Hiroyasu Yoshikawa

A Magnetic Coil Suitable for Magneto-Optical Recording with a Large Working Distance

A first surface magneto-optical (MO) system has a high storage density and a high data-transfer rate. The high-density recording is achieved using a blue laser and lenses that have a high numerical aperture (more than 0.85) and that are integrated with a magnetic-field modulation coil. If the working distance is more than 30 µm, the optical head can be controlled by using a mechanical actuator, for which many existing technologies are available. We have developed a magnetic coil suitable for working distances of up to 30 µm. First, we analyzed the relationship between the properties of a magnetic layer and a magnetic field and then from our analysis derived three design concepts. We then designed a magnetic coil with a fan-shaped magnetic layer composed of stick-shaped magnetic bodies and heat sinks; testing showed that it can be driven at a data transfer rate of 80 Mbps and that it has a wide temperature margin.

Image deblurring using the direction dependence of camera resolution

The blurring that occurs in the lens of a camera has a tendency to further degrade in areas away from the on-axis of the image. In addition, the degradation of the blurred image in an off-axis area exhibits directional dependence. Conventional methods have been known to use the Wiener filter or the Richardson–Lucy algorithm to mitigate the problem. These methods use the pre-defined point spread function (PSF) in the restoration process, thereby preventing an increase in the noise elements. However, the nonuniform degradation that depends on the direction is not improved even though the edges are emphasized by these conventional methods. In this paper, we analyze the directional dependence of resolution based on the modeling of an optical system using a blurred image. We propose a novel image deblurring method that employs a reverse filter based on optimizing the directional dependence coefficients of the regularization term in the maximum a posterior probability (MAP) algorithm. We have improved the directional dependence of resolution by optimizing the weight coefficients of the direction in which the resolution is degraded.

Image enhancement with blurred and noisy image pairs using a dual edge-preserving filtering technique

Taking a satisfactory image using a hand-held camera under dim lighting conditions is difficult because there is a tradeoff between exposure time and motion blur. If the image is taken with a short exposure time, the image is noisy, although it preserves sharp details. On the other hand, an image taken with a long exposure time suffers from motion blur but has good intensity. In this paper, we propose a two-image approach that adaptively combines a short-exposure image and a long-exposure image for the image stabilization function on digital cameras. Our method combines only the desirable properties of the two images to obtain a less-blurry and less-noisy image. Furthermore, the proposed approach uses a dual edge-preserving filtering technique for the edge areas of the long-exposure image, which are more affected by motion blur.

A New Optical System for Holographic Memory Applying a Finite Objective Lens

We have developed a new objective lens for a holographic storage system to increase areal density. To balance focal length and working distance, we adopted a finite objective lens. By incorporating phase conjugate reproduction into the finite objective lens, we obtained practically applicable and adequate signal quality. We measured shift selectivity and the effect of crosstalk and evaluated book size, which was reduced by ca. 0.8. The areal density was estimated to increase by about 1.6 times when using the new objective lens.

Design of an optical pickup using double holographic lenses

An optical pickup system for optical disc drives utilizes optical elements for information signal detection, focus-error detection and track-error detection. We propose an optical pickup using double holographic lenses--replacing conventional optical elements for focus-error detection and track-error detection--to reduce the pickup size. There are two important requirements when using holograms for an optical pickup. One is to compensate for chromatic aberration caused by laser diode wavelength deviation, and the other is to obtain an appropriate beam spot size for focus-error detection. To meet these requirements, we used double holograms, where one has divergent power and the other convergent power. By optimizing the double hologram pattern, the optical system is achromatized. We observed the achromatic effect, 0.07 (lambda) (RMS) for a laser diode wavelength change of +/- 10 nm. The beam position error was less than +/- 1 micrometers . We reduced the NA of the system to give a relatively large spot beam and a large depth of focus.