Optical fiber sensor for simultaneous measurement of refractive index and temperature

An optical fiber sensor based on long period gratings (LPGs) was proposed for simultaneous measurement of refractive index (RI) and temperature. The sensor consists of two LPG sections exciting different order cladding modes and having different cladding diameters. The principle of simultaneous measurement was theoretically analyzed. The two LPGs have different RI sensitivities but similar temperature sensitivities, which construct the sensitivity matrix. The variation of RI and temperature can be deduced by the sensitivity matrix. Two different grating-periods were used to enable the LPGs exciting different order cladding modes and a chemically wet etching was processed on one LPG section to decrease the cladding diameter. The sensor was measured in liquid with different RI. Experimental results show that the LPG based sensor can discriminate between RI and temperature and has a good performance in terms of sensitivity and linearity.