Strain rate is an important basic physical parameter in the fields of deformation observation, geodetic measurement, and geophysical monitoring. This paper proposes a novel fiber optic strain rate sensor (FOSRS) that can directly measure the strain rate through a differentiating interferometer that converts the strain rate to the optical phase. The sensing principle, sensitivity, resolution, and dynamic range of the proposed FOSRS are theoretically analyzed and verified by experiment. The experimental results show that the developed FOSRS with a 12.1 m sensing fiber has a flat sensitivity of 69.50 dB, a nanostrain rate ( n ε / s ) resolution, and a dynamic range of better than 95 dB. An ultrahigh static resolution of 17.07 pε / s can be achieved by using a 25.277 km sensing fiber for long baseline measurements. The proposed method significantly outperforms existing indirect measurement methods and has potential applications in geophysical monitoring and crustal deformation observation.

Jerk is directly related to a physical mutation process of structural damage and human comfort. A fiber optic jerk sensor (FOJS) based on a fiber optic differentiating Mach–Zehnder interferometer is proposed. It can directly measure jerk by demodulating the phase of interference light, which avoids the high-frequency noise interference caused by differentiating the acceleration. The sensing theory and sensor design are given in detail. The experimental and theoretical results agree, demonstrating that the FOJS has a high sensitivity, an ultralow phase noise floor, a wide measuring range, and good linearity. The impact test shows that the FOJS can directly measure jerk and has good consistency with a standard piezoelectric accelerometer. The FOJS has potential applications in earthquake engineering, comfort evaluations, and railway design. This is the first time that directly measuring jerk with an optical sensor is reported.

A high resolution white-light driven strain sensor is proposed based on a differentiating interferometer. The differentiating Mach-Zehnder interferometer with a delay fiber is used as the sensing element. The strain is obtained by demodulating the phase signal using three-channel Kalman filter tracking. The strain frequency response of the differentiating interferometer is analyzed and tested. Increasing the length of the delay fiber can improve the strain sensitivity while sacrificing the measurement frequency band, thereby improving the strain resolution. Experimental results show that the proposed sensor achieves a strain resolution limit of 9 fϵ/√Hz and can sense strains smaller than the nano-strain level. This confirms its potential to be used in applications requiring nano-strain or femto-strain measurements.

Optical interferometers are the main elements of interferometric sensing and measurement systems. Measuring their optical path difference (OPD) in real time and evaluating the measurement uncertainty are key to optimizing system noise and ensuring system consistency. With the continuous sinusoidal wavelength modulation of the laser, real-time OPD measurement of the main interferometer is achieved through phase comparison of the main and auxiliary interferometers. The measurement uncertainty of the main interferometer OPD is evaluated. It is the first evaluation of the impact of different auxiliary interferometer calibration methods on OPD measurements. A homodyne quadrature laser interferometer (HQLI) is used as the main interferometer, and a 3 × 3 interferometer is used as the auxiliary interferometer. The calibration of the auxiliary interferometer using optical spectrum analyzer scanning and ruler measurement is compared. The evaluation shows that the auxiliary interferometer is the most significant source of uncertainty and causes the total uncertainty to increase linearly with increasing OPD. It is proven that a high-precision calibration and large-OPD auxiliary interferometer can improve the real-time accuracy of OPD measurements based on the auxiliary interferometer. The scheme can determine the minimum uncertainty to optimize the system noise and consistency for vibration, hydroacoustic, and magnetic field measurements with OPDs of the ~m level.

This paper proposes a fiber optic multi-parameter seismic observation system based on fiber optic interferometric sensors for seismic wave detection. The multi-parameter fiber optic seismic probe, including a 3-componet fiber optic seismometer a 4-componet strainmeter and a fiber optic temperature sensor, is developed using the fiber optic Michelson interferometers. The interrogator, using phase generating carrier technology, has a high resolution of 10-5 rad/Hz^1/2. The probe is installed in a 60m deep borehole for long-term observation. The observation results show that all sensors can detect the seismic wave signals of the earthquakes, even teleseismic earthquakes from the coast of Taiwan. Moreover, the fiber optic strain sensors also can measure the earth tide. The fiber optic multi-parameter seismic observation system is expected to provide rich data for seismic wave detection in seismology and geophysics.