Highly Sensitive Strain Sensor Based on the Vernier Effect With High Extinction Ratio and Low-Temperature Cross-Sensitivity by Compact Double FPI

In recent years, the Vernier effect has received much attention from researchers in the field of fiber sensing because of its ability to improve measurement sensitivity significantly. However, two prominent problems occur when it is used for strain sensing: low extinction ratio (ER) and high-temperature cross-sensitivity. A compact fiber strain sensor based on the Vernier effect with the advantages of high strain sensitivity, high ER, and low-temperature cross-sensitivity is proposed in this study. The sensing Fabry–Perot interferometer (SFPI) utilizes a hollow-core fiber (HCF) as the FP cavity. HCF can effectively confine the light beam and increase the ER of the interference fringes. Moreover, the second reflecting surface of SFPI is implemented using an indirect fusion method, which increases the active length to improve strain sensitivity. A reference Fabry–Perot interferometer (RFPI) is inserted into a hollow silica capillary (HSC) to isolate it from external environmental effects while serving as one of the reflecting surfaces for SFPI. The factors influencing the strain sensitivity of the sensor are analyzed through theoretical and experimental analyses. The maximum strain sensitivity is
$- 940.02\,\,{{\text {pm}}/\mu \varepsilon }$
, and its temperature sensitivity is only
$0.50 {{\text {pm}}/}{ }{{^{\circ }}}{\text {C}}$
. This sensor also exhibits excellent stability and reversibility. It also has potential applications in the field of strain sensing.