Refractometer based on resonance investigation of capillary cladding modes

A simple refractometer is demonstrated using a fused silica capillary spliced between two single-mode fibers. From a fundamental mode of a lead-in single-mode fiber, a number of cladding modes in the short piece of the capillary are excited. In the transmission spectrum, it could be as a formation of the resonance peaks observed. The experimental results of three different capillary diameter values are described. We presented the sensitivity comparison of all three cases for the liquid analyte refractive index (na) variations in the range from na = 1.33 to na = 1.43. The sensitivity improvement in the range from na = 1.3 to na = 1.35–195.4 nm/RIU is also presented. The proposed refractometer can be further developed as a simple and inexpensive solution for biosensor applications.