A simple fiber tip sensor for refractive index (RI) measurement based on Fabry–Perot (FP) interference modulated by Fresnel reflection is proposed and demonstrated. The sensor head consists of an etching-induced micro air gap near the tip of a single-mode fiber. The microgap and the fiber tip function as two reflectors to form a FP cavity. The external RI can be unambiguously measured by monitoring the fringe contrast of the interference pattern from the reflection spectrum. The experimental results show that the proposed sensor achieves temperature-independent RI measurement with good linear response. The proposed sensor achieves a high RI resolution of up to 3.4 \times 10 5 and has advantages of low cost and easy fabrication.

A novel all-fiber temperature-calibrated refractometer based on a compact fiber Bragg grating (FBG) single–multi–single (SMS) structure is proposed and experimentally demonstrated. The sensor head is composed of a FBG combined with a SMS structure, in which the middle multimode fiber (MMF) section is etched by a time-controlled hydrofluoric. The transmission dip of SMS is extremely sensitive to ambient refractive index (RI) variation, whereas the upstream FBG provides the necessary temperature information for RI calibration. All aforementioned functions are performed via a compact FBG-SMS structure not longer than 25 mm. The proposed sensing device provides a linear RI sensitivity over water or waterbased solutions (RI values near 1.33 at optical wavelengths for most biological and many environmental applications), and has temperature-calibration capability. Hence, the said refractometer is a good candidate for sensing in chemical and biological applications.