KEYWORDS: Optical fibers, Sensors, Scattering, Environmental sensing, Signal attenuation, Gamma radiation, Single mode fibers, Environmental monitoring, Fiber optics sensors, Signal to noise ratio
Brillouin sensing is an interesting technology to monitor strain inside large civil engineering structures like nuclear waste
repository. In radiative environments, performance degradation of Brillouin systems still needs to be investigated. In this
paper, we fully characterized the Brillouin response of two different optical fibers, including the SMF28 fiber from
Corning, under high gamma doses. Radiations increase the fiber linear attenuation and can also lead to a shift in the
Brillouin frequency. We reduced these effects by designing a radiation resistant optical fiber. This fiber is well adapted
for distributed sensing application using Brillouin scattering in MGy dose level environment.
The Laser MegaJoule (LMJ) facility will host inertial confinement fusion experiments in order to achieve ignition by
imploding a Deuterium-Tritium filled microballoon [1]. In this context an X-ray imaging system is necessary to diagnose
the core size and the shape of the target in the 10-100 keV band. Such a diagnostic will be composed of two parts: an X-ray
optical system and a detection assembly. The survivability of each element of this diagnostic has to be ensured within
the mixed pulse consisting of X-rays, gamma rays and 14 MeV neutrons created by fusion reactions.
The design of this diagnostic will take into account optics and detectors vulnerability to neutron yield of at least 1016. In
this work, we will present the main results of our vulnerability studies and of our hardening-by-system and hardening-by-
design studies.
Er/Yb doped fibers and amplifiers have been shown to be very radiation sensitive, limiting their integration in space. We
present an approach including successive hardening techniques to enhance their radiation tolerance. The efficiency of our
approach is demonstrated by comparing the radiation responses of optical amplifiers made with same lengths of different
rare-earth doped fibers and exposed to gamma-rays. Previous studies indicated that such amplifiers suffered significant
degradation for doses exceeding 10 krad. Applying our techniques significantly enhances the amplifier radiation
resistance, resulting in a very limited degradation up to 50 krad. Our optimization techniques concern the fiber
composition, some possible pre-treatments and the interest of simulation tools used to harden by design the amplifiers.
We showed that adding cerium inside the fiber phosphosilicate-based core strongly decreases the fiber radiation
sensitivity compared to the standard fiber. For both fibers, a pre-treatment with hydrogen permits to enhance again the
fiber resistance. Furthermore, simulations tools can also be used to improve the tolerance of the fiber amplifier by
helping identifying the best amplifier configuration for operation in the radiative environment.
A new approach is proposed to monitor in situ the influence of gamma radiations on Brillouin properties of optical fiber
extensometers. Experimental results are illustrated with the characterization of two fibers samples up to total dose of
about 600Gy. The Brillouin frequency shift remains unaffected at such radiations level, as well as the spectral Brillouin
signature or its dependence with strain. Meanwhile, propagation losses increase under radiations with an amplitude
related to fiber dopants. The target application is nuclear wastes repository monitoring where higher doses are expected.
UV radiation preliminary tests show that compaction phenomenon may occur at such high doses, inducing Brillouin
frequency shift up to 20 MHz.
Rare-earth doped optical fibers have been shown to be very sensitive to radiations, limiting the integration of fiber-based
systems in space missions. In this paper, we present the characterization of two amplifiers based on a set of prototype
active Erbium/Ytterbium codoped double clad fibers developed by Ixfiber SAS. One of these fibers has been codoped
with cerium inside its core to enhance its radiation tolerance whereas the other is a classical phosphosilicate Er/Yb fiber.
The two amplifiers based on these fibers have been exposed to γ-rays at a low dose rate (0.3 rad/s) and to doses up to
90 krad. Previous studies indicated that Er/Yb amplifiers using this type of fiber suffered significant degradation for
cumulated dose above 5-10 krad. We observed, on the contrary, that with our radiation hardened fiber, the degradation
of the fiber amplifier's output power can be limited to less than 30% after an exposure dose of ~90 krad.
The integration of rare-earth doped optical fibers as part of fiber-based systems in space implies the development of
waveguides tolerant to the radiation levels associated with the space missions. We report the spatial distribution, the
photoluminescence (PL) properties of color centers and the related changes induced by X-rays radiation at different
doses (50, 500 and 1000 krad) for two different prototypes of Er-doped optical fibers. Each sample (in the version
pristine, X-irradiated and H2 loaded prior to radiation exposure) was characterized by confocal microscopy luminescence
(CML) measurements in Visible range with Visible (488 nm) or UV (325 nm) laser light excitation. The set of tested
fibers allowed us to obtain information on the radiation responses of the silica-based host matrix and on the transitions
between the energy states of rare-earth ions. Under Vis-excitation, the luminescence spectrum of the core revealed the
typical emission pattern of Er3+ ions, with an increase of the emission intensity around 520 nm due to the radiation
treatment; whereas no spectroscopic change induced by radiation was observed when a particular sensitizing element is
added to the core composition or when the fiber was previously H2-loaded. The PL-core spectra under UV-excitation
showed the behavior of the ODC, typical of the silica-based host matrix. For these spectra, addition of the sensitizing
element annihilates the depressions that characterize the profile of ODC emission and that are due to the Er3+ ions
absorption.
In this paper, we reviewed our previous work concerning the responses of rare-earth (RE) doped fibers (Yb, Er and
Er/Yb) to various types of radiations like γ-rays, X-rays and protons. For all these harsh environments, the main
measured macroscopic radiation-induced effect is an increase of the linear attenuation of these waveguides due to the
generation of point defects in the RE-doped core and silica-based cladding. To evaluate the vulnerability of this class of
optical fibers for space missions, we characterize the growth and decay kinetics of their radiation-induced attenuation
(RIA) during and after irradiation for various compositions. Laboratory testing reveals that this class of optical fibers is
very sensitive to radiations compared to passive (RE-free) samples. As a consequence, despite the small length used for
space applications, the understanding of the radiation-induced effects in this class of optical fibers becomes necessary
before their integration as part of fiber-based systems like gyroscopes or communication systems. In this paper, we more
particularly discussed about the relative influence of the rare-earth ions (Er3+ and/or Yb3+) and of the glass matrix
dopants (Al, P, ...) on the optical degradation due to radiations. This has been done by using a set of five prototype
optical fibers designed by the fiber manufacturer iXFiber SAS to enlighten the role of these parameters. Additional
spectroscopic tools like confocal microscopy of luminescence are also used to detect possible changes in the
spectroscopy of the rare-earth ions and their consequences on the functionality of the active optical fibers.
We characterized the responses of different types of rare-earth doped fibers (Yb, Er and Er/Yb) to various types of
radiations like UV, gamma-rays, X-rays and protons. The understanding of the radiation-induced effects in this class of
optical fibers is necessary as they are possible candidates for use as part of fiber-based systems like gyroscopes that will
have to operate in space environment. For all types of irradiations, the main effect is an increase of the linear absorption
of these waveguides due to the generation of point defects in the core and cladding. We characterize the growth and
decay kinetics of the radiation-induced attenuation during and after irradiation for various compositions of optical fibers.
In this paper, we particularly investigate the relative influence of the rare-earth ions (Er, Yb or Er/Yb) and of the glass
matrix dopants (Al, P, ...) on the optical degradation induced by ultraviolet laser exposure at 5 eV. This has been done by
using a set of five prototype optical fibers designed by iXFiber SAS to enlighten the role of these parameters. Additional
spectroscopic tools like confocal microscopy of luminescence are also used to detect possible changes in the
spectroscopy of the rare-earth ions and their consequence on the functionality of the active optical fibers.
The vulnerability of fiber-optic links for high-speed diagnostics to the harsh environments (short pulse of 14 MeV
neutrons, x- and γ-rays) associated with the future Megajoule class lasers (LMJ, NIF) is investigated through
experiments at both ASTERIX and OMEGA facilities. Radiation responses of a Mach Zehnder modulator and single-mode
fiber optics have been characterized with high dose rate x-ray pulses to simulate the conditions expected for the
next generation of pulsed-power facilities. The tolerance of a fiber-optic link to the mixed environment to the present
worldwide most powerful laser neutron source facility (OMEGA, Rochester) is also presented.
The use of rare-earth doped optical fibers in space implies to develop waveguides tolerant to the radiation levels
associated with a space mission. Erbium (Er)- or Ytterbium/Erbium (Yb/Er)-doped fibers have been shown to be very
vulnerable to ionizing radiations especially through the strong increase of their attenuation around the pump and
amplified signal wavelengths. In this paper, we investigate by confocal luminescence microscopy the radiation-induced
spectroscopic changes on prototype Yb or Yb/Er -doped optical fibers. The tested fibers allow us to provide new
insights into the relative influence of the P, Al doping on the radiation responses of their silica-based host matrix and on
the transitions between the energy states of rare-earth ions.
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