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.

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

This review presents the state of the art of the study of germanium-related colour centres in silica glass and silica-based fibres. Most attention is concentrated on the dominant colour centres such as the Ge-related oxygen-deficient centres and paramagnetic Ge(n) centres, all of them having ultraviolet absorption bands. The hypothetical models and formation mechanisms proposed so far for these colour centres are discussed in detail. The origins of the models and formation mechanisms and their weak and strong points are analysed. The origin of the less well studied Ge-related colour centres in the visible spectral range (GeH, GeX, drawing-dependent defects) is also discussed.

https://iopscience.iop.org/article/10.1088/0953-8984/6/35/003/pdf

Colour centres in germanosilicate glass and optical fibres

Overview of radiation induced point defects in silica-based optical fibers

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, the origin of RIA at longer wavelengths (lambda > 1000 nm) is less 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.

Low-Dose Radiation-Induced Attenuation at InfraRed Wavelengths for P-Doped, Ge-Doped and Pure Silica-Core Optical Fibres

We briefly review some of the approaches which have been used to study the distributions of defect properties in amorphous silica and focus mainly on the implementation of the embedded cluster method. We illustrate some of the results obtained using this method and discuss the remaining problems using the example of oxygen vacancy defects in amorphous SiO2. The neutral vacancies are characterized by a wide distribution of formation energies and structural parameters. Our modelling predicts the two major structural kinds of positively charged vacancies (E ' centres): dimers and dangling bond centres. The local structure for both kinds of centres depends on the medium range structure of the surrounding amorphous network. We found that the majority of the dangling bond centres are unpuckered. The optical spectra and electron paramagnetic resonance parameters calculated for all defects are in good agreement with experimental data. The structural criteria which favour the formation of different kinds of centres in the original amorphous structure are formulated in terms of the average Si–O distance of oxygen ion with its two neighbouring silicon ions.

https://iopscience.iop.org/article/10.1088/0953-8984/17/21/007/pdf

Structure and properties of defects in amorphous silica: new insights from embedded cluster calculations

A quantitative simulation in the cluster approximation is made of an autolocalized hole in the amorphous SiO2 network. A model is suggested in which the thermal excitation of holes from Anderson localized states of the top of the silica valence band above the mobility edge corresponds to the thermally stimulated luminescence peak at 125 K, and the analogous optical transition — the radiation-induced low-temperature IR-absorption. The position and form both, of the peak and absorption band and their thermo- and photostability are explained.



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Spectroscopic Manifestations of Self-Trapped Holes in Silica Theory and Experiment

A study was made of the influence of laser light on radiation color centers in glass fiber waveguides of the SiO2–SiO2:B2O3 type. The induced-absorption spectra were determined in the wavelength range 300–1900 nm for fibers subjected to γ-ray doses from 300 to 104 rad at irradiation temperatures 77 and 300 K. Eight types of radiation color centers were identified and the induced-absorption spectra of the waveguides were almost entirely due to these color centers. The centers differed in respect of the stability when subjected to laser light and temperature variations. Infrared absorption appeared when the induced-absorption band at 340 nm was suppressed by λ = 337 nm laser light and then restored at room temperature.

V. N. Karpechev

Papers

Spectroscopic Manifestations of Self-Trapped Holes in Silica Theory and Experiment

Influence of laser light on induced-absorption spectra of fiber waveguides made of pure quartz glass