A Sagnac-based interferometer with optimal polarization control for Lamb wave detection

Sagnac-based interferometers are promising devices for laser ultrasound (LU) inspection with the advantages of being robust, low-cost, and easy to integrate. Compared with all-Polarization Maintaining (all-PM) method, nonPolarization Maintaining (non-PM) method is more cost-effective and has greater compatibility, making it more practical in low-frequency ultrasound detection. The initial pi/2 phase bias (IPB) and high fringe visibility are crucial for realizing the interferometer's full potential. However, a specific process of obtaining the optimal polarization control for non-PM configuration is still absent, leading to the unpredictable fringe states. In this paper, a novel and specific approach to obtaining optimal interference fringes for non-PM configuration based on the polarization control is proposed. Experimentally, the initial pi/2 phase bias and 43.9% fringe visibility (50% is the highest in theory) were obtained. Guided waves of low-frequency A0 mode in aluminum sheets were excited by a Q-switched pulse laser and detected by a non-PM Sagnac-based interferometer to inspect artificial defects on the surface of samples. Such a system provided a cost-effective non-contact detection scheme for out-of-plane Lamb wave which was very difficult to separate from LU signal with complex modes composition using conventional piezoceramics sensors.