In this paper, a modified microfiber-assisted open- cavity (MMA-OC) structure is proposed by introducing two pieces of multimode fiber and a novel sub-millimeter in-line Mach-Zehnder interferometer is theoretically and experimentally completed. The cavity-length dependence of MMA-OC on axial strain sensing is characterized and the comprehensive strain tests are conducted under an optimal waist diameter. The experimental results show that the selective intensity-/wavelength- modulated strain response can be achieved by varying the cavity length. For a shorter cavity length, MMA-OC presents more than 20-dB intensity variation and near-zero wavelength drift. But an apparent and flat red-shift of fringes occurs in the structure with >1000-μm cavity length. Moreover, due to inverse feature of temperature response, the calculated cross-sensitivity is merely 0.106 μϵ/°C. And the detection resolution up to ~0.2 μϵ is gained when the cavity length is equal to 503 μm, with the measurement error of ±0.021 μϵ. Our sensor with the merits of compactness, low-cost and ease of fabrication is very promising for high-precision engineering monitoring and sensing.

In this paper, a thin-core fiber-based in-line Mach–Zehnder interferometer is theoretically and experimentally demonstrated, and a high sensitivity consistency of temperature (>0.995) is obtained owing to the formed single modal interference. Furthermore, the nonlinear wavelength-dependence of elastic-optical coefficient is characterized, and a sharp intensity inversion between the macro- and micro-bending states is experimentally observed. Then, a novel dual-differential compensation (DDC) method is proposed to eliminate the temperature errors in the measured curvature and strain. By means of DDC, the variations of curvature and strain can be simultaneously detected and discriminated without the crosstalk of ambient temperature, and the corrected sensitivities, respectively, reach −17.67 nm/m ⁻¹ and −1.92 pm/ $\mu \varepsilon $ . This fabricated sensor is very practical and promising for the accurate measurement of multiple parameters.

To enhance humidity sensitivity, a parallel Fabry–Perot interferometer (FPI)-based Vernier scheme is theoretically and experimentally developed via the parallel and separated micro-cavities of polyvinyl alcohol (PVA) and hollow core fiber (HCF). Through Au-film assistance and fine attenuation adjustment, the matched light energy of micro-cavities is obtained and high-contrast envelope with ~11-fold improvement is demonstrated within the 200-nm range. Centering the $\sim 77~\mu \text{m}$ length Au-PVA micro-cavity, the comprehensive tests in terms of humidity are conducted. The experimental results show that, owing to the magnification of Vernier effect, ultra-sensitive humidity response of −9.14 nm/%RH is gained. And the crosstalk of temperature is constrained within −0.0843%RH/°C. In addition, 0.505 s dynamic time-response is verified through field tests of breathing. With the merits of stability, compact size, and ease of operation, our scheme is very practical and has potential in the applications of humidity-related biochemical sensing and medical health monitoring.

In this paper, we report a novel in-fiber Mach-Zehnder interferometer based directional torsion sensor, in which a section of two-mode fiber is sandwiched between two single mode fibers by over core-offset splicing technique. The variety of fringe visibility demonstrates the strong dependence on offset and fiber length. For the first time the near zero visibility at 0° rotating state is obtained by fine offset-modulation. The experimental results show that, with 0° turning point, the counter-clockwise to the clockwise direction can be recognized by the reversal from peak to dip of fringes. Moreover, a competitive sensitivity of 21.485 dB/(rad/cm) is gained with high linearity and low-temperature crosstalk in the range from −40 rad/m to 40 rad/m. Without any pre-twisting, our fiber torsion sensor is small size, ease of fabrication, cost efficiency and very potential in the applications of industrial and artificial intelligence.

A novel intensity demodulated refractive index (RI) sensor is theoretically and experimentally demonstrated based on the front-tapered single-mode-multimode-single-mode (FT-SMS) fiber structure. The front taper is fabricated in a section of multimode fiber by flame-heated drawing technique. The intensity feature in the taper area is analyzed through the beam propagation method and the comprehensive tests are then conducted in terms of RI and temperature. The experimental results show that, in FT-SMS, the relative sensitivity is −342.815 dB/RIU in the range of 1.33~1.37. The corresponding resolution reaches 2.92 × 10⁻⁵ RIU, which is more than four times higher than that in wavelength demodulation. The temperature sensitivity is 0.307 dB/°C and the measurement error from cross-sensitivity is less than 2 × 10⁻⁴. In addition, fabricated RI sensor presents high stability in terms of wavelength (±0.045 nm) and intensity (±0.386 dB) within 2 h of continuous operation.