Radiant energy – Photocells; circuits and apparatus – Photocell controls its own optical systems
Reexamination Certificate
2001-08-10
2003-05-13
Pyo, Kevin (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controls its own optical systems
C369S044230, C369S053280
Reexamination Certificate
active
06563099
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical information recording/reproducing apparatus, specifically to a reading optical system of an optical disk drive.
2. Description of the Related Art
FIG. 2
is a schematic drawing of a normal optical system of the optical disk drive. Laser light emitted from a semiconductor laser
1
becomes a collimated beam circular in cross section by a collimator
23
and a prism
2
. Subsequently, the laser light is reflected by a polarizing beam splitter
31
and converted to circularly-polarized light by a quarter-wave plate
21
. This circularly-polarized light is converged on an optical disk
50
by an objective lens
41
. The optical disk
50
has a guiding trench in the interior of the disk
50
and a recorded mark whose reflectivity is different from that of a region without recording mark has been written. The length of the recording mark and the spacing therebetween are encoded according to information to be recorded. Since the optical disk
50
is rotating, the recording mark comes in and comes out of an irradiation position of the laser light and the quantity of reflected light varies with time.
The reflected light carrying information returns to the objective lens
41
, and is converted from the circularly-polarized light to linearly-polarized light by the quarter-wave plate
21
. Since this direction of polarization is orthogonal to a direction of polarization of the emitted light from the semiconductor laser
1
, the light is transmitted by the polarizing beam splitter
31
. This transmitted light is divided into two light beams by a beam splitter
32
. The transmitted light of the beam splitter
32
is blocked into a half by a knife edge
22
and converged onto a split photodetector (a two-part split photodetector)
51
located at a focus position of a converging lens
42
by the converging lens
42
.
When the optical disk
50
deviates in the direction of the optical axis and the focus of the irradiation light is not exactly on a layer bearing the recording marks thereon, the quantities of beams of the light falling on two light detecting elements of the split photodetector
51
become unbalanced. This unbalance is detected by an electronic circuit
60
as a differential signal, which is made to act as a focus error signal
71
. The position of the objective lens
41
is adjusted by a lens actuator
63
using this focus error signal
71
so that the focus position of the emitted light of the objective light
41
is always on the layer bearing the recording marks, to avoid an out-of-focus state. On the other hand, the light reflected by the beam splitter
32
irradiates a split photodetector (a two-part split photodetector)
52
through the converging lens
43
with the light being out of focus to the photodetector
52
. An electronic circuit
61
acts as a differential circuit, and an output therefrom becomes a tracking error signal
72
. Moreover, the signals added up together by an electronic circuit
62
become a data signal
73
.
In the conventional example of the optical disk drive described above, detection of the focus error signal is achieved by the knife edge method, and the tracking error signal is detected by a method called the diffracted light differential system. In addition to these, various methods have been proposed, and there are the astigmatic method, the image rotation method, etc. as typical detection methods of the focus error signal. Regarding detection methods of the tracking signal, there are the triplet-spot method, the wobbling method, and the phase difference method, etc. These methods have drawbacks as well as advantages with respect to optical alignment or efficiency of light, and hence are used properly according to the kind of drives. For these details, a book entitled, “Basis and applications of optical disk storage,” (supervised by Yoshito Tsunoda, edited by a corporate juridical person, Institute of Electronics, Information and Communication Engineers) gives detailed explanation.
At the moment when large-capacity and high-speed data transmission is being made to be possible, the optical disk as an information storage medium needs to be a high-density medium to record large-capacity data of moving picture information etc. To fulfill this task, there is no way except shortening the spacing of the recording marks and the track pitch of the optical information medium. Then, in order to read and write the optical information medium with a small spacing of the recording marks and the track pitch, a minute laser spot is prerequisite, and with this view the laser spot is being made minute down to a size comparable to a wavelength of the laser light with the use of an objective lens having a large numerical aperture (NA). However, there is a lower bound to the spot size that is governed by the diffraction limit, so it cannot be miniaturized interminably. Accordingly, the in-plane recording density that nominally depends on the size of the recording mark or the track pitch has a limit that is governed by a lower bound of the spot size. Then, it has been thought to try to increase the information density per unit area of the optical disk by adopting a layer configuration composed of more than one optical information recording layers.
When information is read from the multilayer optical disk, however, there occurs a crosstalk between the signals of the respective layers and a large error may be generated in the read signal with the conventional type optical head. One of reasons for such malfunction is that the focus position of the laser light deviates away from an optical information medium layer that is intended to be read.
The technology that eliminates interference in the focus error signal is disclosed by JP-A-222867/1998 entitled “Optical Pick-Up Apparatus.” This technology is to provide auxiliary light-receiving regions having small areas on both sides of the split photodetector. This technology stands on a basic idea that a spread of the focus error signal for a single layer is narrowed, but an incident state of the reflected light from a neighboring optical information medium layer on the photodetector is not considered. Therefore, it is necessary to correct electrically the quantities of the signals from the auxiliary light-receiving regions to generate the focus error signal.
SUMMARY OF THE INVENTION
The present invention aims at reducing the crosstalk of the focus error signal originated from the neighboring optical information medium layer in a read optical system of the optical disk drive for reading the multilayer optical disk.
FIG. 3
is a schematic diagram for illustrating the problem of the focus error signal generated from the multilayer disk.
FIG. 3
shows a schematic cross section of a two-layer optical disk
501
having a first optical information medium layer
511
and a second optical information medium layer
512
therein. Irradiation light
80
from a semiconductor laser (not shown in the figure) is converged by the objective lens
41
and is in focus to the first optical information medium layer
511
. Reflected light therefrom is transmitted through the objective lens
41
to become a collimated beam and a part of the collimated beam is blocked by the knife edge
22
on the way. The other part of the light that avoids being blocked by the knife edge
22
falls on the split photodetector
51
through the converging lens
43
, as being in a state
81
.
As shown in
FIG. 4
, the reflected light
81
from the first optical information medium layer
511
is converged to be a small spot on a split position of the split photodetector
51
. In this state, the same quantity of light falls on each of the light detecting elements of the split photodetector
51
, respectively. Since the focus error signal is obtained as a differential signal of the split photodetector, if the light beam is in focus to the optical information medium layer, the focus error signal becomes zero because of a zero differential signal. In the case of a single-lay
Ariyoshi Tetsuo
Kimura Shigeharu
Shimano Takeshi
Pyo Kevin
Sohn Seung C.
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