Dynamic underwater gradient-index lens formed by acoustic-vortex-induced cavitation

Area of Science:

• Acoustic optics
• Fluid dynamics
• Optical engineering

Background:

• Acoustic vortices can generate complex fluid structures.
• Controlling light underwater presents unique challenges due to medium properties.

Purpose of the Study:

• To investigate an acoustic-vortex-based dynamic lens for underwater light manipulation.
• To characterize the pressure-tunable cavitation structures and their optical effects.

Main Methods:

• Generated annular cavitation structures using acoustic vortices.
• Employed high-speed imaging to observe bubble ring dynamics.
• Conducted optical measurements to assess focal length changes.
• Utilized ray-tracing simulations with a gradient-index (GRIN) lens model.

Main Results:

• Stable bubble rings were formed, with thickness increasing from 2.4 to 3.39 mm under pressures from 2.6 to 16.3 MPa.
• Higher input pressures resulted in a shorter effective focal length, indicating a larger refractive-index gradient.
• GRIN lens model simulations accurately reproduced the observed light focusing behavior.

Conclusions:

• The study clarifies the generation mechanism of acoustic-vortex-based dynamic lenses.
• Demonstrated the potential of these lenses as dynamically reconfigurable optical elements for underwater applications.