Temperature compensation of fiber optic unbalanced interferometers for high resolution static strain sensing

Accurate temperature compensation challenges the use of fiber optic unbalanced interferometers in static strain sensing. We propose a temperature compensation theory and scheme of unbalanced interferometers using sensing fibers with different temperature coefficients, aiming at resolving the temperature disturbance of optical fibers and achieving high strain resolution. For the first time, we comprehensively analyze the thermal response of interferometers due to different temperature coefficients, different thermal behaviors, and inconsistent temperature disturbances in the optical fiber arms. A sensing combination and a reference combination are designed. The sensing combination measures a strain and compensates for the temperature disturbance of the sensing arm, and the reference combination provides a reference to compensate for the temperature disturbance of the reference arm and frequency drift of the interferometer in the sensing combination. The proposed scheme can compensate for the temperature disturbances of interferometers in the sensing combination completely. To verify the theory and scheme, bend-insensitive single-mode fibers and Boron/Germanium co-doped fibers are used as sensing fibers in the experiment. The experimental results show that the temperature resolution of the sensing arm is about 3.43 × 10−4 °C, and the static strain resolution of the unbalanced interferometer within 24 h after temperature compensation is about 2.55 nε. The temperature-compensated strain resolution of ∼nε indicates that the proposed temperature compensation theory and scheme can be used in high-resolution, long-term, low-frequency, and static strain sensing, such as crustal deformation observation in geophysical applications.