Holographic sonar imaging via soft meta-structures

Abstract

Autonomous underwater vehicles (AUVs) are used for underwater ranging, exploration, and communication. However, the limited space in AUVs restricts the number of array elements and imaging capability of sonar systems. This paper evaluates imaging speed and error using Fresnel diffraction integration and the iterative angular spectrum algorithm (IASA). Based on the inverse problem of ultrafast angular spectrum imaging, a soft holographic meta-structure designed and fabricated with impedance-matched soft materials is used to modulate sound waves and focus on arbitrary regions at the imaging plane. This flexible design improves imaging clarity and accuracy by enabling dynamic focal control, allowing adjustments within 34.5 mm from the central axis. Underwater tests show that the designed AUV sonar holographic imaging system can make the imaging of the target outlines clearer and more accurate. This approach offers sonar reconstruction capabilities that can be advantageous with respect to phased arrays.