Multi-point fiber-optic refractive index sensor by using coreless fibers

•The multi-point fiber-optic refractive index sensor has high RI sensitivity and the wavelength shift of the resonance peaks as a linear function of RI.•Compared with other aforementioned fiber refractometers, the fabrication of this multi-point fiber optic interferometer is simple and cost-effective, this multi-point sensor also can applicable to on-line detection in any environment containing multiple targets as well as for some measurement processes that required self-inspection.

We present a novel multi-point fiber-optic refractive index (RI) sensor based on two different length coreless fibers spliced between single mode fibers (SMFs). The sensing probe operated based on multimode interference. A multi-point interferometer with 25 mm and 30 mm coreless fiber is fabricated and the measurement of RI is realized by measuring the wavelength shift of resonance dips in the transmission spectrum of the multi-point interferometer. Experimental characterization for a multi-point refractometer is presented. In the RI range of 1.3288–1.3666, the corresponding RI sensitivities are 148.60 nm/RIU and 119.27 nm/RIU for each point, respectively. We demonstrate that this multi-point fiber optic interferometer can be used as a simple transducer for RI sensing with comparable sensitivity.