Optical Fibre Refractive Index Sensor in a Hybrid Fibre Grating Configuration

A temperature compensated Refractive Index (RI) sensor incorporating a Long Period Grating (LPG) and a Fibre Bragg Grating (FBG) in a series configuration has been developed and evaluated. The LPG was used as a RI sensor allowing it to respond to external RI variations while the FBG was used for temperature compensation. The LPG and FBG wavelengths were chosen to be within suitable spectral bandwidths for efficient interpretation of both RI and temperature. The performance of the hybrid sensor was evaluated for use as a chemical sensor by varying sodium chloride (NaCl) concentrations in water from 0.25 – 2%NaCl and temperature from 10-95 oC and by recording its corresponding attenuation band shifts. The sensitivity of the hybrid sensor to NaCl concentration and temperature is calculated to be 0.61 nm/%NaCl and 8 pm/oC respectively.