Liquid refractometric sensors based on optical fiber resonators

A robust, easy-to-fabricate, and sensitive liquid refractometric sensor utilizing optical fiber resonators (OFRs) obtained by simple stripping and cleaning of conventional optical fibers is presented. The sensing scheme is based on recording the spectral changes of the whispering gallery modes (WGMs) observed in the transmission spectrum of an OFR excited with a tunable laser coupled to the OFR through an independent tapered optical fiber. The demonstrated sensor device is tested and fully characterized with water solutions of ethanol and ethylene glycol (EG). Good agreements are obtained with theoretical predictions for both ethanol and EG cases when TE and TM polarized WGMs are considered. The limit of detection of the demonstrated sensor is determined to be between 2.7 and 4.7 × 10−5 refractive index unit (RIU) comparable to the state-of-the-art when precise temperature control of the sample chamber is missing.