production of radiation-excited carriers by an electron beam within a fiber’s volume, played the main role in the spectral transformations to be reported, because high-energy primary electrons are virtually non-dissipating at the propagation through the fiber’s thickness (125 µm). On the other hand, some contribution in ionization might also originate from γ-quanta produced at inelastic scattering of high-energy electrons propagating through the host glass.
The optical transmission spectra of the tested Bi doped fiber samples were obtained using a white-light source with a fiber output and optical spectrum analyzer (OSA) with a 5-nm resolution. The spectra were recorded before (for pristine samples, labeled below by dose “0”) and after each stage (doses “1”, “2”, and “3”) of electron irradiation. The lengths of the fiber samples were chosen to be a few to tens cm to ensure a clear view on the spectra details within the range of main interest (VIS and near-IR), where the most characteristic resonant-absorption peaks of Bi-related centers are localized and where the main spectral transformations take place as the result of electron irradiation. The attenuation spectra presented below have been obtained after recalculating transmission in loss [dB/m].
We also measured the fluorescence spectra and lifetimes before and after irradiation, applying the lateral detecting geometry; the pump wavelength employed (977 nm) was on the Stokes tail of the 750–950 nm absorption band of Bi doped germano-silicate fibers and, correspondingly, on the anti-Stokes slope of the absorption band centered at 1050 nm of Bi doped alumino-silicate fiber (see Fig. 1). Fluorescence emission was collected from surface of a Bi doped fiber sample at the point spaced by approximately 5 mm from its splice with an output fiber of the pump diode laser. We used in this case the same OSA for the fluorescence spectra measurements and a Ge photo detector and oscilloscope for the fluorescence lifetime measurements (a time resolution of the system was ~8 µs).
3. Experimental results
The attenuation spectra of Bi doped germano-silicate fiber sample #48 after electron irradiation with doses “2” and “3” are shown in Fig. 2(a), along with the attenuation spectrum of a pristine (dose “0”) fiber sample of the same type.
A strong irradiation-induced bleaching effect can be revealed from this figure, seen as a drop of magnitude of the absorption peaks labeled “1” (within the 750–950-nm band) and “2” (within the 1250–1450-nm band). This is accompanied by an essential increase of background loss in the fiber at shorter wavelength (refer to the left-hand side of Fig. 2(a)), a well-known feature captured in the experiments on influence of various-type irradiations on the optical properties of purely Ge doped silica fibers (see e.g. Refs. [29–33].). Unfortunately such a drastic growth of background loss didn’t allow us to make well-resolved measurements of the irradiation-induced transformations of the Bi centers band peaked at ~500 nm (see Fig. 1), so we inspect further 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 have been drawn to provide single-mode propagation for the wavelengths in excess of ~1200 nm).]
Of separate interest is the behavior of absorption peak “2”. Since absorption of Bi centers 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 Fig. 2(b). From this figure, it is evident that the contribution in attenuation which comes from contaminating by water (OH groups) is unchanged after irradiation while the one defined by the presence of the Bi dopants is substantially reduced.
One more example of the irradiation-induced bleaching effect is given in Fig. 3 where we make insight to the spectral transformations in the absorption peaks within the 750–950-nm band (“1”) at electron irradiation of two other Bi doped germano-silicate fiber samples, #46 and #52. These two have, in a pristine state, correspondingly a higher and lower initial concentration of active Bi centers than a pristine sample #48 (compare with Fig. 2).
The spectra shown in Fig. 3 have been obtained before (dose “0”: black curves 1 and 3) and after (dose “3”: blue curves 2 and 4) electron irradiation. Apparently, qualitatively the same law, bleaching of the resonant-absorption peaks through the interval 750–950 nm as the result of electron irradiation, can be revealed from the figure, now for fiber samples #46 and #52. Hence, the bleaching effect is found to be a general feature of any of the Bi doped germano-silicate fibers under study (i.e. in this sense initial Bi centers concentration in pristine samples does not have much matter).
The next graphs plotted in Figs. 4(a) and 4(b) demonstrate how the absorption peaks “1” (namely, its main sub-peak centered at 820 nm) and ”2” (the one centered at ~1400 nm) are reduced throughout electron irradiation (these dose dependences are shown for all fibers in the set #46, #48, and #52).
The initial absorption values (in peaks; these are given near each curve in Figs. 4(a) and 4(b)) were taken from the attenuation spectra of pristine (dose “0”) samples. Curves 1 – 3 for the resonant-absorption peaks “1” (Fig. 4(a)) and “2” (Fig. 4(b)), for each fiber sample, were obtained from the spectra shown in Figs. 1 and 2 after subtracting the background loss, which grows throughout irradiation (refer to Fig. 1 and also to Fig. 5 below). Notice that for fiber #52, 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 Figs. 4(a) and 4(b) that bleaching of the resonant-absorption bands as the result of electron irradiation is indeed 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 becomes evident from Fig. 4(c) where we plot the ratio of absorption coefficients in peaks “1” and “2” as a function of irradiation dose for fiber samples ## 46 and 48; see curve I. Apparently, this quantity is kept virtually unchanged throughout irradiation, being equal to its initial value (for pristine samples). 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 Fig. 4(c). This is a manifestation of that resonant-absorption bands peaked at ~500, ~820, and ~1400 nm (and accordingly emission-active Bi centers attributed by these peaks, see Figs. 1–3) being affected by the same or a very similar manner by electron irradiation.
Then, it is seen from Fig. 5 that the background loss (measured in the dip at 700 nm, between the absorption peaks ascribed to Bi centers in germano-silicate fiber; see Fig. 2) monotonously increases with the irradiation dose, a common effect for all-kind Ge doped silica materials. Growth of the background loss becomes even more pronounceable in the UV. Furthermore, attention is to be paid to inset to Fig. 5 which serves to demonstrate that the initial level of the background loss (in pristine samples) correlates with an initial content of emission-active Bi centers, also an interesting peculiarity of the Bi doped germano-silicate fibers.
Figure 6(a) presents the fluorescence spectra of the germano-silicate fiber with the highest Bi centers content (#46) which have been obtained before and after irradiation (see curves 1 and 2, respectively). It was found that these spectra are not affected qualitatively by electron irradiation. Namely, no fluorescence peaks, other than the Bi centers’ emission band, 900–1500 nm, appear after irradiation, but the result being only a decrease of integrated fluorescence power emitted within this broad band. Of worth noticing is the fact that almost no change was detected in the fluorescence kinetics for pristine and irradiated fibers (0.38 ± 0.03 ms). Hence, the changes in the resonant-absorption peaks (refer to Figs. 2–4) should be related to a decrement of the Bi centers concentration in the germanate fibers.
The results of electron irradiation of the Bi doped alumino-silicate fibers (on the example of fiber sample #33) deserve a separate attention. Figure 7 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 in Fig. 7 (obtained before and after irradiation, correspondingly) that in the Bi doped aluminate fiber an opposite (to the case of the Bi doped germanate fiber) trend exists, viz., instead of resonant-absorption bleaching (see Figs. 2–4), weaker but well detectable extra absorption arises in the peaks centered at ~520 and ~700 nm. Inset to Fig. 7 examples the dynamics of the absorption peak at ~700 nm upon irradiation dose; notice that almost the same dose behavior is observed for the peaks at ~520 and ~1050 nm. Since in our experiments there were not detected any qualitative changes in the fluorescence spectra of pristine and irradiated Bi doped alumino-silicate fibers (see curves 1’ and 2’ in Fig. 6(b)) and kinetics (decay times after excitation were measured to be 0.89 ± 0.04 ms), the conclusion can be made that the absorption spectra transformations in Fig. 7 are caused by an increment of Bi(Al) centers concentration as the result of irradiation.
Summarizing the experimental results, one can reveal that Bi(Ge,Si) and Bi(Al) centers (in germano- and alumino-silicate fibers, respectively) are quite different in the sense of their susceptibility to electron irradiation.
First of all, the attenuation spectra of typical pristine Bi-doped germano- and alumino-silicate fibers (see Fig. 1) need an examination. From these spectra that cover an extended wavelengths interval (400 to 1600 nm), one can recognize the “fingerprints” of Bi dopants in the fibers, appeared through the correspondent resonant-absorption bands: These were referred above to as centers Bi(Ge,Si) and Bi(Al). 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), while the ones at 500, 820 (910), and 1400 nm (the Bi doped germano-silicate fiber) – to physically similar Bi(Ge) and Bi(Si) centers. Notice that the peaks at 1400 nm look indistinguishably for both the fiber types; so they can be related to Si forming host of both the glasses; see e.g. Refs. [34,35]. [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 a special design of the fibers (the cutoff peaks).]
As concerned to the experimental results on the high-energy electron irradiation they seem to be remarkable but not enough to make a definite conclusion on real processes involved. The only thing that we can propose is a possible correlation of the rise and decrease of IR-emission-active Bi centers concentration 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 [31,33,36–38]. This difference can substantially influence on the residuary charge state of the Bi specie after the electron irradiation. The process of radiaton-induced charge trapping of both electrons and holes should be accompanied by the formation of different point defects (like Ge(1), Ge(2), GeE’ and Al-E’, Al-oxygen-deficient centers [39–41]) detectable in ESR and optical spectra measurements. In future, together with a study of ESR spectra of radiation-induced defects and of thermal stability of radiation-induced optical spectra, the results obtained in this paper would take a convincing explanation and will allow approaching to a solution of emission-active Bi-related centers enigma.
We have reported a study of the attenuation spectra transformations for a series of Bi doped germano-silicate fibers and, for comparison, Bi doped alumino-silicate fiber as the result of irradiation by a beam of high-energy (6 MeV) electrons. The experimental data allow us to conclude that a considerable decrease in concentration of Bi(Ge,Si) centers with an increase of irradiation dose takes place in the Bi doped germano-silicate fiber. This effect is revealed from irradiation-induced bleaching of the resonant-absorption Bi-related peaks, associated with the presence of Ge and/or Si atoms in the host glass. Such a behavior is remarkable as it was observed in any of the Bi doped germano-silicate fibers, which initially (in the pristine state) had quite different contents of emission-active Bi centers. In contrast, the Bi doped alumino-silicate fibers demonstrate an opposite trend, viz., weak extra absorption arises in the peaks, characteristic to Bi(Al) centers, as the result of electron irradiation. These experimental facts can be useful for future studies with Bi doped silica glasses and fibers, say for those targeting at the development of a more reliable model of emission-active Bi centers.
Authors are thankful to Drs. A.A. Umnikov, N.N. Vechkanov, and A.N. Guryanov (The Institute for Chemistry of High-Purity Substances of the Russian Academy of Sciences, Nizhny Novgorod, Russia) for fabricating Bi doped silica fibers and to Dr. A. Musalityn (The National University of Science and Technology (MISIS), Moscow, Russia) for the help in conducting electron irradiation of the fiber samples.
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