Time-reversal property of acoustic phase conjugate waves and their applications to imaging systems

Abstract

High-efficiency generation of the acoustic phase conjugate waves with nonlinear PZT piezoelectric ceramics was successful. This method has advantages over other methods in its fast response and a weak dependence on incident angle. Efficiency of the phase conjugator, defined as the amplitude ratio of the phase conjugate wave to the incident wave, is strongly dependent on the nonlinear piezoelectric, elastic, and dielectric constants of the materials. These nonlinear constants were measured in some PZT ceramics with different compositions. The efficiency was calculated theoretically from these values. The results were in good agreement with the observed efficiency. In previous studies, a very small efficiency in LiNbO3 crystal was found. The phase conjugation efficiency in PZT ceramics is up to 20%, which is 20 times larger than that of LiNbO3, in which the nonlinear constants are much smaller than those of PZT ceramics. The time-reversal property of the phase conjugate waves was confirmed by visualizing their propagation with a stroboscopic schlieren technique. The phase conjugator was also applied to the acoustic imaging system: The image distortion caused by the inhomogenity in the medium was successfully eliminated.