Pickup device

Details Pickup device Pickup device

2001-07-03

2003-04-01

Edun, Muhammad (Department: 2655)

Dynamic information storage or retrieval

Specific detail of information handling portion of system

Radiation beam modification of or by storage medium

C369S112230, C369S044140, C369S053180

Reexamination Certificate

active

06542454

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a pickup device for recording or reproducing information to or from an optical information recording medium, such as an optical disc, recorded with information on the track. More particularly, the invention relates to a buffer pad attached to an objective lens or a driving part used in the pickup device.
2. Description of the Related Art
In recent years, the optical discs of the optical information recording media are broadly used as means for recording and reproducing data such as images and sound. In a recording/reproducing apparatus, such information is recorded or reproduced to or from a track of the optical disc by the pickup device relatively moving thereto. To the pickup device, there is a demand for further increase of density and reduction of size. In the objective lens of the pickup device, the numerical aperture for example is given 0.45 for the CD (Compact Disc) and 0.6 for the DVD (Digital Versatile Disc). Furthermore, the effective diameter of the objective lens has decreased from nearly 4.5 mm in the conventional to nearly 3 mm. As an influence of size reduction for the pickup device, so-called the working distance of between the tip end of the objective lens and the surface of the optical disc has been decreased. As compared with the conventional structure, there is increased possibility of collision of the objective lens with the optical disc surface.
Furthermore, the thickness of the transmissive substrate, while 1.2 mm for the CD, is 0.6 mm for the DVD. Because the distance is shortened to the recording surface protected by the transmissive substrate, there is increased influence upon the signal based on the injury on the optical disc surface caused by collision of the objective lens.
Meanwhile, recently the advent of the DVD-RW (Digital Versatile Disc-Rewritable) standard has allowed the user to make record onto the optical disc. During data recording by the user, the presence of injury on the disc surface might cause a serious affection.
For a recording/reproducing apparatus for recording/reproducing information on an optical disc loaded therein, 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 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 an 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 a ¼ wavelength plate
18
, 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
. Converged light formed by the detecting lens
7
passes through an astigmatism generating element
8
such as a cylindrical lens, multi lens and the like, to form a spot 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 multi 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
shown in FIG.
1
), 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
drives 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).
In the 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

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