Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
2000-09-12
2004-11-16
Neyzari, Ali (Department: 2655)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S013320, C369S044120, C369S110030
Reexamination Certificate
active
06819637
ABSTRACT:
Generally, the present invention relates to a magneto-optical apparatus and an optical head that uses a magneto-optical storage medium to which data is written to and/or read from by applying a laser beam and a magnetic field thereto. More particularly, the present invention relates to a magneto-optical apparatus and an optical head that irradiates a beam splitter with the divergent spherical light of a laser beam.
BACKGROUND OF THE INVENTION
Optical disks have been widely used as external storage mediums for computers. Within the multi-layered structure of an optical disk, and particularly within the structure of a magneto-optical disk, a magnetic recording layer is provided. Since data is written perpendicularly on the magnetic recording layer, the ability to save the recorded contents is good, making it possible to repeatedly read and write new data many times. In a magneto-optical disk, a sub-micron order mark can be written onto the medium by using a laser beam. When compared with floppy disks, magneto-optical disks have much greater storage capacities, and such capacities are being improved even further. For example, when considering a 3.5-inch magneto-optical (MO) disk, it is now possible to store up to 1.35 GB thereon, whereas earlier MO disks had storage capacities of approximately 128 MB. While high storage capacity is important, it is desirable that the magneto-optical disk apparatus used for reading the MO disk is also compact, lightweight, and of a low cost.
FIG.
13
and
FIG. 14
are explanatory drawings of prior art devices.
FIG. 13
shows a prior art magneto-optical read/write head, and
FIG. 14
is a top view of the polarization distribution surface of FIG.
13
.
In
FIG. 13
, the magneto-optical disk medium
96
includes a recording layer that is formed on the top of a substrate, and is made from a magnetic material. This medium
96
makes use of the changes in the heat of the light from a laser and a magnetic field in order to allow information to be written thereon. There are data tracks on this medium for reading and writing the data. Generally, a spiral shaped groove (a tracking groove) is formed on the medium
96
. The tracks for reading and writing data are formed on the lands between the grooves.
The light beam of an optical head
90
tracks the spiral track. When writing information, the head
90
makes use of the magnetic field and the changes in the heat of the light in order to write the information. Moreover, when reading the information, the head
90
makes use of the magneto-optical effect to read the information from the light beam reflected back from the disk medium. In order to read or write, the laser beam is focused on the writing surface of the medium. Focus servo control is performed such that the laser beam is maintained in the focused state. In addition, it is also necessary for the light beam to follow the data track. Therefore, track servo control is also used.
In this optical head
90
, after the light emitted from the semiconductor laser
91
passes through a beam splitter
93
, the light is arranged to become a parallel beam by a collimator lens
94
, and then it is focused on the magneto-optical disk
96
by an objective lens
95
. On the magneto-optical (MO) disk
96
, in the area of the mark, the polarization angle of the light is turned by the Kerr effect. For example, by using the P-polarized light component for detection, the polarization angle is rotated only at the mark area by &thgr;K by the Kerr effect in order to generate the S-polarized light component.
The reflected light intensity of the S-component and that of the P-component are changed by the beam splitter surface
93
a
, which is part of the polarization beam splitter
93
. Then, the light is separated into the S and P components by another beam splitter (not shown in the figure), such as a Wollaston prism, and then it is input to the light detection devices. The read signal (called the MO signal) is detected from the difference between the strengths of the two signals.
Part of the light returning from the MO disk
96
is projected onto the beam splitter
93
, at which time the focus error signals and the track error signals are detected. In this way, in order to read the signal from the polarized component of the light returned from the MO disk, a polarization separation function is required. When this arrangement of a magneto-optical head is compared with the head for an optical disk, the construction of this arrangement is much more complicated.
In order to simplify the construction of this type of magneto-optical head, the following construction has been proposed in the prior art. Briefly, the divergent spherical wave of a semiconductor laser
91
is projected onto beam splitter surface
93
a
of a beam splitter
93
(for example, as disclosed in Japanese Patent Application H9-231604). In this proposal, the magneto-optical disk signal detection and the servo detection functions are integrated, making it possible to reduce the number of optical components within the optical head. In addition, it is also possible to reduce the number of places requiring adjustment, as well as to make the optical head more compact and of a lower cost.
However, this prior art configuration has the following problems. In order to project the divergent spherical wave of the semiconductor laser
91
onto the beam splitter surface
93
a
of the beam splitter
93
, the beam is projected onto the beam splitter surface
93
a
with an inclination. Therefore, as shown in FIG.
13
and
FIG. 14
, the S-polarized light component is generated by the beam splitter
93
. As a result, after the beam has passed the beam splitter
93
, the linear polarization component of the light emitted from the collimator lens
94
is slightly rotated.
For signals such as the MO signal, which are detected by determining the difference between the S-polarized light component and the P-polarized light component, it is not possible to eliminate this rotation effect. In other words, as shown in
FIG. 14
, this rotated component overlaps the bowl-shaped image B of the diffracted light from the groove, so the MO signal component is overlapped when tracking shifts. The tracking shift is not constant, so the amount of overlap changes corresponding to the tracking shift, and MO errors occur. When this happens, it becomes difficult to reproduce the MO signal.
SUMMARY OF THE INVENTION
The objective of the present invention is to provide an improved magneto-optical apparatus and an optical head for accurately reproducing and/or reading an MO signal even when the apparatus is made more compact.
A second objective is to provide an improved magneto-optical apparatus and an optical head for accurately reproducing and/or reading an MO signal of a relatively simple design.
Another objective of the present invention is to provide a magneto-optical apparatus and an optical head for accurately reproducing an MO signal even when the apparatus is made more compact, and where the apparatus includes structure for projecting the spherical divergent beam of a semiconductor laser onto the beam splitter surface of a beam splitter.
A further objective of the present invention is to provide a magneto-optical apparatus and an optical head of a configuration that eliminates the signal component that is overlapped with the MO signal when track shifting occurs.
Briefly, the magneto-optical apparatus of the present invention includes a beam splitter that has a beam splitter surface for splitting the path of a light beam into a forward path, which is directed toward a magneto-optical storage medium, and a return path that leads from the medium; a light source for emitting a divergent spherical wave onto the beam splitter surface of the beam splitter in order to emit a spot of light onto the magneto-optical storage medium; an MO detection unit for detecting a magneto-optical signal from a light beam on the return path from the beam splitter surface of the beam splitter; a track error detection unit for detecting a tracking error signal from the
Aoyama Nobuhide
Hasegawa Shin-ya
Fujitsu Limited
Greer Burns & Crain Ltd.
Neyzari Ali
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