Temperature sensing utilizing unclad plastic optical fiber with a balloon-like bent structure

Temperature-insensitive refractive index sensor based on a spindle-shaped few-mode fiber modal interferometer

A designed temperature-insensitive modal interferometer for refractive index measurement based on spindle-shaped few-mode fiber (FMF) is proposed and demonstrated. The interferometer consisting of a specific length of FMF fused between two specific lengths of single-mode fibers is bent into a balloon shape and then burned by a flame into a spindle shape to sensitize. Due to the bending of the fiber, part of the light leaks from the core to the cladding and excites the higher-order modes, and the four modes in the core of FMF interfere with the higher-order modes in the cladding. Therefore, the sensor is more sensitive to the surrounding refractive index. The experimental results show that the highest sensitivity is 237.3 nm/RIU from 1.333 to 1.365. The sensor is insensitive to temperature, which solves the problem of temperature cross talk. In addition, with its advantages of a small mechanism, simple fabrication, low loss, and good mechanical strength, the proposed sensor has broad application prospects in chemical production, fuel storage, environmental monitoring, and other fields.

Highly sensitive refractive index sensor based on plastic optical fiber balloon structure

A novel, to the best of our knowledge, design of plastic optical fiber (POF) balloon-based refractive index sensor for the detection of different concentrations of sodium chloride is proposed and experimentally investigated. The experimental characterization supports the finding that the transmission loss is sensitive to the external environment's targeted refractive index changes of the analyte. The proposed sensor achieves a maximum intensity-based sensitivity of 3105 RIU^-1, resolution of 3.22 ×10^-7, and the figure of merit (FOM) is 326 RIU^-1 from 2 to 2.5 Mol/L of the analyte with the chosen refractive index changes at 680 nm for a diameter D = 0.1 cm of the POF balloon structure. Furthermore, a high linear performance of 0.9896 is achieved with good robustness against the fabrication imperfection. The ultra-sensitiveness to the refractive index with a simple demonstration of the POF balloon-based structure has potential applications in the chemical, biological, and food safety sensing fields.