Reflective Fiber-Optic Refractometer Based on a Compact Hi-Bi Fiber Tip

A novel in-fiber refractometer based on a compact Hi-Bi fiber tip working on reflection is proposed and experimentally demonstrated. The fiber sensing head is composed of a short section of PANDA-type Hi-Bi fiber with a length of 0.15 m. The fringe contrast of the interferometer is extremely sensitive to ambient refractive index (RI) variation when the Hi-Bi fiber tip is immersed in water–glycerin solutions with different mass concentration for different RIs. The experimental results show that the proposed sensing device provides a high RI sensitivity of ${-}{\rm 35.34}~{\rm dB/RI}$ unit for resolutions with RI values near 1.33, making it a good candidate for sensing in chemical and biological applications.