Optical pickup device

Details Optical pickup device Optical pickup device

1999-08-03

2002-01-15

Edun, Muhammad (Department: 2651)

Dynamic information storage or retrieval

With servo positioning of transducer assembly over track...

Optical servo system

C369S044120, C369S044250, C369S112010

Reexamination Certificate

active

06339562

ABSTRACT:

TECHNICAL FIELD
The present invention relates to an optical pickup device installed in an optical disk device for optically recording/reproducing information to/from an information recording medium such as an optical disk, or more specifically, relates to an optical pickup device capable of precisely recording/reproducing information to/from an optical disk having a plurality of recording/reproduction layers.
BACKGROUND ART
Recently, practical application of optical disks is promoted in audio, video, computer, and other various fields since an optical disk is capable of recording massive information signals at high density.
In compact disks (CD), video disks, mini disks (MD), computer-use magneto-optical disks, and the like which are now widely put on the market, a 1.2 mm thick substrate is usually used. An objective lens of an optical pickup is also usually designed so as to correct aberration which occurs due to the 1.2 mm thick substrate.
On the other hand, various techniques are examined to increase recording capacity, including a technique of improving optical resolution by increasing a numerical aperture (NA) of an objective lens, and a technique of improving a recording layer is multilaminate.
For example, the Japanese Publication for Laid-Open Patent Application No. 5-151609/1993 (Tokukaihei 5-151609) discloses an optical disk device for reproducing information from an optical disk having a plurality of data layers so that data recorded in the data layers are separately reproduced from the respective data layers. The recording/reproduction layers of the foregoing multilaminate disk are formed by alternately laminating transparent substrates and aerial layers. Information is recorded/reproduced by shifting a focus of an objective lens in an optical axis direction by driving it with an actuator.
In the foregoing example, return light from recording/reproduction layers adjacent to the target layer does not affect since the layers are disposed at sufficient distances. For example, a focus error signal (FES) of an n'th layer becomes 0 when an (n+1)'th or (n−1)'th layer is brought into focus, thereby causing no affect such as offset on FESs of other layers. However, since the layers are disposed at great distances, in the case where focus servo is applied to each layer, a total thickness of the whole disk substrate to be brought into focus greatly varies. Therefore, it is also necessary to correct spherical aberration which is generated on each layer, by using an aberration compensator.
As a disk free from the foregoing problem, a double-layer disk having two data layers at a distance (for example, 40 &mgr;m to 70 &mgr;m) which is very small as compared with a thickness of the substrate has been proposed as a digital versatile disk (DVD) or the like. In this case, spherical aberration occurring due to a difference in the substrate thickness is sufficiently small, and hence no aberration compensator is needed.
However, as to a disk in which recording/reproduction layers are laminated at such a small distance, when a light beam accesses one recording/reproducing surface, a reflected light from the accessed recording/reproducing surface is affected by return light from other recording/reproducing surfaces adjacent to the accessed surface. Therefore, a focus error signal for focus adjustment of the light beam is also affected by the return light, and as a result, precise focus adjustment cannot be conducted.
An example of an optical system for use with the foregoing multilaminate disk in which layers are formed at sufficiently small distances is disclosed, for example, in the Japanese Publication for Laid-Open Patent Application No. 9-161282/1997 (Tokukaihei 9-161282). The optical system is arranged so that, as shown in
FIG. 16
, light from a semiconductor laser
1
is converged onto an optical disk
5
by an objective lens
4
, and a returned light therefrom is led to a light receiving element
6
by a three-division hologram element
2
. The semiconductor laser
1
, the hologram element
2
, and the light receiving element
6
are integrally provided.
As shown in
FIG. 17
, the light receiving element
6
has (i) two main light receiving sections
6
a
and
6
b
for focus error signal (FES) detection use, provided adjacent to each other, and (ii) sub light receiving sections
6
e
and
6
f
for focus error signal compensation use, provided outside the main light receiving sections
6
a
and
6
b,
respectively. Either the main light receiving section
6
a
or
6
b
receives return light, depending on the direction of the focus adjustment. The sub light receiving sections
6
e
and
6
f
are disposed at positions such that they detect light when, in a defocus state, the return light falls also outside the main light receiving sections
6
a
and
6
b.
Thus, by using a pair of beam spots formed with the focus-error-detection-use return light on the sub light receiving sections
6
e
and
6
f,
a focus error is detected and a focus error signal is generated.
More specifically, let output signals of the light receiving sections
6
a,
6
b,
6
e,
and
6
f
be Sa, Sb, Se, and Sf, respectively, then, the focus error signal FES can be computed by (Sa+Sf)−(Sb+Se). As a result, when the light beam is projected to outside the main light receiving section
6
a
(or
6
b
) as well in a defocus state due to a displacement beyond the dynamic range, an output signal of the sub light receiving section
6
e
(or
6
f
) becomes intense, whereby the focus error signal FES becomes weaker. Thus, the focus error signal FES is intensified as a displacement from the just focus position increases, and thereafter abruptly weakens when the displacement exceeds a certain level. Therefore, by appropriately setting the sizes and arrangement of the main light receiving sections
6
a
and
6
b
and the sub light receiving sections
6
e
and
6
f,
only the focus error signal obtained from the recording/reproducing surface scanned can be used as an effective signal, while influences of return light from the recording/reproduction layers adjacent to the scanned layer can be eliminated.
Thus, theoretically, a precise focus error signal is obtained by the foregoing optical pickup device, and therefore, a recording/reproducing operation can be carried out with precision.
In assembling the pickup device, however, an assembly error naturally exists. In the focus adjusting operation, such an error makes the change of shape of the beam spot formed with return light on the light receiving element
6
different from the normal change thereof, thereby causing compensation of the focus error signal to become excessive or insufficient. As a result, it is impossible to obtain an adequate FES curve in recording/reproducing information to/from an optical disk having a plurality of recording/reproduction layers.
Further, in the case where an assembly error occurs, the return light to light receiving sections
6
c
and
6
d
for radial error signal production use is projected thereon with displacement from an ideal position. Therefore, the light receiving sections
6
c
and
6
d
need to be formed to greater sizes with all errors taken into consideration, so that the return light never fails to fall within the light receiving sections
6
c
and
6
d,
even when the displacement is greatest. On the other hand, a signal frequency band of the light receiving element is required to be higher, as capacity of an optical disk such as a DVD increases. To cope with such a requirement, however, it is necessary to reduce a size of the light receiving element, and this is contradictory to the aforementioned requirement of enlarging the light receiving element.
DISCLOSURE OF THE INVENTION
The present invention was made in light with the above-described problems, and the object of the present invention is to provide an optical pickup device capable of performing precise recording/reproducing operations with respect to an optical disk having a plurality of recording/reproduction layers even with as

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