Planar-structure microscope-lens for simultaneous acoustic and optical imaging

The authors propose a combined acoustic and optical microscope-lens of planar structure which is intended to meet the demand for simultaneously obtaining both an acoustic image and its optical counterpart. The lens is created by combining a Fresnel-zone-type acoustic lens with a rod-type gradient-index optical lens (SELFOC). A piezoelectric thin-film transducer having a pinhole at its center is deposited on one end of the rod lens for generation of ultrasound. A number of annular grooves designed according to the theory of the Fresnel zone plate are formed on the other end of the rod for converging the acoustic wave in water. A beam from a He-Ne laser source is guided into the lens through the pinhole and converges owing to the gradient of the refractive index. After passing through the central ungrooved zone of the acoustic Fresnel lens, the optical beam focuses on a spot in water outside the rod. By designing the length of the rod properly, the focal spot of the optical beam is set to coincide with that of the acoustic beam. The design and fabrication of the lens are described, and results of some preliminary experiments on the simultaneous observation of acoustic and optical images of the same portion of a specimen are shown