Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Reexamination Certificate
1999-12-15
2003-11-11
Tran, Thang V. (Department: 2653)
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
Reexamination Certificate
active
06646976
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical pickup apparatus, and more particularly, to an optical pickup apparatus which reduces cross talk caused by adjacent tracks occurring when a high capacity recording medium having a narrow track pitch is recorded or reproduced.
2. Description of the Related Art
In order to increase the capacity of a recording medium, development is centered on using a light source of a shorter wavelength and an objective lens having a larger numerical aperture when recording/reproducing the recording medium. That is, as an optical recording medium, a compact disk (CD) which uses an infrared light source of a wavelength of 780 nm and an objective lens having a numerical aperture of 0.45 is replaced by a digital versatile disk (DVD) which uses a light source of a wavelength of 650 nm and an objective lens having a numerical aperture of 0.6. In addition, to change specifications from the CD to the DVD and enhance the density of recording, development is centered to reduce the track pitch of a recording medium.
However, in the case of a recording medium having a narrow track pitch, the cross talk between adjacent tracks may deteriorate the reproduction of a recorded signal.
When a recording medium is recorded/reproduced by an optical pick-up apparatus, the degree of deterioration of the signal by adjacent tracks is defined by cross talk between the tracks, and an allowable value is set according to a recording medium. For example, in order to acquire a high quality reproduced signal in the case of a DVD-ROM, the DVD is required to have a cross talk value less than −30 dB.
A DVD-ROM uses an objective lens of a numerical aperture of 0.6 and a light source of a wavelength (&lgr;) of 650 nm, is manufactured to have a track pitch of 0.74 &mgr;m, and has a capacity of 4.7 GB. Since the size of a beam spot determining a recording capacity is proportional to &lgr;/NA, in the above case, &lgr;/NA is 1.08. Therefore, the ratio of the track pitch (Tp) to the beam spot is Tp×NA/&lgr;=0.68, and the cross talk between adjacent tracks has a value lower than −30 dB.
Since an HD-DVD which is expected to attract public attention as a high density recording medium in the near future requires a capacity greater than 15 GB, the ratio of the track pitch to the beam spot must be lower than 0.6, and in this case the cross talk caused by adjacent tracks increases. For example, when a HD-DVD uses a light source of a wavelength of 410 nm and an objective lens of a numerical aperture of 0.6, and the track pitch is set to 0.368 &mgr;m so as to have a capacity of 15 GB, cross talk caused by adjacent tracks has a very high value, higher than −20 dB. Therefore, in order to prevent deterioration of a reproduced signal, an optical pickup apparatus which is adapted to reduce cross talk caused by adjacent tracks is required.
FIG. 1
shows a conventional optical pickup apparatus capable of reducing cross talk between adjacent tracks.
Referring to
FIG. 1
, a light emitted from a light source
1
is diffracted by a grating
2
, and is separated into three light beams of a zeroth order diffracted light beam and ± first order diffracted light beams. After the beams are reflected on a beam splitter
3
, the reflected beams are converged by an objective lens
5
and form three light beam spots S
1
, S
2
and S
3
at positions different from each other on an optical disk
10
as shown in FIG.
2
. That is, the zeroth order diffracted light beam forms the light beam spot S
1
on a main track T
1
from which an information signal is reproduced, and the ± first order diffracted light beams form the light beam spots S
2
and S
3
on first and second adjacent tracks T
2
and T
3
adjacent to the main track T
1
, respectively.
At this time, as shown in
FIG. 2
, the light beam spots formed on the optical disk
10
are formed on the main track T
1
and the adjacent tracks T
2
and T
3
to be slanted with respect to each other. That is, the light beam spot S
2
formed on the first adjacent track T
2
precedes the light beam spot S
1
formed on the main track T
1
, and the light beam spot S
3
formed on the second adjacent track T
3
goes behind the light beam spot S
1
. Here, since side portions of the individual light beam spot lie over adjacent tracks due to the narrow track pitch, the zeroth order diffracted light beam and the ± first order diffracted light beams form the light beam spots not only on the corresponding tracks but also on the adjacent tracks.
The light beams which were reflected from the optical disk
10
and passed through the objective lens
5
pass through the beam splitter
3
and are detected by a photodetector
8
which is provided with first, second and third light receiving portions A, B and C performing photoelectrical conversion independently of each other. That is, the three light beams reflected from the respective tracks of the optical disk
10
are received by the respective light receiving portions, A, B and C of the photodetector
8
as shown in FIG.
3
.
In
FIG. 1
, reference number
4
denotes a reflective mirror, and reference number
7
denotes a sensing lens which condenses incident light beams on the photodetector
8
.
In the optical pickup apparatus structured as described above, an information signal to be detected is a signal which is reflected from the main track T
1
and received by the second light receiving portion B. However, since portions of the light beam spot S
1
by the zeroth order diffracted light beam are formed over the first and second adjacent tracks T
2
and T
3
, the detected signal of the second light receiving portion B includes not only the signal of the main track T
1
but also the signals of adjacent tracks, which are intermixed therewith. Therefore, the information signal of the main track T
1
is detected by performing a differential operation on the detected signal of the second light receiving portion B and the signals of adjacent tracks detected by the first and third light receiving portions A and C. When the light receiving portion and the signal detected by the light receiving portion are expressed by the same symbol, the RF signal of the main track T
1
is expressed by the following Equation (1)
RF
signal=
B−K×[A+C]
(1)
where K is a constant which is determined so that vibrations in the time axis of the information signal of the main track T
1
can be minimized, i.e., a constant which is determined so that cross talk by the adjacent tracks T
2
and T
3
can be minimized.
On the other hand, since the three beams diverged by the grating
2
are converged on respective tracks to form the light beam spots which are slanted with respect to each other as shown in
FIG. 2
, the signals of the adjacent tracks which are detected by the first and third light receiving portions A and C are delayed for a constant time with respect to the signals of the adjacent tracks which are included in the signal detected by the second light receiving portion B. Therefore, it is not possible to perform a real-time operation so as to detect the information signal of the main track, and there is a problem in that signals which are delayed for the constant time with respect to the detected signal B are used as the signals A and C of the adjacent tracks.
In addition, in the above method of reducing cross talk by adjacent tracks, since three light beam spots must be formed on respective tracks of the optical disk
10
, the optical efficiency of the light beam spot S
1
for recording/reproducing the information signal of the main track T
1
is lowered, and it is difficult to use the light beam spot S
1
for recording.
SUMMARY OF THE INVENTION
To solve the above problems, it is an objective of the present invention to provide an optical pickup apparatus which detects, in real-time, an information signal of a main track in which cross talk caused by adjacent tracks is reduced, and which reduces cross talk
Ahn Young-man
Chung Chong-sam
Jung Seung-tae
Kim Tae-kyung
Lee Chul-woo
Samsung Electronics Co., Ltd..
Staas & Halsey , LLP
Tran Thang V.
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