Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
2001-05-15
2003-06-17
Edun, Muhammad (Department: 2655)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S044350, C369S044290
Reexamination Certificate
active
06580670
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of Korean Application No. 2000-27749, filed May 23, 2000, in the Korean Industrial Property Office, the disclosure of which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical pickup capable of detecting a tracking error signal with reduced push-pull offset when an objective lens is shifted in a radial direction of an optical disk, and/or a reproduction signal including less crosstalk from adjacent tracks.
2. Description of the Related Art
The capacity of optical recording media for information recording and reproduction is dependent upon the size of a light spot focused on its information recording surface, and its track pitch whose width is determined in consideration of the light spot size. That is, the smaller the light spot and the track pitch, the greater the recording capacity.
As can be inferred from equation (1) below, the size of the light spot is proportional to the wavelength (&lgr;) of the light, and is inversely proportional to the numerical aperture (NA) of an objective lens that focuses incident light to form the light spot on an information recording surface.
size of the light spot∝&lgr;/
NA
(1)
On the other hand, the track pitch (tp) of an optical recording medium is determined by equation (2) so as to minimize track-to-track interference with respect to a reproduction signal.
t
p
>
0.6
×
λ
NA
(
2
)
For example, for a compact disk (CD), which uses a light source having a wavelength of 780 nm and an objective lens having an NA of 0.45, the track pitch of the CD is determined at 1.6 &mgr;m, which is greater than the result of 1.04 &mgr;m found using equation (2) with a &lgr;=780 nm and an NA=0.45. For a digital versatile disk (DVD), which uses a light source having a wavelength of 650 nm and an objective lens having an NA of 0.6, the track pitch of the DVD is determined at 0.74 &mgr;m (equivalent to a recording density of 4.7 gigabytes), which is greater than the result of 0.65 &mgr;m found using equation (2) using a &lgr;=650 nm and an NA=0.6.
Standard CDs and DVDs are designed to have a track pitch that satisfies the condition of equation (2), and thus considerable degradation of a reproduction signal caused by adjacent tracks does not occur during recording/reproduction operation.
Meanwhile, for future generation DVDs, so-called “high-definition DVDs” (HD-DVDs), which have been developed to increase the recording density up to 15 gigabytes or more, equation (2) above cannot be used to determine a standard track pitch of the HD-DVDs. For example, given that an optical pickup using a 400 nm light source and an objective lens having an NA of 0.6 is used with HD-DVDs, the track pitch calculated using the right side of equation (2) above, is 0.4 &mgr;m. However, the track pitch of 0.4 &mgr;m is too wide to achieve the high recording capability of 15 GB or more. Thus, a standard track pitch of HD-DVDs should be 0.4 &mgr;m or less.
When reproducing information from HD-DVDs currently under development, there is also a problem of crosstalk between adjacent tracks. Thus, there is a need to reduce degradation of a reproduction signal caused by interference between adjacent tracks due to the narrow track pitch.
At the same time, for recordable optical disks, a push-pull signal is detected as a tracking error signal. A drawback of the push-pull signal is that a large offset occurs when the objective lens is shifted in the radial direction of an optical disk. In order to reduce the push-pull offset, a conventional technique uses a grating such that a main light spot and first and second sub-light spots are focused on the main track and adjacent tracks of an optical recording medium, thereby detecting a tracking error signal using a differential push-pull technique.
A conventional optical pickup using such a grating is shown in FIG.
1
. Light emitted from a light source
1
is diffracted and split into a 0th-order diffracted beam
1
a
and ±1st-order diffracted beams
1
b
,
1
c
by a grating
2
. The split beams
1
a
,
1
b
,
1
c
are reflected by a beam splitter
3
and a mirror
4
, collimated by a collimating lens
6
, and focused by an objective lens
5
. As a result, light spots of beams
1
a
,
1
b
,
1
c
are formed on an optical disk
10
as shown in FIG.
2
. The main beam
1
a
, which is the 0th-order diffracted beam, is focused on the target track
12
, and the first and second sub-beams
1
b
and
1
c
are focused to be displaced from the main beam
1
a
by ±½ the track pitch in the radial direction of the optical disk
10
, leading and following the main beam
1
a.
After having been reflected from the optical disk
10
, the main beam
1
a
and the first and second sub-beams
1
b
and
1
c
pass the objective lens
5
and the beam splitter
3
, and are then received by a photodetection unit
8
through a sensing lens
7
to condense incident light on the first, second and third photodetectors
8
a
,
8
b
and
8
c
. The photodetection unit
8
, as shown
FIG. 2
, includes a first photodetector
8
a
to receive the main beam
1
a
, and the second and third photodetectors
8
b
and
8
c
to receive the first and second sub-beams
1
b
and
1
c
, respectively. Each of the first, second and third photodetectors
8
a
,
8
b
and
8
c
performs photoelectric conversion, and consists of two split plates arranged in the radial direction of the optical disk.
A tracking error signal detection unit
20
detects a tracking error signal using a differential push-pull technique, which is used in the conventional optical pickup. The tracking error signal detection unit
20
includes first, second, third and fourth differential parts
21
,
23
,
25
and
29
, and first and second amplifiers
27
and
28
, and detects a tracking error signal, which contains no push-pull offset, by the conventional differential push-pull technique.
In particular, the first through third differential parts
21
,
23
,
25
receive the electrical signals from the first through third photodetectors
8
a
,
8
b
and
8
c
, and output first through third push-pull signals, respectively. A first amplifier
27
amplifies the third push-pull signal from the third differential part
25
with a predetermined gain factor G1, and a second amplifier
28
amplifies the sum of the signal output from the first amplifier
27
and the second push-pull signal from the second differential part
23
with a predetermined gain factor G2. The fourth differential part
29
subtracts the signal output from the second amplifier
28
from the first push-pull signal resulting from the main beam
1
a
, which is output from the first differential part
21
, and outputs a tracking error signal. The gain factors G1 and G2 for the first and second amplifiers
27
and
28
are determined based on the intensities of the main beam
1
a
and the first and second sub-beams
1
b
and
1
c
. The fourth differential part
29
outputs a tracking error signal without push-pull offset and thus the tracking error signal detected in the conventional optical pickup includes no push-pull offset even when the objective lens is shifted in the radial direction of the optical disk.
However, the conventional optical pickup detects the tracking error signal by splitting light emitted from the light source
1
into three beams using the grating
2
, so that the light efficiency of the main beam
1
is insufficient to record information on the optical disk. In addition, during a recording operation, information signals recorded on adjacent tracks are possibly erased by the first and second sub-beams
1
b
and
1
c.
SUMMARY OF THE INVENTION
To solve the above and other problems, it is an object of the present invention to provide an optical pickup suitable for high-density optical recording media having narrow track pitch, in which a single light spot is focused on only a main track of an optical recording
Ahn Young-man
Chung Chong-sam
Kim Tae-Kyung
Suh Hea-jung
Edun Muhammad
Staas & Halsey , LLP
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