We experimentally demonstrated a high sensitive gas pressure sensor, which also has more concise processing steps and lower cost. The sensor was fabricated by splicing the single-mode fiber (SMF) and a short section of hollow-core fiber (HCF) which was filled with a Polydimethylsiloxane (PDMS) film to form an enclosed air microcavity with the length of 55 μm. The thin PDMS film also plays a role in reflecting the beam. The thermo-optical effect and the thickness of PDMS film can affect the cavity that is sensitive to external pressure. When the thickness of the PDMS membrane is 3 μm, the measured gas pressure sensitivity reaches to 52.143 nm/Mpa, the linearity is 99.86%, and the cross-sensitivity of the gas pressure over temperature is lower than 2.51 × 10 ⁻³ Mpa/°C. Moreover, the compact structure, high sensitivity, high mechanical strength, and convenient fabrication make it suitable for measuring gas pressure in harsh conditions.

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.

An in-fiber hybrid modal interferometer (HMI) is theoretically and experimentally demonstrated in a section of bias-tapered multimode fiber only fabricated by arc-discharge technique, which consists of multimode interferometer (MMI) and Mach-Zehnder interferometer (MZI). The MZI has the sensitivity as high as -31.08 nm/RIU, but the MMI presents the immunity of both wavelength and intensity to ambient refractive index (RI). In strain test, MMI has a 9.8-dB intensity deduction and MZI shows high wavelength sensitivity (-6.31 pm/με). Thus, our HMI shows a strong multi-parameter discriminative proficiency and simultaneous measurement of RI, strain and temperature is achieved by the means of joint wavelength and intensity demodulation-based inversion-matrix method. Such a compact and low-cost sensor, without the crosstalk from the temperature and the light source, is very promising for the applications of biochemical sensing and structural health monitoring.

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.