Sachiko Hatano

New method of obtaining particle diameter by the fast Fourier transform pattern of the in-line hologram

A new method of determining particle diameter from a farfield in-line hologram is proposed. The method is based on the following properties of an in-line hologram of small particles. The fringe interval of the hologram decreases with radius from the symmetrical axis. Therefore, the corresponding spatial frequency continuously increases with radius. The Fourier transformed (FT) pattern of the hologram preserves the properties similar to those of the original fringe pattern. In the FT plane, the uniform background and scattering term contribute only to the very low frequency region. The interference term, therefore, can be figured out, and it is the only contribution to the higher frequency region. It is shown that the FT pattern has a white ring zone, which corresponds to the zero point of the envelope function of the original fringe pattern. Since the position of the ring contains information on the particle diameter, it is demonstrated that measurement of the diameter of this ring provides a powerful and reliable way of obtaining particle diameter.

Optical method of measuring nanosecond pressure pulse in water

Abstract We have developed a method of recording nanosecond water shock pressure pulses by an optical method. A high-pressure shock front can be detected by the interface between water and a BK7-glass prism, where a He–Ne laser beam is focused. The method utilizes the fact that the critical angle of total internal reflection depends on the density of the medium under compression. After the shock wave front arrives at the water–glass interface, part of the incident light beam is transmitted into the water specimen. The ratio of the light intensity of reflected and transmitted beam depends on the values of the refractive index of water and BK7-glass under shock pressure. In this study, a shock wave in water is generated by focusing a Nd:YAG laser beam to an intentionally roughened surface of PMMA block in water. He–Ne laser beam is incident on the BK7-glass prism interface at an angle slightly less than the critical angle, and an avalanche photodiode monitors the reflected light intensity. The pressure history at the interface can be estimated. Estimated pressure value is very sensitive to the angle of incidence. The time resolution of the method is mainly limited by the focused spot size of He–Ne laser on the interface.

Transient structure of ionized atmosphere in pulsed laser propulsion system

Optical breakdown of air is explored by using high-speed photography for the realization of laser propulsion system in aerospace engineering. The multiple ionization and subsequent pear-shaped emission by laser pulse through a convex lens are recorded and analyzed by image converter camera. The improved pulsed-laser shadowgraphy employed in this system successfully enables us to visualize the transient structures of complicated shock waves more clearly than ever. The ionization just above the surface of an aluminum target is also examined in comparison with the case of no target, which may be the major mechanism of Myrabos demonstration of a small flyer launching by a pulsed carbon dioxide laser. Not only the high- enthalpy states of the ionized atmosphere are calculated but also the precise history of breakdown initiation in the nanosecond laser pulse is obtained.

Nanosecond pressure pulse profile visualization based on the density dependence of the critical angle of total internal reflection

New method is presented to observe the nanosecond pressure pulse field developed by the pulse laser energy deposition into the water. The present method is based on the pressure dependence of the refractive index of water, and the change in pressure can be visualized by the change in the reflectivity of the optical prism-water interface. Two procedures are tested to record the pulse laser induced high-pressure shock wave front in water, that is, (1) monitoring the time evolution of the laser intensity by a photomultiplier reflected from a point on the interface, and (2) taking an instantaneous photograph of the interface. They will give the pressure-time profile by the procedure, and to give the pressure distribution at that instant. Experimental results shows the feasibility of the method.

FFT analysis method for pulse laser in-line hologram of small particles

This paper describes a novel idea of image analysis method of pulse laser in-line hologram of particles to obtain information on particle diameter. Present method is based on the following property of the hologram. Fringe interval of the hologram decreases with radius, and the corresponding spatial frequency increases with radius. Fourier transformed pattern of the hologram, therefore, has the similar properties. We have proposed here the use of this FFT pattern of the hologram for the image analysis. It is shown that the FFT pattern has a white ring zone, which corresponds to the zero-point of the envelope function of original fringe pattern. It is demonstrated that measurement of the diameter of this ring gives a reliable way of obtaining particle diameter. Feasibility of the method has been shown by numerical experiment for Fraunhofer and Fresnel hologram.

Analytical Fourier transform of Fraunhofer hologram pattern of a wire

An exact functional form of the Fourier transform of the Fraunhofer hologram pattern of a wire has been derived. Since the spatial frequency of the fringe pattern increases with distance from the symmetric axis, the same tendency is preserved in the Fourier transformed pattern of it. Although the frequency spectrum of the Fourier transformed pattern resembles that of the original hologram fringe pattern, one gets a quite different impression from the Fourier transformed image, if it is shown as an absolute value. A clear dark zone can be easily observed, which corresponds to the zero-amplitude and extremum points of the sinc function of the original hologram pattern. This property is useful for the estimation of wire diameter.

Observations of bubble motions and distortions by streak camera

Air bubbles are observed at the moment of there emergence from a vertical nozzle in water. We can classify them into two types by formation process. The bubbles broken off from a large air bulk always emit sound pulses, but those not being split but keeping their initial volume hardly produce any sound. By comparing their motions and distortions not only by means of the sequential series of photographs but also by the streak photographs of the vertical linear portion of the axisymmetric bubble, we obtain the difference of the subtle distortion of their surface which causes the emission of sound pulse.

Visualization of submicron particles

A new laser holographic technique has been developed to visualize sub-micron particles. If the size of the particle is comparable with or less than wavelength, it is very difficult to obtain the image of small particles by the conventional optical method.We have obtained the holographic interference fringes by means of superposition of the reference light and the scattered light of comparable intensity by a particle irradiated at the beam waist of focused coherent light. The two-dimensional particle location is measured in terms of the center of holographic fringes, and the three dimensional depth of particle position from the recording plane is obtained by the reconstruction of the hologram.