Finite element analysis of multilevel acoustic Fresnel lenses

Fresnel lenses have recently emerged as viable alternatives to conventional spherical lenses for focusing ultrasonic waves in acoustic microscopy systems. Although these lenses are relatively straightforward to manufacture, their bulk represents a major handicap. A remedy to this problem is to use multilevel acoustic Fresnel lenses. Multilevel lenses are surface relief structures that can be fabricated very precisely using existing VLSI semiconductor technology. However, accurate tools for designing lenses to achieve the desired efficiency and power specifications are not available. This paper presents a finite element study of multilevel acoustic Fresnel lenses. Results showing ultrasonic wave propagation through such surface relief structures together with the resulting diffraction profiles are presented. The high efficiency and focusing power of these lenses are also demonstrated. Simulation results together with a discussion on various multilevel lens design issues are presented. These results confirm the advantages of such lenses, and suggest that the finite element model can serve as a valuable tool for designing, simulating, and studying lens profiles prior to their fabrication.