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.

A dual broadband terahertz bifunction absorber that can be actively tuned is proposed. The optical properties of the absorber were simulated and numerically calculated using the finite-difference time-domain (FDTD) method. The results show that when the conductivity of vanadium dioxide is less than ${\sigma }_0=8.5\times {10}^3$ S/m, the absorptance can be continuously adjusted between 2% and 100%. At vanadium dioxide conductivity greater than ${\sigma }_0=8.5\times {10}^3$ S/m, the absorption bandwidth of the absorber can be switched from 3.4 THz and 3.06 THz to 2.83 THz and none, respectively, and the absorptance remains above 90%. This achieves perfect modulation of the absorptance and absorption bandwidth. The physical mechanism of dual-broadband absorptions and perfect absorption is elucidated by impedance matching theory and electric field distribution. In addition, it also has the advantage of being polarization insensitive and maintaining stable absorption at wide angles of oblique incidence. The absorber may have applications in emerging fields such as modulators, stealth and light-guided optical switches.

In this paper, a novel in-line modal interferometer for refractive index (RI) sensing is proposed and experimentally fabricated by cascading single-taper and multimode-double- cladding-multimode (MDM) fiber structure. Owing to evanescent field in taper area, the ultra-sensitive and linear intensity-responses to the varied surrounding RI are gained in both single- and double-pass structures. Moreover, the crosstalk from temperature can be effectively discriminated and compensated by means of the RI-free nature of MDM. The experimental results show that the RI sensitivities in single- and double-pass structures, respectively, reach 516.02 and 965.46 dB/RIU (RIU: refractive index unit), both with the slight wavelength shift (~0.2 nm). The temperature responses with respect to wavelength and intensity are 68.9 pm°C⁻¹/0.103 dB°C⁻¹ (single-pass structure) and 103 pm°C⁻¹/0.082 dB·°C⁻¹ (double-pass structure). So the calculated cross-sensitivity of intensity is constrained within 8.49 × 10⁻⁵ RIU/°C. In addition, our sensor presents high measurement-stability (~0.99) and low repeatability error (<4.8‰). On account of the ~620 μm size of taper, this compact sensor is cost-efficient, easy to fabricate, and very promising for the applications of biochemistry and biomedicine.

An all-fiber target-type flow velocity sensor was proposed and experimentally demonstrated based on the Michelson interferometer (MI). Commercially available single mode fiber (SMF) was used as the basic material and assisted with the arc discharge technology the sensing structure can be fabricated with high efficiency and quality. The designed sensor structure contains a peanut-shape structure that acts as a coupler/splitter, a SMF that acts as a cantilever beam, and a spherical tip that acts as a target, which all can be realized by arc discharge technology. The simulation verifies the effectiveness and high sensitivity of the peanut-shape structure for flow velocity measurement, and the key parameters were optimized. A comprehensive experiment was carried out on the flow velocity experiment. The experimental results show that the sensor has high sensitivity of -1.30 nm/(cm/s), precision of 1.4% and detection limit (DL) of 0.015 cm/s in the range of 0-11 cm/s. In addition, the temperature sensitivity is 58.5 pm/°C and the flow velocity-temperature cross-sensitivity is less than 0.05 (cm/s)/°C. Such a high-sensitivity, low cross-sensitivity and low-cost sensor, is very promising for the applications on high-precision liquid and abysmal sea flow velocity measurement.

A high sensitivity optical fiber gas pressure sensor based on paralleled Fabry–Pérot interferometers (FPIs) was demonstrated. One micro-cavity FPI is used as a reference FPI (FPI-1) to generate a Vernier effect and the other FPI (FPI-2) is used as a sensing tip. Both FPIs are connected by a 3-dB coupler to form a paralleled structure. The FPI-1 was fabricated by fusion splicing a piece of hollow core fiber (HCF) between two sections of single-mode fibers (SMF), whereas FPI-2 was formed by fusion splicing a section of HCF between SMF and a piece of HCF with a slightly smaller inner diameter for sensing pressure. The gas pressure sensitivity was amplified from 4 nm/MPa of single FPI to 45.76 nm/MPa of paralleled FPIs with an amplification factor of 11.44 and a linearity of 99.9%. Compared with the traditional fiber gas pressure sensors, the proposed sensor showed great advantages in sensitivity, mechanical strength, cost, and temperature influence resistant, which has potential in adverse-circumstance gas pressure sensing.

In this paper, to enhance practicality, a novel tapered thin-core fiber (t-TCF) based modal interferometer is proposed and demonstrated experimentally. The light field distribution of t-TCF structure is investigated by a beam propagation method, and the quantitative relationship is gained between light intensity loss and waist diameter. Under ~30 μm waist diameter, multiple t-TCF based sensor heads are fabricated by arc-discharged splicing and taper techniques, and comprehensive tests are performed with respects to axial strain and temperature. The experimental results show that, with near-zero wavelength shift, obvious intensity strain response is exhibited and negative-proportional to the reduced length of TCF. Thus, the maximum sensitivity reaches 0.119 dB/με when the TCF length is equal to 15 mm, and a sub-micro-strain detection resolution (about 0.084 με) is obtained. Besides, owing to the flat red-shifted temperature response, the calculated cross-sensitivity of our sensor is compressed within 0.32 με/°C, which is promising for high precision strain related engineering applications.