Abstract
With the aim of avoiding the concentration of second harmonic generation at the focus, a modified focusing source designed to minimize the fundamental amplitude at the focus is examined. The present source consists of two coaxially arranged confocal transducers: An inner disk transducer and an outer ring transducer whose areas are the same. They have been polarized in the thickness direction opposite to each other, and are connected in parallel. The theoretical and experimental results show that the second harmonic amplitude attains a maximum at three locations including two positions where the fundamental amplitude takes peak values. A relatively sharp second harmonic beam is demonstrated. Provided the radius of curvature is relatively small, since the three maximum positions are close to each other, the second harmonic beam effectively focuses long. Suppression of side lobes by weighting the source excitation is also discussed.