Marie Tabaru

Measurement of Elastic Properties of Tissue by Shear Wave Propagation Generated by Acoustic Radiation Force

Acoustic radiation force (ARF) imaging has been developed as a novel elastography technology to diagnose hepatic disease and breast cancer. The accuracy of shear wave speed estimation, which is one of the applications of ARF elastography, is studied. The Youngs moduli of pig liver and foie gras samples estimated from the shear wave speed were compared with those measured the static Youngs modulus measurement. The difference in the two methods was 8%. Distance attenuation characteristics of the shear wave were also studied using finite element method (FEM) analysis. We found that the differences in the axial and lateral beam widths in pressure and ARF are 16 and 9% at F-number=0.9. We studied the relationship between two branch points in distance attenuation characteristics and the shape of ARF. We found that the maximum measurable length to estimate shear wave speed for one ARF excitation was 8 mm.

Ultra-Sensitive Strain and Temperature Sensing Based on Modal Interference in Perfluorinated Polymer Optical Fibers

We implement the strain and temperature sensors based on multimode interference in perfluorinated (PF) graded-index (GI) plastic optical fibers, and investigate their sensing performance at 1300 nm. We obtain strain and temperature sensitivities of -112 pm/με/m and +49.8 nm/°C/m, the absolute value of which are 12.9 and over 1800 times as large as those in silica GI multimode fibers, respectively. These ultra-high strain and temperature sensitivities probably originate from the unique core material, i.e., PF polymer.

Measurement of large-strain dependence of optical propagation loss in perfluorinated polymer fibers for use in seismic diagnosis

Brillouin scattering in perfluorinated graded-index (PFGI-) polymer optical fibers (POFs) has been extensively studied for structural health monitoring, including seismic diagnosis. Here, we measure the propagation loss of PFGI-POFs at telecom wavelengths as a function of large applied strain (up to 100%) at three optical powers and as a function of strain rate at a constant optical power. The strain dependence of the propagation loss in PFGI-POFs is found to be independent of the incident power (<27 dBm) and the strain rate (<500%s−1), indicating that PFGI-POF-based Brillouin sensors are potentially applicable to actual seismic monitoring.

Strain and temperature sensing based on multimode interference in partially chlorinated polymer optical fibers

We develop strain and temperature sensors based on multimode interference in a partially chlorinated graded-index polymer optical fiber (PCGI-POF) and experimentally investigate their sensing performance at room temperature using incident light of ∼1300 nm wavelength. The length of the PCGI-POF was 0.7m. The measured strain and temperature sensitivities were −4.47 pm/με and +9.66 nm/°C/m, respectively, the absolute values of which were 0.29 times and over 350 times the values in a silica GI-MMF. This result suggests that the modal interference in PCGI-POFs is potentially applicable to high-sensitivity temperature measurement with low strain sensitivity.

Polymer optical fiber tapering without the use of external heat source and its application to refractive index sensing

We perform a pilot trial of the highly convenient taper fabrication of perfluorinated graded-index polymer optical fibers. Instead of conventional external heating, we utilize internal heating caused by high-power propagating light (500 mW in this experiment). An approximately 4-mm-long section of a polymer fiber is tapered, and the outer diameter of the ~2-mm-long waist around its midpoint is approximately 200 µm, which is quite uniform with a standard deviation of 4.3 µm. The polymer fiber taper fabricated by this technique is shown to be capable of generating evanescent waves and thus measuring the refractive indices of liquids from 1.333 to 1.410.

Ultrasonic motors with polymer-based vibrators

With their characteristics of low density and elastic moduli, polymers are promising materials for making ultrasonic motors (USMs) with high energy density. Although it has been believed for a long time that polymers are too lossy to be applied to high-amplitude vibrators, there are several new polymers that exhibit excellent vibration characteristics. First, we measure the damping coefficients of some functional polymers to explore the applicability of polymers as vibrators for USMs. Second, to investigate the vibration characteristics, we fabricate bimorph vibrators using several kinds of polymers that have low attenuation. Third, a bending mode USM is fabricated with a polymer rod and four piezoelectric plates bonded on the rod as a typical example of a USM. Through an experimental investigation of the motor performance, it was found that the polymer-based USMs exhibited higher rotation velocity than the aluminum-based USM under a light preload, although the maximum torque of the polymer-based USMs was smaller than the aluminum-based USM. Among the tested polymers, polyphenylenesulfide was a prospective material for USMs under light preloads because of the high amplitude and lightweight of polyphenylenesulfide.

Equivalent Circuit Analysis and Design of Multilayered Polyurea Ultrasonic Transducers

In this paper, we carried out equivalent circuit analysis for a multilayered transducer including the backing and load effects. The calculated resonance frequencies showed good agreement with previously measured values. The improved multilayered transducers in this study exhibit good characteristics for output sound pressure when loaded with water. We also demonstrated designs for a four-layer polyurea transducer with a frequency of up to 220 MHz. As a result, designs for a wide frequency range can be obtained by varying the polyurea thickness.

Drastic sensitivity enhancement of temperature sensing based on multimodal interference in polymer optical fibers

It has been reported that temperature sensors based on modal interference in perfluorinated graded-index polymer optical fibers show extremely high temperature sensitivity at room temperature. In this work, we confirm that the temperature sensitivity (absolute value) is significantly enhanced when the temperature increases toward ?70 ?C, which is close to the glass-transition temperature of the core polymer. When the core diameter is 62.5 ?m, the sensitivity at 72 ?C at 1300 nm is 202 nm/?C/m, which is approximately 26 times the value obtained at room temperature and >7000 times the highest value previously reported using a silica multimode fiber.

Measurement of mechanical quality factors of polymers in flexural vibration for high-power ultrasonic application.

A method for measuring the mechanical quality factor (Q factor) of materials in large-amplitude flexural vibrations was devised on the basis of the original definition of the Q factor. The Q factor, the ratio of the reactive energy to the dissipated energy, was calculated from the vibration velocity distribution. The bar thickness was selected considering the effect of the thickness on the estimation error. In the experimental setup, a 1-mm-thick polymer-based bar was used as a sample and fixed on the top of a longitudinal transducer. Using transducers of different lengths, flexural waves in the frequency range of 20-90kHz were generated on the bar. The vibration strain in the experiment reached 0.06%. According to the Bernoulli-Euler model, the reactive energy and dissipated energy were estimated from the vertical velocity distribution on the bar, and the Q factors were measured as the driving frequency and strain were varied. The experimental results showed that the Q factors decrease as the driving frequencies and strains increase. At a frequency of 28.30kHz, the Q factor of poly(phenylene sulfide) (PPS) reached approximately 460 when the strain was smaller than 0.005%. PPS exhibited a much higher Q factor than the other tested polymers, which implies that it is a potentially applicable material as the elastomer for high-power ultrasonic devices.

Traveling wave ultrasonic motor using polymer-based vibrator

With the characteristics of low density, low elastic modulus, and low mechanical loss, poly(phenylene sulfide) (PPS) is a promising material for fabricating lightweight ultrasonic motors (USMs). For the first time, we used PPS to fabricate an annular elastomer with teeth and glued a piece of piezoelectric-ceramic annular disk to the bottom of the elastomer to form a vibrator. To explore for a material suitable for the rotor surface coming in contact with the PPS-based vibrator, several disk-shaped rotors made of different materials were fabricated to form traveling wave USMs. The polymer-based USM rotates successfully as the conventional metal-based USMs. The experimental results show that the USM with the aluminum rotor has the largest torque, which indicates that aluminum is the most suitable for the rotor surface among the tested materials.