Photoinduced and thermal reactions involving hydrogen in high-germania-core optical fibres

We report a Raman scattering study of photoinduced and thermal reactions between H₂ and germanosilicate optical fibres with 22 mol % and 97 mol % GeO₂ in the core (F1 and F2, respectively) after H₂ loading at 150 MPa (1500 atm). The mechanisms of photoreactions are investigated in a wide range of incident laser wavelengths (244, 333, 354, 361 and 514 nm). Thermal reactions are studied at 500 °C. The results indicate that the main mechanism behind the formation of hydrogen-containing defects with Raman bands at 700, 750, 2190, 3600 and 3680 cm^-1 involves ≡Ge – O – Ge≡ or ≡Ge – O – Si≡ bond breaking and formation of hydride and hydroxyl species: =GeH₂ (700, 750 cm^-1), ≡Ge – H (2190 cm^-1), ≡GeO – H (3600 cm^-1) and ≡SiO – H (3680 cm^-1). The key features of the reactions in the F1 and F2 fibres are analysed. In particular, photoinduced reactions give ≡Si – OH groups only in the F1 fibres, whereas the formation of germanium nanoclusters at a relatively low temperature (~500 °C) or ≡GeO – H and ≡Ge – H defects under 514-nm irradiation has only been observed in the F2 fibres.