Design for Underwater Aplanatic Acoustic Mirror

Underwater acoustic lenses are useful devices for underwater imaging because of their small dimension and low power consumption but have a problem under the condition of changing water temperature because the refractive index depends on the water temperature. To solve this problem, we propose aplanatic acoustic mirrors, which consist of two aspherical mirrors and can remove spherical and coma aberrations. These mirrors are designed by a method used in the Ritchey Chretien astronomical telescopes. The aplanatic mirror shows a larger focal sound pressure and better temperature characteristic than an aplanatic acoustic lens but has a narrow field of angle. Then, an off-axis aplanatic mirror, which consists of two aspherical off-axis mirrors, is designed by extraction of parts from a normal aplanatic mirror to obtain a wider field of angle. (C) 2010 The Japan Society of Applied Physics