Nanoscale changes with low temperature annealing inside composite optical fibres

Nanoscale glass milling with mild thermal annealing is proposed and shown to occur within a tri-material composite “W” profile 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 = -8.75×10 pm/K and the strain sensitivity decreases from dλ/de = -(0.11 ± 0.13) pm/μe to dλ/de = 2.21×10 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.