A Macrobending Singlemode Fiber Refractive Index Sensor for Low Refractive Index Liquids

An all-fiber refractive index sensor with a simple configuration is proposed and investigated. The proposed fiber refractometer consists of a full-loop structure of bare high-bend loss singlemode fiber-1060XP, employing the well-known whispering gallery mode (WGM) effect induced by the evanescent wave excitation to measure the refractive index of an external medium. A competitive sensitivity is achieved (average 725.76nm/RIU at a bend diameter of 19.3mm) compared with other fiber-optic sensors, plus benefits resulting from simplicity, low cost and ease of fabrication. media have RI values that lie in the range 1.33~1.40, for example, deionized water (1.333@589.3nm) and acetone (1.359@589.3nm). Such RI values are below the RI of fused silica (1.458@589.3nm), which is the primary material making up the fiber cladding and core, resulting in WGM effects. In this paper, we propose a fiber RI sensor for low RI liquids that operates on the principle of exciting the WGMs formed in the bent bare singlemode fiber, where the peak shift of wavelength at which the WGMs excitation occurs serves as a measure of the RI of the external medium. This involves a significantly different operating principle and measurement approach, by comparison to fiber RI sensors where the RI values under test are greater than those of the cladding.