Optical pickup device minimizing an undesirable astigmatism

Details Optical pickup device minimizing an undesirable astigmatism Optical pickup device minimizing an undesirable astigmatism

1999-10-05

2002-03-26

Huber, Paul W. (Department: 2651)

Dynamic information storage or retrieval

With servo positioning of transducer assembly over track...

Optical servo system

C369S044370, C369S053190, C369S112050

Reexamination Certificate

active

06363038

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to an optical pickup device for use in an optical information recording/reproducing apparatus that records and/or reproduces signals from an optical information medium such as an optical disc.
2. Description of the Related Art
For a recording/reproducing apparatus for recording/reproducing information on an optical disc, loaded therein, such as an optical video disc, a digital audio disc, and so on, a focus servo and a tracking servo are essential for always accurately converging light beams for writing and reading information to a pit train or the like formed spirally or concentrically on a recording surface of the optical disc. The focus servo performs a positional control for an objective lens, used to irradiate a pit train on the optical disc with light beams, in an optical axis direction so as to reduce a focus error, i.e., an error of the position of the objective lens in the optical axis direction with respect to the focus position of the objective lens. The tracking servo performs a positional control for the position of the objective lens, used to irradiate a pit train on the optical disc with a light beams, with respect to a recording track in a radial direction of the optical disc, so as to reduce a tracking error, i.e., an error of the objective lens with respect to the pit train recording track position.
FIG. 1
illustrates a conventional optical pickup device using the astigmatism method.
A laser beam from a semiconductor laser
1
is transformed into a parallel laser beam by a collimator lens
2
, passes through a polarizing beam splitter
3
, and is converged by an objective lens
4
toward an optical disc
5
to form a light spot onto a pit train on an information recording surface of the optical disc
5
.
Light reflected from the optical disc
5
is converged by the objective lens
4
and directed by a beam splitter
3
to a detecting lens
7
. A converged light beam formed by the detecting lens
7
passes through a cylindrical lens
8
, serving as an astigmatism generating element, to form a spot image near the center ‘O’ of a light receiving surface of a quadrant photodetector
9
having four light receiving surface areas (elements) divided by two orthogonal line segments. The cylindrical lens
8
irradiates the quadrant photodetector
9
with a light spot SP in the shape of true circle as illustrated in
FIG. 2A
when the laser beam is converged on the recording surface of the optical disc
5
in focus, and an elliptic light spot SP, extending in an orthogonal direction of the elements as illustrated in
FIG. 2B
or
2
C when the converged laser beam is out of focus on the recording surface of the optical disc
5
(
FIG. 2B
illustrates the light spot SP when the objective lens
4
is too far from the optical disc
5
, while
FIG. 2C
illustrates the light spot SP when the objective lens
4
is too near the optical disc
5
), thus generating so-called astigmatism.
The quadrant photodetector
9
opto-electrically transduces the light spot irradiated to the four light receiving surface areas into respective electric signals which are supplied to a focus error detecting circuit
12
. The focus error detecting circuit
12
generates a focus error signal (FES) based on the electric signals supplied from the quadrant photodetector
9
and supplies the focus error signal to an actuator driver circuit
13
. The actuator driver circuit
13
supplies a focusing driving signal to an actuator
15
. The actuator
15
moves the objective lens
4
in response to the focusing driving signal in the optical axis direction.
The focus error detecting circuit
12
, as illustrated in
FIG. 3
, is connected to the quadrant photodetector
9
, where the quadrant photodetector
9
is composed of four detecting elements DET
1
to DET
4
in first to fourth quadrants which are located adjacent to each other with two orthogonal division lines L
1
and L
2
interposed therebetween and which are independent of each other. The quadrant photodetector
9
is positioned such that the division line L
2
is in parallel with a tangential direction with respect to the extending direction of the recording track, and the other division line L
1
is in parallel with the radial direction of the same. Respective opto-electrically transduced outputs from the elements DET
1
and DET
3
, symmetric with respect to the center ‘O’ of the light receiving surface of the quadrant photodetector
9
, are added by an adder
22
, while respective opto-electrically transduced outputs from the elements DET
2
and DET
4
, also symmetric with respect to the center ‘O’ of the light receiving surface, are added by an adder
21
, and outputs from the respective adders
21
and
22
are supplied to a differential amplifier
23
. The differential amplifier
23
calculates the difference between the supplied signals, and outputs a signal indicative of the difference therebetween as a focus error signal (FES).
As described above, in the conventional focus error detecting circuit
12
, the outputs of the quadrant photodetector
9
are added by the adders
21
and
22
, respectively, and the differential amplifier
23
calculates the difference between the outputs of the adders
21
and
22
to generate a focus error component. In this event, when the light beam is in focus, the light spot in the shape of true circle as illustrated in
FIG. 2A
is formed on the quadrant photodetector
9
, where a spot intensity distribution is symmetric with respect to the center ‘O’ of the light receiving surface of the quadrant photodetector
9
, i.e., symmetric in the tangential direction and in the radial direction, so that the values resulting from the additions of the opto-electrically transduced outputs from the elements on the diagonals are equal to each other, with the focus error component being calculated to be “zero”. On the other hand, when the light beam is out of focus, i.e., an elliptic light spot extending in a diagonal direction as illustrated in
FIG. 2B
or
2
C is formed on the quadrant photodetector
9
, so that the values resulting from the additions of the opto-electrically transduced outputs from the elements on the diagonals are different from each other. Thus, the focus error component output from the differential amplifier
23
exhibits a value corresponding to the focus error. Specifically, assuming that the references designated to the elements of the quadrant photodetector
9
represent the outputs thereof, the focus error signal FES is expressed by the following equation:
FES=(DET
1
+DET
3
)−(DET
2
+DET
4
)
Since the objective lenses of conventional CD players have a small numerical aperture and a large focal depth, slight noise if appeared on a focus error signal (FES) would be negligible as a focus error. In a case where information is read from an optical disc having lands and grooves, such as a DVD-RAM, however, the numerical aperture of the objective lens becomes larger and the focal depth thereof becomes shallower, so that the influence of noise contained in the focus error signal on the focus servo of the objective lens becomes greater.
In conventional optical pickup apparatuses which read information from an optical disc having pre-grooves, therefore, the focus servo system cannot follow up noise on an FES, thus raising problems like oscillation of the focus servo circuit and heating of the actuator.
OBJECT AND SUMMARY OF THE INVENTION
Accordingly, the present invention has been made in view of the problem mentioned above, and thus an object thereof is to provide an optical pickup apparatus capable of adequately eliminating noise from a focus error signal that is generated at the time a light spot crosses a track or groove in the astigmatism method, particularly, the noise component that is originated from a further astigmatism of the optical system or birefringence of the substrate of an optical disc.
An optical pickup device according to the present invention comprises an objective len

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