Focusing efficiency evaluation of ultrasonic energy for fabricated Fresnel lens through surface profile estimation and FEA

High focusing efficiency of a multi-level Fresnel lens is desired when it is designed and fabricated. How to estimate the focusing efficiency is an important issue after the Fresnel lens is made. We propose a method to evaluate the focusing efficiency of the fabricated Fresnel lens based on its surface profile. The profile of Fresnel lens fabricated shows how similar to the designed, which has been theoretically predicted a maximal focusing efficiency such as 81% and 40.5% for a four-phase and two-phase level Fresnel lenses, respectively. This method can be applied to evaluate the focusing efficiency of any fabricated surface profile of Fresnel lenses prior to a direct experimental measurement for energy focusing. The focusing efficiency of a four-phase level Fresnel lens designed to operate at a frequency of 100 MHz of an ultrasonic ejector, which was fabricated by a two-mask process using SU-8 photoresist, approximate 60% is illustrated. Finite element analysis was employed to confirm and observe the energy focusing phenomenon of ultrasonic ejector models.