Optical head and recording/reproducing device

Details Optical head and recording/reproducing device Optical head and recording/reproducing device

2000-07-11

2004-04-13

Tran, Thang V. (Department: 2653)

Dynamic information storage or retrieval

Specific detail of information handling portion of system

Radiation beam modification of or by storage medium

Reexamination Certificate

active

06721259

ABSTRACT:

TECHNICAL FIELD
The present invention relates to an optical head for recording signals on, and reproduce signals from, an optical recording medium such as an optical disk. The invention also relates to a recording/reproducing apparatus having an optical head of this type.
BACKGROUND ART
Hitherto, various types of optical recording media have been proposed as recording media for data such as video data, audio data or computer programs. Among these media are a playback optical disk, a phase-change optical disk, a magneto-optical disk, an optical card and the like. In recent years, there is the increasing demanded that the recording density and storage capacity of these optical recording media be increased.
In order to increase the recording density of such optical recording media, some measure should be taken in an optical head designed to write and read data signals on and from such an optical recording medium. It is effective to increase the numerical aperture (NA) of the objective lens or shorten the wavelength of the light source, thereby to decrease the diameter of the beam spot formed as the objective lens converges a light beam. The numerical aperture of the objective lens is 0.45 and the wavelength of the light source is 780 nm for CDs (Compact Disks) (trademark), i.e., digital optical disk which were put to practical use in the relatively early stage of optical recording. For DVDs (trademark) i.e., digital optical disks on which data is recorded at higher density on CDs, the numerical aperture of the objective lens is 0.6 and the wavelength of the light source is 650 nm.
As mentioned above, it is desired that the recording density and storage capacity of these optical recording media be increased. Therefore, it is desirable to increase the numerical aperture of the objective lens to a value greater than 0.6 and to shorten the wavelength of the light source to a value less than 650 nm.
However, various problems will arise if the numerical aperture of the objective lens is increased and the wavelength of the light source is shorted in the optical head described above. These problems are spherical aberration and chromatic aberration, both being optical problems.
Spherical aberration may occur, mainly due to a manufacturing error in the thickness of the lens or in the thickness of the transparent substrate of the optical recording medium. In the case where the objective lens is composed of a plurality of lenses, the spherical aberration may occur due to an assembling error in the gaps among the lenses. As for the error in the thickness of the transparent substrate of the optical recording medium, for example, the three-dimensional spherical aberration resulting from the error in the thickness of the transparent substrate of a disk of “CD” standards or a disk of “DVD” recording is proportional to the fourth power of the numerical aperture (NA) of the objective lens as seen from the equation [1] given below. Therefore, the influence of the thickness error becomes prominent as the numerical aperture of the objective lens increases.
W
40
=(
t
/8)×(
n
2
−1)/
n
2
×NA
4
  [1]
In the case of disks of the “CD” standards or a disk of the “DVD” standards, the manufacturing tolerances are of such values as to sufficiently reduce the spherical aberration resulting from the error in the thickness of the transparent substrate. In the manufacture of these disks, a specific technique is applied, setting the error in the thickness of the transparent substrate within a prescribed tolerance. It is, therefore, not necessary, in particular, to correct the spherical aberration in the optical system of the optical head. If the numerical aperture of the objective lens is further increased, however, the tolerance for the thickness error of the transparent substrate will become extremely small.
The thickness tolerance &Dgr;t for the substrate of, for example, the “DVD” standards is (0.03 mm. To maintain the same tolerance as this, the following relation expressed by the following equation [2] must be satisfied, as seen from the equation [1]:
dt
≦0.00388
/NA
4
  [2]
From this relation the thickness tolerance for the substrate of the disk may be obtained. The tolerance &Dgr;t is ±0.016 mm if the numerical aperture NA of the objective lens is 0.7, ±0.0095 mm if the numerical aperture NA of the objective lens is 0.8, and ±0.0074 mm if the numerical aperture NA of the objective lens is 0.85.
It is difficult, however, to raise the precision of the thickness of the disk substrate, because the error in the thickness of the substrate thickness depends on the method of forming the disk substrate. For the existing disk substrate it is difficult to reliably reduce the thickness error to 10 &mgr;m or less. If the disk substrate is produced in large quantities, the yield will be low. Hence, the substrate may not be fit for mass- production.
The error in the thickness of the lenses constituting the optical system of the optical head will be considered. For molded lenses it is difficult to reliably attain such a small thickness error as 10 &mgr;m or less. The thickness error of the lenses is almost equivalent to the thickness error of the transparent substrate, if it is regarded as the error in the optical path. In the case where the objective lens used has a large numerical aperture, the thickness error of the lenses will, like the thickness error of the transparent substrate, cause intolerable spherical aberration which would adversely influence the reproduced signals.
An objective lens having a large numerical aperture of 0.8 or more needs to be composed of a plurality of lenses. Even if the spacers made of synthetic resin or metal, which are interposed between the lenses, are process with high precision, it will be difficult to decrease the error in the gap between each lens and another to 10 &mgr;m or less. Such an error in the gap between any two adjacent lenses of the objective lens may cause spherical aberration, like die thickness error of the transparent substrate of the optical recording medium. The gap error therefore adversely influences the signals.
In consideration of the spherical aberration occurring in the optical recording medium and in the optical head, it is virtually difficult to reduce the spherical aberration, in terms of the thickness error of the transparent substrate of the medium, to 10 &mgr;m or less in the case where use is made of an optical system that comprises an objective lens having a numerical aperture of 0.8 or more and composed of a plurality of lenses.
If a semiconductor laser that exhibits a short emission wavelength is used, the problem of spherical aberration will arise, too. It is desired that not only spherical aberration, but also chromatic aberration be corrected.
DISCLOSURE OF THE INVENTION
The present invention has been made in consideration of the foregoing. The object of this invention is to provide an optical head comprising an objective lens composed of a plurality of lenses and having a numerical aperture of 0.8 or more, in which the spherical aberration resulting from the manufacturing errors of the optical recording medium and the lenses, and the chromatic aberration resulting from a semiconductor laser, if used as the light source of the optical head, is sufficiently decreased. Further, the invention aims to reduce the spherical aberration that occurs when the same optical system is used to light beams having different wavelengths.
Another object of this invention is to provide a recording/reproducing apparatus comprising such an optical head as described above, in which the recording density and storage capacity of an optical recording medium can be enhanced and increased.
As indicated above, it is extremely difficult to suppress the manufacturing error of the optical components so much as to reduce the spherical aberration to a negligible degree in an optical system that comprises an objective lens composed of a plurality of lenses and havi

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