Refractive index (RI) represents a fundamental optical parameter extensively utilized in chemical concentration detection, biomedical sensing, and environmental monitoring. Optical fiber RI sensors have garnered significant attention due to their compact structure, high sensitivity, and robust immunity to electromagnetic interference. Various optical fiber RI sensors have been developed, including straight-core, S-shaped, U-shaped, and D-shaped structures. U-shaped fiber sensors have emerged as a prominent research focus due to their enhanced evanescent field interaction, high RI sensitivity, compact structure, straightforward fabrication process, and suitability for localized sensing applications. This study proposes a reflective U-shaped micro-nano optical fiber RI sensor designed to enhance RI sensitivity and spectral performance. The sensor combines a tapered thin-core fiber structure with a U-bending geometry to amplify the evanescent field effect, while incorporating a reflective configuration to establish a dual-path interference mechanism. This design enhances the interaction between guided light and the surrounding medium while improving the spectral extinction ratio, thereby achieving superior RI sensitivity.

The beam propagation method was utilized to simulate optical field energy distribution under different bending diameters and taper waist diameters. A section of thin-core fiber (TCF) with a core diameter of 2.1 μm and a cladding diameter of 125 μm was first fusion-spliced to a single-mode fiber (SMF). The TCF underwent tapering using a hydrogen-oxygen flame tapering system. During the process, the hydrogen flow rate, stepper motor speed, and pulling length of the tapering machine were precisely controlled to achieve accurate regulation of the waist diameter. Subsequently, the micro-nano fiber was bent into a U-shape using a 3D-printed V-groove clamp, enabling precise control of the bending diameter. To examine the influence of structural parameters on sensing performance, two experimental configurations were established. 1) The waist diameter was fixed at 8.41 μm, while the bending diameter varied among 2 mm, 3 mm, and 4 mm. 2) The bending diameter was fixed at 2 mm while the waist diameter varied among 11.51 μm, 7.83 μm, and 5.58 μm. In RI sensing, variations in the surrounding RI modify the phase difference between the core and cladding modes, resulting in shifts in the resonance wavelength of the interference spectrum. A supercontinuum broadband light source (KG-ASE-D-1-1-FA, 1200?1600 nm) was coupled into the sensor via an optical circulator [(1550±50) nm]. The light reflected from the cleaved end face of the fiber re-entered the sensing region and was directed to an optical spectrum analyzer (OSA, Yokogawa AQ6370B, 600?1700 nm), where the reflected spectrum was monitored in real time. The RI sensitivities corresponding to different structural parameters were obtained by tracking the positions of the resonance dips.

Simulation results demonstrate that reducing both the taper waist diameter and the bending diameter substantially enhances the leakage intensity of the evanescent field, thereby strengthening the interaction between the guided mode and the surrounding medium. This theoretical prediction aligns closely with experimental observations. Under a fixed waist diameter of 8.41 μm, decreasing the bending diameter from 4 mm to 2 mm enhances the RI sensitivity from 1894.66 nm/RIU to 2007.79 nm/RIU (Fig. 6). Similarly, with a fixed bending diameter of 2 mm, reducing the waist diameter from 11.51 μm to 5.58 μm results in a significant increase in sensitivity from 1376.22 nm/RIU to 3511.33 nm/RIU (Fig. 7). Within the RI range of 1.3330 to 1.3519, the resonance wavelength exhibits a red shift with increasing RI, with a maximum shift of 67.86 nm. The linear fitting correlation coefficient R2 consistently exceeds 0.99, indicating excellent linear response characteristics. Compared to traditional transmission-type tapered micro-nano fiber sensors, the proposed reflective U-shaped structure improves the extinction ratio by approximately sevenfold, thereby significantly enhancing the interference spectral contrast. Fast Fourier transform (FFT) analysis confirms that the interference spectrum is predominantly governed by low-order modes, ensuring high spectral stability. Moreover, a one-hour stability test in deionized water reveals a maximum wavelength drift of only 0.22 nm, demonstrating excellent environmental stability (Fig. 8). Based on the spectral resolution of the optical spectrum analyzer (0.02 nm), the minimum detectable RI variation is estimated to be approximately 5.7×10?? RIU. These results collectively demonstrate that the sensor exhibits outstanding performance in terms of repeatability, linearity, and sensitivity. Overall, the integration of a U-shaped geometry with a reflection-based interference mechanism enables high-performance RI sensing while maintaining structural simplicity and fabrication feasibility.

This study presents and experimentally validates a reflective U-shaped micro-nano optical fiber RI sensor, integrating the advantages of U-shaped geometry and tapered micro-nano fiber. A systematic investigation examined the influence of bending diameter and waist diameter on sensing capabilities. The U-shaped configuration demonstrates enhanced evanescent field interaction with surrounding media compared to straight micro-nano fiber structures, making it particularly effective for contact sensing of biomolecules and chemical solutions. Additionally, the reflective configuration substantially improves the spectral extinction ratio. These combined features enhance sensitivity, spectral stability, and signal quality. Experimental findings demonstrate that RI sensitivity inversely correlates with both bending diameter and waist diameter. The resonance wavelength exhibits a red shift with increasing RI within the range of 1.330 to 1.3519, maintaining excellent linearity. The sensor achieves a maximum sensitivity of 3511.33 nm/RIU with robust spectral stability. Future research directions include sensitivity enhancement through surface plasmon resonance effects and integration of emerging two-dimensional materials characterized by strong light-matter interaction, large specific surface area, and excellent biocompatibility. These advancements aim to broaden the sensor’s applications in biomedical diagnostics, chemical detection, environmental monitoring, and food safety.