A compact double fiber Bragg gratings(FBGs) accelerometer based on push-pull compliant cylinders is proposed. The mechanical model is demonstrated. The accelerometer contains four parts, double cylinders, double FBGs, inertial mass, shell. Double cylinders and mass block which can reduce the cross-sensitivity and protect the FBGs from being damaged. The material of the cylinders is two-component vulcanized silicone rubber. The amplitude-frequency and sensitivity of the accelerometer are theoretically analyzed and experimental measured. Experimental results show that the resonant frequency of the accelerometer is 640Hz, the sensor has a broad flat frequency range from 20 to 500Hz, the sensitivity of the accelerometer is about 48.5pm/G with a linearity of 0.999, while the two FBGs is contributed to the sensitivity enhancement, temperature and transverse sensitivity independent. The accelerometer has a wide frequency and high sensitivity, making it a good candidate for the cross-well micro-seismic monitoring..
We proposed a fiber-optic sensor implanted with helical seven-core structure based on Mach–Zehnder interference, which can be used for the measurement of tensile strain and extrusion bending. The sensor consisted of a section of seven-core optical fiber with helical structure, which can be described as the SMF-Taper-HSCF-Taper-SMF (HSCF, helical seven-core fiber) sensor. When stretching or bending is applied, the sensor will undergo certain deformation, which will lead to the changes of interference modes in the optical fiber. The tensile strain and extrusion bending can be measured accurately according to the response of transmission spectrum to mode change. The helical seven-core structure can effectively stimulate higher order modes and induced deformation changes. In the experiment, three sensors with different helical periods were fabricated and their spectral characteristics were analyzed. Finally, we selected the sensor with a helical period of 190 μm to conduct a strain and bending test. The results show that the strain sensitivity of the sensor is −21.31 pm / με in the range of 0 to 500 με, and the curvature sensitivity of the sensor is −6.36 nm / m − 1 in the range of 0.16 to 1.6 m − 1. This sensor can detect strain and bending and has stable sensing performance and high sensitivity.