Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Reissue Patent
1998-04-16
2004-11-09
Korzuch, William (Department: 2653)
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
C369S112170, C369S120000, C369S013320, C369S044230, C369S044370
Reissue Patent
active
RE038648
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a pickup device for recording information to and reproducing the same from a magneto-optical recording medium, and also relates to a photo detecting unit in use with the pickup device.
2. Description of the Prior Art
A conventional magneto-optical recording/reproducing system detects a magneto-optical signal by using a magneto-optical light splitting element by utilizing a birefringence, such as a 3-beam Wollaston prism, in an optical system with collimiator, viz., in parallel light beams.
FIG. 1
is a diagram showing an arrangement of a conventional magneto-optical recording/reproducing pickup device. A light beam emitted by a light source
1
, such as a semiconductor laser device, is converted, by a diffraction grating
2
, into at least three spot light beams which will be used for generating a tracking control signal. These light beams are collimated by a collimating lens
3
, usually consisting of two lenses joined together. The collimated light beams are converged on a-magneto-optical disk
6
by an objective lens
5
. To reproduce information, the plane of polarization is turned in accordance with an inverted magnetization pattern corresponding to information written into a vertically magnetized film, in a recording track.
The light beams reflected by the magneto-optical disk
6
are rendered parallel by the objective lens
5
and returned to a beam splitter
4
. The light beams are reflected by the beam splitter
4
toward a Wollaston prism
7
. The Wollaston prism
7
separates the received light beams into an S polarized light component, a P polarized light component, and a light component as the combination of the S and P polarized light components. These polarized light components are incident on a photo detecting unit
12
through a route of a reflecting mirror
8
, a converging lens
9
, a concave lens
10
, and a cylindrical lens
11
for obtaining a focusing error signal. The direction of the magnetization of a readout signal surface is determined by comparing the intensities of the P and S polarized light components. A focusing error signal is obtained, by the astigmatic method, from the light component as the combination of the S and P polarized light components. A tracking error signal is obtained by comparing the intensities of both sub-beams of the three beams derived from the diffraction grating. Thus, the control signals are generated for controlling the focusing and tracking directions.
Another conventional optical pickup device in use with a magneto-optical recording/reproducing apparatus containing the optical system with collimator is disclosed in Unexamined Japanese Patent Publication (Kokai) Sho-63-127436. In the publication, the parallel beams emanating from the collimating lens are optical-axis transformed (reflected) by a polarizing beam splitter. The reflecting light beams are focused on the magneto-optical disk through an objective lens. The reflecting light beams from the magneto-optical disk are converted into parallel light beams by the objective lens. The light beams, after passing through the polarizing beam splitter, are separated, by an analyzer, into an S polarized light component, a P polarized light component, and a light component as the combination of the S and P polarized light components. Finally, a magneto-optical signal and other control signals for controlling the focusing and tracking directions are formed.
The conventional optical pickup devices including the optical system with collimator is employed in order to suppress a variation of the splitting characteristics of the polarizing beam splitter
4
and the Wollaston prism
7
, and to reduce a degree of deterioration of the magneto-optical signals. The objective lenses used in parallel light beams can be more easily designed and manufactured than those used in divergent light beams. For this reason, the collimating lens
3
, usually consisting of two spherical glass-joined lenses, for collimating the divergent light beams, and the converging lens
9
for converging the parallel light beams are indispensable for the conventional pickup devices. Use of those lenses brings about complexity of the construction, increase of the number of the indispensable parts, and increase of the size of the optical pickup device.
To solve the problems, there is proposed an optical pickup device in use with the magneto-optical recording/reproducing apparatus, which is designed on the basis of an optical system without collimator, as shown in
FIG. 2
(“O plus E”, No. 163, 1993, June, pp94 to 95). In the optical pickup device, divergent light beams emitted from a light source
1
pass through a convex lens
13
where a degree of the divergence of the divergent light beams is reduced. The light beams as left divergent are incident, as an S polarized light, on a plate polarizing beam splitter
14
. The divergent light beams reflected by the plate polarizing beam splitter
14
are converged on the recording surface of the magneto-optical disk
6
, through an objective lens
15
. The reflecting light beams are converted, the objective lens
15
, into the convergent light beams which in turn enter the plate polarizing beam splitter
14
. In the plate polarizing beam splitter
14
, the polarizing film allows part of the S polarized light beams and most of the P polarized light beams to pass therethrough. A half wave plate
17
, located on the rear side of the plate polarizing beam splitter
14
, turns the direction of polarization by 45° of the light beam. Thereafter, a plate analyzer
18
splits the light beam into an S polarized light beam, a P polarized light beam, and a light beam as the combination of the S and P polarized light beams. These light beams are converted into electrical signals by a photo detecting unit
19
.
In the pickup device shown in
FIG. 2
, to obtain exact information, it is necessary to accurately adjust the angles of the plate polarizing beam splitter
14
and the plate analyzer
18
. This makes the assembling work difficult. Further, a accurate control of the thickness of the plate analyzer
18
is required. Accordingly, the manufacturing work is difficult. The half wave plate
17
is provided for turning the plane of polarization by 45° and for disposing the plate analyzer
18
on a plane without rotating along the optical axis. This half wave plate
17
is expensive. Provision of the half wave plate
17
runs counter to the cost reduction.
Additional pickup devices based on the optical system without collimator are disclosed in Unexamined Japanese Patent Publication (Kokai) Hei-5-142419, Hei-5-142420, and Hei-5-142421. A Wollaston prism
21
as illustrated in
FIGS. 3A and 3B
is used. The optical system of the optical pickup device is as shown in FIG.
3
C. The Wollaston prism
21
as a multifunctional Wollaston prism includes a polarizing beam splitting
21
c. The polarizing beam splitting film
21
c directs an incident light beam
24
, which is emitted from a light source
1
, toward the objective lens
15
, and allows a reflecting light beam
25
, which comes in through the objective lens
15
, to pass therethrough. (The polarizing beam splitting film
21
c is a multilayer film formed by alternately layering a plural number of dielectric thin films of different refractive indices, and is formed on the incident surface of the Wollaston prism
21
.) The Wollaston prism
21
consists of a first prism
21
a and a second prism
21
b, both being made of crystalline and joined together along their long faces. A plane including the optical axis of the reflecting light beam
25
coming in through the objective lens
15
(the same thing is correspondingly applied to the optical axis of the incident light beam
24
emitted from the light source
1
) and the optic axis
21
d of the first prism
21
a, is at an angle, not a right angle, to a plane including that optical axis and the optic axis
21
e of the second prism
21
b. The Wollaston prism
21
thus constructed is disposed slanted with
Hirose Kazunori
Kinouchi Mitsuru
Chu Kim-Kwok
Korzuch William
TDK Corporation
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