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• An optical fiber strain sensor based on metal welding has been presented. • The proposed fiber sensor has the strain sensitivity of 9.11 pm/με. • The fiber sensor has the advantages of easy fabrication and low cost. • The relationship between capillary length and strain sensitivity is investigated. This paper presents a strain sensor based on Fabry-Perot (F-P) interference. The F-P strain sensor is composed of a single-mode fiber (SMF) and a silica capillary, which are encapsulated by metal welding. The relationship between the cavity length and sensitivity is analyzed theoretically, then the F-P strain sensor is calibrated and tested. The experiment results show that the strain sensitivity of the fabricated sensor reaches 4.4 pm/με when the length of the silica capillary is 12 mm and the F-P cavity length is 436 μm. In addition, the temperature sensitivity of the F-P sensor is 11.7 pm/°C. With the increase of the capillary, the strain sensitivity increases. When the capillary length is 20 mm and the cavity length is 341 μm, the sensitivity of the sensor reaches 9.11 pm/με. The sensor proposed fabricated by metal welding, has the advantages of high sensitivity, simple fabrication, simple structure, and low cost. [ABSTRACT FROM AUTHOR]

• A compact concentration sensor based on the modified thin-diameter polarization-maintaining fiber is demonstrated. • The relationship between the measurement sensitivity and the length and corroded time of the TDPMF is discussed. • The SC concentration sensor exhibits a high linear sensitivity response up to 4.65 nm/%, and the temperature sensitivity is 0.08179 nm/°C. • The MMF-MTDPMF-MMF structure sensor has excellent repeatability and stability in SC concentration sensing. Sodium citrate, as strong alkali and weak acid salt, play an essential role in the food and pharmaceutical industries. However, lack of detection method selectivity, limited biocompatibility and complexity of the manufacturing process are challenges that hinder the practical application of the sensors. In this paper, a improved-sensitivity sodium citrate concentration sensor with simple and compact interrogation setup is proposed. The sensor is fabricated by fusion splicing the sandwich structure of modified thin-diameter polarization-maintaining fiber, two pieces of multimode fiber, and single-mode fiber. In theoretical modeling, the properties of the structure are investigated. In the experiment, the evanescent field generated by the modified thin-diameter polarization-maintaining fiber plays a vital role in the detection of different sodium citrate concentrations. The sodium citrate concentration sensor exhibits a high linear sensitivity response up to 4.65 nm/%. The sensor provided refractive index sensitivity of 3173.2 nm/RIU. In addition, the response, repeatability, and stability are also evaluated by the experiments. Our results show that the sodium citrate concentration sensor holds great promise for serving as a soft and biocompatible probe for monitoring sodium citrate in situ. [ABSTRACT FROM AUTHOR]

The all-fiber voltage sensor is difficult for application because of its complex structure, difficult signal demodulation, and sensitivity to the environment. In this paper, a wavelength modulated all-fiber voltage sensor with a simple structure is designed and proposed using straight tapered single-mode fiber and stacked piezoelectric crystal as a voltage sensing unit. The simulation analysis of its performance shows that it can realize the voltage measurement of 0 kV ∼ 10.41 kV, and the accuracy can reach 0.2%. Additionally, the tapered fiber has good strain sensitivity and temperature insensitivity, and its sensitivity is up to 0.03206 nm / μ ε with low bending loss. [ABSTRACT FROM AUTHOR]

• All fiber-optic sensor based on a Balloon-like interferometer for pH measurement was demonstrated. • The Mach-Zehnder interferometer was fabricated by bending standard single-mode fiber into a Balloon shape. • In addition to the high sensitivity, the proposed sensor exhibits a fast response time with excellent stability. In this paper, we present the fabrication of a new fiber-optic pH sensor based on the macro-bending loss properties in a droplet-shaped single-mode fiber (SMF) bending structure for the first time. To form the droplet-shaped probe, a segment of straight SMF is properly bent into a droplet shape using a short length of a thin silica capillary tube and without any splicing. By gradually varying the bending diameter of the droplet-shaped structure, interference results between the core and cladding modes, and a Mach–Zehnder interferometer (MZI) is effectively introduced within the bending section. Sodium hydroxide (NaOH) or hydrochloric acid (HCl) were added to the pH indicator utilizing buffers solution to achieve a wide pH range from 1 to 14. Experimental results show that the proposed sensor exhibits maximum sensitivity of 3.264 nm/pH and 5.336 nm/pH for acidic and alkaline solutions, respectively. In addition to the high sensitivity, the proposed sensor structure provides high sensitivity with a high linear response and good repeatability. [ABSTRACT FROM AUTHOR]

• Recycling fused optical fiber to develop a miniature self-referred Fabry-Perot microcavity sensor. • The physical theory and Finite Element Method (FEM) of the Fabry-Perot microcavity were considered to fabricate an efficient sensor. • This sensor simultaneously measurements the refractive index and temperature. • This sensor has the advantages of low cost, high repeatability, and small dimension, making it a desirable sensor for chemical and biomedical applications. In this paper, a fiber-optic Fabry-Perot microcavity sensor is presented based on splicing a catastrophic fused fiber (CFF) to a single-mode fiber (SMF) for the simultaneous measurement of the refractive index (RI) and temperature. CFF formed by the catastrophic fuse effect consists of a periodic void in the fiber core that creates a teardrop shape or elliptical air microcavity after splicing. By simulation, the elliptical microcavity sensor is selected to measure external RI changes by Fast Fourier Transform (FFT) analysis. The RI sensitivity was obtained to be −2.5 ± 0.0074/RIU with a resolution of about 4 × 10−5 RIU. Also, the experimental results showed that temperature is obtained independently from the FFT parameter without being affected by the RI. The sensor temperature was determined by tracking the wavelength shift of the interference spectrum. Consequently, the temperature sensitivity of 7 pm/°C in the range of 20 °C to 95 °C was achieved. Therefore, the RI and the temperature were simultaneously measured by analyzing the spatial frequency and wavelength shift. The proposed dual-microcavity FP sensor can compensate for any power fluctuation in the input light source. [ABSTRACT FROM AUTHOR]

This paper proposes a biosensor based on side-hole fibers (SHF) formed by the structure of single-mode fiber (SMF)-coreless fiber (CLF)- SHF-CLF-SMF for the detection of bovine serum albumin (BSA). Among the fibers, the SHF's misalignment splicing makes one hole exposed, which is very sensitive to the change of the external environment. Due to the unique structure of the SHF, two Mach-Zehnder interferences (MZI) are generated in this fiber sensor, which further improves the sensitivity of the sensor. Experiments show that the refractive index (RI) sensitivity of this sensor reaches 13421.638 nm/RIU, and for the BSA concentration detection, the sensitivity reaches 2.413 nm/(mg/ml). The proposed biosensor is simple, easy to manufacture, cost-effective, and can be used for many chemical and biological analyses. [ABSTRACT FROM AUTHOR]