Akira Kokubo

Acoustic phase conjugation by nonlinear piezoelectricity. II. Visualization and application to imaging systems

Phase conjugate waves of ultrasound were generated in PZT ceramics through nonlinear piezoelectric interaction between an incident ultrasonic field at ω and an electric field at 2ω. The amplitude reflectivity of the phase conjugator was 23% at 10 MHz. The behavior of the incident waves and the phase conjugate waves were visualized by stroboscopic schlieren technique. Time-reversal property and the automatic correction of wavefront distortion in the phase conjugate process were confirmed. A scanning ultrasonic imaging system with a PZT phase conjugator has been built. This system was used to visualize test samples composed of solid plates and phase disturbers. Images by phase conjugate reflection yielded clear figures of the solid plates in spite of the existence of phase disturbers made of agarose gel with rough surfaces, whereas conventional images showed serious distortion.

Acoustic phase conjugation by nonlinear piezoelectricity. I. Principle and basic experiments

The principle and the basic experimental results of the acoustic phase conjugation by nonlinear piezoelectricity are described. Acoustic phase conjugate waves at frequency ω are generated as a result of the interaction between incident acoustic waves at ω and pump electric fields at 2ω. An original explanation based on the modulation of sound velocity by the electric field together with the concept of time grating is given. Coupling equations for the PZT ceramics as a nonlinear material are derived. Experimental results of the sound velocity modulation and the phase conjugate generation in nonlinear piezoelectric PZT ceramics are shown. Amplitude reflectivity of the phase conjugation was 23% for the acoustic incidence at 10 MHz.

Nonlinear piezoelectricity in PZT ceramics for generating ultrasonic phase conjugate waves

We have succeeded in the generation of acoustic phase conjugate waves with nonlinear PZT piezoelectric ceramics and applied them to ultrasonic imaging systems. Our aim is to make a phase conjugator with 100% efficiency. For this purpose, it is important to clarify the mechanism of acoustic phase conjugation through nonlinear piezoelectricity. The process is explained by the parametric interaction via the third-order nonlinear piezoelectricity between the incident acoustic wave at angular frequency omega and the pump electric field at 2 omega. We solved the coupling equations including the third-ordered nonlinear piezoelectricity and theoretically derived the amplitude efficiency of the acoustic phase conjugation. We compared the efficiencies between the theoretical and experimental values for PZT ceramics with eight different compositions. Pb[(Zn1/3Nb2/3)(1 - x)Tix]O3 (X = 0.09, PZNT91/9) piezoelectric single crystals have been investigated for high-performance ultrasonic transducer application, because these have large piezoelectric constants, high electrical-mechanical coupling factors and high dielectric constants. We found that they have third-order nonlinear piezoelectric constants much larger than PZT and are hopeful that the material as a phase conjugator has over 100% efficiency.

Nonlinear Piezoelectricity of PZT Ceramics and Acoustic Phase Conjugate Waves

We succeeded in generating acoustic phase conjugate waves with nonlinear PZT (Pb(Zr x Ti 1-x )O 3 ) ceramics. The efficiency of the phase conjugator, which is defined as the amplitude ratio of the phase conjugate wave to the incident wave, is found to be proportional to the variation in sound velocity under an applied electric field. We measured the sound velocity in PZT ceramics with different compositions by varying the applied electric field. The results correlated well with the observed efficiency. The PZT ceramics are generally more capable than LiNbO 3 crystal of generating phase conjugate waves. We confirmed the time-reversal property of the phase conjugate waves by visualizing its propagation with the stroboscopic schlieren technique.

Reflection-Type Ultrasonic Phase Conjugate Imaging System with a Waveguide

A reflection-type ultrasonic C-mode imaging system with an ultrasonic phase conjugator and a waveguide has been built, and images at 10 MHz have been obtained for metal samples with surface undulations. Surface or sub-surface elastic properties were visualized with a small influence of the surface roughness. The waveguide worked to enlarge the effective aperture of the phase conjugator, to smooth the incident-angle dependence of the phase conjugate reflectivity, and to improve the signal-to-noise ratio of the phase conjugate signals. Schlieren images of the ultrasonic fields in the system are also presented.

Estimation of Nonlinear Piezoelectricity of 0.5Pb(Ni1/3Nb2/3)O3–0.35PbTiO3–0.15PbZrO3 and 0.64Pb(Ni1/3Nb2/3)O3–0.36PbTiO3 Ceramics

The 3rd-order nonlinear piezoelectric constants of relaxor-type perovskite ceramics, 0.5Pb(Ni1/3Nb2/3)O3–0.35PbTiO3–0.15PbZrO3 (50PNN–35PT–15PZ) and 0.64Pb(Ni1/3Nb2/3)O3–0.36PbTiO3 (64PNN–36PT), were estimated by the dynamic measuring of velocity variation under an alternating electric field. Their nonlinearity was much larger than that of the various types of current lead zirconate titanate (PZT) ceramics. We have succeeded in the generation of acoustic phase conjugate waves through the nonlinear properties of PZT ceramics and obtained an efficiency over 100%, the efficiency being defined as the intensity ratio of the phase conjugate wave to the incident wave. We theoretically calculated the phase conjugate efficiencies in both 50PNN–35PT–15PZ and 64PNN–36PT ceramics from the measured 3rd-order nonlinear piezoelectric constants, and found very large values that could yield a phase conjugator with 300% to 500% efficiency.

Incident-angle dependence of the phase conjugate reflectivity by nonlinear piezoelectric interaction in PZT ceramics.

The reflectivity of an acoustic phase conjugator by nonlinear piezoelectric interaction in PZT ceramics was measured as a function of the incident angle. The phase conjugator was designed as a solid block with a flat surface for the acoustic incidence from water. The phase conjugate reflectivity showed incident angle dependence, which is mainly determined by the transmissivity of the acoustic waves, together with some effects of anisotropy. The field-reconstructing ability of this phase conjugator was simulated for focused incident beams.

Aberration-free ultrasonic C-mode imaging system with an ultrasonic phase conjugator

An ultrasonic C-mode imaging system incorporating an ultrasonic phase conjugator has been built and images have been obtained at a frequency of 10MHz. This system is free from the influence of wavefront distortion due to the reflection or the refraction at the rough surface of the samples and visualizes the net attenuation of ultrasonic waves.

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

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 th...

Ultrasonic micrometer with submicron precision

A new system of VHF ultrasonic resonance was developed for the accurate measurement of plate thickness. A PVDF film excites 40 to 60 MHz waves in a plate under study, and the thickness resonance is detected via the same transducer. This technique is useful for plates of 0.1 to 10 mm thickness and yields relative accuracy as high as 0.1 μm when the plate thickness is 0.1 mm. Two examples of industrial measurements are given: thickness of a rolled invar plate and an aluminium beverage can.