Takeaki Yoshimura

Statistical properties of dynamic speckles

The statistical properties of dynamic speckles produced by a moving diffuse object were reviewed by providing the space–time correlation function and the power spectrum of speckle-intensity fluctuation for five combined cases of both the optical configuration and the illumination light. In the optical configuration, three kinds of geometry (free-space, single-lens, and double-lens) were taken, and three kinds of illumination light (a Gaussian beam, a plane-wave beam, and a Gaussian Schell-model beam) were used. Consequently, it was shown that the cross-correlation function and the power spectrum are both Gaussian under some assumptions. From the dynamic properties, two types of speckle motion, boiling and translation, were also evaluated for various conditions of object motion, optical configuration, and illumination light.

Investigation of calcium-iron-silicate glasses by the Mössbauer method

Abstract The oxidation-reduction and coordination of iron atoms in calciumsilicate glasses has been studied as functions of the basic oxide content and partial oxygen pressure on the basis of Mossbauer spectra. It was found that the equilibrium concentration ratio NFe3+/NFe2+ increased as the CaO content or partial oxygen pressure increased. The coordination behavior of iron atoms was complicated. In the glasses containing a large amount of Fe2O3, the Fe2+ ion was always present in the octahedral site, while the Fe3+ ion showed amphoteric behaviour. The ratio in number of tetrahedrally coordinated to octahedrally coordinated ferric ions did not exhibit any remarkable variation for larger partial oxygen pressures (1 and 0.21 atm), but increased slightly with the CaO/SiO2 ratio for the small oxygen pressure (3 × 10−7 atm). In the glasses containing a small amount of Fe2O3, the Fe2+ ion was present in both tetrahedral and octahedral sites. However, the coordination state of the Fe3+ ion was not sufficiently clear in such glasses.

Surface-roughness dependence of the intensity correlation function under speckle-pattern illumination

The surface-roughness dependence of the intensity correlation function of the speckle pattern, produced in the Fresnel region with fully developed speckle-pattern illumination, has been theoretically investigated and has been discussed as follows. This correlation function is represented by two correlation functions of scattered and unscattered components. As the diffuse object becomes rough, the speckle size varies from the speckle size of the illumination light to that obtained with the condition, so that the object is a deep phase screen. The speckle contrast, however, is always one.

Reflection-type holographic disk memory with random phase shift multiplexing

A reflection-type holographic disk memory system with random phase shift multiplexing is proposed. The experimental results show that a binary data page of 18x17 bits is recorded successfully at intervals of 4 mum in a Fe:LiNbO3 crystal with a thickness of 0.5 mm when six data pages are superimposed. Numerical results show that random phase modulation can improve the shift selectivity in shift multiplexing recording as well as in data security. Experimental and numerical results show that reflection-type holographic disk memory has a high potential for terabyte storage capacity as in transmission-type memory.

Statistical properties of doubly scattered image speckle

Statistical properties of laser light doubly scattered by two diffusers ordered along the optical axis are studied. If the first and the second diffusers are both in motion, it is well known that the doubly scattered amplitude does not generally obey Gaussian statistics. However, if the first diffuser remains in motion and the second diffuser is fixed at the experimental system, then the doubly scattered amplitude is shown to obey circular Gaussian statistics. Although the moment of the intensity fluctuation is independent of the surface roughness of the second diffuser, it is shown that the spatial-intensity correlation function depends on the surface roughness.

Dynamic properties of three-dimensional speckles

Dynamic properties of three-dimensional speckles formed in free space on a diffuse object by an illuminating Gaussian beam have been investigated by evaluation of the space–time cross-correlation function. It has been found that the major axis of three-dimensional static speckles is always directed to the center of the beam spot on the object but that dynamic speckles are directed to a position other than the center according to the velocity of the object. Moreover, the motion of dynamic speckles is composed of two types: shifting and tilting.

Fast acquisition system for digital holograms and image processing for three-dimensional display with data manipulation

A three-dimensional (3D) digital holographic display system with image processing is presented. By use of phase-shifting digital holography, we obtain the complex amplitude of a 3D object at a recording plane. Image processing techniques are introduced to improve the quality of the reconstructed 3D object or manipulate 3D objects for elimination and addition of information by modifying the complex amplitude. The results show that the information processing is effective in such manipulations of 3D objects. We also show a fast recording system of 3D objects based on phase-shifting digital holography for display with image processing. The acquisition of 3D object information at 500 Hz is demonstrated experimentally.

Optimum determination of speckle size to be used in electronic speckle pattern interferometry

Characteristics of the fringe pattern detected by an electronic speckle pattern interferometer, in conditions in which a test object deforms in an arbitrary direction and the speckle intensity is detected over a pixel area in the TV camera to be used, have been investigated from two aspects: speckle noise reduction and fringe contrast. The main result is that the fringes are obtained with high contrast and low speckle noise, if the speckle size is selected by the optical system so as to be smaller than the pixel size. This result is applicable to highly accurate measurements of the out-of-plane displacements of the test object, whose in-plane displacement is small.

Simultaneous measurements of three-dimensional reflectivity distributions in scattering media based on optical frequency-domain reflectometry.

A new type of tomography system based on optical frequency-domain reflectometry is presented. Using an area sensor, the system can simultaneously measure three-dimensional reflectivity distributions in scattering media without the need for mechanical scanning. In preliminary experiments we demonstrate that a target (resolution chart) placed behind biological tissue can be imaged with high depth resolution (47 microm) for a short measurement time.

Image-based numerical evaluation techniques in volume holographic memory systems

A simulation technique for a volume holographic memory system based on binary page data is proposed to evaluate quantitative diffraction efficiency, bit error rate, and signal-to-noise ratio. In this simulator, propagation of a signal and reference beams and diffraction at gratings are calculated quantitatively by use of numerical Fresnel propagation and Kogelniks coupled-wave theory. To apply Kogelniks wave theory to arbitrary signal and reference beams, both beams at the hologram plane are decomposed into plane waves. Numerical results show good agreement with the exact solution under the Born approximation in a transmission-type volume hologram. We also apply the simulator to reflection-type holographic memory.