Enhanced refractive index sensing characteristics of optical fibre long period grating coated with titanium dioxide thin films

Abstract

A new type of fibre-optic refractive index sensor based on a long period fibre grating (LPFG) coated with a titanium dioxide (TiO2) thin film was demonstrated.The wavelength shift of the attenuation bands of this LPFG sensor to changes in the refractive index of the external media from 1.30 to 1.64RIU was investigated. In order to optimize the sensor the TiO2 thin film thickness deposited around the LPFGs was varied from 10 to 80nm.It was found that the TiO2 thin film increases the wavelength sensitivity of the LPFG to changes in the surrounding refractive index for values lower and higher than the cladding refractive index. As opposed to the bare LPFG it was shown the possibility to monitor refractive indices lower and higher than cladding refractive index tailoring the TiO2 thickness.An average wavelength sensitivity of 5250nm/RIU was achieved in the range 1.444 to 1.456RIU for a TiO2 thickness of 50nm. In the region between 1.46 and 1.48RIU the average sensitivity of about 825nm/RIU was measured for a 40nm thick film.