Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Reexamination Certificate
2001-06-06
2003-04-29
Edun, Muhammad (Department: 2655)
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
C369S112010, C369S112050
Reexamination Certificate
active
06556532
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to optical pickup devices and particularly to optical pickup devices used for magneto-optical disc apparatuses.
2. Description of the Background Art
In recent years there has been developed a magneto-optical disc reproduction apparatus capable of repeatedly recording and reproducing audio and video data, documents and other similar data. An optical pickup device is used as a main component of the magneto-optical disc reproduction apparatus and its miniaturization is considered important.
The present inventors have proposed a miniaturized optical pickup device in Japanese Patent Laying-Open No. 2001-034989.
With reference to
FIG. 17
, Japanese Patent Laying-Open No. 2001-034989 discloses an optical pickup device including a light source
103
, a collimator lens
108
and an objective lens
109
collecting and passing light from light source
103
onto a magneto-optical (MO) disc
110
, a photodetector
124
detecting light reflected from MO disc
110
. This optical pickup device further includes a polarization separating prism
105
formed of anisotropic optical member
101
and anisotropic optical member
102
, arranged on an optical path extending from light source
103
to collimator lens
108
and having an optical branching function to internally reflect light from light source
103
to guide it to collimator lens
108
and to transmit light reflected from MO disc
110
.
Isotropic and anisotropic optical members
101
and
102
are arranged to have a boundary surface
105
a
with an inclination of 45° to an optical axis
114
of reflected light.
Thus a light beam B
1
from light source
103
is directed parallel to optical axis
114
and a light beam B
2
reflected by a plane of reflection
101
c
is directed perpendicular to optical axis
114
.
In this configuration, an isotropic optical member
102
is formed for example of LiNbO3, which is stable in electrochemistry, has a large difference in refraction index, and can be produced in a large amount at one time and thus available at low cost, and isotropic optical member
101
is for example TaFD30, a dense and inexpensive optical glass available from HOYA CORP.
Light transmitted through boundary surface
105
a
has aberration. Thus on photodetector
124
a spot has a geometry larger than the source light, as shown in FIG.
18
. Light transmitted through boundary surface
105
is separated into an ordinary ray R
1
and an extraordinary ray R
2
. Ordinary ray R
1
provides a spot having a length of 80 &mgr;m in a direction T
1
and a length of 100 &mgr;m in a direction orthogonal to direction T
1
. Extraordinary ray R
2
provides a spot having a length of 60 &mgr;m in both of a direction T
2
and a direction orthogonal to direction T
2
. Herein, T
1
and T
2
are directions corresponding to a guide groove of the MO disc. A beam enlarged by aberration is detected by a light receiving portion
126
divided in four, to provide focus servo through astigmatism, and a beam is detected at a light receiving portion
125
divided in two, to provide tracking servo by a push-pull method (a 1-beam method).
Generally, if the 1-beam method is employed, shifting objective lens
109
causes unbalance of light, (hereinafter referred to as a “radial offset”) on photodetector
125
. As such, if the 1-beam method is implemented, the radial offset also needs to be prevented. However, to prevent the radial offset a high level of technique is required, and most of manufactures thus avoid adopting the 1-beam method. Thus the method is less prevalent in the market.
By contrast, a 3-beam method eliminates the necessity of accommodating the radial offset. As such, a simple technique is sufficient to provide stable tracking servo, which makes optical pickup devices more prevalent.
With reference to
FIG. 19
is shown a spacing between a main beam and a subbeam in an optical pickup device of a typical 3-beam system. If the main beam and the subbeam are spaced by 17 &mgr;m on an MO disc
131
in a tangential direction, the beams are spaced on a photodetector
132
by approximately 50 to 60 &mgr;m, attributed to a constant of collimator lens
133
and that of objective lens
134
.
From a different point of view the present inventors are also currently studying and developing a more miniaturized and highly integrated optical pickup device, as has already been proposed in Japanese Patent Laying-Open No. 2000-348374. Reference will now be made to
FIGS. 20 and 21
to describe a configuration of the optical pickup device. This optical pickup device includes a stem
111
, a semiconductor laser
103
provided on stem
111
to serve as a light source, a cap
113
covering stem
111
, and an optically transmissive substrate
114
attached on cap
113
. Furthermore the optical pickup device also includes a ½ wavelength plate
115
attached on optically transmissive substrate
114
, an optical element
105
attached on ½ wavelength plate
115
, and a collimator lens
108
and an objective lens
109
collecting on a magneto-optical recording medium
110
a beam of light emanating from laser diode
103
. Furthermore the optical pickup device also includes a photodetector
124
arranged on stem
111
to detect light reflected from magneto-optical recording medium
110
and branched by optical element
105
. On optically transmissive substrate
114
are arranged first and second diffraction elements
121
and
122
.
A beam of light emanating from laser diode
103
passes through the second diffraction element
122
and is separated into transmitted light and three, positive and negative first-order diffracted beams of light. The light then passes through ½ wavelength plate
115
and it is reflected by optical element
105
at first and second surfaces
101
c
and
105
a
, passes through collimator lens
108
and objective lens
109
and is then collected on magneto-optical recording medium
110
. Magneto-optical recording medium
110
provides a reflection of light, which beam is separated into an ordinary ray and an extraordinary ray at an angle of refraction determined by a ratio of a refraction index of the first member to an ordinary index of the third member, and a ratio of the refraction index of the first member to an extraordinary index of the third member. The rays then arrive at the first diffraction element
121
underlying optical member
105
and they are further separated into transmitted light and diffracted light and thus collected on photodetector
111
.
Optical element
105
is formed of a first member
101
formed of an isotropic glass material and a third member
102
formed of an anisotropic material (a birefringent material). The first and third members
101
and
102
have therebetween a boundary surface serving as a polarization separating surface. A reflection of light from magneto-optical recording medium
110
that refracts at the second surface
105
a
has a wave aberration, which is compensated for by forming the optical element from materials so selected that an average of ordinary and extraordinary indexes of refraction of the birefrigent material forming the third member
102
, and an index of refraction of the glass forming the first member
101
substantially match in value. For example, the glass material for the first member
101
may be LF
5
, a product of Schott Group with n of 1.5722, and the birefringent material for the third member may be lithium tetraborate having no of 1.605 and ne of 1.549.
The first diffraction element
121
is divided into first to third regions. Furthermore, photodetector
124
is structured as shown in FIG.
21
. Light transmitted through the first diffraction element
121
is collected on each of photodetection portions
124
e
-
124
h
. Light diffracted in the first diffraction element at the first region is collected on a boundary surface of photodetection portions
124
c
and
124
d
. Light diffracted at the second region is collected on photodetection portion
124
a
. Light diffracte
Ogawa Masaru
Saeki Tetsuo
Conlin David G.
Edun Muhammad
Edwards & Angell LLP
Jensen Steven M.
Sharp Kabushiki Kaisha
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