Optical pickup device

Details Optical pickup device Optical pickup device

1999-08-05

2002-05-21

Edun, Muhammad (Department: 2651)

Dynamic information storage or retrieval

With servo positioning of transducer assembly over track...

Optical servo system

C369S044230, C369S044250, C369S112010

Reexamination Certificate

active

06392965

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an optical pickup device for use in optical disk devices which optically record information in and/or reproduce information from a data recording medium such as an optical disk. Specifically, the present invention relates to an optical pickup device which enables accurate recording and reproducing operations using an optical disk having a plurality of recording and reproducing layers.
BACKGROUND OF THE INVENTION
Since optical disks are capable of recording large quantities of information signals at high density, they are being increasingly used in recent years in fields such as audio, video, computers, etc. Recent innovations include recording media which aim to increase recording capacity by recording signals on a plurality of recording layers, and optical systems which aim to reproduce recorded signals at high speed by simultaneously reading signals from a plurality of tracks using a plurality of light beams.
In the foregoing recording media provided with a plurality of recording layers, if the respective recording and reproducing surfaces are too close together, when the light beam is accessing a given recording and reproducing surface, light reflected from that recording and reproducing surface is influenced by light reflected from adjacent recording and reproducing surfaces. In this case, a focus error signal, for focusing adjustment of the light beam, is also subject to the foregoing influence, and thus accurate focusing adjustment cannot be performed.
In an attempt to provide an optical system able to resolve the foregoing problem, the present Applicant has previously proposed the optical pickup device shown in
FIG. 11
(Japanese Unexamined Patent Publication No. 9-161282/1997 (Tokukaihei 9-161282), published on Jun. 20, 1997).
In the optical pickup device shown in
FIG. 11
, light projected by a semiconductor laser
1
passes through a holographic element
2
, a collimating lens
3
, and an objective lens
4
, and is converged on an optical disk
5
. Light reflected therefrom passes through the objective lens
4
and the collimating lens
3
, and is directed to the holographic element
2
.
As shown in FIG.
12
(
b
), the holographic element
2
is divided into three divisions
2
a,
2
b,
and
2
c
by a dividing line
2
g,
running in a y direction corresponding to the radial direction of the optical disk
5
, and a dividing line
2
h,
running from the center of the dividing line
2
g
in an x direction perpendicular to the radial direction of the optical disk
5
, i.e., a direction corresponding to the track direction of the optical disk
5
.
As shown in FIG.
12
(
a
), a photoreceptor element
7
includes four rectangular photoreceptive domains
7
a,
7
b,
7
c,
and
7
d
arranged along the x direction corresponding to the track direction of the optical disk
5
. The central photoreceptive domains
7
a
and
7
b
(photoreceptive domains for focusing) are divided from one another by a dividing line
7
y
running in the y direction corresponding to the radial direction of the optical disk
5
, and auxiliary photoreceptive domains
7
e
and
7
f
are provided on the outer sides of the photoreceptive domains
7
a
and
7
b.
The foregoing photoreceptive domains are arranged such that when the light beam is focused on a recording surface of the optical disk
5
, reflected light diffracted by the division
2
a
of the holographic element
2
forms a beam spot P
1
on the dividing line
7
y,
and reflected light diffracted by the divisions
2
b
and
2
c
forms beam spots P
3
and P
2
on the photoreceptive domains
7
c
and
7
d,
respectively.
Then, if Sa, Sb, Sc, Sd, Se, and Sf are output signals from the photoreceptive domains
7
a,
7
b,
7
c,
7
d,
7
e,
and
7
f,
respectively, then a focusing error signal FES is calculated as (Sa+Sf)−(Sb+Se). By this means, an FES curve can be corrected so as to be optimum for a recording medium with a plurality of recording layers.
The following will explain in detail, with reference to FIGS.
13
(
a
) through
13
(
e
), only the photoreceptive domains
7
a,
7
b,
7
c,
and
7
d
and the beam spot P
1
, which relate to FES. In a focused state, as shown in FIG.
13
(
a
), the beam spot P
1
, which is reflected light for focusing, is focused on the dividing line
7
y.
As the optical disk
5
gets farther away, as shown in FIGS.
13
(
b
) and
13
(
c
), the beam spot P
1
first spreads into the photoreceptive domain
7
b,
and is finally incident on the photoreceptive domain
7
f
as well; as the optical disk
5
gets closer, as shown in FIGS.
13
(
d
) and
13
(
e
), the beam spot P
1
first spreads into the photoreceptive domain
7
a,
and is finally incident on the photoreceptive domain
7
e
as well.
In
FIG. 14
, a curve of the focusing error signal FES=(Sa+Sf)−(Sb+Se) is shown as a solid line. Here, outside the pull-in range between −d
1
and +d
1
where the curve converges with zero, the curve can be brought back to
0
more steeply than the focusing error signal FES when the auxiliary photoreceptive domains
7
e
and
7
f
are not provided (=Sa−Sb), shown as a broken line, which returns to
0
more gradually. In this case, when reproducing, for example, a two-layer optical disk
5
in which the distance between the layers is d
2
, the FES curve will be as shown in
FIG. 15
, giving two independent FES curves (for the two layers) having a sufficiently small FES offset, and thus enabling normal focus servo to be performed.
However, when assembling the optical pickup, there is naturally some assembly error. If the optical pickup is ideally assembled, it is possible, as above, to reduce offset of the focusing error signal by means of light reflected from adjacent recording and reproducing layers, but in the event of assembly error, this changes the shape of the light reflected to the photoreceptor element when focusing operations are performed, which changes the focus error signal correction quantity and makes it impossible to obtain a good FES curve when reproducing an optical disk having a plurality of recording and reproducing layers.
Accordingly, in order to resolve the foregoing difficulties, the present Applicant proposed an optical pickup which, by optimizing the shape of the auxiliary photoreceptive domains, enables accurate recording and reproducing operations on an optical disk having a plurality of recording and reproducing layers, even if assembly error arises during assembly of the optical pickup (Japanese Unexamined Patent Publication No. 10-222867/1998 (Tokukaihei 10-222867), published on Aug. 21, 1998).
The optical pickup disclosed in Japanese Unexamined Patent Publication No. 10-222867/1998 has the same structure as the optical pickup discussed above, but differs in that the shape of the auxiliary photoreceptive domains is optimized by setting their width in the x direction.
The following will explain disturbance of the FES curve which arises in the conventional optical pickup discussed above (Japanese Unexamined Patent Publication No. 9-161282/1997) due to error in assembly.
In particular,
FIG. 16
shows an FES curve when the holographic element
2
shown in FIGS.
11
and
12
(
b
) is misadjusted with an offset in the +x direction with respect to an optical axis determined by the semiconductor laser
1
and the collimating lens
3
. In this case, in a greatly defocused state, a large peak is produced when defocusing is in the far direction (at around +d
2
). In this state, reproducing, for example, a two-layer optical disk in which the distance between the layers is d
2
gives rise to an FES offset of &Dgr;d
1
, and a correctly focused state cannot be obtained.
Since FES offset due to incorrect positioning of the holographic element, tolerance of the various members, laser wavelength aberrance, etc. is generally adjusted to
0
by rotation adjustment of the holographic element
2
, it does not create a problem in single-layer optical disks, but in a greatly defocused state, the

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