Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
1999-06-01
2002-07-09
Huber, Paul W. (Department: 2653)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S112030
Reexamination Certificate
active
06418098
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical head apparatus for recording/reproducing information by irradiating a light beam onto an optical recording medium such as an optical disk and an optical card and the like, and more particularly to an optical head apparatus which is capable of detecting a focus error without being influenced by noise, caused by diffraction from pits or recorded marks on a recording plane of the recording medium, or caused by diffraction from grooves on the recording plane of the recording medium while a light beam crosses across tracks thereof, thereby achieving a high utilization efficiency in detecting a focus error of the light beam.
2. Description of the Related Art
It is important, in an optical head apparatus for recording/reproducing information by using an optical disk, to coincide a focus position of an objective lens with a recording surface of the optical disk, so as to irradiate a light beam from a light source onto the recording surface. Therefore, detecting a focus error of the objective lens against the recording surface and thereby adjusting the objective lens along a beam axial direction of the objective lens is required as a control method called “focusing servo”. There are many known ways of detecting the focus error, however, an astigmatic method is well known as one of the conventional ways thereof.
FIG. 1
is a diagram showing the system of detecting the focus error using the astigmatic method, as explained in Japanese Patent Disclosure No. H04-364231. The light beam generated from the light source
1
is condensed to the optical disk
6
, through a collimator lens
2
, a beam shaping prism
3
, a beam splitter
4
and the objective lens
5
. The reflected beam from the optical disk
6
is guided to a cylindrical lens
14
to give astigmatism, then the reflected beam is further guided to a four-divided photo-detector
15
. The four-divided photo-detector
15
is arranged in a position where the section of the reflected beam becomes circular in an in-focus condition, that is, the condition when the focus position of the objective lens
5
coincides with the recording surface of the optical disk
6
. The output from the four-divided photo-detector
15
is input to an arithmetic circuit
13
through a current-voltage convert amplifier array
12
, and a calculation, such as (A+C)−(B+D), concerning each output of the four-divided photo-detector
15
is carried out in an arithmetic circuit
13
, thereby obtaining a signal of the focus error.
In the focus error detecting system using the conventional astigmatism method, the focus error signal in the in-focus condition is always calculated to be zero, if the optical system is an ideal system and there is no misalignment between each optical element. However, the optical elements can not be disposed into the optical head apparatus without any misalignment. In case when there is misalignment of the optical element, the focus error signal in the in-focus condition is not calculated to be zero, under the influence of diffraction caused by pit or recorded mark, thereby making it difficult to realize a stable focusing servo.
FIG. 2A
is a diagram showing the intensity distribution of the light beam on the four-divide photo-detector
15
, and
FIG. 2B
is a diagram showing the movement of a light spot on the optical disk
6
. Each figure is illustrated in case when the four-divided photo-detector
15
is disposed with misalignment. Here, each reference number
151
to
153
in
FIG. 2A
indicates the detecting surface of the four-divided photo-detector
15
, and each circle thereof schematically shows the light beam entered at the four-divided photo-detector
15
, and each dark portion thereof schematically shows the diffracted images generated by the pits or the recorded marks on the optical disk
6
respectively. Additionally, each circle
171
to
173
illustrated by short dashes of line shows the optical spot and the reference number
16
indicates the pit or the recorded mark on the optical disk
6
.
As the optical beam, which is condensed by the objective lens
5
, scans along the pits or recorded marks
16
on the optical disk
6
like spots
171
,
172
and
173
, as shown in
FIG. 2B
, the intensity distribution on the four-divided photo-detector
15
changes
151
,
152
and
153
as shown in FIG.
2
A. Here, the dark portions in the figure correspond to shade portions by the influence of diffraction caused by the pits or the recorded marks
16
.
As shown in
FIGS. 2A and 2B
, when the four-divided photo-detector
15
has misalignment, the focus error signal F (=−(A+C)−(B+D) ) on the four-divided photo-detector
15
becomes positive in the intensity distribution on the receiving plane
151
, or becomes zero in the intensity distribution on the receiving plane
152
, or becomes negative in the intensity distribution on the receiving plane
153
. Therefore, it is difficult to realize an accurate focusing control.
Further, there is another case in which the influence of the diffraction caused by grooves on the optical disk exists, other than those caused by the pits and recorded marks.
That is, in the focus error detecting system in the conventional astigmatism method, the focus error signal becomes zero all the time in the in-focus condition. However, the optical elements in the optical head apparatus can not be fixed without misalignment. In the case where such misalignment exits, the focus error signal does not become zero, by influence of the diffraction caused by the grooves, thereby making it difficult to realize the stable focusing servo.
FIG. 3A
is a diagram showing the intensity distribution of the light beam on the four-divide photo-detector
15
, and
FIG. 3B
is a diagram showing the movement of a light spot on the optical disk
6
. Each figure is illustrated for the case when the four-divided photo-detector
15
is disposed with misalignment, similar to
FIGS. 2A and 2B
. Here, each reference number
251
to
253
in
FIG. 3A
indicates the detecting surface of the four-divided photo-detector
15
, and each circle thereof schematically shows the light beam entered at the four-divided photo-detector
15
, and each dark portion thereof schematically shows the diffracted images generated by the pits or the recorded marks on the optical disk
6
respectively. Additionally, each circle d to f shows the optical spot and the reference number
116
indicates the grooves on the optical disk
6
.
The intensity distribution on the four-divided photo-detector
15
changes as reference numbers
251
to
253
as shown in
FIG. 3A
by scanning the beam spot, condensed by the objective lens
5
, as d, e, and f on the grooves
116
in FIG.
3
B. Here, the dark portions thereof show the diffracted images generated by the grooves
116
.
As shown in
FIGS. 3A and 3B
, when the four-divided photo-detector
15
has misalignment, the focus error signal F (=(A+C)−(B+D) ) on the four-divided photo-detector
15
becomes positive in the intensity distribution on the receiving plane
151
, or becomes zero in the intensity distribution on the receiving plane
152
, or becomes negative in the intensity distribution on the receiving plane
153
. Therefore, it is difficult to realize an accurate focusing control.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above-mentioned circumstances and is intended to solve the above-mentioned problems. In particular, one purpose of the present invention is to provide an optical head apparatus in which the focus error signal thereof is less influenced by diffraction caused by the pits or the recorded marks, or caused by the grooves, on the optical disk, even though the optical elements have misalignment.
The present invention provides an optical head apparatus that is capable of reproducing information from an optical recording medium, including: an objective lens that receives a light beam from a light source and co
Iwata Katsuo
Tanaka Masahiko
Yamamoto Yuichiro
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