Objective Fiber optic strain sensors, with their immunity to electromagnetic interference, small size, light weight, and high stability in harsh environments, offer potential applications in numerous fields such as aerospace, biomedicine, and frozen soil monitoring. Recent research has demonstrated various fabrication techniques for these sensors, including fiber Bragg grating (FBG), long period fiber grating (LPFG), Fabry-Perot- Perot interferometer (FPI), tapered fiber, and diverse fiber optic interferometers. Some researchers have developed fiber optic sensors utilizing FP cavities and Mach-Zehnder- Zehnder interferometer (MZI) cascaded, achieving a strain sensitivity of 4.80 pm/mu epsilon over a 0 to 600 mu epsilon range, indicating low sensitivity. Others have introduced a novel parallel structure of fiber FPIs leveraging the cursor effect, comprising an open cavity FPI with a single- mode optical fiber (SMF)-SMF-SMF- SMF- SMF structure and a closed cavity FPI with an SMF-hollow- hollow core optical fiber (HCST)-SMF- SMF structure in parallel. This enhances the strain sensitivity of the sensor to -43.20 pm/mu epsilon, which is 4.6 times higher than that of a single open chamber. However, despite its high strain sensitivity, this sensor is not widely adopted due to its large dislocation amplitude, manufacturing challenges, and low repeatability. In this study, we propose and prepare a vernier- sensitized fiber Fabry-Perot- Perot strain sensor to achieve high- sensitivity strain measurements. Methods In the high- sensitivity vernier sensitizer fiber Fabry-Perot- Perot strain sensor, both the sensor cavity and reference cavity employ an SMF-HCF-SMF- HCF- SMF structure for FPI. By adjusting the cavity lengths of both FPIs, two similar yet distinct free spectrum ranges (FSRs) are achieved, generating a vernier effect. As the external strain on the sensor cavity changes incrementally, the reflection spectrum of the sensor shifts, allowing for the measurement of the sensor's strain sensitivity. Subsequently, the strain sensitivity of the single sensing cavity is compared with that of the two samples in parallel, resulting in a significant enhancement in sensitivity. Results and Discussions Within the strain range of 0-900 mu e, the strain sensitivity of a single sensor cavity is 1.31 pm/mu e. After parallel connection, the strain sensitivity of the sensor reaches -11.50 pm/mu e and -12.76 pm/mu e, respectively, amplifying the sensitivity by 8.70 times and 9.74 times and significantly improving the sensor's strain sensitivity. Conclusions In this paper, we fabricate a vernier sensitized fiber Fabry-Perot- Perot strain sensor and improve the sensitivity of strain measurement by keeping the sensing cavity unchanged and altering the length of the reference cavity. The sensor consists of two FPIs with an SMF-HCF-SMF- HCF- SMF structure connected in parallel by 3 dB couplers. During preparation, the length of the hollow core fiber is controlled as closely as possible so that the sensing and reference cavities have similar FSRs, enabling the superimposed spectrum to produce a vernier effect. The experimental results show that within a strain range of 0-900 mu e, the sensitivity of a single sensing cavity is 1.31 pm/ mu e, the length of the sensing cavity remains unchanged, and the length of the reference cavity is changed by changing the amplification factor of the strain sensitivity. The strain sensitivity of the sensor can be improved to -11.50 pm/mu e and -12.76 pm/mu e by using the cursor effect demodulation in parallel. This method yields a strain sensitivity 8.70 and 9.74 times higher than that of a single sensing cavity FPI, significantly enhancing strain sensitivity. Producing two samples for strain testing with different strain sensitivity amplifications can expand the sensor's measurement range in the future, improving measurement precision and accuracy to meet various strain conditions. The sensor also offers advantages such as low production cost, simple operation, and high sensitivity, making it applicable in fields like aerospace, frozen soil monitoring, and biomedicine.
