We proposed and demonstrated a compact inline optical fiber sensor for curvature and temperature measurement with low cross sensitivity. The device consists of a 5 mm long hollow-core fiber (HCF) spliced between two single-mode fibers. Two up-tapers were fabricated at each splicing joint forming a Mach-Zehnder Interferometer (MZI). The HCF acted as the anti-resonant reflecting optical waveguide (ARROW), giving periodic dips at resonant wavelengths in the optical transmission spectrum. The cross sensitivity of curvature and temperature problem is solved by demodulating the wavelength shift of the MZI for temperature sensing and intensity variation of ARROW dips for curvature sensing. The curvature and temperature sensitivities are experimentally measured to be -4.28 dB/ $\text{m}^{-1}$ and 25.76 pm/°C, respectively. The cross sensitivities for ARROW is measured to be 0.0056 $\text{m}^{-1}$ /°C. The structure of the sensor is simple and compact, which can be used for structural health monitoring in a complex environment.

Yucheng Li, Shun Wang, Chenghua Lin, Frank Guerin, Loic Barrault. Proceedings of the 17th Conference of the European Chapter of the Association for Computational Linguistics. 2023.

Translating interferometric applications into practical field use with the required flexible precision and measurement range, is a recognized challenge. We report an in-situ adjustable fiber-optic piezometer based on parallelly structured external Fabry-Perot interferometers (EFPIs) with the Vernier effect and its harmonics. By accommodating the EFPI structure with an adjustable and a fixed cavity length, the EFPIs are utilized as a referencing fiber piezometer (RFP), and a sensing fiber piezometer (SFP), respectively. The Vernier effect with amplified sensitivity is formed by connecting the RFP and SFP in parallel via a 3 dB optical coupler. By simply tuning the cavity length of the RFP, the magnification factor M of the Vernier effect is in-situ continuously adjustable. Using the two prototypes SFP1 and SFP2, water level measurement is performed in the fundamental Vernier effect (FVE) mode and the harmonic Vernier effect (HVE) mode. Experimental results demonstrate that in the FVE mode, the sensitivity can be predictably tuned from −0.15 nm/cm to −7.02 nm/cm with M=1∼48 for SFP1, and −0.31 nm/cm to −7.22 nm/cm with M=1∼23 for SFP2. In the HVE mode, the sensitivity can be further enhanced as high as −9.08 nm/cm, while the sensing performance fluctuates during the adjusting process. Benefiting from the merits of in-situ adjustable sensitivity and measurement range, simplicity of composition, robustness, and remote sensing capability, the proposed scheme can be applied to various practical applications.

A novel fiber sensor for curvature and acoustic wave measurement which is based on a thin core ultra-long period fiber grating (TC-ULPFG) has been proposed and experimentally demonstrated. By tracking the power variation of different resonant wavelength caused by TC-ULPFG, high curvature sensitivity of 97.77dB/m-1 is achieved, to best of our knowledge, which is highest than other structures at the same measurement range. Thus, the desired curvature property of the TC-ULPFG is used for acoustic measurement. The polyethylene terephthalate (PET) film is selected as a transducer, on which TC-ULPFG is tightly pasted. The acoustic pressure sensitivity of 1.89V/Pa is two orders higher than other structures based on the diaphragm transducer, and the noise-limited minimum detectable pressure is 1.94mPa/Hz1/2 at 200Hz. In addition, the frequency fluctuations are nearly ±0.4dB from 70Hz to 200Hz and ±0.2dB from 1kHz to 3kHz, respectively. Therefore, the proposed optical fiber acoustic sensor (OFAS) has a flat frequency response in relatively lower frequency. The TC-ULPFG shows many advantages including high sensitivities of curvature, high acoustic pressure sensitivity, easy fabrication, simple structure and low cost.