Photo- and radio-darkening of ytterbium-doped silica optical fibres – PARADYSIO

Under irradiation, silica-based optical fibres suffer from a considerable excess optical loss that develops across ultra-violet, visible and infra-red spectral ranges. This damage encompasses two major effects. The first is specific to rare-earth-doped fibres and is due to the absorption of the pump photons. Known as photo-darkening, it has become critical in high-output power (HOP) fibre lasers and amplifiers. The second degradation effect is related to fibres operated in harsh environments, due to external ionizing radiations. We refer to this damage as ‘radio-darkening’. Darkening of silica-based fibres affect a variety of fields involving harsh environments or/and HOP fibre-based lasers: fibre sensors (for monitoring purposes in nuclear plants or deep repository of radioactive waste), lasers considered for space-based (optical inter-satellite links, OISL, and LIDAR), industrial (laser marking, cutting, welding…) or medical (myringotomy, surgery…) applications. HOP fibre lasers are considered a critical technology offering the great advantages of reduced weight, size, power consumption, cost, and greater efficiency compared with solid-state lasers. Though, darkening effects strongly limit their technological development. OISL and space-based LIDAR are the most challenging applications because they combine a HOP laser source with the radiative space environment (protons, electrons), thus raising the question of the coexistence of the photo- and radio-darkening (photo-radio-darkening, PRD). In addition, space radiations are characterized by very low dose rates (10-4¬-10-2 Gy h-1) acting over 10-15 years. The radiation resistance of components must therefore be assessed by accelerated tests, conducted at much higher dose rate, which should produce equivalent effects as a typical space mission. PRD and the design of relevant ground test protocols for silica fibres are two crucial problems that have never been explored. In this project, we propose to tackle both problems. Basic objectives consist in determining proper accelerated test conditions and in developing PRD-resistant fibres. We focus on ytterbium-doped fibres (YDF) because YDF are the basic option in the design of HOP fibre lasers. All results gained from the understanding of the darkening mechanisms and mitigation will have consequences on the other medical and industrial applications of HOP fibre lasers. Researches concerned with radiation damage in optical fibres most often use commercial fibres. Even if the fibre composition is known, the impossibility of making quick attempts on new designs and doping strongly limits the development of radiation-hardened fibres. The team of the PARADYSIO project offers a unique combination of academic expertises in fibre fabrication, irradiation, characterization and modelling. The research program is based on Yb-doped aluminosilicate fibres with systematic variations in dopant concentrations. First, the basic mechanisms responsible for PRD will be investigated through pumping and electron or proton irradiations. Equivalent irradiations by soft x rays will be also established for routine measurements. Radiation-induced centres will be characterized by coupling radiation-induced attenuation, thermally stimulated luminescence (TSL) and electron paramagnetic resonance (EPR) measurements, whereas PRD kinetics will be pointed out by pump-probe techniques. Then, a theoretical model will be developed to simulate PRD and find space-equivalent conditions for accelerated irradiations. A fibre laser source, based on a master oscillator power amplifier scheme will be finally built to test the PRD-resistance of fibres according to the proposed protocol in CW and pulsed operations. Simulations, fibre fabrications and accelerated tests will run according to a predictive-corrective feedback loop that is expected to converge towards PRD-resistant fibre compositions.