High-resolution, wide-area underwater object shape imaging using ultrasonic diverging wave transmission and laser-deflection-based echo reception

Abstract During the ongoing decommissioning of the Fukushima Daiichi Nuclear Power Plant, accurate visualization of debris shapes during fuel debris cracking operations is essential. To image objects with complex surface geometries such as debris, this study developed a convex ultrasonic transmitter to generate a wide imaging area and an omni-directional echo receiver—the laser-deflection-based echo receiver (LDER)—to achieve high-resolution imaging. These two components were integrated into a single system. The resulting LDER-diverging wave imaging (DWI) system was then experimentally validated. Notably, the LDER functions as an ultrasonic receiver that detects echoes through the deflection of laser light by sound waves, an acousto-optic effect. Namely, the LDER receives echoes by capturing the spatial density changes caused by sound waves as changes in transmitted light intensity via laser light deflection. Images were reconstructed from the received echo signals using the delay-and-sum algorithm and circular coherence factor. Underwater wire imaging confirmed that the system’s resolution and contrast ratio were approximately twice those obtained using a conventional ultrasonic element with a width equal to the wavelength at a viewing angle of approximately 60°. Furthermore, imaging of rock surfaces—used to simulate debris shapes—demonstrated that the LDER-DWI system enables clear visualization of complex geometries with enhanced resolution and contrast.