Static and Ultrasonic Structural Health Monitoring using Phase-Shifted Fiber Bragg Gratings with Controlled Polarization

When a fiber Bragg grating, particularly a phase-shifted fiber Bragg grating (PSFBG), is used for embedded structural health monitoring, the distortion of its spectrum owing to transverse strain difference affects the measurement accuracy. In this study, using the sensing principles and transfer matrix method, the spectrum of the PSFBG under the influence of birefringence, and its static and ultrasonic sensing performance under three typical polarization states were simulated. Furthermore, a polarization-controlled static and ultrasonic sensing technique was proposed to experimentally verify theory and simulations. The results showed that the axial strain can be accurately measured when the different polarization states were distinguishable and stable. The ultrasonic sensitivity of the PSFBG improved when a spectrum with a narrow transmission peak was reconstructed via polarization control. These results indicate that the proposed method can eliminate the negative effects induced by birefringence on axial strain and ultrasonic sensing.