Design, Simulation, and Experimental Validation of an Optimized Bi-Fresnel Zone Plate Acoustic Lens

Acoustic lenses based on Fresnel Zone Plates provide an efficient and low-cost approach for ultrasound focusing, with potential applications in Focused Ultrasound, High-Intensity Focused Ultrasound, and blood–brain barrier opening. This work presents the design, numerical analysis, and experimental validation of a bifocal Fresnel acoustic lens capable of simultaneously generating two independent acoustic focal spots. The lens was designed by combining two optimized Fresnel zone distributions under spherical-wave incidence conditions, introducing geometric tuning parameters (c1=0.97 and c2=0.40) to maximize the effective aperture and balance the acoustic energy delivered to both foci. The acoustic response was investigated using finite element simulations with an axisymmetric model. Numerical results predicted well-defined focal regions with FLHM values of 2.25λ and 3.15λ, and FWHM values of 0.73λ and 0.75λ. The lens was subsequently manufactured from brass and experimentally characterized in an automated immersion tank using a piston transducer and a needle hydrophone. Experimental measurements confirmed the simultaneous generation of both focal spots. The measured lateral beam widths agreed well with the numerical predictions, with deviations below 11% and an average discrepancy of approximately 7%. Furthermore, the spatial acoustic gain maps showed that the acoustic energy remained concentrated within the intended focal regions despite the complex interference pattern inherent to the multifocal field. These results demonstrate the feasibility of the proposed optimized bifocal Fresnel acoustic lens as a passive multifocal ultrasound focusing device for future biomedical applications.