Optical disk apparatus

Details Optical disk apparatus Optical disk apparatus

1999-10-29

2001-10-02

Hindi, Nabil (Department: 2753)

Dynamic information storage or retrieval

With servo positioning of transducer assembly over track...

Optical servo system

C369S044340, C369S047270, C369S047550, C369S053230

Reexamination Certificate

active

06298019

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an optical disk apparatus that optically records signals in a recording medium and reproduces the recorded signals using a laser beam emitted from a light source, such as a laser. More specifically, the invention relates to an optical disk apparatus equipped with a focus control system for controlling focusing of the light beam irradiating the recording medium so that the light beam is focused on a prescribed position of the recording medium.
BACKGROUND OF THE INVENTION
There is an optical disk apparatus in which a light beam emitted from a light source, such as a semiconductor laser, is focused on a disk type recording medium rotating at a prescribed speed, and signals are recorded in, or reproduced from, the recording medium, as described in Japanese Published Patent Application No. Hei. 7-129968. The disk type recording medium has a spiral or concentric tracks. The width of the tracks is about 0.6 &mgr;m, and the pitch of the tracks is about 1.5 &mgr;m. In order to record signals on the tracks or reproduce signals recorded on the tracks, a focusing of the light beam irradiating the recording medium is controlled so that the light beam is focused on a prescribed position of the recording medium.
FIG. 19
is a block diagram illustrating an example of an optical disk apparatus including a focus control system.
The apparatus shown in
FIG. 19
comprises a light source
1
, such as a semiconductor laser, that emits a light beam
8
toward a disk
7
(recording medium), a coupling lens
2
, a polarization beam splitter
3
, a polarizing plate
4
, a focusing lens
5
, and a disk motor
6
for rotating the disk
7
at a prescribed speed. A light beam
8
emitted from the light source
1
is collimated by the coupling lens
2
, and reflected by the polarization beam splitter
3
to the polarizing plate
4
. The light beam then travels through the polarizing plate
4
and the focusing lens
5
, and is focused on the disk
7
rotated by the disk motor
6
.
This apparatus further comprises a condenser lens
9
and a split mirror
10
as elements receiving a light beam reflected at the disk
7
. The reflected light beam travels through the focusing lens
5
, polarizing plate
4
, the polarization beam splitter
3
, and the condenser lens
9
, and is then split into two beams
11
and
15
by the split lens
10
. The light beams
11
and
15
are applied to a focus control system and a tracking control system, respectively.
The focus control system comprises a two-element photodetector
12
, a preamplifiers
13
A and
13
B, a differential amplifier
14
, a phase compensator
18
, a linear motor
19
, a switch
33
, a driving circuit
35
, a focus control element (focus actuator)
36
, a logic circuit
40
, a comparator
41
, and a chopping wave generator
42
. The two-element photodetector
12
has two light responsive parts A and B. Output signals from the light responsive parts A and B are amplified by the preamplifiers
13
A and
13
B, respectively, and are inputted to the differential amplifier
14
. A knife edge detection is realized by the condenser lens
9
and the split mirror
10
, and a signal output from the differential amplifier
14
is a focus error signal (hereinafter referred to as FE signal).
The phase of the FE signal in the focus control system is compensated by the phase compensator
18
, and the switch
33
opens or closes a loop of the focus control system. When the focus control system is closed by the switch
33
, the FE signal output from the phase compensator
18
is sent to the driving circuit
35
through the switch
33
. The driving circuit
35
amplifies the FE signal and sends the FE signal to the focus control element
36
. In this structure, when the focus control system is in the closed state, the focus control element
30
is driven so that the light beam is always focused on a prescribed position of the disk
7
. Further, an output signal from the chopping wave generator
42
is also input to the switch
33
. The FE signal is also input to the logic circuit
40
through the comparator
41
. The logic circuit
40
controls the opening and closing operation of the switch
33
.
The linear motor
19
moves the focusing lens
5
, the focus control element
36
, the polarization beam splitter
3
and the like in the direction transverse to the tracks on the disk
7
. The linear motor
19
is operated when the focal point of the light beam is moved to a prescribed track.
On the other hand, the light beam
15
from the split mirror
10
is input to the two-element photodectector
16
in the tracking servo system. The photodetector
16
has two light responsive parts C and D, and a difference between output signals from the respective light responsive parts C and D becomes a track error signal. The light beam on the disk
7
in controlled by this track error signal to correctly scan the tracks on the disk
7
. Since the present invention does not relate to the tracking control, a detailed description of the tracking control is omitted here.
In the optical disk apparatus with the focus control system shown in
FIG. 19
, the focus control is performed as described below.
Initially, the disk
7
is rotated by the disk motor
6
. When a prescribed rotating speed is reached, the switch
33
selects the chopping wave generator
42
, and the focus control element
36
is operated in response to a signal output from the chopping wave generator
42
, whereby the focusing lens
5
is moved up and down, i.e., in the direction perpendicular to the recording face of the disk
7
. Thereby, the focal point of the light beam on the disk
7
moves up and down. At this time, an S-shaped FE signal (hereinafter referred to as S signal), which appears when the focal point of the light beam passes through the recording face, is detected by the comparator
41
. By the detection of the S signal, the logic circuit
40
knows whether the focal point of the light beam is positioned in the vicinity of the recording face or not. When the focal point is positioned in the vicinity of the recording face, the logic circuit
40
controls the switch
33
to select the phase compensator
18
. In this way, the focus servo loop is closed, and the focus control (focus lead-in) is performed so that the light beam is focused on a prescribed target position.
The focus lead-in will be described with reference to FIGS.
20
(
a
),
20
(
b
),
21
, and
22
. FIGS.
20
(
a
) and
20
(
b
) illustrate a waveform of a focusing lens driving signal and a waveform of an FE signal having S signals, respectively, at the focus lead-in.
FIG. 21
illustrates a waveform for explaining the relationship between the focus lead-in level and S signals that appear in the FE signal at a protection film at the surface of the disk
7
and at the recording film when the focusing lens
5
comes close to and goes away from the disk
7
.
FIG. 22
is a flow chart showing a fundamental focus leadin procedure in the focus control system.
As shown in
FIG. 22
, when the reading and reproducing apparatus is turned on, the disk motor
6
is turned on and the disk
7
is rotated (step S
21
). When the disk
7
reaches a prescribed rotating speed, the light source
1
is turned on, and the semiconductor laser emits light (step S
22
). Subsequently, the linear motor
19
is driven to move the focusing lens
5
toward the inner circumference of the disk
7
(step S
23
). The above-mentioned initial operation is followed by the focus lead-in operation.
In the focus lead-in operation, initially, the focusing lens
5
is moved down away from the disk
7
, in response to an output signal from the chopping wave generator
42
(step S
24
). Thereafter, the focusing lens
5
is moved up toward the disk
7
(step S
25
). While repeating the up and down movement of the focusing lens
5
, it is detected that the S signal reaches a prescribed lead-in level (step S
26
). After the prescribed lead-in level is reached, the logic circuit
40
controls the switch
33
to select the phase compensato

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