Aziz Boukenter

Radiation hardening techniques for Er/Yb doped optical fibers and amplifiers for space application

We investigated the efficiencies of two different approaches to increase the radiation hardness of optical amplifiers through development of improved rare-earth (RE) doped optical fibers. We demonstrated the efficiency of codoping with Cerium the core of Erbium/Ytterbium doped optical fibers to improve their radiation tolerance. We compared the γ-rays induced degradation of two amplifiers with comparable pre-irradiation characteristics (~19 dB gain for an input power of ~10 dBm): first one is made with the standard core composition whereas the second one is Ce codoped. The radiation tolerance of the Ce-codoped fiber based amplifier is strongly enhanced. Its output gain decrease is limited to ~1.5 dB after a dose of ~900 Gy, independently of the pump power used, which authorizes the use of such fiber-based systems for challenging space missions associated with high total doses. We also showed that the responses of the two amplifiers with or without Ce-codoping can be further improved by another technique: the pre-loading of these fibers with hydrogen. In this case, the gain degradation is limited to 0.4 dB for the amplifier designed with the standard composition fiber whereas 0.2 dB are reported for the one made with Ce-codoped fiber after a cumulated dose of ~900 Gy. The mechanisms explaining the positive influences of these two treatments are discussed.

/spl gamma/-rays and pulsed X-ray radiation responses of germanosilicate single-mode optical fibers: influence of cladding codopants

The radiation-induced attenuation (RIA) in germanosilicate single-mode optical fibers was measured at 1.55 and 1.31 mm after a pulsed X-ray irradiation and at 1.55 mm during and after a steady-state g-ray irradiation. The influence of codoping the fiber cladding with germanium (Ge), phosphorus (P), and fluorine (F) on the sensitivity of Ge-doped core fibers was characterized. P-codoping makes it possible to decrease the RIA for short times (10/sup *6/ s*10/sup *3/ s) post- pulse. However, P-codoped fibers exhibit larger values of permanent RIA than P-free fibers after transient exposure and are inadequate for a steady-state environment. The impact of F-codoping depends on the other codopants incorporated in the fiber cladding, but its addition seems to be deleterious for the radiation hardening of the germanosilicate fiber at the two tested wavelengths. Ge-codoping increases the sensitivity of P-, F-codoped fibers under X-rays and steady-state g-ray irradiation, whereas it decreases the RIA in F-doped ones. Some hypotheses on the creation mechanisms and properties of the color centers related to these three codopants are proposed.

Sol-gel derived ionic copper-doped microstructured optical fiber: a potential selective ultraviolet radiation dosimeter

We report the fabrication and characterization of a photonic crystal fiber (PCF) having a sol-gel core doped with ionic copper. Optical measurements demonstrate that the ionic copper is preserved in the silica glass all along the preparation steps up to fiber drawing. The photoluminescence results clearly show that such an ionic copper-doped fiber constitutes a potential candidate for UV-C (200-280 nm) radiation dosimetry. Indeed, the Cu⁺-related visible photoluminescence of the fiber shows a linear response to 244 nm light excitation measured for an irradiation power up to 2.7 mW at least on the Cu-doped PCF core. Moreover, this response was found to be fully reversible within the measurement accuracy of this study ( ± 1%), underlying the remarkable stability of copper in the Cu⁺ oxidation state within the pure silica core prepared by a sol-gel route. This reversibility offers possibilities for the achievement of reusable real-time optical fiber UV-C dosimeters.

Evolution of Photo-induced defects in Ge-doped fiber/preform: influence of the drawing

We have studied the generation mechanisms of two different radiation-induced point defects, the Ge(1) and Ge(2) centers, in a germanosilicate fiber and in its original preform. The samples have been investigated before and after X-ray irradiation using the confocal microscopy luminescence and the electron paramagnetic resonance techniques. Our experimental results show the higher radiation sensitivity of the fiber as compared to the perform and suggest a relation between Ge(1) and Ge(2) generation. To explain our data we have used different models, finding that the destruction probability of the Ge(1) and Ge(2) defects is larger in fiber than in preform, whereas the generation one is similar. Finally we found that the higher radiation sensitivity of the fiber at low doses is essentially related to the presence of germanium lone pair center generated by the drawing.

Nanosize structural modifications with polarization functions in ultrafast laser irradiated bulk fused silica

Laser-induced self-organization of regular nanoscale layered patterns in fused silica is investigated using spectroscopy and microscopy methods, revealing a high presence of stable broken oxygen bonds. Longitudinal traces are then generated by replicating static irradiation structures where the nanoscale modulation can cover partially or completely the photoinscribed traces. The resulting birefringence, the observed anisotropic light scattering properties, and the capacity to write and erase modulated patterns can be used in designing bulk polarization sensitive devices. Various laser-induced structures with optical properties combining guiding, scattering, and polarization sensitivity are reported. The attached polarization functions were evaluated as a function of the fill factor of the nanostructured domains. The polarization sensitivity allows particular light propagation and confinement properties in three dimensional structures.

High γ-ray dose radiation effects on the performances of Brillouin scattering based optical fiber sensors.

The use of distributed strain and temperature in optical fiber sensors based on Brillouin scattering for the monitoring of nuclear waste repository requires investigation of their performance changes under irradiation. For this purpose, we irradiated various fiber types at high gamma doses which represented the harsh environment constraints associated with the considered application. Radiation leads to two phenomena impacting the Brillouin scattering: 1) decreasing in the fiber linear transmission through the radiation-induced attenuation (RIA) phenomenon which impacts distance range and 2) modifying the Brillouin scattering properties, both intrinsic frequency position of Brillouin loss and its dependence on strain and temperature. We then examined the dose dependence of these radiation-induced changes in the 1 to 10 MGy dose range, showing that the responses strongly depend on the fiber composition. We characterized the radiation effects on strain and temperature coefficients, dependencies of the Brillouin frequency, providing evidence for a strong robustness of these intrinsic properties against radiations. From our results, Fluorine-doped fibers appear to be very promising candidates for temperature and strain sensing through Brillouin-based sensors in high gamma-ray dose radiative environments.

Fluorescence line narrowing study of Cr3+ ions in cordierite glass nucleating MgAl2O4 nanocrystals

Abstract The luminescence of Cr 3+ ions in cordierite glass nucleating MgAl 2 O 4 nanocrystallites has been investigated. The time resolved fluorescence line narrowing measurements and the temperature dependence of the homogeneous line width show that most Cr 3+ ions are inside the nanocrystallites. Cr 3+ ions occupy non-equivalent crystal sites, due to the Mg 2+ –Al 3+ inversion effect. The values of the homogeneous line width compare well with those of previous studies in crystals. No surface effect has been observed due to the large size of the crystals.

Properties of phosphorus-related defects induced by γ-rays and pulsed X-ray irradiation in germanosilicate optical fibers

Abstract Time dependent radiation-induced attenuation changes of germanosilicate (∼13 wt%) optical fibers have been measured under steady-state γ-ray and pulsed X-ray irradiations. Particularities of phosphorus (P)-codoped cladding fibers for these environments are presented. For each irradiation type, spectral attenuation measurements show that permanent radiation-induced losses at 1.55 μm are due to the infrared absorption band of P 1 centers. We assume that particular kinetics of recovery and an increase of attenuation measured for P-codoped fibers, after pulsed X-ray irradiation, are induced by a charge retrapping phenomenon from phosphorous oxygen hole (POH) centers to P 1 ones. With this mechanism, we explain the P-codoped fiber characteristics after both types of irradiation. Our study shows that the effect of P-related color centers is increased when the P-codoped cladding (∼0.3 wt%) contains also germanium (∼0.3 wt%).

Radiation tolerant fiber Bragg gratings for high temperature monitoring at MGy dose levels

We report a method for fabricating fiber Bragg gratings (FBG) resistant to very severe environments mixing high radiation doses (up to 3 MGy) and high temperatures (up to 230°C). Such FBGs have been written in two types of radiation resistant optical fibers (pure-silica and fluorine-doped cores) by exposures to a 800 nm femtosecond IR laser at power exceeding 500 mW and then subjected to a thermal annealing treatment of 15 min at 750°C. Under radiation, our study reveals that the radiation induced Bragg wavelength shift (BWS) at a 3 MGy dose is strongly reduced compared to responses of FBGs written with nonoptimized conditions. The BWS remains lower than 10 pm for temperatures of irradiation ranging from 25°C to 230°C without noticeable decrease of the FBG peak amplitude. For an applicative point of view, this radiation induced BWS corresponds to an additional error on the temperature measurements lower than 1.5°C, opening the way to the development of radiation-tolerant multi-point temperature sensors for nuclear industry.

Spatial distribution of the red luminescence in pristine, γ rays and ultraviolet-irradiated multimode optical fibers

In this letter, a confocal microscopy setup was used to evaluate, with a resolution of 2 μm, the nonuniform spatial distributions of the red photoluminescence (640–750 nm) in pristine, γ rays (∼1 MeV, 1.2 kGy, and 0.33 Gy/s) and ultraviolet (244 nm and 127 J/cm2) irradiated multimode optical fibers. In pristine samples, the Raman scattering is predominant and the emitting centers are only present at low concentration in the fiber cladding. However, these centers are generated by both irradiations in the whole fiber cross sections, in particular near the core–cladding interface. The spectral deconvolution of the luminescence spectra showed that the Non-Bridging Oxygen Hole Centers are mainly responsible for this red luminescence. For both irradiation types, these centers seem to be predominantly created from precursor sites: strained Si–O–Si bonds which could be induced during the fiber drawing process.