Fiber optic sensors based on circular and elliptical polymer optical fiber for measuring refractive index of liquids

Abstract

Measurement of refractive index (RI) is required in laboratory and industry for many applications, especially that of liquids. Though there have been commercial products for precision RI measurement, miniaturized and low-cost solutions are still desired and thus have attracted a great number of researchers’ attention. Among various competitive technologies, fiber optic sensors based on macro-bending loss (e.g. U-shaped fibers) exhibit advantages including facile fabrication and low cost. In order to make this kind of sensors more reliable for real applications, this study proposes to bend a section of pristine polymer optical fibers around circular or elliptical solid slices to form the sensor tip for sensing RI of liquids. Such configuration can precisely and firmly fix the geometry of the bent fiber, while most bent fibers in U-shaped sensors previously reported are free-standing and their shapes are prone to change in long-term usage. Impacts of bending shape and radius on sensors’ sensitivity to RI are investigated. Among all fabricated sensors, the highest sensitivity is −66.58 dB/RIU in the RI range of 1.33–1.46, which is yielded by the sensor consisted of POF in elliptical shape in diameters of 5 and 3 mm. It is lower than those of U-shaped silica fibers bent in smaller diameters or those of cladding-processed U-shaped sensors. However, the proposed sensor has advantages in precise geometry control and facile fabrication, making it competitive to other RI sensing techniques for those cost-sensitive applications. Besides, the geometry of the fiber can be optimized to further enhance sensors’ sensitivity in future.