Optical pickup

Details Optical pickup Optical pickup

1999-12-23

2003-03-25

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

C369S112190, C369S112280

Reexamination Certificate

active

06538975

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical pickup capable of recording or reproducing information on or from an optical recording medium, and more particularly, to an optical pickup capable of reducing cross-talk by signal interference of adjacent tracks during playback of information from a main track.
2. Description of the Related Art
Recently, for high-density recording media, development of an optical pickup adopting a light source having a relatively short wavelength and an objective lens having a relatively large numerical aperture (NA), is increasing. For example, due to an increasing tendency toward use of a digital video disc (DVD), rather than a compact disc (CD), the configuration of an optical pickup has also changed. That is, development of an optical pickup adopting a light source having a wavelength of about 650 nm and an NA of 0.6, rather than an optical pickup adopting a light source having a wavelength of about 780 nm, i.e., infrared rays, and an NA of 0.45, has increased. By replacing the optical pickup, a high-density optical recording medium having a narrow track pitch can be adopted for information recording/playback.
As a track pitch which is the interval between adjacent tracks become narrow, there is concern about deterioration of a playback signal due to signal interference of the adjacent tracks. Such playback signal deterioration due to signal interference of adjacent tracks is referred to as “cross-talk”, the allowable range of which differs according to the type of optical recording medium. For instance, a DVD-ROM requires a cross-talk level of −30 dB or less.
As shown in
FIG. 1
, a conventional optical pickup capable of reducing cross-talk includes a light source
11
, a grating
13
for diffractingly transmitting a light beam emitted from the light source
11
to produce 0th-order and ±1st-order diffracted beams, a beam splitter
15
for changing the traveling path of the beam incident thereto, an objective lens
16
for converging the incident light to form an optical spot on the optical recording medium
10
, a photodetector
19
for receiving beams that have been reflected by the optical recording medium
10
and passed through the beam splitter
15
, and a light receiving lens
19
arranged between the beam splitter
15
and the photodetector
19
.
The 0th-order and the 1st-order diffracted beams, which have been diffracted by the grating
13
, are simultaneously condensed as three optical spots S
1
, S
2
and S
3
on different positions of the optical recording medium
10
, as shown in FIG.
2
. That is, the 0th-order diffracted beam forms the optical spot S
1
on a main track T
1
from which an information signal is reproduced, while the 1st-order diffracted beams form the optical spots S
2
and S
3
on first and second adjacent tracks T
2
and T
3
adjacent at either side of the main track T
1
. For reference, the optical spots S
1
, S
2
and S
3
are formed beyond the corresponding tracks. That is, due to the narrow width of the tracks, the optical spots S
1
, S
2
and S
3
partially extend to the adjacent tracks.
Also, as shown in
FIG. 2
, the optical spots S
1
, S
2
and S
3
are formed on the optical recording medium
10
with a time lag. In other words, the optical spot S
2
formed on the first adjacent track T
2
precedes the optical spot S
1
formed on the main track T
1
, and the optical spot S
3
formed on the third adjacent track T
3
is delayed relative to the optical spot S
1
.
The optical spots S
1
, S
2
and S
3
enter the photodetector
19
through the objective lens
16
, the beam splitter
15
and the light receiving lens
17
. As shown in
FIG. 3
, the photodetector
19
includes first through third light receiving portions A, B and C for respectively receiving the 0-th order and 1st-order diffracted beams, which have been reflected by the optical recording medium
10
, and for photoelectrically converting the received 0-th order and 1st-order diffracted beams, respectively.
In the optical pickup having the above configuration, an information (radio frequency, RF) signal to be reproduced is reflected by the main track T
1
and then received by the first light receiving portion A. Also, a part of the optical spot S
1
of the 0-th order diffracted beam is formed on the first and second adjacent tracks T
2
and T
3
, wherein RF signals of the first and second adjacent tracks T
2
and T
3
, which are received by the first light receiving portion A, can be detected based on the RF signals of the second and third light receiving portions B and C, respectively.
In other words, the RF signal of the main track T
1
is detected through operation with the signals detected from the first and second adjacent tracks T
2
and T
3
, which is expressed by the following formula
RF
signal=
RF
signal(first light receiving portion)−
K×[RF
signal(second light receiving portion)+
RF
signal(third light receiving portion)]  (1)
where K is an operation constant which minimizes jitter of the RF signal, that is, cross-tack due to signal interference of adjacent tracks.
In the optical pickup having the above configuration, the signals detected by the second and third light receiving portions B and C precedes or are delayed relative to the signal detected by the first light receiving portion A. That is, the signals of the first and second adjacent tracks T
2
and T
3
, which are received by the first receiving portion A, are detected ahead or behind the detection of the optical spot S
1
formed on the main track T
1
. Thus, it is basically impossible to operate the RF signals in real-time.
Meanwhile, as shown in
FIG. 4
, when an optical pickup is configured such that a time lag in forming the optical spots S
1
, S
2
′ and S
3
′ on the main track T
1
and the first and second adjacent tracks T
2
and T
3
, does not occur, as shown in
FIG. 5
, all three optical spots are received by the first light receiving portion A of the photodetector
19
. In such a case, it is impossible to selectively detect the RF signal recorded on the main track T
1
.
Another conventional optical pickup capable of reducing cross-talk during playback due to signal interference of adjacent tracks is disclosed in Japanese Patent Publication No. Hei 6-150363 (dated May 31, 1994).
The disclosed optical pickup is characterized in that an optical spot formed on a main track and optical spots formed on adjacent tracks have a phase difference. As shown in
FIG. 6
, the optical pickup includes first and second light sources
21
and
22
, a polarization beam splitter
24
, a beam splitter
25
, a phase plate
23
disposed between the second light source
22
and the polarization beam splitter
24
, an objective lens
26
, a polarization hologram optical element (HOE)
27
, and a photodetector
28
for receiving beams that have been emitted from the first and second light sources
21
and
22
and reflected by an optical recording medium
20
.
The first light source
21
emits a linearly polarized coherent light beam. The traveling path of the beam emitted from the first light source
21
is changed via the polarization beam splitter
24
and the beam splitter
25
toward the optical recording medium
20
. The beam that has passed through the beam splitter
25
is converged by the objective lens
26
on the main track of the optical recording medium
20
. The second light source
22
emits a linearly polarized coherent light beam having a polarization perpendicular to the direction of the beam from the first light source
21
. The phase plate
23
transmits the incident beam from the second light source
22
. The phase plate
23
is stepped with a different thickness d such that a transmission beam has a beam intensity distribution having at least two peaks at the center of the optical axis. In the optical pickup, the beam emitted from the first light source
21
is used as a primary beam, while that emitted from the second light source
22
is used as a s

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