Effect of Dopant Diffusion on the Long-Term Stability of Fabry-Pérot Optical Fibre Sensors

We present a research to determine the long -term
stability of intrinsic Fabry-Pérot (F-P) optical fibre sensors in
high temperature environments. In-fibre sensors were create d in
125μm diameter single mode fibre (Corning SMF28 Ultra) and a
125µm diameter PCF ESM-12B pure SiO2 fibre spliced to a
SMF28 fibre with a low reflectivity Cr layer at the interface. The
outcome is a low finesse optical cavity either formed by a short
length of Ge doped SMF fibre, or a short length of pure and
undoped SiO2 core PCF fibre. We demonstrate the
manufacturing technique required for these intrinsic FP sensors
as well as the stability of their optical characteristics at
temperatures up to the range of 850°C to 1050°C. We re port on
the effect of annealing on stability after exposing sensors to
temperatures of 1000°C above nominal working temperatures. In
the temperature range above 900°C we observe increasing le vels
of non-reproducible drift characteristics. Stability is
demonstrated up to 1000°C. After extended exposure of s e nsors
to high temperatures we observe deviations from the initial
smooth second order response of phase versus temperature which
has been attributed to a change in core diameter in the fibre
leading to the sensor at the distal end due to Ge diffusion at the
high temperatures. The down lead is exposed to over a le ngth of
17cm. The dopant diffusion of an SMF28 ultra fibre has been
studied using Energy Dispersive X-rays analysis (SEM/EDX), to
measure the radial distribution of Ge concentration before and
after being heated for a period of 100 days.