All-Fiber Modal Interferometer Based on a Joint-Taper-Joint Fiber Structure for Refractive Index Sensing With High Sensitivity

We propose and demonstrate an all-fiber modal interferometer (MI) formed by sandwiching a thinned fiber taper (TFT) between two core-offset joints (COJs) for refractive index sensing. The COJs are fabricated using the splicing method on a standard single-mode fiber and their core-offset distances are over 10 $\mu{\rm m}$ . The TFT is made by the flame-heated drawing technology, and has a very thin waist with only several microns in diameter for which satisfies the core-mode cutoff condition. The all-fiber MI device with a TFT waist diameter of 6.8 $\mu{\rm m}$ has extremely high sensitivities because of the strong evanescent field interaction, for example, 5339 and 10377 nm/refractive index unit at the surrounding refractive indexes of 1.334 and 1.35, respectively. In addition, the device has the advantages of compact size, simple fabrication, and low cost.