Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
2000-12-12
2002-01-15
Young, W. R. (Department: 2651)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S044410, C369S112280
Reexamination Certificate
active
06339564
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the invention
The present invention generally relates to optical information recording/reproducing apparatuses, and more particularly to an optical information recording/reproducing apparatus which optically records information on a recording medium and/or optically reproduces the information from the recording medium.
2. Description of the Related Art
An optical disk unit is an example of a unit which uses an optical information recording/reproducing apparatus. The optical disk unit can be used as a storage unit of a file system or the like, and is suited for storing programs and large amounts of data. In such an optical disk unit, it is desirable that an optical system thereof can accurately record and/or reproduce the information, and that the number of parts thereof is minimized so as to reduce the cost of the optical disk unit as a whole.
Various techniques have been proposed to detect a focal error in the optical disk unit. Generally, the astigmatism technique and the Foucault technique are well known. The Foucault technique is sometimes also referred to as the double knife edge technique.
Compared to the astigmatism technique, the Foucault technique is less affected by the external disturbance that occurs when a track on an optical disk is traversed, the birefringence of the optical disk. Accordingly, the mixture of the external disturbance into a focal error signal when the Foucault technique is employed is extremely small compared to the case where the astigmatism technique is employed. In addition, the Foucault technique detects a reflected light beam from the optical disk by a photodetector which is arranged in a vicinity of an image formation point of the optical beam, and for this reason, an abnormal offset is unlikely generated in the focal error signal even if the reflected light beam shifts from an optical axis. Because of these advantageous features obtainable by the Foucault technique, it is desirable to employ the Foucault technique as the focal error detection technique.
First, an example of an optical information recording/reproducing apparatus within a conventional magneto-optic disk unit which employs the Foucault technique will be described with reference to FIG.
1
.
In an optical system of the optical information recording/reproducing apparatus shown in
FIG. 1
, a laser beam which is emitted from a laser diode
201
is formed into a parallel beam having an oval cross section in a collimator lens
202
, and is thereafter formed into a light beam having a circular cross section in a true circle correction prism
203
. The light beam from the true circle correction prism.
203
is transmitted through a beam splitter
204
, reflected by a mirror
205
, and is converged on a disk
207
via an objective lens
206
. A reflected light beam from the disk
207
enters the beam splitter
204
via the objective lens
206
and the mirror
205
, but this time the reflected light beam is reflected by the beam splitter
204
and is directed towards a beam splitter
208
. The beam splitter
208
splits the reflected light beam into two light beams, and supplies one light beam to a magneto-optic signal detection system and the other light beam to a servo signal detection system.
The magneto-optic signal detection system includes a Wollaston prism
209
, a lens
210
and a 2-part photodetector
211
. One of the two light beams output from the beam splitter
208
is input to the 2-part photodetector
211
via the Wollaston prism
209
and the lens
210
, and the 2-part photodetector
211
detects the magneto-optic signal, that is, the information signal, based on the input light beam.
The servo signal detection system includes a condenser lens
212
, a beam splitter
213
, a 2-part photodetector
214
, a composite prism
215
and a 4-part photodetector
216
. The other of the two light beams output from the beam splitter
208
is input to the 2-part photodetector
214
via the condenser lens
212
and the beam splitter
213
on one hand, and is input to the 4-part photodetector
216
via the composite prism
215
on the other. The 2-part photodetector
214
forms a tracking error detection system in the servo signal detection system, and generates a tracking error signal by obtaining a difference between the outputs of the 2-part photodetector
214
according to the push-pull technique. The composite prism
215
and the
4
part photodetector
216
form a focal error detection system in the servo signal detection system, and generates a focal error signal based on outputs of the 4-part photodetector
216
according to the Foucault technique. A focus servo operation controls the relative positional relationship of the objective lens
206
and the disk
207
based on the focal error signal, so that an in-focus position is located on the disk
207
.
Next, a description will be given of the push-pull technique, by referring to
FIGS. 2 and 3
. FIGS.
2
(
a
),
2
(
b
) and
2
(
c
) show the relative positional relationship of the light beam which is irradiated via the objective lens
206
and the track on the disk
207
, and FIGS.
3
(
a
),
3
(
b
) and
3
(
c
) show a spot of the reflected light beam which is formed on the 2-part photodetector
214
in correspondence with FIGS.
2
(
a
),
2
(
b
) and
2
(
c
).
FIG.
2
(
b
) shows a case where the spot of the light beam is positioned at the center of a guide groove
207
a
of the disk
207
. In this case, the spot of the reflected light beam on the 2-part photodetector
214
is formed as shown in FIG.
3
(
b
), and a light intensity distribution b is symmetrical to the right and left. If the outputs of the 2-part photodetector
214
are denoted by A and B, a tracking error signal TES is generated based on the following formula (1).
TES=A−B (1)
In this case, the tracking error signal TES is 0.
If the spot of the light beam in FIG.
2
(
b
) shifts to the right as shown in FIG.
2
(
a
), a light intensity distribution a of the reflected light beam becomes unbalanced and the light intensity at the left detector part of the 2-part photodetector
214
becomes larger as shown in FIG.
3
(
a
). For this reason, the tracking error signal TES in this case takes a positive value.
On the other hand, if the spot of the light beam in FIG.
2
(
b
) shifts to the left as shown in FIG.
2
(
c
), a light intensity distribution c of the reflected light beam becomes unbalanced and the light intensity at the right detector part of the 2-part photodetector
214
becomes larger as shown in FIG.
3
(
c
). For this reason, the tracking error signal TES in this case takes a negative value.
Accordingly, if the spot of the light beam on the disk
207
shifts to the right or left with respect to the central position of the guide groove
207
a
, the tracking error signal TES which is obtained in the above described manner changes to a more positive or negative value. Thus, it is possible to carry out an appropriate tracking control operation based on the tracking error signal TES.
FIG. 4
shows an example of the shapes of the composite prism
215
and the 4-part photodetector
216
. The 4-part photodetector
216
includes detector parts
216
a
,
216
b
,
216
c
and
216
d
. A focal error signal FES is generated from outputs A, B, C and D respectively output from the detector parts
216
a
,
216
b
,
216
c
and
216
d
of the 4-part photodetector
216
, based on the following formula (2).
FES=(A−B)+(C−D) (2)
Ideally, the focal error signal FES is
0
in a state where the spot of the light beam is in focus on the disk
207
. In this case, the focal error signal FES having an S-curve as shown in
FIG. 5
is obtained depending on the distance between the objective lens
206
and the disk
207
. In
FIG. 5
, the ordinate indicates the focal error signal FES, and the abscissa indicates the distance between the objective lens
206
and the disk
207
. The origin (
0
) on the abscissa corresponds to the in-focus position, and the above distance becomes smaller tow
Fujimaki Tohru
Itami Satoshi
Miyabe Kyoko
Tezuka Koichi
Uno Kazushi
Fujitsu Limited
Young W. R.
LandOfFree
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