Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
2000-09-27
2004-03-02
Korzuch, William (Department: 2653)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S053220, C369S053350, C369S124120
Reexamination Certificate
active
06700842
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical head used for at least either recording or reproducing information by light to/from a recording medium, an optical information recording and reproducing apparatus configured by using such an optical head, and a focus error detecting method for detecting a focus error of light condensed onto a recording medium.
2. Description of the Related Art
In recent years, various optical information recording and reproducing apparatuses for at least either optically recording or optically reproducing information by using an optical information recording medium such as an optical disk have been being commercially available. Among them, an optical disk drive using an optical disk as a recording medium has been remarkably spread and its recording density is becoming higher and higher. As a playback-only optical disk drive, for example, a DVD drive capable of reproducing a DVD of which recording capacity has been increased to 4.7 GB that is about 7 times as large as that of a CD-ROM (Compact Disk-Read Only Memory) is already commercially available. The DVD has the same size (diameter of 120 mm) as a CD-ROM having a recording capacity of about 650 MB.
Generally, in an optical disk, a recording surface is formed on a transparent substrate and light for recording or reproducing emitted to an optical disk via an objective lens passes through the transparent substrate and is condensed on the recording surface. For example, a DVD drive adopts a differential phase difference method (DPD method) using a photodetector which is divided into four parts by a cross lattice. On the other hand, in order to minimize a beam spot on the recording surface, focusing error detection is performed. As the detection method, an astigmatism method, in which the same photodetector as that used for the tracking error detection is usable, is often employed.
In a CD drive, the NA (Numerical Aperture) of an objective lens is set to 0.45. In a DVD drive, in order to make the recording density higher, the NA of the objective lens is increased to 0.60. When the NA is increased, generally, aberration (mainly, coma aberration) in the case where a disk is tilted increases and a reproduction signal is degraded. Since the aberration is usually almost proportional to the cube of the NA and the thickness of the substrate, in the DVD, in order to suppress the aberration, the thickness of a disk is set to 0.6 mm which is the half of that of a CD (=1.2 mm).
The DVD is expected as an optical disk which succeeds the CD. It is desired that the DVD drive is usable for reproducing information from a CD. In the case of reproducing a CD by an optical system including an objective lens optimized for reproducing a DVD of a higher density, spherical aberration occurs due to different thickness of the substrate. A good reproduction signal cannot be therefore obtained.
Under present circumstances, the DVD drive is further expected to reproduce also a CD-R (CD recordable) as a write-once CD. The CD-R is, however, usually made of materials including a coloring agent sensitive to light of a relatively long wavelength. Consequently, it is difficult to reproduce a CD-R by using a light source which emits light of 650 nm used in the DVD drive. In each of a DVD-ROM reproducing apparatus and a DVD-Video reproducing apparatus which can reproduce a CD-R as well, two kinds of light sources of a light source for emitting light of 650 nm and a light source for emitting light of 780 nm are used and an optical head (optical pickup) devised to suppress also signal degradation caused by different substrate thickness is adopted. An optical head for use in the DVD-ROM apparatus capable of reproducing not only a DVD-ROM but also a CD and a CD-R will be described hereinbelow. In the following description, a CD and a CD-R will be simply described as a CD, and a DVD-ROM and a DVD-Video will be simply described as a DVD.
FIG. 1
is a plan view showing the structure of a photodetector used for an optical head of a DVD apparatus capable of reproducing both a DVD and a CD.
First, the photodetector will be described. As shown in
FIG. 1
, a photodetector
19
comprises: a substrate
191
; a photoreceiving part
192
for a main spot disposed in the central region of the substrate
191
; and two photoreceiving parts
193
and
194
for side spots disposed at equal intervals on both sides of the photoreceiving part
192
for the main spot. The photoreceiving part
192
for the main spot has a rectangular shape as a whole and is divided in four photoreceiving regions
192
A,
192
B,
192
C and
192
D of almost the same shape by dividing lines of a cross. Each of the photoreceiving parts
193
and
194
for side spots is a single region which is not divided.
At the time of reproducing a CD, a light beam emitted from the light source for a CD (not shown) is split into three beams by a diffracting optical system (not shown). The three beams are condensed by an objective lens onto the recording surface of the CD as a recording medium. As shown in
FIG. 1
, the three light beams reflected by the recording surface of the CD enter the center areas of the photoreceiving part
192
for the main spot and the photoreceiving parts
193
and
194
for side spots in the photodetector
19
and form beam spots
196
,
197
and
198
, respectively.
On the other hand, at the time of reproducing a DVD, a light beam emitted from a light source for DVD (not shown) is condensed by the objective lens onto the recording surface of a DVD as a recording medium. The light beam reflected by the recording surface of the DVD passes through a predetermined optical system, is incident on the central area of the photoreceiving part
192
for the main spot in the photodetector
19
, and forms the beam spot
196
. The center of the beam spot
196
is adjusted to almost coincide with the center of the photoreceiving part
192
for the main spot (that is, the intersecting point of the four photoreceiving regions
192
A,
192
B,
192
C and
192
D).
FIG. 1
shows the case where the beam spot
196
on the photoreceiving part
192
for the main spot has an almost circle shape, that is, the optical head is in the focusing state. The focusing state denotes a state where a light beam is focused by the objective lens to form the minimum spot on the recording surface of the recording medium. When the optical head goes out of focus, the beam spot
196
of the photoreceiving part
192
for the main spot changes its shape to either an oval shape having the major axis whose upper part is inclined to the left from the vertical line by 45 degrees and whose lower part is inclined to the right from the vertical line by 45 degrees as shown in
FIG. 2A
or an oval shape having the major axis whose upper part is inclined to the right from the vertical line by 45 degrees and whose lower part is inclined to the left from the vertical line by 45 degrees as shown in FIG.
2
B. Each of
FIGS. 2A and 2B
enlargedly shows only the photoreceiving part
192
for the main spot in the photodetector
19
.
When photoreception signals which go out from the four photoreceiving regions
192
A,
192
B,
192
C and
192
D of the photoreceiving part
192
for the main spot are designated by reference characters a, b, c and d, respectively, and photoreception signals which go out from the photoreceiving parts
193
and
194
for side spots are designated by reference characters e and f, respectively, a focus pull-in signal FPI, a reproduction signal RF, a focus error signal FE, and a tracking error signal TE are expressed by the following equations (1) to (4). The focus pull-in signal FPI is a signal used to regulate a range in which a focusing control is performed on the basis of the focus error signal FE and is obtained by, for example, eliminating high frequency components of the reproduction signal RF by using a predetermined low pass filter (not shown).
focus pull-in signal FPI=reproduction signal RF=
a+b+c+d
&
Chu Kim-Kwok
Frommer William S.
Frommer & Lawrence & Haug LLP
Korzuch William
Polito Bruno
LandOfFree
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