Franck Mady

Thermoluminescence characterization of traps involved in the photodarkening of ytterbium-doped silica fibers

The photodarkening (PD) mechanisms of ytterbium-doped silica optical fibers have still not been elucidated, although hardening routes have been proposed. Most basic questions are still under debate about the assignment of the darkening excitation bands into the UV range, the nature of absorbing centers (photoionized centers or trapped carriers?), or of traps accepting photo-released carriers (electron or hole traps?). We used thermoluminescence measurements to characterize traps populated by different radiation types. It is notably demonstrated that photodarkening involves silica hole traps. The popular idea that color centers are formed upon carrier trapping is not consistent with our observations.

Performance of Ge-Doped Optical Fiber as a Thermoluminescent Dosimeter

We have investigated the thermoluminescent response of three Germanium-doped silica-based optical fibers obtained by varying the drawing parameters from a unique preform. We compared under X-ray irradiation, their dosimetric properties to those of two widely used commercial dosimeters based on different technologies. Then, we investigated the potential of these optical fibers to monitor gamma rays at different doses and dose-rates, and to different fluences of particles (0.8 and 14 MeV neutrons and 63 MeV protons). Our results show that the thermoluminescence response of the Ge-doped optical fibers depends linearly on the direct ionizing dose (gamma-, X-rays) or on the indirect ionizing dose (protons, neutrons). As a consequence, this class of fibers is an excellent candidate for passive dosimetry in various fields from medical applications to high-energy physics.

Global View on Dose Rate Effects in Silica-Based Fibers and Devices Damaged by Radiation-Induced Carrier Trapping

We give a novel and very general treatment to a standard model describing dose rate effects in systems damaged by carrier trapping. This model is well adapted for optical fibers, but the lessons we draw may also be helpful to discuss dose rate effects (DRE) in electronic devices. By highlighting the few determinant ratios of physical parameters that govern the system behavior, we clarify when, how and how much dose rate effects affect trap filling and radiation-induced degradation. Critical dose rate, marking the demarcation between low and high dose rate regimes, is also estimated as a function of these parameters. Taking this step back is important to enlighten contradictory results reported on DRE behaviors. The dose and dose rate dependencies of trap filling measured on silica optical fibers, as well as the critical dose rate, are successfully reproduced with a single set of a few adjustable physical parameters.

Thermal quenching investigation in chemical vapor deposited diamond by simultaneous detection of thermally stimulated luminescence and conductivity

Chemical vapor deposited (CVD) diamond is a very promising material that is extensively studied in many technologies. Thermally stimulated luminescence (TSL) experiments demonstrate that an important thermal quenching phenomenon occurs with microwave plasma chemical vapor deposited diamond. This phenomenon leads to a large discrepancy on the determination of electronic trap parameters. Thermally stimulated conductivity phenomenon is not affected by thermal quenching and allows an easier determination of trap parameters than TSL measurements. The difference between the two ways of computing trap energy is found to be around 4% instead of 28%, which is obtained when thermal quenching fails.

Interplay between photo- and radiation-induced darkening in ytterbium-doped fibers

This Letter demonstrates a remarkable interplay between photo- and radiation-induced darkening of ytterbium-doped alumino-silica optical fibers operated in amplifying conditions and harsh environments (as, e.g., in space-based applications). Influences of the pump power, ionizing dose, and dose rate on this interaction are characterized. The pump is capable of accelerating or slowing down the radiation-induced darkening build-up depending on the ionizing dose. The steady-state photo-radio-darkening level is independent of the dose and at least equal to the equilibrium level of pure photo-darkening. This lower limit is notably reached at low dose rates, including those encountered in space. We, therefore, argue that photo-resistant ytterbium-doped fibers will resist against a space mission, whatever the dose.

A physical model of the photo- and radiation-induced degradation of ytterbium-doped silica optical fibres

We propose a model to describe the photo- or/and the radiation-induced darkening of ytterbium-doped silica optical fibers. This model accounts for the well-established experimental features of photo-darkening. Degradation behaviors predicted for fibers pumped in harsh environments are also fully confirmed by experimental data reported in the work by Duchez et al. (this proceeding), which gives a detailed characterization of the interplay between the effects of the pump and those of a superimposed ionizing irradiation (actual operation conditions in space-based applications for instance). In particular, dependences of the darkening build-up on the pump power, the total ionizing dose and the dose rate are all correctly reproduced. The presented model is a ‘sufficient’ one, including the minimal physical ingredients required to reproduce experimental features. Refinements could be proposed to improve, e.g., quantitative kinetics.

Mobility reduction and apparent activation energies produced by hopping transport in the presence of Coulombic defects

A Monte Carlo simulation is proposed to study the mobility reduction due to Coulombic defects for hopping transport in a one-dimensional regular lattice. Hops between energetically equivalent sites and within an exponential distribution of energy levels are considered. In the absence of Coulombic wells, the calculations reproduce the well known features of Gaussian and highly dispersive transport respectively. When the field due to Coulombic potential wells is superimposed on the applied one, the macroscopic conduction features change dramatically. The computed apparent mobilities or transit times exhibit a Poole-Frenkel character and a modified Arrhenius temperature dependence. Their activation energy differs from the mean energy characterizing hops at the microscopic scale and it is found to depend on parameters such as the defect charge. This has important practical consequences for data interpretation.

In situ observation of the Yb 2+ emission in the radiodarkening process of Yb-doped optical preform

This Letter relates the clear evidence of Yb2+ formation under 2.5 MeV electron irradiation in optical fiber preforms showing a darkening of the core. We thus detected by in situ photoluminescence measurements the green emission of divalent Yb2+ under the 355 nm excitation. Moreover, we showed the existence of two types of Yb2+ ion species with different stabilities. We demonstrated that the radiodarkening mechanism is based on a pair association of Yb2+ with aluminum oxygen hole center point defects.

The incorporation site of Er in nanosized CaF2

The incorporation site of Er dopants inserted at high and low concentration (respectively 5 and 0.5 mol%) in nanoparticles of CaF2 is studied by x-ray absorption spectroscopy (XAS) at the Er L III edge. The experimental data are compared with the results of structural modeling based on density functional theory (DFT). DFT-based molecular dynamics is also used to simulate complete theoretical EXAFS spectra of the model structures. The result is that Er substitutes for Ca in the structure and in the low concentration case the dopant ions are isolated. At high concentration the rare earth ions cluster together binding Ca vacancies.

Equilibrium Degradation Levels in Irradiated and Pumped Erbium-Doped Optical Fibers

This paper investigates the local interplay between radiation-induced darkening and pump-induced bleaching in erbium-doped silica optical fibers. Thanks to proper characterization conditions using short fiber samples, we demonstrate that degradation levels can be tuned reversibly between equilibrium levels by varying the pump power and/or the dose rate while the dose is being accumulated. This novel finding inspires a local physical model of the radiation-induced degradation of erbium-doped fibers in active conditions. This model is shown to reproduce all empirical features, including dose rate and pump power dependencies. This validated local model could be implemented in long fiber amplifier simulations.