Optical fiber waveguide refractive index (RI) sensors have potential applications in environmental monitoring and biochemical sensing due to their low cost, small size, easy operation and integration, high sensitivity and stability, anti-electromagnetic interference, and corrosion resistance. At present, different optical fiber waveguide RI sensors have been proposed, among which U-shaped optical fiber waveguide has become a research hotspot in optical fiber waveguide sensing because of its compact structure, simple preparation, and high sensitivity. On one hand, the preparation parameters of optical fiber waveguide structures are designed and optimized, and on the other hand, the sensitivity of the sensing unit to the external environment is improved by combining new two-dimensional materials to further improve the sensitivity of U-shaped optical fiber waveguide sensors. However, precious metal materials have high losses and preparation costs, with limited response wavelength of two-dimensional materials. Realizing optical fiber waveguide RI sensors featuring low cost, easy operation, and high stability and sensitivity still poses significant challenges. Thus, we propose and implement an interference enhanced hybrid optical waveguide (IE-HOW). The sensor based on IE-HOW is characterized by high sensitivity, low loss, compact structure, and good stability, and it has broad application prospects in biochemical detection, clinical diagnosis, and other fields.

The beam propagation method is employed to simulate the energy distribution of the optical field before and after the cascade. To prepare IE-HOW, we first fuse a micro-bottle cavity (MBC) on the surface of single mode fiber (SMF) by arc discharge, and then place the SMF containing MBC on the hydrogen-oxygen flame tapering machine for melting tapering. During the manufacturing, precise control of the discharge frequency and propulsion amount of the fusion welding machine controls the MBC size, and parameters such as hydrogen flow rate, step speed, and stretching length of the hydrogen oxygen flame cone pulling machine are controlled precisely. Finally, the bending diameter is controlled reasonably to bend the tapered hybrid optical waveguide into a U-shaped one, and the length and diameter of the MBC are characterized by an optical microscope. The MBC size and bending diameter are kept unchanged, and the stretching lengths are changed (h=16000 μm, h=18000 μm, and h=20000 μm) to prepare ordinary non-interference enhanced micro-nano U-shaped optical waveguide (UOW) and IE-HOW structures with different cone diameters respectively. Meanwhile, RI sensing tests are performed on ordinary UOW without cascaded structures and IE-HOW with cascaded structures to compare their sensitivity. For RI sensing, the change of external environment RI will alter the phase difference between the core mode and the cladding mode, leading to accordingly changed spectrum wavelength. The broadband light source (BBS, from 1250 to 1650 nm) is connected to the IE-HOW input end, and the output transmission spectrum of the output section is recorded in real time by an optical spectral analyzer (OSA, AQ6370D).

Solutions with different RI are prepared by NaCl solution and calibrated with Abbe refractometer, with an RI range of 1.333-1.362. In UOW-based RI sensing measurements, the maximum sensitivities of RI sensor tests with cone diameter d₁=12.30 μm, d₂=7.44 μm, and d₃=4.88 μm are 640.06 nm/RIU (1507.5 nm), 913.25 nm/RIU (1487.10 nm), and 2750.32 nm/RIU (1412.70 nm) respectively, with the linearity R² >0.9 (Fig. 5). In IE-HOW-based RI sensing measurements, the maximum sensitivities of RI sensor tests with cone diameter d₁=12.30 μm, d₂=7.44 μm, and d₃=4.88 μm are 905.21 nm/RIU (1428.10 nm), 2587.22 nm/RIU (1432.50 nm), and 8813.26 nm/RIU (1406.50 nm) respectively, with the linearity R²> 0.9 (Fig. 6). The smaller cone diameter leads to greater RI sensitivity. When the cone diameter of the micro-nano U-shaped fiber is 4.88 μm, the RI sensitivity of IE-HOW is three times higher than that of UOW (Fig. 7). Temperature and stability tests are carried out on IE-HOW with a cone diameter of 4.88 μm respectively. The sensing unit is placed in a temperature control box, and spectral data are recorded at intervals of 10 °C and stabilized for 10 min within the range of 20-80 °C. The wavelength variation within the range of 60 °C is less than 0.02 nm (Fig. 8). The spectral data are recorded every 10 min at room temperature, and the maximum wavelength change within 1 h is less than 0.02 nm (Fig. 8). Therefore, the high-sensitivity RI sensor based on IE-HOW has high-temperature stability.

We propose and implement a highly sensitive RI sensor based on IE-HOW. By cascading UOW and MBC to construct IE-HOW, interference enhancement and low loss are achieved based on the strong focusing effect of MBC and large bending diameter respectively. Compared to UOW, under the same cone diameter of 4.88 μm, the RI sensitivity is increased by 3 times, reaching 8813.26 nm/RIU, and R²> 0.9. This sensor has advantages such as high sensitivity, low loss, all fiber, low cost, and good stability, with practical significance in biosensing, environmental monitoring, and other fields that require high RI sensitivity.