High Strain Sensitivity Fiber-Optics Bolt Sensor Based on Vernier Effect

In this paper, a high strain sensitivity fiber-optics bolt sensor based on the Vernier effect is proposed. Four weak reflection fiber Bragg gratings (WFBGs) are inscribed along the fiber-optics axis based on femtosecond laser direct writing, and the two cascaded Fabry-Perot cavities with similar free spectral ranges (FSRs) composed of WFBGs can generate the Vernier effect. Based on the Vernier effect fiber-optics (VEFO) sensing unit and the temperature-compensated fiber Bragg grating (TCFBG), a temperature-compensated Vernier effect fiber-optics (TCVEFO) bolt sensor was designed and fabricated. For comparison, according to the current common structure of fiber-optics bolt sensor, a single FBG fiber-optics (SFFO) bolt sensor was also fabricated. The experimental results show that both sensors have good linear responses, and the maximum sensitivity of the TCVEFO bolt sensor is 17.42 pm/με. Compared with the SFFO bolt sensor, the strain sensitivity is improved by a factor of 23.23. A simple structure of the sensor eliminates the need for complicated fabrication processing, thus it can be a promising candidate for high sensitivity structural health monitoring.