Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Reexamination Certificate
1999-10-08
2002-04-16
Huber, Paul W. (Department: 2651)
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
Reexamination Certificate
active
06373808
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an optical pickup apparatus used for recording and reproducing an optical disc, and more particularly to an optical pickup apparatus that is capable of eliminating the crosstalk effectively.
2. Description of the Prior Art
Generally, an optical pickup apparatus irradiates a light on the recording face of an optical disc to detect the reflected light, thereby performing the information recording and reproducing operation. To this end, the optical pickup apparatus is composed of a laser diode for emitting a light, an objective lens for focusing the emitted light on the recording face of the disc, and other optical system components required for concentrating and receiving the light.
An optical disc, such as a compact disc(CD) or a digital versatile disc(DVD) having more improved recording capacity, has been commercially available. This optical disc has wider use for recording and reproducing audio and video data and computer data, etc. Recently, an optical disc having much more improved recording capacity is expected owing to the development of a blue laser generating a short wavelength of light. There have been suggested several schemes for increasing the numerical aperture(NA) of an objective lens or narrowing the track pitch along with the use of a light source such as a blue laser, etc. so as to enlarge the recording capacity of the optical disc. However, it has a problem in that cross talk is caused between the adjacent tracks when there is narrowing of the track pitch of the optical disc.
More specifically, since an optical spot irradiated so as to reproduce a certain pit P
1
, it has a greater width than a width of the pit shown in
FIG. 1
, and it is also irradiated onto the pits in the adjacent tracks. As a result, a cross talk component caused by the pits in the adjacent tracks is involved in the reproduced signal. A strategy employing a polarizing phase plate as shown in
FIG. 2
has been known as one of the methods used for eliminating such a cross talk component.
FIG. 2
shows the conventional optical pickup apparatus for eliminating the cross talk component. In
FIG. 2
, the optical pickup apparatus includes a light source
12
for generating a light beam, an objective lens
20
for focusing a light beam from the light source
12
on the recording face of an optical disc
22
, first and second photo detectors
26
and
28
for converting a reflective light beam from the optical disc
22
into an electrical signal, a beam splitter
18
arranged among the light source
12
, the objective lens
20
and the first and second photo detectors
26
and
28
, a polarizing beam splitter(PSB) arranged among the beam splitter
18
and the first and second photo detector
26
and
28
, a polarizing phase plate
14
arranged between the light source
12
and the beam splitter
18
, and a collimator lens
16
arranged between the polarizing phase plate
14
and the beam splitter
18
. The light source
12
generates two polarized beams having a polarizing characteristic moving perpendicularly to each other. The collimator lens
16
converts a divergent light beam progressing from the light source
12
, via the polarizing phase plate
14
, toward the beam splitter
18
into a parallel light beam to prevent leakage of the light beam. The beam splitter
18
passes a light beam from the collimator lens
16
in such a manner as to be progressed toward the objective lens
20
, and reflects a reflective light beam reflected from the recording face of the optical disc
22
, and passing through the objective lens
20
in such a manner as to be progressed toward the PSB
24
. The objective lens
20
focuses an incident light beam from the beam splitter
18
on the recording face of the optical disc
22
. The polarizing phase plate
14
consists of two phase segments(0, &pgr;) having a phase difference of 180° in the left and right phases thereof as shown in
FIG. 3A
, and which is responsible for selectively changing a phase of the light beam from the light source
12
in accordance with a polarizing characteristic thereof. More specifically, the polarizing phase plate
14
passes the first polarized beam in any one direction of two polarized beams emitted from the light source
12
as it is, thereby allowing the first polarized beam to be irradiated, via the collimator lens
16
, the beam splitter
18
and the objective lens, on the recording face of the optical disc
22
as a main beam as shown in FIG.
2
A. On the other hand, the polarizing phase plate
14
passes the second polarized beam in a direction perpendicular to the first polarized beam of two beams from the light source
12
with a phase being modulated, thereby allowing the second polarized beam to be irradiated on the recording face of the optical disc
22
as a sub-beam having a double-mountain shape superposed at each side of the main beam as shown in FIG.
2
A. The main beam in the light beams irradiated on the recording face of the optical disc
22
in this manner is irradiated on the signal track to be accessed and is used to reproduce an information signal. On the other hand, the sub-beam is irradiated on the adjacent tracks and is used to detect a cross talk component included in the reproduced signal. The PSB
24
passes the main beam in a reflective light beam reflected from the disc
22
and received via the objective lens
20
and the beam splitter
18
as it is in such a manner to be progressed toward the first photo detector
26
, whereas it reflects the sub-beam in such a manner to be progressed toward the second photo detector
28
. The first photo detector
26
detects a main beam received from the PSB
24
and detects a sub-beam received from the PSB
24
, thereby converting them into electrical signals. In other words, the first photo detector
26
detects a radio frequency signal including an information signal from the main beam while the second photo detector
28
detects a cross talk component in the adjacent tracks from the sub-beam.
Further, the optical pickup apparatus includes an amplifier
30
connected to the second photo detector
28
, and a differential amplifier
32
connected to the first photo detector
26
and the amplifier
30
. The amplifier
30
amplifies and outputs a crosstalk component in the second photo detector
28
, and the differential amplifier
32
eliminates and outputs a crosstalk component output from the amplifier
30
from a radio frequency signal output from the first photo detector
26
.
The optical pickup apparatus must have the ability to vary a distance between the sub-beams irradiated on the adjacent tracks so as to access all of the optical discs having a different track pitch accurately. In the above mentioned optical pickup apparatus, however, a degree of freedom for its design is deteriorated because a wavelength(&lgr;) of a beam determining a distance between the sub-beams or the numerical aperture(NA) of the objective lens must be controlled such that the distance between the sub-beams can be controlled.
More specifically, assuming that a distance extending from the center of the double-mountain shaped sub-beam as shown in
FIG. 3B
into a peak thereof, that is, a position of the sub-beam should be x, x is equal to fsin&thgr;(wherein f is a focus length of the objective lens, and &thgr; is an angle at which a line linking the sub-beam with the center of the objective lens makes an optical axis of the objective lens). Also, assuming that a diameter of the objective lens is a and a wavelength of a beam be &lgr;, asin&thgr; is equal to &lgr; when considering the diffraction equation. In consideration of said relationship, a position value(x) of the sub-beam can be derived from the following equation:
X
=
f
λ
a
=
λ
2
NA
(
1
)
wherein f is a focus length of the objective lens, a is a diameter of the objective lens, &lgr; is a wavelength of the beam, and NA is the numerical aperture of the objective lens. It can be seen from the equation (1) that a position value(
Birch & Stewart Kolasch & Birch, LLP
Huber Paul W.
LG Electronics Inc.
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