A depolarized Sagnac-based interferometer with a stable quadrature phase bias for laser ultrasound detection

This study proposes an all-fiber-optic design with an optimal fringe in non-polarization-maintaining (non-PM) depolarized Sagnac-based interferometers (SIs) for laser ultrasound detection. We theoretically demonstrate that the rotation-time (RT) symmetry property of the depolarized SIs is a fundamental principle of the stable phase bias, and experimentally verify that our design simultaneously provides a stable quadrature phase bias and the highest visibility. As the system stabilizes at the "zero amplitude"point, the phase error and signal fading challenges caused by the polarization nonreciprocal effect in the conventional non-PM setup are not serious anymore. In the experiment, a visibility of 48.4% (50% is the highest in theory) with the stable quadrature phase bias is obtained. This method extends SIs to non-PM applications with a more straightforward adjustment process compared to the method used in previous work using polarized light and is necessary to meet the demand of large-scale applications. The SI system developed here provides a cost-effective, high-sensitive, and easy-to-use scheme for non-contact laser ultrasound detection.