Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Reexamination Certificate
2000-12-18
2004-10-12
Hindi, Nabil (Department: 2655)
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
C369S110040, C369S112160
Reexamination Certificate
active
06804185
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical pickup and a recording and/or reproducing apparatus for optically recording and/or reproducing information on an information medium.
2. Description of the Related Art
An optical disc such as a CD (Compact Disc) or DVD (Digital Video Disc or Digital Versatile Disc) is known as an information medium used for optical information recording and reproduction.
The optical disc has a multi-layered structure including a recording layer, a light transmitting layer, and a protection layer. The recordation and reproduction of information data is done by irradiating a light beam onto the recording layer through the light-transmitting layer.
FIG. 1
illustrates a configuration of a known optical pickup for such recordation and/or reproduction of information as mentioned above. Referring to the figure, the optical pickup includes a light source
100
for emitting a linear polarization laser beam, a collimator lens
101
, a beam splitter
102
, a quarter-wave plate
103
, an objective lens
104
, a condenser lens
105
, and an optical detector
106
.
A linear polarization laser beam (i.e., laser beam for information recording) which is modulated by information data to be recorded is emitted from the light source
100
to record the information data on the optical disc. The laser beam is, then, collimated by the collimator lens
101
to pass through the beam splitter
102
. Subsequently, the laser beam is circularly polarized by the quarter-wave plate
103
and then converged by the objective lens
104
. The laser beam is thereby provided with a small diameter for radiation (hereinafter referred to as the main beam) to radiate the recording layer of the optical disc
107
through the light transmitting layer. This allows the information data to be recorded on the recording layer by the optical energy of the main beam.
In addition, a laser beam for reading information is emitted from the light source
100
to read information on the optical disc. The laser beam for reading information has a small diameter of radiation to form the main beam and is allowed to radiate the recording layer of the optical disc
107
through the light transmitting layer as in the case of recording information. Moreover, reflected light produced by radiating the optical disc
107
with the aforementioned main beam passes through the objective lens
104
and is then polarized by the quarter-wave plate
103
to be orthogonal to the laser beam emitted from the light source
100
. The reflected light is thereby reflected by the beam splitter
102
toward the condenser lens
105
and then converged by the condenser lens
105
to be received by the optical detector
106
, thereby allowing the information to be read.
An information recording medium having a higher recording density and a larger capacity has been in increasing demands. To respond to the demands, an attempt has been made to increase the numerical aperture NA of the objective lens.
However, an increase in the numerical aperture NA of the objective lens would not allow the aforementioned known pickup to detect the reflected light with high accuracy due to birefringence resulting from the optical characteristics of the light transmitting layer and the recording layer of the optical disc. Thus, different difficulties to be solved have occurred, such as in implementing appropriate reading of information and implementing appropriate servo control for recording information.
More specifically, the optical pickup shown in
FIG. 1
allows a linear polarization laser beam to be emitted from the light source
100
and pass through the beam splitter
102
. Then, the laser beam is circularly polarized by means of the quarter-wave plate
103
to form a circularly polarized laser beam with which the optical disc
107
is radiated. Subsequently, circularly polarized reflected light produced by the radiation of the laser beam is polarized by the quarter-wave plate
103
to form a linear polarization laser beam. The resulting laser beam is reflected by the beam splitter
102
and then detected by the optical detector
106
via the condenser lens
105
.
The presence of birefringence in the optical disc
107
, however, causes the reflected light to be elliptically polarized whereas a circularly polarized light beam is to radiate the optical disc
107
. Accordingly, every light beam is not linearly polarized when the elliptically polarized reflected light passes through the quarter-wave plate
103
. Part of the reflected light passes through the beam splitter
102
to return to the collimator lens
101
, whereas the remaining linear polarization reflected light is reflected by the beam splitter
102
to reach the optical detector
106
. This makes it impossible to detect the due amount of reflected light on the optical detector
106
, thus resulting, for example, in a decrease in S/N. In addition, it is necessary to take countermeasures against an adverse effect to be exerted, by the part of the reflected light through the collimator lens
101
, on the semiconductor laser diode provided in the light source
100
.
In particular, an increase in numerical aperture NA of the objective lens
104
to provide the optical disc
107
with a higher density would cause the range of the angle of the light beam incident to the optical disc
107
to become larger. This causes the distribution of birefringence, which is dependent on the incident angle, to be more spread in the pupil plane of the disc, thus making the effect of the birefringence greater to such an extent that it cannot be neglected. In addition, with the aforementioned prior-art optical pickup, use of a short-wavelength laser beam to implement a higher density would make it difficult to reduce the effect of the birefringence in the optical disc
107
.
OBJECT AND SUMMARY OF THE INVENTION
The present invention was made in view of the above problems. It is therefore an object of the present invention to provide an information recording and/or reproducing apparatus and an optical pickup, which can prevent the effect of birefringence to allow highly accurate recording and reading of information.
It is another object of the present invention to provide an information recording and/or reproducing apparatus and an optical pickup, which can prevent the effect of birefringence to allow highly accurate recording and reading of information even when an objective lens with a greater numerical aperture or a laser beam of a shorter wavelength for reading or recording information is employed.
According to the present invention, there is provided an optical pickup including a light source, an optical element having a polarization characteristic of allowing transmittance or reflectance to be changed according to a polarization of incident light and an objective lens, wherein a light beam emitted from the light source is transmitted or reflected in one direction of the optical element and converged by the objective lens to radiate an information medium therewith, and the reflected light beam from the information medium passes through the objective lens to be incident on the optical element in another direction thereof as well as being reflected by the optical element to perform optical detection, which comprises a variable optical unit, disposed between the optical element and the objective lens, for providing incident polarized light with a phase difference.
According to another aspect of the present invention, the optical pickup further comprises control means for variably controlling a phase-shift amount of the variable optical unit, wherein the control means controls the phase-shift amount of the variable optical unit according to birefringence of the information medium.
According to another aspect of the present invention, the control means controls the phase-shift amount of the variable optical unit according to birefringence of each information medium.
According to another aspect of the present invention, the variable optical unit is formed of a
Iwasaki Masayuki
Kikuchi Ikuya
Maeda Takanori
Sato Mitsuru
Hindi Nabil
Morgan & Lewis & Bockius, LLP
Pioneer Corporation
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