Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Patent
1993-08-26
1995-08-15
Epps, Georgia Y.
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
369 4432, 369 58, 369112, G11B 709
Patent
active
054426154
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to an optical disc apparatus and more particularly, is suitable to apply to, for example, a computer memory, music information recording apparatus, and an image information recording apparatus.
BACKGROUND ART
Nowadays information expands, so it is necessary to use a high density optical disc as a recording or memorizing device of music and image information of the computer memories, compact discs, video discs and the like.
As a one method of making high density, there is a process of enlarging an open number NA of the object lens of an optical pick up optically recording and reproducing information more than that of the conventional compact discs and video discs.
When the open or aperture number NA of the object lens is enlarged as described above, a comatic aberration of a spot focussed on the optical disc is enlarged in case the optical disc skewed. As a result, for example reproducing wave form strain is enlarge or increases, symbols are connected, and crosstalk is generated between memory tracks on the optical disc.
When a shape of the optical spot on an information recording surface of the optical disc reproducing apparently changes to an oval one along not only a radial direction but also a recording track direction, and further a long axis direction of the oval light spot coincides with an extending direction of the recording track, between-signals-interference is generated in a reproduced signal. Consequently, two pits of sequential pits are made near to each other in a high density recording resulting in an impossibility of correct or effective reproducing the recorded information.
On the contrary, when the long axis of the oval is coincided with a radial direction of the optical disc, crosstalk is generated between recording tracks. Consequently, when the recording is done at a high density narrowing the track pitch, it becomes impossible to correctly reproduce recorded information.
In addition, in the optical disc reproducing apparatus, a comatic aberration makes a point spread function and a line spread function of the light spot non-symmetrical. So, it becomes impossible to effectively correct any reproducing wave form strain by means of, for example, an ordinal fixed three-tap type equalizer.
That is, when the optical disc is used as a compact disc, for example, it has a transparent substrate of a thickness of about 1.2 mm. Light beam is shone and focussed on the information recording surface through the transparent substrate, and, for example, a reflected light from the recording surface reproduces information signals.
Consequently, when the information recording surface of the optical disc is skewed or inclined relative to light axis of the object lens, comatic aberration is generated proportioning to about three power of the aperture number NA and about one power of skew value.
The comatic aberration is expressed by Zaidel (phonetic)aberration coefficient equation as follows. ##EQU1##
Equation (1) is transferred to the following equation (2), ##EQU2## wherein, W31d is a ternary comatic aberration, Td is a thickness of the transparent substrate of the optical disc, Nd is a refractive ratio of the transparent substrate of the optical disc, and NAO is an aperture number of the object lens.
When NA is 0.6 and the aperture number is so large as 1.33 times of the compact disc, the comatic aberration generated is 2.37 times if the disc skew value is identical with that (aperture number NA=0.45) of the compact disc. In other words, when NA=0.45 of the compact disc, it is supposed that 0.6.degree. of the disc skew is permittable value of the disc skew value becomes so small considerably as of 0.25.degree..
By the way, it is practically difficult to make any disc skew completely remove or small because that warp or wave of the optical disc along its radial direction and tangential direction (circumferential direction), and inclination of the optical disc owing to the tilt of the disc mounting surface from the mounting standard surface.
When a wave fa
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patent: 4780865 (1988-10-01), Yamakawa
patent: 4807212 (1989-02-01), Kaneda et al.
patent: 4847478 (1989-07-01), Sugiura
patent: 5027336 (1991-06-01), Sugiura
Patent Abstracts of Japan, vol. 12, No. 1978 (P-708), 26 May 1988 & JP-A-62 287 484 (Sony Corp.), 14 Dec. 1987.
Eguchi Naoya
Ohsato Kiyoshi
Uemura Kamon
Benotsson W. Patrick
Chu Kim-Kwok
Epps Georgia Y.
Sony Corporation
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