Induced nanoscale changes with low temperature annealing inside composite optical fibres probed by strain-insensitive long period gratings

Abstract

Nanoscale glass milling with mild thermal annealing is proposed and shown to occur within a tri-material composite optical fibre. Evidence of glass relaxation with annealing is inferred directly through measured diameter changes in the core and inner cladding of the long period gratings (LPGs) (Δϕcore = (0.13 ± 0.05) μm; Δϕinner cladding = -(0.5 ± 0.2) μm) using scanning electron microscopy (SEM) before and after annealing at T ~300 °C for 1 hour. Large reductions in the magnitudes of both the temperature and strain coefficients are observed after annealing. The temperature sensitivity drops from dλ/dT = -(111.2 ± 2.4) pm/K to dλ/dT = (1.3 ± 0.3) pm/K and the strain sensitivity decreases from dλ/de = -(0.11 ± 0.13) pm/μe to dλ/de = (0.02 ± 0.11) pm/μe. The fabrication of LPGs using 193 nm radiation is shown to produce measurable increases in the core dimensions within tri-material composite fibres whereas no changes are observed under similar conditions for commercial bi-material single mode fibre.