Optical pick-up and optical recording system

Details Optical pick-up and optical recording system Optical pick-up and optical recording system

1997-11-26

1999-11-16

Edun, Muhammad

Dynamic information storage or retrieval

Specific detail of information handling portion of system

Radiation beam modification of or by storage medium

369121, 369110, 369 4423, 369 4412, G11B 712

Patent

active

059869968

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to an optical pick-up and an optical recording system for performing a process such as a recording process or a reproducing process to an optical recording medium such as an optical disk.
2. Description of the Related Art
As an optical disk which is recently available, a compact disk (CD) based on the compact disk standards (CD standards) is known. The substrate of the compact disk has a thickness of 1.2 mm, and a track pitch formed on the surface of the substrate to record information is 1.6 mm. In order to reproduce the CD, an optical pick-up comprising a laser diode for emitting a laser beam having a wavelength of 0.78 mm and an objective lens having a numerical aperture of 0.45 is used. In recent years, various developments were made to increase the recording density of the optical disk, and it is examined in the developments to increase the numerical aperture of the objective lens. When the numerical aperture of the objective lens is increased, the optical resolution is improved. For this reason, the linear recording density on the optical disk can be increased, and the pitch of recording tracks can be decreased. Therefore, the recording density can be considerably increased.
However, when the numerical aperture of the objective lens is increased, a phenomenon that the aberration of a light spot focused by the objective lens occurs. More specifically, due to axial runout of the optical disk and axial runout of a turntable which has the optical disk attached thereto and is rotated, inclination (so-called tilt) of the optical disk and the objective lens occurs, and a coma aberration occurs in this light spot due to this tilt. For this reason, when only the numerical aperture is increased, focusing performance is not improved, and an optical disk having a high recording density cannot be realized.
For this reason, it is considered that, by decreasing the substrate of the optical disk in thickness, the numerical aperture of the objective lens is increased while preventing degradation of an aberration (coma aberration) caused by tilt. On the bas is of this consideration, standardization of the high-density optical disk advances. One of the standards is the standard of a digital video disk (DVD). In this DVD standard, the thickness of the optical disk is set to 0.6 mm, and the track pitch is set to 0.74 mm. To this DVD, a recording or reproducing operation is performed by using an optical pick-up for irradiating a laser beam having a wavelength of 0.65 mm or 0.635 mm through an objective lens having a numerical aperture of 0.6.
When t he optical disk having a thin substrate based o n this standard is used, a recording density can be increased. However, since the objective lens of the DVD optical pick-up is suitably designed for a thin optical disk, a spherical aberration increases when the objective lens is applied to an optical disk having a conventional thick substrate, thereby degrading imaging performance. For this reason, the DVD optical pick-up cannot perform a recording/reproducing operation to a CD. Further development has been made to prevent the above disadvantages. It is considered that a laser beam emitted from the optical pick-up is split into two focusing points to be converged, thereby coping with optical disks having different thicknesses. For example, Japanese Unexamined Patent Publication No. 7-65407 describes the following technique. That is, a light flux emitted from a laser diode is split by adding a prism or a hologram to an optical system, and the light fluxes are converged by different optical lengths, respectively, thereby forming two beam spots. In Japanese Unexamined Patent Publication No. 7-98431, a composite objective lens is formed by combining a deflective objective lens and a hologram lens to form two beam spots.
However, in the optical pick-up described above, in order to make it possible to process optical disks having different thicknesses, one laser beam is split into two beam spots to be

REFERENCES:
patent: 4566088 (1986-01-01), Yoshida et al.
patent: 4841514 (1989-06-01), Tsuboi et al.
patent: 5483511 (1996-01-01), Jewell et al.

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