A. Boukenter

Radiation Effects on Silica-Based Optical Fibers: Recent Advances and Future Challenges

In this review paper, we present radiation effects on silica-based optical fibers. We first describe the mechanisms inducing microscopic and macroscopic changes under irradiation: radiation-induced attenuation, radiation-induced emission and compaction. We then discuss the influence of various parameters related to the optical fiber, to the harsh environments and to the fiber-based applications on the amplitudes and kinetics of these changes. Then, we focus on advances obtained over the last years. We summarize the main results regarding the fiber vulnerability and hardening to radiative constraints associated with several facilities such as Megajoule class lasers, ITER, LHC, nuclear power plants or with space applications. Based on the experience gained during these projects, we suggest some of the challenges that will have to be overcome in the near future to allow a deeper integration of fibers and fiber-based sensors in radiative environments.

Correlation effects on Raman scattering from low‐energy vibrational modes in glasses. II. Experimental results

New experimental results of low‐frequency Raman scattering from different glasses are presented. They are compared to the vibration density of states obtained from inelastic neutron scattering to deduce the light‐vibration coupling coefficient C(ω) as a function of the vibration frequency. It was found that C(ω)∝ω2 at very low frequency ω<20 cm−1 in inorganic glasses (SiO2,B2O3) and that C(ω)∝ω at low frequency in polymer glasses and in inorganic glasses for ω≳20 cm−1. Our experimental results for glasses are interpreted by assuming no correlation at very low‐frequency and only radial correlation at low‐frequency. A full correlation would exist in silica‐aerogels in which the effect of random fluctuations would be negligible.

Dynamical structure of water: Low‐frequency Raman scattering from a disordered network and aggregates

Low‐frequency inelastic light scattering of water is observed from a temperature equal to 80 down to −20 °C in the supercooled regime. For energies higher than 3 cm−1 it is shown that the major part of the light scattering is Raman scattering. A broadband with a maximum at 50 cm−1 is interpreted as scattering from transverse acoustic modes of a disordered network. A weaker scattering which shifts towards the Rayleigh line when the temperature decreases is well explained by Raman scattering from the oscillations of water molecule aggregates which grow when the temperature decreases.

Proton- and Gamma-Induced Effects on Erbium-Doped Optical Fibers

We characterized the responses of three erbium-doped fibers with slightly different concentrations of rare-earth ions (240-290 ppm) and Al2O3 (7-10 wt.%) during proton and gamma-ray exposures. We have simultaneously measured the radiation-induced attenuation (RIA) around the Er3+ ion pumping wavelength (980 nm) and the associated changes of the Er3+ emission around 1530 nm. The three erbium-doped fibers show similar radiation responses. All fibers exhibit RIA levels between 9 times 10-3 and 1.7 times 10-2 dB m-1 Gy-1 at 980 nm and between 4 times 10-3 and 1.1 times 10-2 dB m-1 Gy-1 at 1530 nm. Protons and gamma-rays lead to similar radiation damages, with small differences between the protons of different energies (50 MeV and 105 MeV). Furthermore, we have performed online measurements of the spectral dependence of RIA from 600 to 1600 nm and offline measurements from 1200 to 2400 nm. The three fibers exhibit the same spectral response. Losses decrease monotonically from the visible to the infrared part of the spectrum. We have performed spectral decomposition of these RIA curves with the help of absorption bands previously associated with radiation-induced point defects. Our analysis shows that the main part of the RIA (600-1700 nm) in erbium-doped glass can be explained by the generation of Al-related point defects. The other defects related to the germanium and phosphorus doping of the silica seem to have a lower contribution to the induced losses. The Er3+ ion properties seem to be mainly unaffected by proton exposure, suggesting a solvation shell around the Er3+ ion formed by Al2O3 species.

Low‐frequency Raman scattering and structure of amorphous polymers: Stretching effect

Low‐frequency Raman scattering of amorphous polymers [polyethyleneterephthalate (PET) and polymethylmethacrylate] is investigated. The low‐frequency Raman band called ‘‘Boson peak’’ is interpreted in terms of a noncontinuous structure, similar to that of inorganic glasses, i.e., solid amorphous polymers are composed of 50 A blobs. These are related blobs to transient entanglements in the polymer melt. Modifications induced by stretching are described for a PET film. The nature of the blob structure is discussed.

Ultrafast laser induced electronic and structural modifications in bulk fused silica

Ultrashort laser pulses can modify the inner structure of fused silica, generating refractive index changes varying from soft positive (type I) light guiding forms to negative (type II) values with void presence and anisotropic sub-wavelength modulation. We investigate electronic and structural material changes in the type I to type II transition via coherent and incoherent secondary light emission reflecting free carrier behavior and post-irradiation material relaxation in the index change patterns. Using phase contrast microscopy, photoluminescence, and Raman spectroscopy, we determine in a space-resolved manner defect formation, redistribution and spatial segregation, and glass network reorganization paths in conditions marking the changeover between type I and type II photoinscription regimes. We first show characteristic patterns of second harmonic generation in type I and type II traces, indicating the collective involvement of free carriers and polarization memory. Second, incoherent photoemission ...

Gel-to-glass transformation of silica a study by low-frequency Raman scattering

Abstract The effects of heat treatment on silica gels were studied by low-frequency Raman scattering. The results obtained from a base-catalyzed aerogel and described in more detail. Three stages of the transformation to glass are distinguished: (1) modification of the particle surface; (2) coalescence of the particles; and (3) change of the internal structure and glass relaxation. The difference between the gel and glass structures is emphasized.

Radiation-hard erbium optical fiber and fiber amplifier for both low- and high-dose space missions

We present a new structure for erbium-doped optical fibers [hole-assisted carbon-coated, (HACC)] that, combined with an appropriate choice of codopants in the core, strongly enhances their radiation tolerance. We built an erbium-doped fiber amplifier based on this HACC fiber and characterize its degradation under γ-ray doses up to 315 krad (SiO2) in the ON mode. The 31 dB amplifier is practically radiation insensitive, with a gain change of merely -2.2×10(-3) dB/krad. These performances authorize the use of HACC doped fibers and amplifiers for various applications in environments associated with todays missions (of doses up to 50 krad) and even for future space missions associated with higher dose constraints.

Combined High Dose and Temperature Radiation Effects on Multimode Silica-Based Optical Fibers

We investigate the response of Ge-doped, P-doped, pure-silica, or Fluorine-doped fibers to extreme environments combining doses up to MGy(SiO 2) level of 10 keV X-rays and temperatures between 25 °C and 300 °C. First, we evaluate their potential to serve either as parts of radiation tolerant optical or optoelectronic systems or at the opposite, for the most sensitive ones, as punctual or distributed dosimeters. Second, we improve our knowledge on combined ionizing radiations and temperature (R&T) effects on radiation-induced attenuation (RIA) by measuring the RIA spectra in the ultraviolet and visible domains varying the R&T conditions. Our results reveal the complex response of the tested fibers in such mixed environments. Increasing the temperature of irradiation increases or decreases the RIA values measured at 25 °C or sometimes has no impact at all. Furthermore, R&T effects are time dependent giving an impact of the temperature on RIA that evolves with the time of irradiation. The two observed transient and stationary regimes of temperature influence will make it very difficult to evaluate sensor vulnerability or the efficiency of hardening approaches without extensive test campaigns.

Correlation effects on Raman scattering from low-energy vibrational modes in fractal and disordered systems. I: Theory

In disordered fractal or nonfractal systems, the disorder induces rapid fluctuations of the acoustical vibrational wave functions, so that statistics can be used to resolve the problem of Raman scattering from localized low‐energy vibrations. We derive the expressions of light‐vibration coupling coefficient, C(ω), as a function of the frequency ω, in the case of the dipole‐induced dipole mechanism of susceptibility fluctuation and for different types of correlation, from full correlation to no‐correlation. The results are compared to the existing numerical calculations. The expressions of C(ω) are determined for other mechanisms of susceptibility fluctuations, depending on an interaction propagating in the fractal.