Optics and Photonics Journal, 2011, 1, 155-166
doi:10.4236/opj.2011.14026 Published Online December 2011 (http://www.SciRP.org/journal/opj)
Electron-Irradiation and Photo-Excitation Darkening and
Bleaching of Yb Doped Silica Fibers: Comparison
Alexander V. Kir’yanov
Centro de Investigaciones en Optica, Leon, Mexico
Received September 25, 2011; revised October 28, 2011; accepted November 10, 2011
Abstract
We report a comparative experimental study of the attenuation spectra transformations for a series of Yb
doped alumino-germano silicate fibers with different contents of Yb3+ dopants, which arise as the result of
irradiation either by a beam of high-energy electrons or by resonant (into the 977 nm absorption peak of Yb3+
ions) optical pumping. The experimental data obtained reveal that in the two circumstances, substantial and
complex but different in appearance changes occur within the resonant absorption band of Yb3+ ions and in
the off-resonance background loss of the fibers. Possible mechanisms responsible for these spectral changes
are discussed.
Keywords: Ytterbium Doped Silica Fibers, Photodarkening, Electron Irradiation
1. Introduction
Yb3+ doped silica fibers (YFs) with different core glass
hosts co-doped with aluminum, germanium, or phos-
phorous 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 con-
figurations (core and cladding) and pump wavelengths
were extensively examined so far which resulted in rec-
ognition of optimal arrangements for multi-watt release
from YF based lasers with high optical efficiency ~70 -
75% and perfect beam quality [1,2]. However, in spite of
remarkable progress in the field, there remain certain
obstacles that limit the performance of YF based lasers,
one of them being the photodarkening (PD) effect [3], i.e.
long-term (minutes to hours) degradation of laser power
from units to tens %. This hardly mitigated disadvantage
becomes notable when dealing with a laser based on
heavily-doped YF where high Yb3+ population inversion
is created, either at high-power continuous-wave or mod-
erate-power pulsed lasing. A number of studies during
the past years were aimed to understand the PD phenom-
enon which has remained unclear, although a few hypo-
theses have been proposed for its explanation [4-12].
Meanwhile, on one hand, the characterization procedures
have been specified to quantify the PD phenomenon in
YFs [9,10], allowing a proper choice among YFs for a
certain application. And on the other hand, some ways to
enhance resistance of YFs to PD or at least to minimize
its consequences have been suggested [13,14].
In the meantime, a few studies aimed at the charac-
terization of susceptibility of YFs having different che-
mical compositions under such irradiations as x-rays, 
quanta, and UV have been reported recently [15-17]. The
main motivation for these works was an inspection of
resistance of YF employed in telecommunications and
space technologies 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 (0.9 µm - 1.0 µm) band. This
interesting fact undoubtedly deserves attention and veri-
fication by an experiment where YF would be subjected
to other types of irradiation (say, by high-energy elec-
trons) and the result of this be directly compared with the
PD consequences in the same YF.
Here we report on two sets of experiments where sus-
ceptibility of YFs with similar alumino-germano (Al,Ge)
silicate glass core but with different Yb3+ ions’ concen-
trations is inspected under irradiation either by an elec-
tron beam or by resonant (into Yb3+ resonant band) opti-
cal pumping. For both circumstances, qualitatively simi-
lar trends are revealed: Strong and monotonous changes
in attenuation loss of the fibers in VIS (darkening) ac-
companied by more complex transformations (an initial
decrease followed by increase) within the resonant ab-

A. V. KIR’YANOV
156
sorption band of Yb3+ ions upon dose (the case of elec-
tron irradiation) or time (the case of optical pumping at
977-nm wavelength). However, these trends are shown
to be peculiar in details. Below, we compare and discuss
the experimental data obtained and propose preliminary
explanations.
2. Experimental Arrangement
2.1. Fibers Characterization
YFs inspected in these experiments were drawn from
Al,Ge co-doped silicate glass preforms fabricated using
the MCVD and solution-doping processes. The atenua-
tion spectra in a pristine (as-received) state of the fibers
are shown in Figure 1(a). The concentrations of Yb3+
ions through a set of these YFs differed by more than an
order of magnitude, so there were expected differences in
the consequences of electron irradiation (further-e-irra-
diation) and optical pumping at 977 nm wavelength (fur-
ther-OP) on the fibers’ posterior properties. The fiber sam-
ples, having correspondingly the lowest, the intermediate,
and the highest Yb3+ doping level, are referred further to
as YF-1, YF-2, and YF-3.
2.2. Experimental Methods
A controllable linear accelerator of the LU type which
sources mono-energetic (~6 MeV) electrons was used in
experiments where a pulsed (~5-µs) e-irradiation mode
has been realized. The experiments were conducted at
room temperature. YF samples were irradiated by plac-
ing them in the accelerator chamber during various time
intervals, which provided growing irradiation doses. In-
dices “1”, “2”, and “3” label, on some of the figures be-
low, doses 2 × 1012, 1 × 1013, and 5 × 1013 cm-2, respec-
tively. The irradiated fibers were leaved for 10 days in
advance to the measurements to avoid possible short-
living instabilities in the host glass, known for the fibers
containing Al and Ge. Notice that ionization, i.e. the
production of irradiation-excited carriers by an electron
beam within a fiber’s volume, plays the main role in the
spectral transformations being reported. This is because
high-energy primary electrons are virtually non-dissi-
pating at the propagation through the fibers with an outer
diameter of 125 µm. Meanwhile, some contribution in
ionization might be produced by γ-quanta born at inelas-
tic scattering of high-energy electrons propagating throu-
gh the host glass.
Experiments on OP of YF at 977 nm were made by a
similar way as described in Ref. [8]. YF samples were
pumped using a standard 300 mW 977 nm laser diode
(LD). The pump light was launched from the LD to
SMF-28 fiber through a splice and then through one
more splice to an YF sample. The end of the latter was
spliced to another piece of SMF-28 fiber that was con-
nected to an optical spectrum analyzer (OSA) for the
transmission spectra measurements. In these experiments,
we dealt with short pieces of YFs, of a cm range, to en-
sure no-lasing 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 for a spectral range 400 nm - 1200
nm, where the most interesting spectral transformations
occur as the result of e-irradiation or OP. The output of
the white-light source was spliced to a fiber set containing
(a) (b)
Fi gur e 1. ( a) A ttenuation (small-signal absorption) spectra of fibers w ith low (YF-1), i ntermediate (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 figure (a). Inset in figure (b) shows “cooperative” fluorescence in VIS.

157 A. V. KIR’YANOV
an YF sample (pristine or subjected to e-irradiation or to
OP) and white-light attenuation within the sample was
recorded using the OSA. The attenuation spectra were
recorded before (using pristine samples) and after each
stage of e-irradiation (“doses”) or OP at 977 nm pum-
ping (“times”). Lengths of the fiber samples were chosen
to be short enough, from less than 1 cm (YF-3) to tens of
cm (YF-1), to provide the final spectra free of spectral
noise artifacts. We measured the attenuation spectra from
VIS to near-IR, i.e. within the range where the main
spectral transformations resulting from e-irradiation or
OP occur. In some of the figures below, the difference
spectra are demonstrated which were obtained after sub-
traction of the attenuation spectra of pristine samples
from the ones taken after some dose (time) of e-irradia-
tion (OP). This allows insight to the net spectral loss re-
sulting from the fibers darkening. All the spectra pre-
sented below have been obtained after formal recalculat-
ing transmission coefficients in losses [dB/cm].
We also measured fluorescence spectra and fluores-
cence kinetics of Yb3+ before and after e-irradiation or
after OP of the fibers, applying the lateral detecting ge-
ometry [18]. Fluorescence emission was collected from
surface of an YF sample at the point spaced by approxi-
mately 5 mm from its splice with an output fiber of the
LD. We used the same OSA for the fluorescence spectra
measurements and a Ge photo-detector and oscilloscope
for the fluorescence decay measurements. In the last case,
LD power was modulated by a driver controlled by a
function generator’s signal to achieve square-shaped
pulses with sharp rise and fall edges. The time resolution
of the entire experimental setup was approximately 8 µs.
3. Experimental Results
3.1. E-irradiation experiments
The results of these experiments are highlighted by Fig-
ures 2, 3, 4(a)-(b) and 7.
The attenuation spectra of samples YF-3 and YF-1,
having correspondingly the highest and lowest Yb3+ con-
centrations, after different doses of e-irradiation along
with the attenuation spectra of the samples in a pristine
(dose “0”) state are shown in Figure 2(a )- (b ).
First, a notable increase of background loss in the fi-
bers in VIS with increasing e-irradiation dose is revealed
from Figure 2 (see main frames). Also notice a specific
spectral character of this loss for both fibers, that is, a
drastic rise of its magnitude towards shorter wavelengths.
This is a trend well-known for the experiments on influ-
ence of various-type irradiations upon optical properties
of Yb3+-free silica fibers. At the same time, the apparent
differences are seen in magnitude of e-irradiation in-
duced loss in these two samples, i.e. a much higher level
of darkening in YF-3 than in YF-1. [The data for sample
YF-2, intermediate in Yb3+ doping level, demonstrate
similar but intermediate growth of the background loss in
VIS as compared to samples YF-3 and YF-1.]
Second, definitive but less pronounced spectral trans-
formations are revealed for the resonant-absorption band
of Yb3+ (850 - 1100 nm); see insets to Figures 2(a)-(b).
The insets show the difference spectra obtained as it is
described in Section 2. Vastly small in sample YF-1 (Fig.
ure 2(b)), the spectral transformations become signifi-
cant in sample YF-3 (Figure 2(a)) and they have a complex
(a) (b)
Figure 2. 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” to “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 schematically show the positions of wave-
lengths for which the data in Figure 3 are built.

A. V. KIR’YANOV
158
character regarding e-irradiation dose growth: Compare
the difference spectra obtained after doses “2” and “3”.
The revealed behavior seems to be a consequence of
some process associated with e-irradiation which affects
the concentration of Yb3+ ions.
More details are seen from Figure 3 where we plot the
results obtained for samples YF-1 (a) and YF-3 (b), taken
from the whole set of e-irradiation doses. Figures 3 (a)-(b)
demonstrates how attenuation within the resonant- absorp-
tion of Yb3+ (peaks at 920 and 977 nm, see also Figure.
1(a)) changes through e-irradiation: See curves 1 (for the
977 nm peak) and curves 2 (for the 920-nm peak), respec-
tively. A decrease followed by increase of the magnitude
of small-signal absorption coef ficient arises in both
peaks with increasing e-irradiation dose in YF-3 (heavier
doped with Yb3+); notice that this behavior is much less
expressed in YF-1 (lower doped with Yb3+).
For comparison purposes, we also plot in Figure 3 the
changes in attenuation of samples YF-3 (c) and YF-1 (d)
in VIS where background (non-resonant) losses arise as
the result of e-irradiation. Here we limit ourselves by
giving the data for a couple of wavelengths in VIS, at
500 (curve 3) and 633 (curve 4) nm. It is seen that back-
ground loss monotonously (almost linearly) grows with
e-irradiation dose, a common effect for silica based fi-
bers. Importantly, the rate of growth is higher in YF-3
than in YF-1. Furthermore, it deserves attention that an
initial level of background loss in pristine YF samples
correlates with an initial content of Yb3+ ions. [The data
for sample YF-2 are similar to the ones shown in Figure
3 for samples YF-3 and YF-1, but the magnitude of spe-
ctral transformations in YF-2 is intermediate when com-
paring those in YF-3 and YF-1.]
In Figures 4(a) and (b), we gather the experimental
results from Figures. 2 and 3 for samples YF-1 and YF-3
and add the data for sample YF-2. This allows seeking the
concentration dependences of the resonant (Yb3+) and
background loss induced in the fibers at e-irradiation upon
(a) (b)
(c) (d)
Figure 3. 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).

159 A. V. KIR’YANOV
(a) (b)
(c) (d)
Figure 4. The results of experiments with fibers YF-1, YF-2, and YF-3, which were obtained for differe nt 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 circles) (a, c). Dotted
lines are for visual purposes only.
the value of small-signal absorption coefficient at 977 nm.
From Figure 4(a), one can first reveal a monotonic in-
crease of background (non-resonant) loss in VIS (dark-
ening), exampled by wavelengths 500 and 633 nm, with
increasing Yb3+ concentration, in turn proportional to YF
small-signal absorption at 977 nm. This demonstrates
that the presence of Yb3+ dopants in the fibers gains their
degradation at e-irradiation. Here we show the results
obtained for e-irradiation dose “3” only since for the o-
ther doses the dependences are similar, given by a mo-
notonic dose dependence of the induced loss in VIS (re-
fer e.g. to Figures 3 (c)-(d)).
Then, from Figure 4(b) it is seen that the lowest levels,
to which the values of the resonant absorption in the 977
nm peak approach through e-irradiation (minima of curves
1 in Figure 2), decrease with increasing Yb3+ concentra-
tion through a set of samples YF-1, YF-2, and YF-3. 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 the result of e-irradiation (at its pri-
mary stage). However, it should not be overlooked that at
the following stages of e-irradiation Yb3+ concentrations
are re-established on the levels comparable with those in
pristine YFs (refer to Figure 3(a)).
The remainder of Figure 4, graphs (c) and (d), gives
the results obtained in the experiments on spectral trans-
formations in YFs at OP which are reported in the next
sub-section.
3.2. OP experiments
The results of these experiments are highlighted by Fig-
ures 4(c)-(d) and Figures 5-8.

A. V. KIR’YANOV
160
We limit ourselves by reporting here the results of OP
experiments for sample YF-3 mainly (see Figures 5-7
below), having the highest content of Yb3+ ions in a pris-
tine state. Meanwhile, we summarize all of the results
(for samples YF-1, YF-2, and YF-3) in Figure 4(c)-(d).
Figure 5 shows the attenuation spectra of sample
YF-3 (having a short length of 0.8 cm) after 40 and 120
min. of OP. LD power was fixed in these experiments at
approximately 300 mW, providing the highest attainable
level of Yb3+ population inversion. For comparison, the
Figure 5. 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 wave-
lengths for which the data in Figure 6 are built.
attenuation spectrum of a pristine (0 min) sample YF-3 is
shown in Figure 5, too. Once compared with the ate-
nuation spectra after e-irradiation (refer to Figure 2(a)),
these spectra are seen to be similar in appearance. That
is,a substantial increase of background loss arises in VIS
with increasing OP time (the PD effect). Notice that the
spectral signature of PD resembles the one at darkening
after e-irradiation (refer to Figure 2). The spectral changes
that occur within the resonant (Yb3+) absorption band at
PD of are discussed below; see Figure 7.
In Figure 6(a), we demonstrate the results of the ex-
periments 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
3(a). From Figure 6(a), it is seen how attenuations in the
two absorption peaks of Yb3+ ions (at 977 and 920 nm)
change through OP; see curves 1 and 2, respectively. The
time dependence of OP induced changes at 977 nm re-
sembles the dose dependence at e-irradiation of sample
YF-3 (see curve 1 in Figure 3(a)). However, curve 1 in
Figure 6(a) has an “asymmetric” shape versus OP time,
differing from a “symmetric” shape of the dose depend-
ence at e-irradiation given by curve 1 in Figure 3(a).
Furthermore, the time dependence of OP induced changes
at 920 nm, see curve 2 in Figure 6(a), is seen to be very
weak, being completely different from curve 2 in Figure
3(a) (e-irradiation). Therefore, we can propose that
somewhat different mechanisms are involved in these
two (e-irradiation and OP) treatments of the fibers which
are responsible for the induced changes within the reso-
nant-absorption band of Yb3+ at 977 and 920 nm.
(a) (b)
Figure 6. 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.

161 A. V. KIR’YANOV
In Figure 6(b), we demonstrate the results of all of the
spectral transformations that occur in sample YF-3 in
VIS, where non-resonant background loss arises as the
result of OP. This can be also seen by referring to Figure
5 and compared with the results of e-irradiation shown in
Figure 3(c). Again, we provide in Figure 6(b) the data
for a couple of wavelengths only, 500 (curves 3) and 633
(curves 4) nm, as the representatives. In contrast to the
dose dependences at e-irradiation, long-term OP at the
resonant wavelength 977 nm results in completely dif-
ferent dynamics of background loss with time. Indeed, it
is essentially nonlinear in time: There is a short time in-
terval in the beginning (few minutes) where PD increases
dramatically while in the remainder of OP (tens of min-
utes) PD slows down and tends to saturate. Note that the
described time dependences are very similar to the ones
commonly met at PD experiments with YFs at 633-nm
probing (a He-Ne laser).
Let’s now consider the results shown in Figures 7 and
8 where we make insight to the difference spectra.
Figure 7 allows a direct comparison of the attenuation
spectra for sample YF-3 after dose “3” of e-irradiation
(curve 1) and after 2 hours of OP (curve 2). The spectra
look qualitatively similar which could validate the
mechanisms that stand behind the spectral transforma-
tions to be basically similar. At the same time, if one
spectrum is formally subtracted from another, the result
(curve 3 in Figure 7) shows a definitive difference. That
is, apart from the difference in the background loss
which ought to be present in anyway, there is a feature
within the Yb3+ resonant band: Although no deviation
from a “plain” behavior of curve 3 is seen nearby 920 nm
peak of Yb3+, there is a well-pronounced 977 nm
Figure 7. Difference attenuation spectra after dose “3” of
e-irradiation (curve 1) and after 2 hours of OP at 977 nm
(pump power is 300 mW) (curve 2); curve 3 is the differ-
ence of spectra 1 and 2. The data are for sample YF-3.
peak (it is marked by a dotted ring on Figure 7). This
detail seems to be important because it lightens non-
homogeneity within the Yb3+ resonant-absorption band
nearby 977 nm, present at long-term OP but not—at
e-irradiation of the fiber.
This becomes more apparent when an analysis of the
results of OP-induced PD of the other samples, YF-1 and
YF-2, has been made. Indeed, from Figures 8 (a)-(b),
where we plot the difference spectra obtained for these
fibers, a firm spectral detail is seen to appear exactly
within the 977 nm peak of Yb3+ ions (it is marked by a
dotted ring in plots (a) and (b)). This fact can be inter-
preted as follows: At OP-induced PD, that is, at rising of
background loss tailing from VIS towards near-IR (see
the left part of Figures 8(a)-(b)), drastic decreasing of
the resonant absorption occurs within a narrow 977-nm
peak. Some more assertions on this feature are made in
the Discussion section.
In Figures 4(c)-(d), we gather the results of OP experi-
ments for an entire set of YF samples, seeking the concen-
tration dependences of the resonant (within the Yb3+ band)
and background non-resonant spectral transformations.
In contrast to the results of e-irradiation (refer to Fig-
ures 4(a)-(b)), one can firstly reveal essentially nonlin-
ear growth of background loss at wavelengths 500 and
633 nm with increasing Yb3+ concentration (Figure 4(c)).
Obviously, it is different from linear growth of back-
ground loss at e-irradiation of the fibers (Figure 4(a)).
Secondly, it is seen that instead of a linear decrease of
resonant absorption peaks at 977 and 920 nm with dose,
occurring at primary stages of e-irradiation (see Figure
4(b)), a strongly nonlinear law is seen to fit a decrease of
the resonant absorption peak at 977 nm while almost no
change to be present at 920 nm; see Figure 4(d ).
Hence, the situation with OP induced spectral trans-
formations in our YFs is more 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 pres-
ence in the fibers of centers others than Yb3+ dopants, but
closely related to them and spectrally matching them
nearby 977 nm. Moreover, partial weight of such centers
in YF core ought to increase with increasing Yb3+ ions
concentration. The nonlinear behavior of non-resonant
background loss versus OP time, revealed above (see
Figure 6(b) ), seems to be a closely related phenomenon.
It should be noted that at further exposure of YFs to
OP an initial state of the resonant absorption peaks tends
to restore (Figure 6(a)), thus demonstrating the behavior
quite similar to e-irradiation of the fibers (Figure 3(a)).
3.3. Fluorescence Measurements
The fluorescence spectra obtained for pristine samples

A. V. KIR’YANOV
162
(a) (b)
Figure 8. Difference (loss) spectra of samples YF-1 (a) and YF-2 (b), obtained after 1-hour of OP at 977 nm (pump power is
300 mW).
YF-1, YF-2, and YF-3 at 977-nm pumping are shown in
Figure 1(b). All of these are similar in appearance and
their intensities are proportional to concentrations of
Yb3+ in the fibers. [The measurements were made at the
same conditions and for same pump powers.]
We also measured the fluorescence spectra of the fi-
bers after irradiation by an electron beam or after
long-term OP at 977 nm, but we couldn’t capture any
qualitative spectral change within the Yb3+ fluorescence
band; so, we don’t provide them here. We could only see
a small decrease in the fluorescence intensity as the re-
sult of irradiations but obviously this trend could not be
quantified.
For the fluorescence decays measured at modulated
pumping of the YFs, it was found that the characteristic
Yb3+ fluorescence decay time slightly decreases through
the set of pristine YF samples. This is a result of the
presence of two exponents in the fluorescence kinetics
measured by ~0.7 ms (main) and ~0.2 - 0.3 ms (“auxi-
liary”). Notice that insignificant growth of the latter con-
tribution was detected for the fiber with the highest Yb3+
content (YF-3); see also Refs. [18-20]. However, the
time constants obtained for fitting the decays were found
to be non-affected neither after e-irradiation nor after
long-term OP. A lone novelty found was that both the
treatments gave rise to growth of scattering in the fibers
at the pump wavelength. This extra scattering looked as
an instantaneous (within the resolution limit of 8 s)
drop of a signal from the photo-detector, followed by
slow Yb3+ fluorescence decay. Since such a scattering
signal was virtually not present in pristine samples, this
observation might deserve attention.
Concluding, we can reveal that none, or vastly small,
changes occurred with our YFs in the sense of Yb3+
fluorescence properties.
4. Discussion
4.1. Interpretation of Experimental Results
Summarizing all the data reported in Section 3, we notice
that either at e-irradiation or at resonant OP of YFs sub-
stantial and complex but different in appearance changes
arise within the resonant absorption band of Yb3+ ions
(“reversible bleaching”) while monotonous growth of
non-resonant background loss occurs in VIS (“darken-
ing”). Furthermore, these trends are revealed to originate
from the changes in concentrations of Yb3+ ions and
seemingly of other centers, closely related to them and
spectrally matching them nearby 977 nm. This is the
main result of our experiments. In the meantime, in vir-
tue of importance of the details figured out above a few
more assertions can be made.
A general consequence of the experiments on e-irra-
diation, a rise of background non-resonant loss in YFs in
VIS (see Figures 3(c)-(d)), is not surprising. This loss
correlates by a spectral signature with the excess loss that
arises in optical fibers after other types of irradiation
(x-rays,
 quanta, UV [15-17]). Some other aspects are as
follows:
(1) A monotonic increase of the background loss in
VIS (darkening) with increasing Yb3+ content in the YFs
which demonstrates, as it was already mentioned, that
the presence of Yb3+ dopants leads to a higher degree of

163 A. V. KIR’YANOV
degradation of the fibers at e-irradiation; see Figure 4(a);
(2) A notable decrease followed by equally notable
increase that arise in the resonant-absorption peaks of
Yb3+ (at 920 and 977 nm) with increasing e-irradiation
dose (Figure 3(a)-(b)), the effect also dependent on Yb3+
concentration; see Figure 4(b).
Thus, the presence of Yb3+ dopants in the fibers results
in a more pronounceable degradation as the result of
e-irradiation, with a probable reason being that Yb3+ ions
are powerful sources of secondary carriers (electrons and
holes) born at e-irradiation. That is, the changes within
the resonant-absorption band of Yb3+ may stem from
e-irradiation induced excitation of inner-shell (f) elec-
trons of Yb3+ and their valence transformation through
the charge-transfer (CT) processes (direct and return),
sketched by the following reactions [8,21]: e– + Yb3+ →
Yb2+; e+ + Yb2+ → Yb3+, where e- and e+ are the notations
for secondary (irradiation induced) electrons and holes
and Yb2+ is the notation for Ytterbium ions in va-
lence-two state. In turn, the presence in the fibers of
secondary carriers as the result of e-irradiation can cause
formation of such defects as oxygen-deficit (ODC) and
non-bridging oxygen-hole (NBOHC) centers [19,22].
These centers are known to be responsible for the wide
excess-loss spectral bands similar to the ones formed in
our darkened fibers; see Figures 2 and 7.
(2) Qualitatively similar observations can be made re-
garding the spectral transformations in the YFs as the re-
sult of OP at 977 nm; refer to Figures 4(c)-(d) and Fig-
ures 5-8.
Analogously, the following trends are revealed:
(3) Background loss in VIS significantly grows at
long-term OP (see Figures 5 and 6(b)) and its character
is typical for the PD effect in YFs [3-13]. At the same
time, an increase of this loss in VIS with increasing
small-signal absorption has, in contrast to e-irradiation, a
strongly nonlinear law (see Figure 4(c)), thus revealing
an almost quadratic dependence versus Yb3+ concentra-
tion in the fibers [23];
(4) The dependences of resonant absorption, measured
in the peaks of Yb3+ at 977 and 920 nm upon OP time,
have essentially different characters (see Figure 6(a)). If
the absorption coefficient in the 977 nm peak changes by
a law similar to the one at e-irradiation (a primary de-
crease followed by increase versus time), the absorption
coefficient in the 920 nm peak is virtually constant
through long-term OP. The concentration dependences
shown in Figure 4(d) tell us more: The changes in these
peaks with increasing content of Yb3+ ions in the fibers
are also different. We cannot interpret these details in
terms of simple concentration dependences with regard
to Yb3+ ions. Otherwise, an assumption should be made
instead that the changes in the 977 nm peak are related to
the changes in concentration of some centers others than
Yb3+ ions but spectrally matching them nearby the 977
nm peak;
(5) The spectral signature of the latter is seen from
Figures 7 and 8 where the difference attenuation spectra
after OP are presented. One can capture from Figures 7
and 8 that the PD effect (growth of non-resonant loss in
VIS) is accompanied by bleaching of the resonant peak
at 977 nm whereas none occurs with the peak at 920 nm.
Notice that a similar feature was reported earlier for
other type of YF, fabricated by the DND method [8].
All the facts (3-5) being gathered together, tell us that
PD in YF at high-power long-term OP at 977 nm occurs
among the centers the concentration of which is a non-
linear (almost quadratic) function of Yb3+ ions concen-
tration. These are most probably the centers composed of
couples of Yb3+ ions (“pairs”). Furthermore, similar
reactions: e- + Ybp
3+ → Ybp 2+; e+ + Ybp 2+ → Ybp 3+ (see
above) can be written to address these transformations at
OP, where index p stands to emphasize that a pair of
Yb3+ ions is involved in the processes and notations e-
and e+ are used for an electron and hole, free or trapped
by the nearest ligand, say oxygen [8,24]. Such reactions
can also go at the assistance of CT processes between ion
pairs where both the constituents are in the excited state.
Hence, the spectrally wide background loss (PD) in the
fibers, see Figures 5 and 7, can stem from producing of
Ybp
2+ and of e–/e+ -related centers (say, ODC and
NBOHC) at OP like this takes place at e-irradiation.
It is currently believed that PD occurs among clusters
of Yb3+ ions (obviously, pairs are their kind). However, a
meaningful novelty found in the present study for the
first time is the spectral feature, occurring at OP (see
dotted rings in Figures 7 and 8 but not - at e-irradiation.
4.2. Pre-concluding Remarks
There are evidences for that the PD process can be asso-
ciated with non-binding oxygen near surfaces of Yb/Al
clusters that can be formed in alumino-silicate glass (our
case). The non-binding oxygen originates from Yb3+
substituting Si4+ sites. When subjecting an YF to 977-nm
OP, the excess energy is radiated as phonons, causing a
lone electron of a non-binding oxygen atom to shift to a
nearest neighbour non-binding oxygen atom with crea-
tion 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. The conversion of such an un-
stable center to a semi-stable center requires the shifting
of one electron of the lone electron pair to a nearest
neighbour site [25]. As a result of this, the formation of
Yb- (and probably Al-) related ODC can occur. On the
other hand, PD in alumino-silicate YFs may take place

A. V. KIR’YANOV
164
through the 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 of similarity of the spectral
transformations in YFs at e-irradiation (creation of “sec-
ondary” carriers in the core glass by an electron beam)
and at OP (creation of carriers and color centers by the
pump light).
II. Some more assumptions can be made in attempt to
understand the PD phenomenon at OP. Of course, dif-
ferent “paired” complexes (say, Yb-O-Yb or more com-
plicate clusters of such kind) can be thought to be in-
volved at PD, but we suggest here Yb2O3 “molecules”
(or their agglomerates), the presence of which in YFs at
increasing Yb3+ ions concentration is quite probabilistic
[26]. It is also worth noting that CT transitions are
well-known for Yb-doped sesquioxides (Yb2O3 is one of
them) while these are still debated for single Yb3+ ions
“dissolved” in YF core glass [8,12,27,28]. So, Ybp
3+ in
the form of inherent centers Yb2O3, absorption spectrum
of which matches well the spectral feature ringed in
Figure 8, seem to be relevant candidates to explain PD:
Refer e.g. to the works [29,30] where the presence of the
strong 977 nm peak and the absence (vanishing) of the
920-nm peak was revealed to be characteristic for Yb2O3
(see also Ref. [31] where the peaks at 977 nm from sin-
gle and paired Yb3+ are shown to match spectrally). Fur-
thermore, on one hand, the presence of cooperative VIS
fluorescence at 977 nm excitation (see inset in Figure
1(b)) is typical for Yb2O3 [32], while on the other hand,
PD was proved to be associated with the presence of
cooperative processes in YFs [7,33]. (The spectral fea-
ture seen in Figure 1(b) in VIS is undoubtedly ascribed
by us to the cooperative fluorescence of Yb3+ since no
other spectral features were detected which would origi-
nate from traces of un-wanted rare-earth dopants [34,35]
like Tm3+.)
One more assumption can be made that Yb2O3 is a
typical defect center in the core glass network which can
be formed at high Yb3+ concentrations. Probably, namely
this “color” center, firstly detected yet in 1997 [36], is
responsible for the presence of non-saturated (by 977-nm
radiation) resonant absorption in heavily Yb3+ doped
fibers [18,36-38]. This non-saturated absorption can be
understandable in virtue of extremely high absorption
coefficient at 977 nm (~1x10-20 cm2) and almost
quenched fluorescence (~10 µs), characteristic for Yb2O3
centers [28,32,39]. Apparently, these values are com-
pletely incompatible with those known for single Yb3+
ions dissolved in the core glass: ~1x10-21 cm2 and ~1 ms,
respectively; see e.g. Ref. [40]. Further, possible pre-
sence of Yb2O3 centers in heavily-doped YF, which in-
tensively absorb the pump light but are non-fluorescent
(“quenched”), may cause an excessive temperature rise
in YF core.
It is logical to bridge here to the papers [41,42] where
the idea of an intrinsic color center, like Yb2O3, has been
proposed to address some of the concentration phenom-
ena in heavily rare-earth doped materials. So, the physi-
cal essences given by doping YF with Yb2O3 (non-in-
tentionally or intentionally [43]) can be of importance.
III. We didn’t discuss above a possible role of the
spectral changes in refractive index of YFs at OP and
e-irradiation which ought to be induced as well, accor-
ding to the Kramers–Kroënig relations. A study of these
changes can be the substance of a future work.
5. Conclusions
We report a comparative experimental study of the ate-
nuation spectra transformations for a series of Yb doped
alumino-germano silicate fibers with various Yb3+ con-
tents, occurring as the result of irradiation either by a
beam of high-energy electrons or at in-band optical
pumping at 977 nm wavelength. Substantial and complex
but different in appearance changes are found to arise
within the resonant absorption band of Yb3+ ions (re-
versible bleaching) while monotonous growth of non-
resonant background loss to occur in VIS (darkening).
Both the trends are shown to originate from the changes
in concentrations of either Yb3+ ions (at electron irradia-
tion) or other centers, seemingly Yb3+ clusters, closely
related to single Yb3+ ions and spectrally matching them
at 977 nm (at optical pumping). So, in both cases, i.e. at
photodarkening, observed in heavily Yb3+ doped fibers at
resonant (977 nm) optical pumping, and at electron irra-
diation-induced darkening of the fibers, we can capture a
notable role of Yb3+ dopants as the agents, creating
high-energy radiation, responsible for formation of color
centers in the fibers, and at the same time their role as the
sensitizers of these processes.
6. Acknowledgements
Author thanks Dr. N.S. Kozlova (MISIS, Moscow, Rus-
sia) and Dr. A.D. Guzman Chavez (CIO, Leon, Mexico)
for help in making e-irradiation and photodarkening
measurements, respectively, and Dr. Yu.O. Barmenkov
(CIO, Leon, Mexico) and Dr. N.N. Il’ichev (GPI, Mos-
cow, Russia) for useful discussions.
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