Shogo Yagi

Crystallization process of Sb‐Te alloy films for optical storage

Sb‐Te alloy films are developed as rewritable optical recording materials based on amorphous crystalline phase transformations. The crystallization process of Sb‐Te sputtered films is systematically studied through measurement of recording characteristics to solve the trade‐off problem between data (amorphous) stability and erasing sensitivity. Sb2Te3 is shown to be the best practical phase change medium, having room‐temperature stability in amorphous states, short erasing times, and potentially good reversibility. The carrier‐to‐noise ratio of 50 dB in writing and a decrease in the carrier level of over 30 dB in erasing are achieved in dynamic measurement with a single beam optical head. These favorable properties are attributable to the wide composition margin for a single phase formation in the Sb‐Te alloy system.

New Beam Scanning Model for High-Speed Operation Using KTa1-xNbxO3 Crystals

We propose a new beam scanning model that is applicable to electrooptic materials with electron traps. With this model, we can achieve both high-speed operation and wide-angle scanning, because the operating speed is limited not by the electron mobility but by the frequency limit of the electrooptic effect of the materials. The voltage dependence of the scanning angle at 100 kHz using a KTa1-xNbxO3 crystal is consistent with the property predicted by the proposed model.

Fast Response Variable Focal-Length Lenses Using KTa1-xNbxO3 Crystals

We fabricated variable focal length lenses with fast responses by using the strong Kerr effect of KTa1-xNbxO3 single crystals. We observed focus shifts of about 40 mm with the assistance of a 250-mm-focal-length lens, which corresponds to a focus shift from infinity to 1.56 m without the assisting lens. The time response was as fast as 1 µs.

Fast response varifocal lenses using KTa 1−x Nb x O 3 crystals and a simulation method with electrostrictive calculations

We fabricated cylindrical varifocal lenses with fast responses by using the strong Kerr effect of KTa(1-x)Nb(x)O(3) (KTN) single crystals. We observed focus shifts of up to 87 mm with the assistance of a 250 mm focal length lens, which corresponds to a focus shift from infinity to 720 mm by the KTN lens itself. The response time was as fast as 1 μs. We also present a simulation method for calculating refractive index distributions in KTN single crystals, which is essential when designing the lens. The method is characterized by the strain contribution, which has not conventionally been typical of electro-optic simulations. We used this method to explain the refractive index modulations that are characteristic of the varifocal lenses.

Digital Holographic Memory Using Two-Dimensional Modulation Code

The degradation of reconstructed images from a digital holographic memory that uses highly sensitive strontium barium niobate as a recording medium was reduced using two-dimensional modulation codes that have low average intensity and high coding efficiency. The signal-to-noise ratios of the reconstructed images and hence the bit error rates of the data retrieved from the images were improved without reducing the coding efficiency.

Effects of Heat Treatment on Photorefractive Sensitivity of Ce- and Eu-Doped Strontium Barium Niobate

We investigated the effects of heat treatment on the data storage performance of strontium barium niobate doped with cerium and europium. The photorefractive writing sensitivity was increased by elevating the annealing temperature. When the annealing was undertaken at a high temperature, for example 900°C, the sensitivity reached a value about ten times that obtained at an annealing temperature below 600°C. This increase was caused by a photoconductivity increase which was probably due to an increase in the oxygen vacancy concentration. The page multiplexing performance of the crystal is also discussed in relation to the photorefractive trap density.

Orthogonal aperture multiplexing for multilayered waveguide holographic read-only memories

A multilayered waveguide holographic read-only memory is a promising candidate for the next generation of optical data storage systems. We improved the data density of the memory by using a multiplexing method with a set of orthogonal optical masks. We multiplexed as many as nine images into one waveguide hologram, and all the observed images had negligible cross talk. This made it possible to achieve a ninefold increase in data density. We provide experimental results for both metallic and liquid-crystal masks.

Resolution Enhancement of KTa1-xNbxO3 Electro-Optic Deflector by Optical Beam Shaping

We enhance the resolution of a KTa1-xNbxO3 (KTN) deflector by shaping the optical beam. Controlling the beam size by placing a lens in front of the deflector is essential for the efficient utilization of the deflector aperture. The beam-shaping optics were developed based on beam propagation analysis by ray tracing. We demonstrated a 1.5-fold enhancement of the resolution at a repetition rate of 100 kHz.

Improvement of coherence length in a 200-kHz swept light source equipped with a KTN deflector

We are developing a new light source for swept-source OCT, namely, an external-cavity LD equipped with a KTN electro-optic deflector. Being free from mechanical resonance, our 1.3-μm laser exhibits scanning range of almost 100 nm up to 200-kHz under a ±300 V deflector driving voltage. Using a semi-empirically derived equation, we find that KTNs convex lens power degrades the coherence length, and this can be compensated with a cylindrical concave lens. Such compensation was experimentally confirmed by observing reduction of elliptical beam divergence. OCT images of a human fingernail are obtained using the swept source.

Time-sharing display approach using liquid crystal light valve and a photoreractive crystal for electroholography

A new electroholography system is introduced that uses a liquid crystal display, a photorefractive crystal, and a time- sharing display method. By using the photorefractive crystal as a kind of screen through which the hologram is recreated, we can realize high quality, off-axis, and computer-generated holograms. The time sharing approach enables an electronic display device to display 3D objects clearly by breaking them into parts that are displayed in sequential frames. If the fames are cycled at a high enough speed, we see these parts as a single object due to the afterimage effect. This approach relaxes the strict dynamic range requirements that would normally be placed on the electronic display device. The photorefractive crystal suppresses the flicker caused by the time-sharing display method. This paper first describes the time-sharing method. Next, a new experimental 3D-TV system with a photorefractive crystal, strontium barium niobate, is demonstrated. This system confirms the exciting future of electroholography.