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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 controllable cavity length, high sensitivity, simple to fabricate and cost-effective axial strain sensor based on optical fiber Fabry-Perot microcavity is presented. • To further improve the sensitivity of axial strain, a sensing structure based on parallel Vernier effect is proposed. • These two FPIs are paralleled by a coupler, and the sensitivity of axial strain can be amplified by 22 times. • The axial strain sensitivity of the proposed sensor can reach 33.5 pm/με owing to the parallel Vernier effect with excellent linear property (R-square is 0.9981). In this paper, a Fabry-Perot interferometer (FPI) based on the fusion structure of hollow-core fiber (HCF) and single-mode fiber (SMF) is reported. Its length can reach a level of 200 μm, which has the advantage of axial strain sensitivity, low temperature crosstalk, and a wide free spectrum range (FSR). The experimental results show that the axial strain and temperature sensitivity of FPI with the length of 200 μm is 1.50 pm/με (R-square is 0.9999) and 1.5 pm/°C during the range of 20 °C–100 °C. A sensing structure based on a parallel Vernier effect is proposed to improve axial strain sensitivity and eliminate temperature crosstalk. Two FPIs with similar lengths are fabricated in this experiment. A 2*2 coupler parallels these two FPIs, the sensitivity of axial strain can be amplified by 22 times, and the optical path difference caused by temperature change can be eliminated. The axial strain sensitivity of the proposed sensor can reach 33.5 pm/με owing to the parallel Vernier effect with excellent linear property (R-square is 0.9981) and negligible crosstalk of temperature. In addition, the parallel sensing structure has the advantages of repeatability, reproducibility, flexible collocation, low cost, easy preparation. [ABSTRACT FROM AUTHOR]