Simultaneous measurement of refractive index and temperature using SMP in Sagnac loop

A novel sensor is proposed and demonstrated for simultaneous measurement of refractive index (RI) and temperature by splicing a 9.5 mm claddingless fiber to a 20.3 mm polarization-maintaining fiber in a Sagnac loop. Only two extinct dips are observable from 1400 nm to 1700 nm in the transmission spectrum of the sensor. One dip induced by Sagnac interference of the Sagnac loop is located near 1520 nm, while the other one near 1630 nm is the result of multimode interference of the single mode-multimode-polarization maintaining fiber (SMP) structure. Since the dip induced by Sagnac interference is immune to RI, temperature can be solely measured and the RI information can be extracted from the wavelength shift of the other dip with temperature effect compensated. Thanks to the high temperature-sensitivity of Sagnac loop and the high RI sensitivity of the SMP structure, a temperature-sensitivity of 1.929 nm/°C is achieved and an average RI-sensitivity reaches up to 235.3 nm/RIU.