Ultrasonic Imaging by the Phase Conjugate Resonance System

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This study is an attempt to improve ultrasonic C-mode imaging systems by using phase conjugate waves. An ultrasonic phase conjugator was devised making use of nonlinear piezoelectricity of PZT ceramics. This phase conjugator was built in an ultrasonic C-mode imager together with an waveguide. Since phase conjugate waves act as time reversed waves for arbitrary incoming waves, the initial ultrasonic field keeps its wavefront shape as if it is in a resonator. This effect provides ultrasonic images that are less sensitive to ultrasound's wavefront deformation but still sensitive to ultrasonic damping. With this principle, new ultrasonic images were successfully obtained. Those images showed surface or subsurface elastic structures of metal samples, even if their surfaces were not flat.At the same time, investigation of the fidelity of ultrasonic phase conjugate waves was carried out. The ultrasonic fields at several different stages in the process of phase conjugation were visualized by an schlieren optical system. The optical images of ultrasound revealed that, in our system, the shape of wavefronts of phase conjugate waves agreed with those of the incident waves with errors less than a wavelength.