IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 56, NO. 6, DECEMBER 20093293
Radiation Effects on Ytterbium- and
S. Girard, Member, IEEE, Y. Ouerdane, B. Tortech, C. Marcandella, T. Robin, B. Cadier,
J. Baggio, Member, IEEE, P. Paillet, Senior Member, IEEE, V. Ferlet-Cavrois, Senior Member, IEEE,
A. Boukenter, Member, IEEE, J.-P. Meunier, Member, IEEE, J. R. Schwank, Fellow, IEEE,
M. R. Shaneyfelt, Fellow, IEEE, P. E. Dodd, Senior Member, IEEE, and E. W. Blackmore, Member, IEEE
the radiation effects on ytterbium- (Yb) and ytterbium/erbium
(Yb/Er)-doped optical fibers. Their vulnerability to the environ-
attenuation (RIA) measurements during and after exposure to
10 keV X-rays, 1 MeV
-rays, and 105 MeV protons. These fibers
present higher levels of RIA (1000×) than telecommunication-type
fibers. Measured RIA is comparable for -rays and protons and is
on the order of 1 dB/m at 1.55
m after a few tenths of a kilorad.
Their host matrix codopedwithaluminum (Al) and/orphosphorus
(P) is mainly responsible for their enhanced radiation sensitivity.
Thanks to the major improvements of the Er-doped glass spec-
troscopic properties in case of Yb-codoping, Yb/Er-doped fibers
appear as very promising candidates for outer space applications.
Intheinfrared partofthespectrum, losses inP-codoped Yb-doped
fibers are due to the P?center that absorbs around 1.6
are very detrimental for the operation of Er-codoped devices in
a harsh environment. The negative impact of this defect seems
reduced in the case of Al and P-codoping.
tons, radiation effects, ytterbium.
tigated as these waveguides offer numerous advantages in this
harsh space environment . Optical fibers, however, suffer
from the radiations encountered during the mission lifetime.
The main fiber degradation mechanism is the generation, at
the microscopic scale, of point defects that increases its linear
absorption [radiation-induced attenuation (RIA)] . The
amplitude and kinetics of this degradation greatly differ from
HE vulnerability of a large variety of silica-based optical
fibers to the environment in outer space has been inves-
Manuscript received July 16, 2009; revised September 23, 2009. Current ver-
sion published December 09, 2009.
Bruyères-le-Châtel, France (e-mail: firstname.lastname@example.org).
Y. Ouerdane, B. Tortech, A. Boukenter, and J-P. Meunier are with Labora-
toire Hubert Curien, UMR-CNRS 5516, 42000 Saint-Etienne, France.
T. Robin and B. Cadier are with IXFiber SAS, F-22300 Lannion, France.
V. Ferlet-Cavrois was with CEA DIF, F91680 Bruyères-le-Châtel, France.
She is now with the European Space Agency, The European Space Research
and Technology Centre, 2200 AG Noordwijk, The Netherlands.
J. R. Schwank, M. R. Shaneyfelt, and P. E. Dodd are with Sandia National
Laboratories, Albuquerque, NM 87185 USA.
E.W. Blackmore is with TRIUMF, Vancouver, BC V6T2A3, Canada.
Color versions of one or more figures in this paper are available online at
Digital Object Identifier 10.1109/TNS.2009.2033999
one fiber type to another, depending on composition (core and
cladding dopants) , elaboration process parameters (fiber
drawing, so on) , or profile of use (signal wavelength, power
level, so on) . Rare-earth (RE)-doped optical fibers are en-
visaged for use as part of amplifiers or of more complex optical
systems like fiber optic gyros for space missions. Incorporated
in a silica-based matrix, RE-ions act as laser-active dopants and
can be used to design high-power lasers. Previous studies have
shown that RE-doped fibers are very sensitive to radiations
and exhibit high RIA levels . Despite the small lengths of
RE-doped fibers used for space applications, there is then a
strong need to evaluate this fiber sensitivity and to improve the
understanding of radiation effects on these complex glasses. In
this work, we investigate radiation effects on ytterbium (Yb)
and ytterbium-erbium (Yb/Er)-doped optical fibers that have
been studied less. Possible space applications differ for Yb-
and Yb/Er-doped fibers due to differences in the spectroscopic
properties of these two RE ions. A simplified description of the
energy levels of these ions and their main interactions is given
in Fig. 1(a). Yb
ions can be pumped at wavelengths around
940 nm ( F
F transition) and then emit laser light
in the 1 to 1.1
m range. This class of fibers can be used for
example for deep-space optical communications  or low-loss
power-conserving optical systems . Er
pumped in the near-infrared around 980 nm ( I
transition) and then emit around 1540 nm ( I
transition). This mechanism is used to obtain high-power fiber
lasers in the third Telecom window . Er-doped fibers then
become key components of fiber amplifiers or broadband laser
source to be used in satellites as parts of high-speed data
links or fiber gyroscopes , . The association of the two
rare-earths has been shown to significantly improve the gain of
Er-related glasses by limiting the quenching effect due to the
appearance of ion clusters at increasing erbium concentrations
. The codoping with ytterbium overcomes this limit through
a transfer of energy between the energy levels of the two ions
I transition). Furthermore, Yb
rize to dope the glass with a larger concentration of RE ions.
Furthermore, they present a larger absorption cross section
compared to Er
ions, strongly improving the pumping effi-
ciency without affecting the absorption in the emission range
In this work, we estimate the radiation vulnerability of
RE-doped fibers by studying their response in a passive config-
ions can also be
0018-9499/$26.00 © 2009 IEEE
3294 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 56, NO. 6, DECEMBER 2009
Fig. 1. (a) Energy levels of Erand Yb ions. (b) Structure of tested Yb-doped double-clad optical fibers.
TESTED PROTOTYPE OPTICAL FIBERS
uration, it means by characterizing them like passive Telecom
optical fibers. Our objectives are to compare the intrinsic
response of these particular glasses with RE-free glasses and to
identify the relative influence of the various parameters on their
radiation sensitivities. The most promising fibers will then be
tested in a future paper in their active configuration, it means
by directly pumping the RE ions with at 940 nm and recording
the impact on gain curves, on amplified spontaneous emission,
II. EXPERIMENTAL DETAILS
A. Tested Optical Fibers
We characterize the five prototype samples described in
Table I. They were developed by iXFiber SAS, France with
well-known composition and modified chemical vapor depo-
sition process parameters . Their cores are doped with
ions) at concentrations ranging from 1.2 to
4.6 wt.%. The cores of #4 YbErP and #5 YbErPAl samples
also contain erbium (Er
ions). To improve the incorporation
and the efficiency of the active ions, the matrix is codoped
with P and/or Al . All fibers have bare fiber geometry
with octagonal pure-silica double-clad (DC) as illustrated in
Fig. 1(b). The laser pump is injected into the DC and its partic-
ular octagonal structure ensures a rapid and efficient coupling
between the pump and the rare-earth ions of the fiber core.
B. Irradiation Facilities
Proton tests (at 104 MeV) were performed at room temper-
ature (RT) at TRIUMF PIF, Canada . Our fiber samples
(3 m length) were arranged as “one layer” to ensure an uni-
form proton exposure. The fluence was measured using a cal-
ibrated ion chamber. The equivalent deposited dose in Si evalu-
ated using the conversion factor
proton MeV cm rad Si
Proton experiments were performed up to equivalent doses of
krad with a dose rate of
are representative of those associated with space applications
but lower dose rate (
rad/min) is expected in space .
rad/s. These dose levels
Gamma-Ray Tests: These tests were conducted at RT with
Co source at CEA, France. We used 5 m long fiber coils
(60 mm diameter) and the same experimental setups as for the
proton experiments. Our goal was to verify that both 105 MeV
protons and 1 MeV photons lead to comparable degradation
levels. Irradiations were performed to doses of up to
with a dose rate of
10 keV X-Ray Tests: These tests were done at RT with an
ARACOR machine . The interest of these tests consists in
the possibility to irradiate a small length of fiber (
an UV-improved experimental setup that allows us to record
the RIA in the 200–1100 nm range for doses between
Mrad at dose rates ranging from
C. Experimental Setups for RIA Measurements
Two different experimental setups were used to characterize
the Yb-doped fibers in their passive configuration. Both of them
allow an in situ recording of the transmitted power changes
during or after irradiation. The difference is that setup A uses
an excitation of the double-clad (DC) to inject the white light
into the RE-doped core whereas setup B allows a direct injec-
tion through the RE-doped core.
Setup (A): White light from a deuterium-halogen source
(DH2000 Ocean Optics) is injected through multimode fiber
pigtails in the DC of the tested samples and then to their cores.
As pure-silica is radiation-tolerant compared to RE-doped
silica , we assume that the fiber degradation will be mainly
GIRARD et al.: RADIATION EFFECTS ON YB- AND YB/ER-DOPED DC OPTICAL FIBERS3295
caused by RIA of the active core. The RIA in the 850–1700 nm
range are recorded with an Ocean Optics NIR512 spectrometer.
Setup (B): Additional measurements were done with sole
excitation of the fiber core to unambiguously distinguish be-
tween the contributions of the DC and the RE-doped cores.
This was achieved with a supercontinuum source (SuperKred
by Koheras) that enables to inject directly the white light
(550–2500 nm) into the small cores. Setup B is much more
complicated to use than setup A due to the lower stability
of supercontinuum sources. Furthermore, another important
difference between the two setups is that the used light power
is estimated to be less than 100 W for setup A and more than
10 mW with setup B. The influence of photobleaching , if
any, could be different for the two setups.
D. Confocal Microscopy of Luminescence (CML)
As pointed out by Fox et al. recently , , there exists
strong needs for complementary spectroscopic analysis of point
defects and energy levels of RE-ions to fully characterize the
radiation-induced effects in RE-doped glasses. We previously
showed the interest of the CML technique to determine the spa-
tial distribution of point defects inside the fiber cross section
, . We applied this technique to our set of Yb-doped
fibers. Preliminary results obtained under excitation at 488 nm
we complete our previous work with results acquired with a
325 nm ultraviolet excitation (spatial resolution of
III. EXPERIMENTAL RESULTS
We present first typical RIA growth and decay during and
after exposure to protons, -rays or X-rays to estimate the fiber
vulnerability. Second, we present spectral RIA measurements
used to analyze and discuss the origin of the fiber degradation.
We then briefly discuss the influence of the irradiation nature
(proton or gamma-rays) and of the experimental setup (A or
B) on the fiber degradation. Finally, we present CML results
that enable us to identify some of the radiation-induced point
A. Time and Dose Dependences of RIA
Fig. 2 shows at several different wavelengths the time de-
pendence of the 105 MeV proton RIA during and after the
irradiation for the #2 YbAl fiber. Its transmission capability
continuously decreases leading to RIA in the order of 3 dB/m
after a dose of 50 krad (
decrease of the transmitted signal. Another important point,
similar for all tested Yb-doped fibers, is that the phenomenon
of RIA bleaching at the end of the proton exposure appears to
be very limited, leading these fibers quasi-permanently affected
by an excess of losses. Due to this limited recovery, we can also
expect that varying the dose rate will not drastically change
the RIA values, meaning that these losses will not be strongly
reduced at a lower dose rate. Finally, we tested the fiber with an
injected white light power of about 100
applications using an active pumping of the ions at 940 nm
can use very high power of laser light (
s) corresponding to 50%
W. Some fiber-laser
kW) that may affect
Fig. 2. Time dependence of radiation-induced attenuation at selected wave-
lengths before, during, and after a 105 MeV proton exposure of #2 YbAl fiber
Fig. 3. Dose dependence of radiation-induced attenuation during the 105 MeV
proton exposure of the samples (A) at 1 and 1.1 ?m for #1 YbP to #3 YbPAl
fibers (B) 1.2 and 1.55 ?m for #4 YbErP and #5 YbErPAl fibers (setup A).
the fiber radiation sensitivity. Increasing the power could be
beneficial through photobleaching effect that can decrease the
RIA values .
In Fig. 3(A), we plot the dose dependence of the RIA at 1 m
applications take place in this spectral domain. For Er-doped
samples, we compare in Fig. 3(B) the RIA dose dependence at
1.2 and1.55 m. Thefirst wavelengthillustrates theresponseof
the response in the spectral domain of absorption and emission
for the Er
The whole set of fibers exhibits a significant increase of RIA
with the dose. For doses up to 50 krad, RIA is in the order of
a few dB/m depending on the wavelength of interest. These in-
duced losses are non-negligible even if only a few meters of
3296 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 56, NO. 6, DECEMBER 2009
Fig. 4. Spectral dependence of radiation-induced attenuation during the 105
MeV proton exposure of the #1 YbP sample at selected doses (1, 5, 10, 25, and
50 krad) (setup A).
RE-doped fibers are usually needed for most of the space appli-
cations. For example, such excess losses would be a huge hand-
icap and very perturbing for deep-space communications over
large interplanetary distances.
B. Spectral Dependence of RIA
Fig. 4 illustrates the spectral dependence of the infrared in-
duced losses in #1 YbP fiber at different doses.
In the tested dose range, the spectral dependence of RIA re-
mains unchanged, showing that the point defects at the origin
of the fiber degradation are generated with comparable effi-
ciencies and exhibit similar RT stabilities. The sudden change
in the RIA around 1050 nm is explained by changes in the
modes of light propagation in the fiber (setup A) between these
two spectral region (single mode for
nm). As a consequence, losses measured
nm will be mainly related to the core whereas at
shorter wavelengths, the contribution of the DC increases.
Fig. 5 compares the RIA spectral dependence of the five
Yb-doped samples (105 MeV protons, 25 krad, setup A).
All the fibers globally present similar induced loss levels al-
though some differences are visible in the RIA spectral depen-
nm, losses are quite comparable as ex-
pected as the whole set of fibers has been designed with the
same DC. Infrared induced losses in P-codoped fibers #1 YbP
and #4 YbErP are clearly related to an absorption band cen-
tered near 1600 nm. This band seems to contribute not or be
absent in the other fibers even those containing P (#3 YbPAl
and #5 YbErPAl). For the two Er-doped samples (#4 YbErP
and #5 YbErPAl),weobserve noticeabledecrease of RIAin the
in this paper.
nm and multi-
C. Comparison Between
and Proton Irradiations
105MeV protonswithslightly different doserates (
rad/s for photons). Globally, similar RIA levels
Fig. 5. Comparison of the spectral dependence of radiation-induced attenua-
tion for the five Yb-doped optical fibers for an equivalent dose of 25 krad (dose
rate of ?? rad/s, setup A).
Fig. 6. Comparison of the dose dependence of the radiation-induced attenua-
tion at 1.55 ?m and 1.2 ?m in the fiber #4 YbErP for a 105 MeV proton expo-
sure (8 rad/s) and a steady state ?-ray irradiation (0.35 rad/s).
were measured during and after exposure for the whole fiber
set, as expected from our previous studies on Er-doped fibers
 and other studies by Williams et al. and Rose et al. ,
. Fig. 6 illustrates the RIA growth with the dose at 1.2 m
and around 1.55 m for the #4 YbErP fiber during both expo-
sures. Comparable dose dependences of RIA are measured for
the two irradiation types. RIA seems slightly lower for -rays
but the differences can be explained by the uncertainties of our
experiments. Our tests confirm that -rays tests are still able to
reproduce the space environments for Yb-doped glasses.
D. Influence of the Experimental Setup on RIA.
We compared the RIA results obtained with setups A and
B during 105 MeV proton exposure. Similar results are ob-
tained for the different samples, as illustrated in Fig. 7 for fiber
GIRARD et al.: RADIATION EFFECTS ON YB- AND YB/ER-DOPED DC OPTICAL FIBERS3297
Fig. 7. Comparison between the spectral dependence of the radiation-induced
attenuation for fiber #1 YbP for a 105 MeV proton exposure (?? rad/s,
25 krad) measured with the experimental setups A and B. In the inset, kinetics
of RIA growth with dose measured with the two setups at 1200 and 1550 nm
irradiated (30, 100, 300, 1000 krad) samples of #2 YbAl fiber. The inset illus-
trates the lower emission measured in the 350–800 nm spectral domain.
Despite the reduced signal-to-noise ratio of RIA measured
with setup B, our tests confirm that similar RIA levels are mea-
sured withthetwosetups,meaningthatthedopedcore response
explains the measured RIA in the 1100–1700 nm. This is an
important result to understand the results of a future complete
characterization of this class of fibers with an active scheme
(pumping of the ions with diodes around 950 nm and measure-
ments of the resulting emission around 1
a first approximation, the DC (if made of pure-silica) will not
drive the fiber sensitivity.
m or 1.55m). As
E. CML Results
We compare in Fig. 8 the emission spectra recorded by ex-
citation at 325 nm of the core center of pristine and X-ray ir-
radiated samples of the #2 YbAl fiber at different dose levels.
We measured a strong decrease with the dose of the ampli-
tude of the emission from 950 to 1100 nm that is typical of the
along a diameter of a 1 Mrad irradiated sample of fiber #1 YbP. The center of
the fiber core is at ??.
. Our results also show an increase with dose of the lumi-
nonbridging oxygen hole centers (NBOHCs) .
We associated these spectral measurements with spatially
resolved analysis of the luminescence distribution in the fiber
cross section. An example of the luminescence distribution
alonga fiber diameteris giveninFig.9 forthe 1Mrad irradiated
sample of fiber #1 YbP.
The CML results confirm the generation of radiation-induced
point defects in both the fiber cladding and core. Most of the
emission bands are localized in the RE-doped core where
several dopants and rare-earth were incorporated to design
the laser-active glass: Yb and/or Er ions, aluminum, and/or
phosphorus. As an example, the emission associated to the
FF transition is clearly limited to the fiber core
as the two emission bands centered around 400 and 530 nm
that are probably related to color centers. The mapping of
Fig. 9 reveals the generation of NBOHCs in both the fiber core
and cladding, with an enhanced generation in the core. By
comparing the results of the measurements made on the whole
fiber set, we note for fiber #3 YbPAl a very large emission
around 650 nm from NBOHCs compared to fibers #1 YbP
and #2 YbAl. The generation of NBOHCs may be enhanced
in Al/P-codoped glasses. Previous measurements with 488 nm
excitation also reveal the presence of other radiation-induced
defects (Si-NBOHC, another Si-related defect and maybe
a P-related defect in P-doped samples). Radiation-induced
changes were also measured for the emission amplitudes be-
tween the different energy levels of Er
and #5 YbErPAl samples .
ions for #4 YbErP
A. Origin of RIA in Yb- and Yb/Er-Doped Optical Fibers
The Yb-doped samples present higher RIA levels (1 dB/m
after 50 krad) than pure-silica-core or germanium-doped cores
fibers. For comparison, Mula et al. from NUFERN  re-
ports RIA levels at 1.57
m for two passive optical fibers of
dB/km after a 50 krad proton. This can be explained by the
3298 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 56, NO. 6, DECEMBER 2009
Fig. 10. Comparison between the spectral responses under 105 MeV proton
fibers (#1 YbP and #2 YbAl), a P-doped multimode fiber and an Al/Er-doped
SM fiber .
the matrix to optimize the resulting gain of the fiber laser. This
is mostly achieved through P or Al doping of the host matrix.
To distinguish between the relative influences of the different
species (RE ions, and codopants) we compared in Fig. 10 the
degradation of samples #1 YbP and #2 YbAl with those of a
P-doped silica-based MM fiber  and with the response of
the Al/Er-doped single-mode (SM) fiber . In addition to the
105 MeV proton results, we plotted in this figure the RIA mea-
sured in ultraviolet-visible range under 10 keV X-rays at the
same dose of 10 krad.
The comparison shows that the doping with Yb
not drastically change the radiation response as it was shown
for the Er
ions . The dopants (P, Al) are mainly respon-
sible for the infrared radiation-induced attenuation as shown
by the comparison between the P-doped fibers. We obtained
roughly comparable results for the #1 YbP fiber (P/Yb) and
P-doped MM fiber even if the P-doping was different (respec-
wt. % and wt.%). At shorter wavelengths,
the RIA difference becomes more pronounced. It is mainly ex-
plained by the convolution between the response of the pure-
silica DC and the P-doped core of the Yb-doped fiber as pre-
viously mentioned. Their broad absorption in the 300–550 nm
range is related to the phosphorus oxygen hole centers (POHC)
, . The origin of the broad absorption around 550 nm in
the two Yb-doped fiber is unclear, no Si-related defects absorbs
at these wavelengths, the absorption may be due to P-defects in
fiber #1 YbP (POHC) and Al-defects in #2 YbAl as an absorp-
tion band at 550 nm was reported by Amosov et al. in KI glass
The comparison between RIA levels in #2 YbAl sample
(Yb/Al) and Er/Al SM fiber  reveals comparable spectral
dependence of losses, equivalent to a fiber doped with only
Al characterized by Brichard et al. . Small differences
cannot be discussed due to the differences in Al concentrations
between the two fibers (respectively
The negligible role of Yb
ions in the fiber degradation is
a very important result, as these ions clearly improve the
performances of the Er-doped fibers by increasing the gain
allowing the reduction of the fiber length for a given applica-
tion, reducing by the way the global vulnerability of the system
facing the radiations. Our study shows that Yb/Er fibers present
very interesting behavior in radiation environments compared
to Er-fibers as if their intrinsic performances (gain) are well
optimized by the presence of Yb, their radiation sensitivity
appears to be not increased.
Relative Influence of Al and P on the Radiation Responses
of Yb-Doped Samples: By comparing the radiation response of
#1 YbP to #3 YbPAl samples, we evaluate the influence and
possible interactions between these two co-dopants. Clearly,
RIA of #1 YbP fiber is mainly explained by the generation
under irradiation of the P defects that absorb around 1.6
This defect, previously studied by Griscom et al.  was
shown to be generated in all irradiated P-doped or codoped
samples , . It corresponds to the phosphorus E’-center
and can be generated by several mechanisms, including a
conversion mechanism after irradiation from POHCs . The
comparison between the response of #1 YbP and P-doped MM
fiber clearly confirms their main contribution to RIA. The ad-
dition of Al to the P-doped glass (comparison between #1 YbP
and #3 YbPAl; #4 YbErP and #5 YbErPAl) results in a clear
decrease of P -related IR absorption. This can be explained
by the fact that during the fiber manufacturing process, Al
preferentially bonds to P, as shown by Vienne et al.  re-
ducing the concentration of phosphorus-oxygen double-bonds
that act as precursors sites for the generation of POHC and
then P defects. This positive influence of Al-doping is more
beneficial for PYbEr-doped samples as P
near the infrared absorption and emission bands of Er
around 1550 nm than for PYb-doped fiberswhere losses around
1050 nm seem mainly related to Al defects, discussed in .
We present a complete characterization of the radiation-in-
duced effects on Yb and Yb/Er-doped optical fibers in their
passive configuration by providing experimental values of RIA
after 105 MeV proton-exposures gamma-ray tests on a set of
five different double-clad Yb and Yb/Er-doped optical fibers.
Our measurements provide evidence for a significant decrease
in the fiber transmission (on the order of dB/m for
doses) that is a factor
1000 more than the losses measured in
Telecom-type or radiation-tolerant fibers. This excess of losses
seems related to the host matrix that is necessary to incorporate
the rare-earth ions into silica. An important consequence of this
propertiescompared to Er-dopedfibers are veryinterestingcan-
didates for EDF amplifiers or sources. We explain some of the
microscopic mechanisms and defects explaining the losses re-
lated to aluminum and phosphorus-doping. The phosphorus-re-
fibers around 1550 nm, and its generation efficiency under radi-
ation may be decreased by appropriate codoping that provides
more attractive traps for released charges, those traps being re-
sponsible for absorption bands in the UV rather than in the
GIRARD et al.: RADIATION EFFECTS ON YB- AND YB/ER-DOPED DC OPTICAL FIBERS3299 Download full-text
Future work has to be done to test this kind of optical fiber
within an active scheme with the pumping of the double clad
with a rack of diodes and simultaneous recording of the radi-
ation-induced changes in the spectral region of laser emission
(1000–1100 nm for Yb-doped fibers and 1500 nm–1600 nm for
Yb/Er fibers) in order to consider the possible effects of photo-
bleaching and photodarkening associated with the fiber profile
The authors thank TRIUMF’s committee for providing the
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