▎ 摘　　要

Refractive index (RI) sensing is a promising technique in a variety of different scientific and industrial applications. In this paper, we theoretically and experimentally propose a newly-designed RI sensor. The proposed sensor consists of a particular long-period fiber grating inscribed in a two-mode fiber (LPFG-TMF) coated with tunable graphene oxide (GO), and the special structure allows an extremely high sensitivity in the transition region. The mode coupling between the LP11 core mode and the sixth-order cladding mode in the LPFG-TMF offers an apparently higher sensitivity feature. And the sensitivity is higher than that of a single-mode fiber-based LPFG (LPFG-SMF), which originates from the coupling between the core mode and the sixth-order cladding mode. Furthermore, the ultrasonic treatment time of GO, which impacts on the properties and performance of RI sensing, is also studied and investigated. Proof-of-concept results demonstrate significantly that comparing with the bare LPFG-TMF (621.66 nm/RIU) and LPFG-SMF coated with GO ultra-sonicated 5 hours (323.68nm/RIU), the LPFG-TMF coated with GO ultra-sonicated 5 hours performs optimized sensitivity of 11605.79 nm/RIU when surrounding RI ranges from 1.4558 to 1.4577. With its premium ability to operate high-sensitivity measurements, the RI sensing is expected to be useful for broad applications across the chemical fields.