Optical pickup for adjusting focus error signal offset and having widely separated photodetector elements

An optical pickup which include a light source; an optical path changing portion to change a path of light emitted from the light source; an objective lens to converge the light from the optical path changing portion to form a light spot on a recording surface of a disk; and a photodetector having at least two light-receiving regions, preferably four light-receiving regions in a 2.times.2 matrix, which separately perform photo-electric conversion, for receiving the light which has passed through the optical path changing portion and the objective lens and been reflected from the disk. The light-receiving regions are separated by a predetermined distance, such that a center region of the light is not received by the light-receiving regions.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a compatible optical pickup which is 
compatible with disks having different thicknesses, and more particularly, 
to an optical pickup enabling adjustment of a focus error signal offset, 
and reducing the focus error signal offset during recording/reproducing 
disks having different thicknesses. 
2. Description of the Related Art 
An optical pickup device is used for recording and reproducing an image, 
sound or data on and from an optical disk at a high density. The optical 
disk includes a substrate made of plastic or glass having a predetermined 
thickness, through which incident light passes, and an information 
recording layer formed behind the substrate. The size of the optical disk 
is standardized, and efforts are now concentrated on increasing the 
information recording density on the information recording layer, to 
record more information on the optical disk having a predetermined size. 
A digital versatile disk (DVD) and high definition (HD)-DVD, released 
recently, are high-capacity optical disks capable of recording massive 
amounts of information. The thickness of the DVD is standardized to be 
different from that of a compact disk (CD), taking account of an allowance 
error in mechanical disk slant and numerical apertures of objective 
lenses. 
That is, an optical pickup for recording/reproducing information on/from a 
CD has an objective lens having a numerical aperture of 0.45, while an 
optical pickup for recording/reproducing information on/from a DVD has an 
objective lens having a numerical aperture of 0.6, to increase 
recording/reproducing density. The thickness of the CD is 1.2 mm while the 
thickness of the DVD is 0.6 mm due to the allowance error of disk slant, 
caused by using the objective lens having a larger numerical aperture. It 
is likely that the thickness of the HD-DVD will be standardized to 0.6 mm. 
Also, wavelengths of the light source for reproduction are different in the 
DVD and CD. That is, while the wavelength of the light source for 
reproduction of a CD is approximately 780 nm, the wavelength of the light 
source for reproduction of a DVD is approximately 650 nm. Also, for 
reproduction in the HD-DVD, a light source emitting light having a short 
wavelength of approximately 420 nm will be required. 
Because of the difference of thickness between the CD and DVD, spherical 
aberration occurs when information is recorded/reproduced on/from a CD 
using an optical pickup for the DVD, so that enough light intensity for 
recording information cannot be obtained or a reproduced signal 
deteriorates. 
Referring to FIG. 1, which shows the optical arrangement of a general 
optical pickup for DVD, the optical pickup includes a light source 1, a 
light beam splitter 7 for changing the path of incident light, a mirror 8 
to reflect the incident light from the beam splitter, an objective lens 12 
for converging incident light reflected from the mirror 8 to form a light 
spot on the recording surface of a disk 19, and a photodetector 18 for 
detecting an error signal and a radio frequency signal. Here, the light 
source 1 emits a light having a wavelength of approximately 650 nm, and 
the numerical aperture of the objective lens 12 is 0.6. 
The light emitted from the light source 1 passes through the beam splitter 
7 and is reflected by the mirror 8 to be condensed by the objective lens 
12 and form a light spot on the recording surface of the disk 19. 
Then, light reflected from the disk 19 passes through the objective lens 
12, is reflected by the mirror 8, passes through the beam splitter 7 to be 
incident on the photodetector 18. As shown in FIG. 2, the photodetector 18 
includes four light-receiving regions A, B, C and D which separately 
undergo photo-electric conversion to allow the detection of a focus error 
signal (FES) by an astigmatism method. Here, a distance d' between the 
light-receiving regions is less than approximately 10 .mu.m. In FIG. 1, 
reference numeral 3 represents a grating for diverging the incident light 
by diffraction to detect a tracking error signal by a three-beam method, 
and reference numeral 16 represents a condensing lens for condensing the 
incident light to form a light spot on the photodetector 18. 
In operation, the beam splitter 7 is astigmatic, which affects the light 
reflected from the disk 19 as it passes through the beam splitter 7. Thus, 
a light spot formed on the photodetector 18 during the 
recording/reproduction of a DVD changes according to the distance between 
the objective lens 12 and the disk 19 as shown in FIGS. 3A through 3E. 
Here, FIGS. 3A and 3B show a light spot received by the photodetector 18 
in the case where the distance between the objective lens 12 and the disk 
19 is longer than the focal distance of the objective lens 12. FIG. 3C 
shows a light spot received by the photodetector 18 in the case where the 
distance between the objective lens 12 and the disk 19 is equal to the 
focal distance, that is, under an on-focus state, and FIGS. 3D and 3E show 
a light spot received by the photodetector 18 in the case where the 
distance between the objective lens 12 and the disk 19 is shorter than the 
focal distance. 
In the optical pickup, the FES is obtained from the difference between the 
sums of signals of diagonally opposite light-receiving regions. That is, 
the FES is obtained by the difference between the sum of signals of the 
light-receiving regions A and C, and the sum of signals of the 
light-receiving regions B and D. In an on-focus state as shown in FIG. 3C, 
the value of a reproduced signal becomes the maximum, and the FES is 
approximately 0. 
When recording/reproducing information on/from a CD using an optical pickup 
device for DVD, the light spot formed on the photodetector 18 is distorted 
due to the spherical aberration caused by the difference between the 
thickness of the DVD and CD. In this case, the FES is also distorted, so 
that focusing is difficult. Accordingly, it is difficult to 
record/reproduce information on/from a CD. 
To overcome the above problems, an objective lens 12' having an annular 
light control pattern 13 is adopted as shown in FIG. 4, so that the FES as 
shown in FIG. 5 is obtained when recording/reproducing information on/from 
the CD. In FIG. 5, the horizontal axis represents the distance between the 
objective lens 12' and the disk 19, that is, the moving amount of the 
objective lens 12', and the vertical axis represents an FES. 
The distance between the disk 19 and the objective lens 12' is controlled 
to achieve focus according to the FES, thereby recording/reproducing 
information on/from the CD. 
However, even though the distance between the CD and the objective lens 12' 
is correct for focus, the FES is not equal to 0 but has a predetermined 
FES offset value due to the effect of the spherical aberration caused by 
the difference in thickness between the CD and the DVD. As shown in FIG. 
5, the focus error signal offset is expressed as a percentage of the focus 
error signal value Wb in an on-focus state where the value of the 
reproduced signal is the maximum, with respect to the maximum amplitude Wa 
of the focus error signal. If the focus error signal offset is over 10%, 
compatibility with the CD is impossible. In particular, when reproducing 
information from the CD using an optical pickup for HD-DVD 
recording/reproduction, adopting a light source emitting light of 
approximately 420 nm and an objective lens having the numerical aperture 
of 0.6, the focus error signal offset becomes 30% due to the effect of the 
spherical aberration, so that it is impossible to reproduce information 
from the CD. 
SUMMARY OF THE INVENTION 
To solve the above problems, it is an object of the present invention to 
provide an optical pickup for adjusting a focus error signal offset, 
having a photodetector of a structure to reduce a focus error signal 
offset, so as to be compatible with disks of different thicknesses. 
Additional objects and advantages of the invention will be set forth in 
part in the description which follows and, in part, will be obvious from 
the description, or may be learned by practice of the invention. 
To achieve the above and other objects of the present invention, there is 
provided an optical pickup including a light source; optical path changing 
means for changing the path of light emitted from the light source; an 
objective lens to converge the light from the optical path changing means 
to form a light spot on a recording surface of a disk; and a photodetector 
having at least two light-receiving regions, which separately perform 
photo-electric conversion, and receive the light which has passed through 
the optical path changing means and the objective lens and been reflected 
from the disk, wherein the light-receiving regions are separated by a 
predetermined distance, such that a portion of the light which is exposed 
to a great effect of spherical aberration is not received. 
The at least two light-receiving regions preferably are four 
light-receiving regions arranged in a 2.times.2 matrix. Further, 
preferably, the distance between the light-receiving regions is in the 
approximate range of 23 to 35 .mu.m.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Reference will now made in detail to the present preferred embodiments of 
the present invention, examples of which are illustrated in the 
accompanying drawings, wherein like reference numerals refer to the like 
elements throughout. The embodiments are described below in order to 
explain the present invention by referring to the figures. 
Referring to FIG. 6, an optical pickup for adjusting a focus error signal 
offset, according to the present invention, includes a light source 1, 
optical path changing means 27 for changing the path of incident light, a 
mirror 17 to reflect the incident light from the optical path changing 
means 27, an objective lens 15 for condensing incident light reflected 
from the mirror 17, and a photodetector 28 having at least two 
light-receiving regions A, B (not shown in this FIG.) for receiving light 
via the optical path changing means 27 after being reflected from the disk 
19. The optical pickup having the above structure is suitable for DVD 
reproduction. 
The objective lens 15 can be the objective lens 12' having the light 
control pattern 13 illustrated with reference to FIG. 4. 
The optical path changing means 27 may be a flat type beam splitter for 
passing the light reflected from the disk 19 and introducing astigmatism. 
Alternatively, the optical path changing means 27 may be various other 
optical devices for diverging the path of incident light. Here, an 
astigmatic lens (not shown) according to the distance between the 
objective lens 15 and the disk 19 may be included in the optical path 
between the optical path changing means 27 and the photodetector 28. 
As shown in FIG. 7, the photodetector 28 includes light-receiving regions 
A, B, C and D in a 2.times.2 matrix, which separately perform 
photo-electric conversion to detect the FES by the astigmatism method. 
According to the characteristic of the present invention, the distance "d" 
between the light-receiving regions A, B, C and D becomes several tens of 
.mu.m, which is greater than the conventional distance of less than 10 
.mu.m. 
With the above configuration, when recording/reproducing information 
on/from the disk 19 when the disk 19 is a thick disk, e.g., a CD, the 
light at the center of the light spot incident onto the photodetector 28, 
which is less affected by the astigmatism, due to the large spherical 
aberration, is not received by the photodetector 28. Thus, the effect of 
the spherical aberration, caused when using the disk 19 having the 
different thickness, is minimized, so that the FES offset of the FES 
obtained from the difference between the sums of the signals of the 
diagonally opposite light-receiving regions is decreased. 
When reproducing information from a CD using an optical pickup for DVD 
according to the present invention, the FES offset according to the change 
of the distance between the light-receiving regions A, B, C and D of the 
photodetector 28 is shown in FIG. 8. 
In FIG. 8, the wavelength of the light source 1 is 420 nm and the numerical 
aperture of the objective lens 15 is 0.6. 
As shown in FIG. 8, when the distance between the light-receiving regions 
is approximately 25.about.30 .mu.m, the FES offset is reduced to .+-.5%. 
Also, when the distance between the light-receiving regions is 
approximately 23.about.35 .mu.m, the FES offset is still within .+-.10%, 
which ensures compatibility with CDs. 
According to the present invention, the distance "d" between the 
light-receiving regions A, B, C and D of the photodetector 28 can be 
controlled to a suitable value within the range in which the photodetector 
28 does not receive the part of the light spot which is exposed to a great 
effect of the spherical aberration. 
The optical pickup according to the embodiment of the present invention 
includes a photodetector which has four light-receiving regions in a 
2.times.2 matrix, and detects a focus-error signal by an astigmatism 
method. However, the photodetector may instead include at least two 
light-receiving regions arranged to reduce the focus error signal offset, 
and detect the focus error signal by another method, e.g., a beam size 
method. 
As described above, the optical pickup according to the embodiment of the 
present invention includes a photodetector in which the distance between 
the light-receiving regions is property adjusted to minimize the effect of 
the spherical aberration caused by the difference in thickness when 
information is recorded/reproduced on/from a disk having a different 
thickness, so that the FES offset is reduced. As a result, the optical 
pickup is compatible with disks having different thicknesses.