Katsuhiro Sasaki

Low-Frequency Air-Coupled Ultrasonic System beyond Diffraction Limit Using Pinhole

A novel noncontact air-coupled ultrasonic system introducing a conical acoustic probe (CAP) with a pinhole is constructed at low frequency. A 40 kHz ultrasonic wave in air goes through the pinhole, which has a diameter much smaller than the ultrasonic wavelength λ (9 mm), and forms a unique sound field. The lateral -3 dB widths of both theoretical and experimental sound fields approximately coincide with the pinhole diameter. A lateral resolution of λ/30 (0.3 mm) beyond the diffraction limit has been achieved using this system. As an application of the system, the surface profile for a 0.5 mm (λ/18) drilled hole in an aluminum plate was successfully imaged. The surface topographical profile of a step difference of 1 µm (λ/9000) was also measured.

Nanometer-Order Resolution Displacement Measurement System by Air-Coupled Ultrasonic Wave Introducing Maximum Phase-Sensitivity Tuning

An improved system introducing an automatic tuning of the maximum phase sensitivity of ultrasonic signals is constructed for detecting displacements with a nanometer-order resolution. The phases of multiple-reference waves introduced in our previous system are controlled and appropriately set to tune the phase sensitivity. To evaluate our system, the displacement of a high-precision mechanical stage was measured using 40 kHz air-coupled ultrasonic waves. The resolution of the phase detection for our system is improved more than ten times our previous system. A resolution of 40 nm (λ/200,000) displacement has been achieved with a measurable displacement range of 8 mm (λ).

Precise Displacement Measurements Using Phase Information of 40 kHz Ultrasonic Waves in Pinhole-Based Air-Coupled Ultrasonic System

Noncontact precise displacement measurements, including surface profiling, were demonstrated using the phase information of narrow-band 40 kHz continuous waves (wavelength λ=9 mm) in a pinhole-based air-coupled ultrasonic system beyond the diffraction limit. Real-time displacement measurements with a distance resolution of λ/900,000 (10 nm) were accomplished using a highly sensitive phase variation when the distance between the pinhole and the surface of an object was in the range from approximately 5 to 10 µm. To extend the working distance of our system while maintaining a high resolution, an automatic distance control system was constructed and introduced in our system. Using this advanced system, a surface topographical profile of 16 µm height was successfully measured, while maintaining maximum phase sensitivity. These results demonstrate the usefulness of our system when applied to noncontact precise displacement measurements.

Submicrometer-order displacement measurements using an air-coupled ultrasonic transducer at frequencies of 40 and 400 kHz

Noncontact air-coupled ultrasonic systems for measuring micrometer- or submicrometer-order displacements are newly constructed at frequencies of 40 and 400 kHz. The phase detection accuracy of the displacement measurements under several thousandths of ultrasonic wavelengths λ is markedly improved by the introduction of phase differences between the reference waves and the transmitting wave signal. To verify our system, the displacement of the reflector, which is translated by a high-precision mechanical stage, is measured using a single piezoelectric ceramic transducer in the reflection mode. Resolutions of displacement of better than 1 µm or 0.1 µm (λ/8500) using ultrasonic waves of 40 or 400 kHz in air, respectively, are achieved.

Air-coupled ultrasonic time-of-flight measurement system using amplitude-modulated and phase inverted driving signal for accurate distance measurements

This paper proposes an amplitude-modulated and phase-inverted (AMPI) ultrasonic driving signal for accurate distance measurements. Undesirable part of the ultrasonic wave was canceled using the phase-inverted wave and its active canceling effect was enhanced by the amplitude-modulation, so that a unique ultrasonic waveform having a sharp envelope could be generated even with a narrow-band transducer of frequency 40kHz (wavelength λ ≅ 9mm). The sharp envelope generated by the AMPI signal determined a zero-crossing time without any uncertainty. As a result, an accuracy better than 0.02mm was achieved in the range of 0.1 - 0.5m.

Observation of Hexatic Vortex Glass in Al-Doped YBa2Cu3O7-x Single Crystals

The high-resolution Bitter pattern technique has been used to observe the vortex structure in 4% Al-doped YBa2Cu3O7-x single crystals at low magnetic fields in the range of 5-20 G. Analysis of the images showed development of both translational and orientational order in the vortex arrangement with an increase of the magnetic field. However, in all fields, the translational order decayed much faster than the orientational order, indicating the existence of a hexatic vortex glass state. Comparison of the results obtained with the existing data on Ba2.1Sr1.9Ca0.9Cu2O8+x and twin-free regions of YBa2Cu3O7-x suggests a similar behaviour of vortex ordering in all three materials.

Detection of second harmonic components of Lamb waves passed through fatigue tested magnesium plate

Lamb waves are transmitted into pure magnesium plate which is carried out fatigue test and the second harmonic components generated from the plates are detected. Second harmonic components had increased as the number of fatigue test cycles producing the stress faults was increased. At 100 thousand and 200 thousand cycles, second harmonic components had increased by approximately 3 dB and 6 dB respectively compared with an unstressed plate.

Analysis of sound field profile formed by conical acoustic probe with pinhole

This paper analyzes sound pressure field profiles formed by a conical acoustic probe (CAP) with a pinhole much smaller than the ultrasonic wavelength λ. The analysis is done using the Rayleigh integral and the finite element method (FEM). Both calculated results reveal that lateral -3dB widths near the pinhole are approximately equal to the pinhole diameter and that the shape of the CAP does not affect the lateral width. A lateral resolution of the CAP, which is much smaller than λ, is verified by our analysis.