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, non-Polarization Maintaining (non-PM) method is more cost-effective and has greater compatibility, making it more practical in low-frequency ultrasound detection. The initial π/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 π/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.

基于 Sagnac 的干涉仪是用于激光超声 (LU) 检查的有前途的设备，具有坚固、低成本和易于集成的优点。与全极化保持（all-PM）方法相比，非极化保持（non-PM）方法性价比更高，兼容性更强，在低频超声检测中更实用。初始 π/2 相位偏置 (IPB) 和高条纹可见度对于实现干涉仪的全部潜力至关重要。然而，仍然缺乏获得非 PM 配置的最佳偏振控制的特定过程，导致不可预测的边缘状态。在本文中，提出了一种基于偏振控制获得非 PM 配置的最佳干涉条纹的新颖而具体的方法。实验上，获得了初始 π/2 相位偏差和 43.9% 的条纹可见度（50% 是理论上的最高值）。低频导波A铝板中的₀模式由 Q 开关脉冲激光激发，并由基于非 PM Sagnac 的干涉仪检测以检查样品表面上的人工缺陷。这种系统为平面外兰姆波提供了一种具有成本效益的非接触式检测方案，使用传统的压电陶瓷传感器很难从具有复杂模式组成的 LU 信号中分离出来。