Review: Digital acousto-optic holography

Digital acousto-optic holography (DAOH) integrates ultrasonic excitation with digital holographic measurement to enable full-field, non-contact detection of subsurface defects in opaque solids. This review outlines the operating principles of DAOH and compares its two primary optical recording modes—television holography for displacement measurement and digital shearography for strain-gradient detection. The complex acoustic field obtained at the surface is numerically propagated into the material interior using Rayleigh–Sommerfeld diffraction, enabling volumetric reconstruction of internal discontinuities. Detection performance, including fringe visibility, sensitivity, robustness, and depth resolution, is systematically evaluated. Current challenges related to mode conversion, attenuation, and material heterogeneity are discussed, along with recent advances in high-speed acquisition, multi-wavelength deformation measurement, and dual-mode optical architectures. Prospects such as multi-mode excitation, super-resolution acousto-optic imaging, and physics-informed reconstruction are highlighted as key pathways toward real-time, high-fidelity three-dimensional non-destructive testing imaging.