Temperature-insensitive optical fiber sensor based on SMF-NCF-FMF-NCF-SMF spindle-shaped structure for refractive index measurement

A spindle-shaped sensor with temperature insensitivity based on single-mode fiber (SMF) - no-core fiber (NCF) - few-mode fiber (FMF) - NCF - SMF (SNFNS) for refractive index is proposed. An interferometer with SNFNS structure can be formed by fusing, and then the fiber can be shrunk to form a spindle shape through burning, which is known as an SNFNS spindle-shaped sensor. The introduction of NCF and curvature causes partial light transmitted in the core to enter the cladding, which results in the emergence of phase differences. The interference dips formed by modal interference have different sensitivities to the outside refractive index. The experimental results show that the maximum refractive index sensitivity of sensors is 258.5 nm/RIU in the range of 1.333–1.365. And the sensors have the characteristic of temperature insensitivity in the range of 10–70℃. Accordingly, the proposed sensors are ideal for applications in biomedicine, environment monitoring and other fields.