Refractive Index Detection of Liquid Analyte in Broad Range Using Multimode Fiber Speckle Sensor

The multimode fiber (MMF) speckle sensor applied to measure the refractive index (RI) of liquid analyte is investigated. The change of refractive index is detected by the changes of speckle patterns with image correlation technology. Tapered multimode fibers are immersed in liquids analytes with different refractive index, and different speckle patterns are formed at the output of the fibers. Using zero-mean normalized cross-correlation coefficient (ZNCC) to calculate of correlation degree between different speckles can obtain the relationship between the refractive index and the speckle pattern correlation, so as to achieve the detection of the refractive index. Our results exhibit a linear correlation over the large range of refractive index variations, i.e., from 1.3449 to 1.4323. The sensitivity of the speckle sensor is 2.56 RIU ^-1 .