IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 54, NO. 4, AUGUST 2007
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Low-Dose Radiation-Induced Attenuation at InfraRed
Wavelengths for P-Doped, Ge-Doped and Pure
Silica-Core Optical Fibres
Elise Regnier, Ivo Flammer, Sylvain Girard, Frans Gooijer, Frank Achten, and Gerard Kuyt
Abstract—Exposing optical silica fibers to radiative environments leads to an increase of fiber attenuation. This gamma-sensitivity of the fibers is strongly wavelength dependent. Many papers
already mentioned the strong Radiation-Induced Attenuation
(RIA) in the UV and Visible ranges, which is explained by radiation-induced defects absorbing in these spectral ranges. However,
1000 nm) is less
the origin of RIA at longer wavelengths (
clear. An exception is phosphorous-doped fibers for which P1
defects absorbing around 1700 nm have already been highlighted.
For fibers with no phosphorus, the RIA at Near-InfraRed (NIR)
wavelengths is usually assumed to be small as it results from
the UV-visible absorption tail, which decreases with increasing
wavelength. In this paper, we study three prototype silica based
optical fibers and show that the RIA does not monotically decrease
with increasing wavelength, highlighting RIA-contributions
having their origins at NIR-wavelengths. We show that these
NIR-absorbing defects are generally the main contributor to RIA
at telecommunication wavelengths (1310 nm and 1550 nm), the
impact of UV-visible absorption tail being secondary only. The
nature of defects involved in these NIR absorptions depends on
fiber composition. For fibers with no phosphorous, we propose
Self Trapped Hole defects (STH) as origin.
Index Terms—Fiber, infrared, pure and doped silica, radiation.
I. INTRODUCTION
L
IGHT transmission in optical fibers exposed to radiative
environment is strongly affected by the formation of
point defects in silica network. The nature of radiation-induced
defects depends mainly on glass composition, but also on other
parameters like glass thermal history or residual stress [1]. Each
defect has specific absorption properties, growth and recovery
kinetics. The Radiation-Induced Attenuation (RIA) therefore
depends on many parameters: fiber composition, irradiation
conditions (dose-rate, temperature, total dose) and operating
conditions (wavelength, injected power, measurements under
radiation or after a few weeks during which unstable defects
anneal). Thus the correct choice of a radiation resistant fiber
depends on the irradiation conditions and the fiber’s working
conditions.
Manuscript received October 6, 2006; revised January 26, 2007. This work
was supported by Draka Comteq.
E. Regnier and I. Flammer are with Draka Comteq France, 91460 Marcoussis,
France (e-mail: [email protected]; [email protected]).
S. Girard is with the CEA DIF, 91680 Bruyères-le-Châtel, France (e-mail:
[email protected]).
F. Gooijer, F. Achten, and G. Kuyt are with Draka Comteq Fibre BV, 5651
Eindhoven, The Netherlands (e-mail: [email protected]; frank.achten@
draka.com; [email protected]).
Digital Object Identifier 10.1109/TNS.2007.894180
Long-Term Radiation-Induced Attenuation (i.e., RIA that remains even a long time after the end of the radiation, when all the
unstable defects have been annealed) has been widely studied
over the past few decades, and it is now a well-known fact that
RIA generally increases with shorter wavelengths, as many radiation-induced defects absorb in the ultraviolet (UV) and visible ranges. These UV-visible absorbing defects have already
been studied in detail [2]–[4]: Ge(1), Ge(2), GEC, GeE’ with
regard to Germanium-related defects, POHC, P2 and P4 defects
with regards to Phosphorus-related defects, and also NBOHC,
POR, SiE’ and Self-Trapped-Hole (STH) defects. Absorption
bands of these defects are located from 195 nm (GeE’) up to
Visible wavelengths (550 nm for the POHC, 630 nm for the
NBOHCs, 540 nm, 660 nm and 760 nm for the STHs). Hownm), opever, at Near InfraRed (NIR) wavelengths (
tical fibers sometimes exhibit RIA increasing with wavelength
[5], [6]. Such behavior reveals the existence of absorption bands
centered in the NIR. Contrary to UV-defects, these defects are
not so well-known, except for P-doped fibers, in which a NIR
-induced absorption has already been attributed to P1 defects
[7]. For fibers of Pure Silica Core Fiber (PSCF) type (pure or
fluorine-doped silica cores), a broad absorption band centered
near 1600–1700 nm has been attributed to STH defects [8], [9].
However, these defects are thought to exist only at low temperatures [8]. In [10], Griscom proposes that STH defects may exist
at room temperature and may therefore lead to InfraRed absorption excesses. In Ge-doped fibers, the existence of a Long-Term
NIR-RIA has been shown in [5] and [6], as irradiated Ge-doped
single-mode fibers (SMF) and multimode fibers (MMF) showed
very broad -induced bands centered around 2050 nm and at a
wavelength 2500 nm, respectively. To our knowledge, neither
of these two bands have been identified yet.
The purpose of this paper is to clarify the Long-Term NIRpart of the RIA, as well as to quantify and to compare it to
the UV-visible contribution. We compare the RIA spectra in
three prototype silica-based fibers gamma-irradiated at low dose
levels (from 5 Gy to 100 Gy) and with three different core compositions: one is P-doped, the second one is of PSCF type, and
the last one is Ge-doped. We show that for telecommunication
applications it is the NIR-absorption band which mainly determine the RIA level.
II. EXPERIMENTAL PROCEDURE
For this study, three prototype singlemode fibers were realized: Fiber1, Fiber2 and Fiber3. Fiber1 is P-doped in core
wt%) and Fiber3 Ge-doped ( wt%). Fiber2 is of PSCF
(
0018-9499/$25.00 © 2007 IEEE
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IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 54, NO. 4, AUGUST 2007
The sum of these three components results in a minimumRIA window around 1000 nm for Fiber1, 1100–1200 nm for
Fiber2, and 1200–1300 nm for Fiber3.
Fig. 1 also confirms the well-known very poor radiation-resistance of P-doped fibers compared to Ge-doped and undoped
silica fibers over the whole spectral range [1], [4], [11]: despite
the low gamma-dose (5 Gy) applied to the P-doped fiber compared to the dose applied to the Ge-doped and undoped fibers
(100 Gy), the RIA is much higher for the P-doped fiber.
IV. DISCUSSION
A. Method to Extract the NIR-RIA Contribution
Fig. 1. Normalized RIA spectra for the P-doped fiber (after 5 Gy), the
Ge-doped fiber, and the undoped fiber (after 100 Gy).
type and contains only fluorine to lower its refractive index
profile. The three fibers have been made using CVD technique.
We conducted the irradiation experiments within the CEA,
Bruyères-le-Chatel, France, using a Co source at room temperature. We irradiated a few kilometers of each fiber. The P-doped
fiber (Fiber1) has been irradiated at a total dose of 5 Gy and
Fiber2 and Fiber3 at 100 Gy. We performed spectral attenuation measurements both before and after the gamma-exposure.
Attenuation measurements performed after the radiation have
been made a few weeks after the irradiation, which means that
most of the unstable defects were already annealed. These attenuation measurements were performed at room temperature,
from 450 nm up to maximum 2100 nm. For NIR-measurements, we took care of minimizing bending losses by zero-stress
spooling of the fiber. Uncertainty of the attenuation mea% for
800 nm (home-made
surements is maximum
nm (optimized Bentham-bench). For
bench) and
nm (commercial PK2200 apparatus),
dB/km.
attenuation uncertainty is smaller:
III. EXPERIMENTAL RESULTS
Fig. 1 shows the normalized Radiation-Induced Attenuation
spectra for the three fibers studied. Normalization has been
made so that RIA at 1550 nm for the undoped fiber is equal
to 1 dB/km. Note that the largest wavelength up to which we
could determine the RIA spectra depends on the RIA level.
Indeed, due to the measurement uncertainty on attenuation
% for
spectra both before and after the radiation exposure (
nm) and due to the high attenuation level of silica
at long wavelengths (higher than 7 dB/km for
nm),
only RIA larger than 1.7 dB/km can be measured for these
wavelengths. This explains why RIA could not be determined
nm for Fiber2 and Fiber3. Because RIA of Fiber1
for
is larger, we could measure it up to 2100 nm.
One can see from Fig. 1 that for all three types of fibres, RIA
curves result from three main contributions:
— an UV-absorption tail that strongly increases with decreasing wavelength,
— absorption bands located in the Visible range (especially
strong for Fiber1 and Fiber2),
— an IR-contribution that increases with increasing wavelength.
In order to get more information on the NIR-part of the RIA,
we have extracted the NIR-contribution by subtracting all the
UV and visible contributions from the RIA-spectra. To do that,
we fitted the radiation-induced visible bands with Gaussian
band shapes, as widely assumed for defects in inhomogeneous
solids like vitreous silica (see for example [7], [12]). Local
electrical fields existing in the disordered silica matrix indeed
impact the electronical environment of absorbing defects,
and therefore impact their absorption energy, shifting it to
lower or upper energies with a certain distribution. Gaussian
functions with quite large width generally well fit the resulting
absorption bands around the maximum of absorption. In addition to these bands located in the visible range, we chose
to consider an absorption contribution of an exponential form
, assuming that this contribution
includes the sum of all the long absorption tails of radiation-induced defects absorbing in the VUV-UV range. The values
of C and K depend on both the total dose and the fiber type.
The fit of this UV contribution has been performed based on
RIA values around both 450 nm and 1100 nm, i.e., where the
UV-tail is predominant. Even if the fits of the UV and visible
contributions are quite raw, this allowed us to determine the
main contribution of the RIA (UV or NIR contribution) at both
1310 and 1550 nm (uncertainty on the contribution smaller
%). Having fitted the UV-contribution with another
than
wavelength dependency could have led to slightly larger uncertainty, but would not have changed the conclusion concerning
the main RIA contribution.
B. Phosphorus-Doped Fibers
For the P-doped fiber, RIA spectrum can be fitted by a UV-tail
and a sum of two Gaussian curves centered around 540 nm
(attributed to POHC defects [7]) and 1620 nm (attributed to
P1 defects [7]) (see Fig. 2). From Fig. 2, one can see that at
telecom wavelengths, RIA of P-doped fibers is due almost entirely to P1 defects: at both, 1310 nm and 1550 nm, radiation-induced absorption due to P1-absorption band represents the main
origin of RIA, with more than 80% and 90% of the total RIA,
respectively.
We deduce from these RIA decomposition that the P1 band
is well fitted by a Gaussian function, with a maximum located
eV (
nm) and a FWHM of
at
eV. These values agree with literature data that state the
band maximum of P1 defects between 0.76 and 0.79 eV [7] and
a FWHM from 0.26 to 0.35 eV [7]. Our very good precision
REGNIER et al.: LOW-DOSE RADIATION-INDUCED ATTENUATION AT INFRARED WAVELENGTHS
Fig. 2. Normalized RIA (after 5 Gy) of the P-doped fiber can be fitted by a
UV-tail, and two absorption bands located around 540 nm and 1620 nm.
Fig. 3. Normalized RIA (after 100 Gy) decomposition for Fiber2: we can see
a UV-tail, at least two absorption bands located in the Visible range (around
650 nm and 750 nm), and one absorption band in the NIR.
on P1 peak position and peak width is due to the fact that at
such long wavelengths, the RIA is almost exclusively due to
P1-defects and because we took into account the wavelength
dependency of the mode power fraction in the P-doped part of
the fiber (as already suggested in [13]). The calculation of this
power fraction is based on Maxwell propagation equations applied to a cylindrical dielectrical waveguide in which the usual
weakly guiding approximation was made. Features of the waveguide are determined by measuring the refractive index profile
of the preform. Not taking this effect into account would have
led to an error on the peak maximum of about 90 nm (maximum
would have been found around 1530 nm instead of 1620 nm). In
addition to that, the shape of the band-fit would also have been
somewhat distorted, with an underestimation of P1-contribution
at long wavelengths.
C. Undoped Fibers
The RIA-decomposition for Fiber2 is shown in Fig. 3. It allowed us to highlight the existence of absorption bands located
in the visible range (at least two bands around 650 nm and
750 nm) and a broad NIR-absorption band, which is responsible
for more than 90% of total RIA at 1550 nm, and more than 75%
of total RIA at 1310 nm. Gamma-sensitivity of the undoped fiber
at telecommunication wavelengths thus mainly results from this
NIR-band. By fitting this band by a Gaussian function, we find
a peak maximum around 1800 nm (about 0.7 eV,
eV) (see Fig. 3).
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Fig. 4. Intensity of the NIR-absorption band attributed to STH defects. Data at
77 K, 130 K, 145 K, and 160 K are extracted from [1]. Data at 300 K has been
obtained in this study.
Because this band maximum is very close to that of the absorption band attributed to STH defects by Chernov et al. [11],
we think that this NIR-band is related to STHs. In addition to
that, the presence of a second absorption band located around
750 nm, which is characteristic for STH defects too, tends to
confirm the presence of these defects. However, the presence
of STHs in our fiber (irradiated and measured at room temperature) is surprising as STHs are known to be unstable at room
temperature. Up to now, the NIR-band attributed to STHs has
been observed up to 160 K only [10], but not at higher temperatures. Chernov et al. showed that this NIR-band decreases when
increasing temperature. As shown in Fig. 4, RIA-measurements
performed on our undoped fiber irradiated at the same total dose
as that used by Chernov show that the NIR-band intensity is
more than two orders of magnitude smaller than that measured
at low temperature in [10]. The fact that we still observe STH,
even at room temperature, is therefore probably due to our very
high measurement sensitivity obtained by using fibers of several
km in length (note that Chernov et al. used fiber samples of only
4 m in length). This therefore tends to show that STH defects
exist at room temperature, but at very extremely low contents.
The appearance of STHs in pure silica tends to show that their
formation is related to the glass matrix properties. To reduce
the formation of these defects, Yamaguchi et al. [14] propose to
reduce the glass disorder.
D. Ge-Doped Fibers
For the Ge-doped fibre, the RIA spectrum suggests the existence of absorption structures between 550 nm and 850 nm,
and a very broad absorption band having its origin in the NIR
range, at
nm (see Fig. 5). With the available data (
nm), we can’t have much precision about the exact location of this band. Measurements performed after higher dose
would be needed to determine the exact location. Despite this
low precision, we can clearly see that the impact of this NIRband at 1310 nm is not so strong as for the P-doped and undoped
fibers: at this wavelength, the NIR-band contribution is secondary only (around 40%). However, at 1550 nm, the NIR-contribution becomes bigger and even the main contribution, with
more than 80% of the total RIA. The origin of this NIR absorption band might be linked to some Ge-impacted STH defects,
with optical properties slightly different from “regular” STH
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IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 54, NO. 4, AUGUST 2007
Fig. 5. Normalized RIA (after 100 Gy) decomposition for the Ge-doped fiber:
we can see the presence of the UV-absorption tail, some absorption bands in the
Visible range, and a broad band centered in the NIR range.
defects. Such defects have actually already been mentioned in
literature. For example, Sasajima showed by calculation that
Ge-defects called Small Polaron defects (SP(Ge)) are similar
to STH defects, but have slightly different optical absorption
properties: he measured an STH-absorption band around 2.6 eV,
and an absorption band related to SP(Ge) around 2.2 eV [15].
Also Pacchioni published some calculations on Ge-Lone-Pair
showing that such deCenter with a trapped hole
fects have energies similar to that of STHs [16]. At last, Nishii
et al. [17], [18] show by ESR measurements that STH defects
also exist in Ge-doped silica irradiated at low temperature. We
thus propose that the NIR-band observed in our Ge-doped fiber
is due to some STH species, whose absorption properties could
be slightly changed due to the presence of Germanium atoms in
their neighborhood.
Comparing the RIA data of Fiber 2 and Fiber 3, we see that the
contribution of the NIR-absorption band at 1310 nm and 1550 nm
is slightly lower in Ge-doped fibres than in fibres with no Ge. This
can be explained firstly by the fact that the UV-absorption tail is
somewhat stronger in the Ge doped fiber, and secondly because
the NIR-absorption band in Ge-doped SMF is centered at higher
wavelengths than the NIR-band in undoped SMF.
For higher doses (up to 100 000 Gy), we observed that
UV-tail and NIR-contribution increase much more in the
Ge-doped fibre than in the undoped fiber. In this case, RIA for
Ge-doped fibers is larger over the whole studied wavelength
range (from 450 nm up to 2000 nm). At such high doses, fibers
with no Ge should thus be preferred to Ge-doped fibers.
At last, we can note that our results differ from measurements
performed on erbium-doped fibers, where the UV absorption
tail seems to be the main contribution, even at NIR wavelengths
[19]. Erbium-doped fibers however usually show much higher
RIA values than fibers studied here [20]. The strong UV-contribution can be explained by the presence of specific dopants like
Aluminium [21]. Also the role of Erbium ions is discussed by
certain authors [20].
V. CONCLUSION
We have seen in this paper that attenuation properties of optical fibers exposed to radiative environments are impacted by
the formation of defects absorbing not only in the UV-visible
range, but in the NIR range too. We even show that this NIR-
contribution mainly determines the RIA level at both standard
telecommunication wavelengths (1310 nm and 1550 nm). This
is the case for all three studied fibers: the P-doped silica fiber, the
Ge-doped silica fiber and the undoped silica fiber. It is therefore
very important to be aware of the existence of these bands and to
study their properties in more details. In fibers with no P, we propose that the NIR-absorption bands are related to STH species.
Note that such defects are likely to exist not only in silica fibers
exposed to radiations, but also in all today’s technologies based
on silica material and exposed to radiative environments. Moreover, the appearance of STHs in pure silica tends to show that
their formation is related to the glass matrix properties. To our
knowledge, it is the first time that STH’s NIR-band has been observed at room temperature. This was possible because we used
very long fiber samples (a few kilometers), allowing thus to detect very low defect concentrations. The sum of the UV-visible
components and the NIR component leads to a RIA minimum
between 1000 nm and 1300 nm. As a consequence, fibers used
under radiative environment should rather be used in this wavelength range. We also found that at low irradiation doses, fibers
of PSCF type are more radiation-resistant than Ge-doped fibers
in the visible range, but not in NIR range.
ACKNOWLEDGMENT
The authors would like to thank the Alcatel R&I team for
having supplied some of the studied samples.
REFERENCES
[1] S. Girard, “Analyse de la réponse des fibres optiques soumises à divers
environnements radiatifs,” Ph.D. dissertation, Univ. Jean Monnet de
Saint-Etienne, St. Etienne, France, 2003.
[2] K. Nagasawa, Y. Hoshi, Y. Ohki, and K. Yahagi, “Improvement of
radiation resistance of pure silica core fibers by hydrogen treatment,”
Jpn. J. Appl. Phys., vol. 24, no. 9, pp. 1224–1228, 1985.
[3] V. B. Neustruev, “Colour centres in germanosilicate glass and optical
fibres,” J. Phys.: Condens. Matter, vol. 6, pp. 6901–6936, 1994.
[4] P. Lu, X. Bao, K. Brown, and N. Kulkarni, “Gamma-induced attenuation in normal single-mode and multimode, Ge-doped and P-doped optical fibers: A fiber optic dosimeter for low dose levels,” Can. J. Phys.,
vol. 78, pp. 89–97, 2000.
[5] H. Henschel and O. Köhn, “Radiation-induced optical fibre loss in the
far IR,” Proc. RADECS 99, Abbaye de Fontevraud, pp. 466–470, 2000.
[6] M. Kyoto, “Gamma-ray irradiation effect on loss increase of single
mode optical fibers (I)—Loss increase behavior and kinetic study,” J.
Nucl. Sci. Technol., vol. 26, no. 5, pp. 507–515, 1989.
[7] D. L. Griscom, E. J. Friebele, and K. J. Long, “Fundamental defect
centers in glass: ESR and optical spectroscopic studies of irradiated
P-doped silica glass and optical fibers,” J. Appl. Phys., vol. 54, no. 7,
1983.
[8] D. L. Griscom, “The nature of point defects in amorphous silicon
dioxide,” Defects in SiO and related dielectrics: Sci. Technol., pp.
117–159, 2000.
[9] D. L. Griscom, “Radiation hardening of pure-silica-core optical fibers:
Reduction of induced absorption bands associated with self-trapped
holes,” Appl. Phys. Lett., vol. 71, no. 2, pp. 175–177, 1997.
[10] P. V. Chernov, “Spectroscopic manifestations of self-trapped holes in
silica,” Phys. Stat. Sol., vol. B115, pp. 663–675, 1989.
[11] E. J. Friebele, M. E. Gingerich, and K. J. Long, “Radiations damage of
optical fiber waveguides at long wavelengths,” Appl. Opt., vol. 21, no.
3, pp. 547–553, 1982.
[12] L. Skuja, “Optically active oxygen-deficiency-related centers in amorphous silicon dioxide,” J. Non-Crystalline Solids, vol. 239, no. 1–3, pp.
16–48, 1998.
[13] E. W. Mies and L. Soto, “Characterization of the radiation sensitivity
of single-mode fibers,” Proc. IOOC-ECOC’85, pp. 255–258, 1985.
[14] M. Yamaguchi, K. Saito, and A. J. Ikushima, “Fictive-temperature-dependance of photoinduced self-trapped holes in a-SiO2,” Phys. Rev. B,
vol. 68, 2003.
REGNIER et al.: LOW-DOSE RADIATION-INDUCED ATTENUATION AT INFRARED WAVELENGTHS
[15] Y. Sasajima and K. Tanimura, “Optical transitions of self-trapped holes
in amorphous SiO2,” Phys. Rev. B, vol. B68, 2003.
[16] G. Pacchioni, L. Skuja, and D. L. G, “Defects in SiO2 and related dielectrics: Science and technology,” NATO Sci. Series, 2000.
[17] J. Nishii, K. Fukumi, and H. Yamanaka, “Photochemical reactions in
GeO2-SiO glasses induced by ultraviolet irradiation: Comparison between Hg lamp and excimer laser,” Phys. Rev. B, vol. 52, no. 3, pp.
1161–1165, 1995.
[18] J. Nishii and K. K, “Pair generation of Ge electron centers and selftrapped hole centers in GeO2-SiO2 glasses by KrF excimer-laser irradiation,” Phys. Rev. B, vol. 60, no. 10, pp. 7166–7169, 1999.
1119
[19] T. S. Rose, D. Gunn, and G. C. Valley, “Gamma and proton radiation
effects in erbium-doped fiber amplifiers: Active and passive measurements,” J. Lightw. Technol., vol. 19, no. 12, pp. 1918–1923, Dec. 2001.
[20] G. M. Williams, M. A. Putman, C. G. Askins, M. E. Gingerich, and E.
J. Friebele, “Radiation effects in erbium-doped fibers,” Electron. Lett.,
vol. 28, no. 19, pp. 1816–1818, 1992.
[21] B. Brichard, A. F. Fernandez, H. Ooms, M. Van Uffelen, and F. Berghmans, “Study of the radiation-induced optical sensitivity in erbium and
aluminium doped fibres,” in Proc. IEEE RADECS Conf., 2003, pp.
35–38.