FINITE-ELEMENT ANALYSIS OF MULTILEVEL ACOUSTIC FRESNEL LENSES

Fresnel lenses have recently emerged as viable alternatives to convent ional spherical lenses for focusing ultrasonic waves in acoustic micro scopy 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 lens es 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 spec ifications are not available. This paper presents a finite element stu dy of multilevel acoustic Fresnel lenses. Results showing ultrasonic w ave propagation through such surface relief structures together with t he resulting diffraction profiles are presented. The high efficiency a nd focusing power of these lenses are also demonstrated. Simulation re sults together with a discussion on various multilevel lens design iss ues are presented. These results confirm the advantages of such lenses , and suggest that the finite element model can serve as a valuable to ol for designing, simulating, and studying lens profiles prior to thei r fabrication.