Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Patent
1992-10-13
1995-04-18
Epps, Georgia Y.
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
369 4428, 369 4441, G11B 700
Patent
active
054084520
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to an optical information recording/reproducing system for an optical information carrier such as an optical magnetic disk or a write-once optical disk provided with grooves for tracking, and more particularly to the detection of tracking information from such an optical information carrier.
BACKGROUND ART
As a background of an optical information carrier, such as an optical magnetic disk or a write-once optical disk, a technique is known in which guide grooves are provided on the optical information carrier and are utilized to control the tracking of beams for recording and reproduction of information. A conventional system mainly used as a tracking information detecting system is called a push-pull system. In the push-pull system, tracking information is obtained by using one light beam and detecting a difference in intensity between .+-. 1st order diffracted lights which are produced when the light beam extends across the guide groove.
However, since the push-pull system utilizes a change in light amount distribution of a far field pattern of reflected light from the optical information carrier corresponding to the difference in intensity between the .+-. 1st order diffracted lights which is produced in accordance with the degree of extension of the light beam across the guide groove, this system is greatly affected by the inclination of the optical information carrier with respect to the optical axis of beam, the form of the guide groove, the deviation of the optical axis of lenses from the optical axis of beam, the positional deviation of light detectors and so on. Therefore, the requirements for the mechanical characteristic of the optical information carrier are severe. As for an optical information recording/reproducing system, sufficient attention must be paid to the precision of assembling/adjusting an optical head, the positional deviation of parts attendant upon the lapse of time, and so on. In addition, even if sufficient attention is paid, it is difficult to reduce the offset of the light beam with respect to the center axis of the track into zero. Also, in order to minimize a lens shift, a two-stage servo system is employed in which a control is made interlocking a coarse actuator and a fine actuator with each other. There results in an optical information recording/reproducing system which is complicated and high in cost.
As a system for solving the above problems there is known a three-beam system in which three beams are used. In the following, the conventional optical information recording/reproducing system for an optical magnetic disk using the three-beam system will be explained by use of FIG. 4. In the figure there is seen a laser diode, a collimator lens 2, a diffraction grating 3, beam shaping prisms 4 and 5, beam splitters 6 and 7, a riser mirror 8, a focusing lens 9, a 1/2 wavelength plate 10, a polarized beam splitter splitter 11, a mirror 12, a converging lens 13, a cylindrical lens 14, light detectors 15 to 17, and an optical magnetic disk 18.
In FIG. 4, a laser beam emitted from the laser diode 1 in a horizontal direction is collimated by the collimator lens 2 and is then developed by the diffraction grating 3 into a zero order diffracted light and .+-. 1st order diffracted lights. Each of those diffracted lights is shaped by the beam shaping prisms 4 and 5 into a light beam having a circular spot form. The zero order diffracted light will be called a main light beam, and the .+-. 1st order diffracted lights will be called subsidiary light beams. Also, the main and subsidiary light beams will generally be termed a light beam.
The light beam shaped by the beam shaping prisms 4 and 5 into a circular spot form is passed through the beam splitters 5 and 6, risen by the riser mirror 8 in a vertical direction and focussed onto the optical magnetic disk 18 by the focusing lens 9.
The light beam impinging upon the optical magnetic disk 18 is reflected by the optical magnetic disk 18. The light beam with which the optical magnetic disk
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Nakamura Shigeru
Ojima Masahiro
Sakemoto Akito
Sugiyama Toshinori
Tsutsumi Zenji
Epps Georgia Y.
Hindi Nabil
Hitachi , Ltd.
Hitachi Maxell Ltd.
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