Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Reexamination Certificate
2000-10-24
2004-08-10
Huber, Paul W. (Department: 2653)
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
C369S109010
Reexamination Certificate
active
06775221
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical pickup and a recording and/or reproducing apparatus focusing a laser beam on an optical disk.
2. Description of the Related Art
FIG. 1
is a view of the configuration of an optical pickup of the related art.
This optical pickup
1
has a semiconductor laser
4
, a collimator lens
5
, a beam splitter
3
, an objective lens
2
, a focus lens
6
, a cylindrical lens
7
, and a photo detector
8
.
The semiconductor laser
4
outputs a linear polarized laser beam and supplies the same to the collimator lens
5
.
The collimator lens
5
collimates the laser beam from the semiconductor laser
4
to a parallel beam and supplies the same to the beam splitter
3
.
The beam splitter
3
passes the laser beam from the collimator lens
5
therethrough and supplies it to the objective lens
2
.
The objective lens
2
condenses the laser beam from the beam splitter
3
and supplies it to a track of an optical disk
80
.
Further, the objective lens
2
returns the laser beam reflected at the optical disk
80
to the beam splitter
3
.
The beam splitter
3
receives the laser beam from the optical disk
2
and reflects the incident laser beam and supplies it to the focus lens
6
.
The focus lens
6
focuses the laser beam from the beam splitter
3
and supplies it to the cylindrical lens
7
.
The cylindrical lens
7
passes the laser beam from the focus lens
6
and supplies it to the photo detector
8
.
The photo detector
8
receives the laser beam from the cylindrical lens
7
at its light receiving portion and generates an output signal.
FIG. 2
is an explanatory diagram of the configuration of the light receiving portion of the photo detector
8
.
The photo detector
8
is a four-divided photo detector obtained by equally dividing a light receiving portion
8
S to four by two division lines
8
Sx and
8
Sy.
The light receiving portion
8
S has four divided regions
8
A to
8
D. In the light receiving portion
8
S of
FIG. 2
, a beam spot MS is formed by the laser beam from the cylindrical lens
7
.
The direction of the generatrix of the cylindrical lens
7
forms an angle of about 45 degrees or about 135 degrees with respect to the direction of the division line
8
Sx or division line
8
Sy of the light receiving portion
8
S.
The intersecting point of the division lines
8
Sx and
8
Sy is located at the center or substantially the center of the laser beam passing through the cylindrical lens
7
.
The shape of the beam spot MS formed at the light receiving portion
8
S changes in a diagonal direction in accordance with the distance between the optical disk
80
and the objective lens
2
, so it is possible to detect focus deviation at the optical disk
80
by an astigmatism method based on output signals generated by the divided regions
8
A to
8
D.
A focus error signal FE is expressed by the following equation (1) by using output signals SA to SD generated by the divided regions
8
A to
8
D.
FE=SA+SC−(SB+SD) (1)
Summarizing the problem to be solved by the invention, in an optical disk of a structure having lands and grooves, the focus error signal FE becomes a different value depending on whether the position of the focus is on a land or is in a groove. This will be explained by referring to FIG.
3
.
FIG. 3
is a graph showing various focus error signals with respect to the lands and the grooves. A focus error signal in the case where the track is a land is indicated by a dotted line LA, while a focus error signal in the case where the track is a groove is indicated by a dotted line GR.
The curves LA and GR of the focus error signals shown in
FIG. 3
are expressed as values of percentage (100×FE/&Sgr;) obtained by standardizing (normalizing) the value obtained by equation (1) by a sum &Sgr; (=SA+SB+SC+SD) of the output signals SA to SD for convenience. Further, the relative distance from the focus position (focused position) where the laser beam from the objective lens is condensed to a recording surface of the optical disk is defined as an amount of defocus.
In a mirror disk, an optical disk of a structure with no grooves, when the recording surface is located at the focused position, the value of the focus error signal becomes 0.
In an optical disk having a land and groove structure, however, when the recording surface of the optical disk is located at the focused position and the beam is focused (where the amount of defocus is 0), the value of the focus error signal does not become 0. This is because diffraction of light due to the lands and/or grooves occurs when the laser beam is reflected at the track, interference of the light occurs at the light receiving portion of the photo detector, and an offset occurs in the focus error signal at the time of focusing.
Further, when the ratio of the widths of the lands and the grooves, the depth of the grooves, etc. differ due to the differences of the optical disks, the value of the detection error in focus detection at the focused position changes. Namely, accurate detection of the focus error has been difficult in the method of the related art not only in optical disks of the land and groove structure, but in all optical disks having any groove widths and depths.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an optical pickup and a recording and/or reproducing apparatus capable of preventing interference of light in a photo detector.
A laser beam reflected at a track of an optical disk having land and/or grooves includes 0-th order diffraction light and +1st order diffraction light created due to the diffraction by the lands and/or grooves.
It can be considered that, by passing this laser beam containing the 0-th order diffraction light and the +1st order diffraction light through a cylindrical lens and supplying this to the photo detector, interference of light occurs between a region in which the 0-th order diffraction light and the +1st order diffraction light overlap and a region in which they do not overlap, beam spots of the light receiving portion form an asymmetric intensity distribution with respect to the division lines at the time of focus, and therefore the focus error signal does not become 0 at the time of focus as shown in
FIG. 3
described above.
FIG. 4
is an explanatory view of the state of the laser beam (returned laser beam) reflected at the beam splitter
3
and supplied to the focus lens
6
in the optical pickup
1
of
FIG. 1
, in which a contour of the 0-th order diffraction light is indicated by a solid line ring, and the contour of the +1st order diffraction light is indicated by a broken line ring.
A distance d
0
up to the center of the +1st order diffraction light with respect to the center of the 0-th order diffraction light is expressed by the following equation (2) by a pitch p in a radius direction of the groove, a wavelength &lgr; of the laser beam, and a numerical aperture NA of the objective lens
2
:
d
0
=(&lgr;/NA)/p (2)
Here, the radius of the 0-th order diffraction light determined by the radius of the objective lens
2
was standardized (normalized) and defined as 1. The radius of this 0-th order diffraction light is equal to the radius of the +1st order diffraction light and corresponds to the radius (or aperture) of the objective lens
2
.
In the explanatory diagram of
FIG. 4
, the case where d
0
=1, that is the case where &lgr;/NA=p, is exemplified, and the contours of the +1st order diffraction light contact each other. If parameters such as the wavelength &lgr;, numerical aperture NA, and the pitch p are determined, the region in which the 0-th order diffraction light and the +1st order diffraction light overlap (overlapping region) and the region where they do not overlap (nonoverlapping region) are uniquely determined.
In the optical pickup and the recording and/or reproducing apparatus according to the present invention, the ov
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