Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
1998-12-30
2002-06-18
Young, W. R. (Department: 2653)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S112110, C369S112120, C369S120000, C369S112260
Reexamination Certificate
active
06407967
ABSTRACT:
The present invention relates to an optical apparatus including a photodetector having a plurality of light receiving sections to receive a condensed light beam, a tracking apparatus including the optical apparatus, and a disk apparatus including the optical apparatus. The present invention particularly relates to an optical apparatus which compensates for offset resulting from displacement of an optical axis of a condensed light beam with respect to a photodetector.
BACKGROUND OF THE INVENTION
A photodetector for receiving a condensed light beam is generally designed as a two-divided detector or a four-divided detector. The two-divided detector has a light receiving section which is divided into a couple of light receiving regions to receive a condensed spot of a condensed light beam. The four-divided detector has a light receiving section divided into four light receiving regions. The two-divided detector is arranged in such a manner that a spot of a condensed beam equally illuminates both light receiving regions. Moreover, the four-divided detector is arranged in such a manner that a spot of a condensed beam illuminates equally the four light receiving regions.
These two-divided detectors and four-divided detectors are used in an optical disk apparatus. The two-divided detector is typically used as a detector for tracking error detection and as a detector for reading information. The four-divided detector is typically used as a detector for focus error detection.
FIG. 1
shows an optical system of an optical disk apparatus. The optical system shown in
FIG. 1
includes a detector for tracking error detection and a detector for focus error detection. The light beam emitted from a light source
7
becomes a coherent light beam through a collimator lens
12
. The coherent light beam passes a beam splitter and is then focused by an objective lens
20
. The focused light beam illuminates a disk
9
. The objective lens
20
forms a light beam spot on the disk
9
.
The disk
9
is an optical disk on which many information tracks or other targets are formed. In
FIG. 1
, the optical system structured by including the collimator lens
12
, the beam splitter
13
, and the objective lens
20
is called a light focusing optical system
8
. The light beam focused on a recording surface of the disk
9
is also reflected by the recording surface. Moreover, the light beam is diffracted depending on the information recorded on the disk
9
. The reflected light beam is converted to a parallel beam passing through the objective lens
20
. Thereafter, the reflected light beam is bent in its light path by 90 degrees with the beam splitter
13
. The light beam of which the light path direction has been changed is then passed into a beam splitter
14
.
This coherent light beam is split, by this beam splitter
14
, to a light beam directed to the optical system
10
for focus error detection and a light beam directed to the optical system
11
for tracking error detection. The optical system
10
for focus error detection comprises a four-divided photodetector
5
, an optical system
10
a
introducing an asymmetrical spot shape of the condensed light beam, and a condenser lens
10
b
for condensing the light beam on the four-divided photodetector
5
. The optical system
11
for tracking error detection comprises a two-divided photodetector
6
and a condenser lens
11
a
for condensing the light beam on this two-divided photodetector.
The light beam which is deflected by the beam splitter
13
is divided into one light beam input to a reproducing optical system (not illustrated) for reproducing information recorded on the disk
9
, and another light beam entered into the beam splitter
14
.
In the optical system
11
for tracking error detection, deviation between the information track formed on the disk
9
and the beam spot formed on the disk
9
by the objective lens
20
is detected. As the detecting method, various methods are known. The most popular method utilizes the diffraction phenomenon generated on the recording surface of the disk
9
. In a rewritable magneto-optical disk, a guide groove is formed between adjacent tracks. This groove is arranged as many grooves formed in the equal interval in the track width having constant width. Many grooves enables the disk surface to work as the diffraction grating. When a beam spot having a diameter almost equal to the track width is focused on the disk surface and is then reflected therefrom, the mirror-surface reflected light beam and diffracted light beam are generated. The diffracted light beam interferes with the light element reflected by the mirror-surface to largely change the distribution of light beam intensity reflected from the disk
9
. The beam could also be directed through the disk
9
if the disk were transparent.
FIG. 2
shows the distribution of reflected beam intensity influenced by the diffracted light beam. FIG.
2
(
a
) shows the distribution of intensity when the beam spot is located at the center of the track. FIG.
2
(
b
) shows the distribution of strength when the beam spot is located at the position deviated by about ¼ the width of the track from the center of the track. Distribution of the light beam intensity is largely different at the portions where the mirror-surface reflected light beam and diffracted light beam are overlapped or not overlapped. In this case, the reflected light beam is divided into a left side region and a right side region with a straight line (broken line A in the figure) which passes the center of the reflected light beam and is parallel to the track. The beam intensity in the left side region of the straight line A is compared with that in the right side region. In the case of FIG.
2
(
a
), the beam intensity in the left side region is equal to that in the right side region. Meanwhile, in the case of FIG.
2
(
b
), since the diffracted beam is unbalanced, a certain difference is generated in the beam intensity of the left side and right side regions. This difference in intensity is detected by the two-divided photodetector having the dividing light parallel to the track on the disk
9
. Namely, the difference of output of the two-divided photodetector
6
corresponds to the difference of beam strength and also to the amount of deviation of the spot from the center of the track. Therefore, such amount of deviation is detected in higher accuracy. This method is called a push-pull method.
This push-pull method detects the amount of deviation revealed by the difference of strength of the two-divided photodetector. Therefore, when the beam spot center is deviated from the dividing line of the two-divided photodetector, offset is generated in the differential output of the two-divided photodetector.
FIG. 3
shows the condition where deviation of the optical axis occurs. In
FIG. 3
, a chain line indicates the main beam
4
a
of the light beam when the optical axis
20
a
of the objective lens is aligned with the optical axis of the light beam
4
. When the objective lens
20
moves upward (direction of arrow mark B) and is located at the position indicated by a broken line, the main beam of the light beam is input to the objective lens
20
, but the main beam of the light beam is deviated from the optical axis
20
a
of the objective lens
20
. The light beam is refracted by the objective lens
20
and is then directed to the disk
9
. The light beam is reflected by the disk
9
and is then input to the objective lens
20
. The optical axis
4
b
of the main beam of the incident light beam of the objective lens
20
is deviated upward (direction of arrow mark B) from the optical axis
20
a
of the objective lens
20
. This reflected light beam is indicated by a broken line.
Thereafter, the light beam passes the beam splitters
13
,
14
and is then input to the optical system
11
for tracking error detection. Next, this light beam is input to the condenser lens
11
a
at the position where the main beam thereof is deviated downward (direction of arrow mark C) for the optical axis of th
Hasegawa Shin-ya
Odajima Wataru
Greer Burns & Crain Ltd.
Young W. R.
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