Optical pickup device

An optical pickup device which is capable of reproducing and recording information from at least two discs having different thicknesses includes a light source, an objective lens provided along the light path from the light source facing the plane of a disc and having a predetermined effective diameter, a beam splitter provided between the objective lens and the light source, a photodetector for detecting the beam split from the light splitter and reflected from the disc, and light controller provided along the light path facing the photodetector lens for controlling the light of the intermediate region between near- and far axis regions of an incident light beam. The optical pickup device is simplified and the manufacturing cost therefor is low. Also, by reducing the spherical aberration effect for the light, discs having different thicknesses can be used in a single disc drive.

BACKGROUND OF THE INVENTION 
The present invention relates to an optical pickup device, and more 
particularly, to an optical pickup device which is capable of reproducing 
and recording information from/onto at least two kinds of discs having 
different thicknesses. 
An optical pickup records and reproduces information such as video or audio 
data onto/from recording media, e.g., discs (or disks). A disc has a 
structure including an information-recorded surface formed on a substrate. 
For example, the substrate can be made of plastic or glass. In order to 
read or write information from a high-density disc, the diameter of the 
optical spot must be very small. To this end, the numerical aperture of an 
objective lens is generally made large and a light source having a shorter 
wavelength is used. However, when using the shorter wavelength light 
source and an objective lens having a large numerical aperture (NA), a 
tilt allowance of the disc with respect to optical axis is reduced. The 
thus-reduced disc tilt allowance can be increased by reducing the 
thickness of the disc. 
Assuming that the tilt angle of the disc is .theta., the magnitude of a 
coma aberration coefficient W.sub.31 can be obtained from: 
##EQU1## 
where d and n represent the thickness and refractive index of the disc, 
respectively. As understood from the above relationship, the coma 
aberration coefficient is proportional to the cube of the numerical 
aperture (NA). Therefore, considering that the NA of the objective lens 
required for a conventional compact disc (CD) is 0.45 and that for a 
conventional digital video disc or digital versatile disc (DVD) is 0.6 (to 
accommodate the higher information density), the DVD has a coma aberration 
coefficient of about 2.34 times that of the CD having the same thickness 
for a given tilt angle. Thus, the maximum tilt allowance of the DVD is 
reduced to about half that of the conventional CD. In order to conform the 
maximum tilt allowance of the DVD to that of the CD, the thickness d of 
the DVD could be reduced. 
However, such a thickness-reduced disc using a shorter wavelength (high 
density) light source, e.g., a DVD, cannot be used in a 
recording/reproducing apparatus such as a disc drive for the conventional 
CDS using a longer wavelength light source because a disc having an 
non-standard thickness is influenced by spherical aberration to a degree 
corresponding to the difference in disc thickness from that of a normal 
disc. If the spherical aberration is extremely increased, the spot formed 
on the disc cannot have the light intensity needed for recording 
information, which prevents the information from being recorded precisely. 
Also, during reproduction of the information, the signal-to-noise (S/N) 
ratio is too low to reproduce the recorded information exactly. 
Therefore, an optical pickup adopting a light source having a short 
wavelength, e.g., 650 nm, which is compatible for discs having different 
thicknesses, such as a CD or a DVD, is necessary. 
For this purpose, research into apparatuses which can reproduce and record 
information from/onto at least two kinds of discs having different 
thicknesses with a single optical pickup device adopting a shorter 
wavelength light source are underway. Lens devices adopting a combination 
of a hologram lens and a refractive lens have been proposed in, for 
example, Japanese Patent Laid-Open Publication No. Hei 7-98431. 
FIGS. 1 and 2 show the focusing of zero-order and first-order-diffracted 
light onto discs 3a and 3b having different thicknesses, respectively. In 
each figure, a hologram lens 1, provided with a pattern 11, and a 
refractive objective lens 2 are provided along the light path in front of 
discs 3a and 3b. The pattern 11 diffracts a light beam 4 from a light 
source (not shown) passing through hologram lens 1, to thereby separate 
the passing light into first-order-diffracted light 41 and zero-order 
light 40 each of which is focused to a different point on the optical axis 
with a different intensity by the objective lens 2. The two different 
focal points are the appropriate focus points on the thicker disc 3b and 
the thinner disc 3a, respectively and thus enable data read/write 
operations with respect to discs having different thicknesses. 
However, in using such a lens system, the separation of the light into two 
beams (i.e., the zero order and first order light) by the hologram lens 1 
lowers the utilizing efficiency of the actually used (reflected and 
partially twice diffracted, 1st order) light to about 15%. Also, during 
the read operation, since the information is included in only one of the 
two beams and the beam carrying no information is likely to be detected as 
noise. Moreover, the fabrication of such a hologram lens requires a 
high-precision process used in etching a fine hologram pattern, which 
increases manufacturing costs. 
FIG. 3 is a schematic diagram of another conventional optical pickup device 
as disclosed in U.S. Pat. No. 5,281,797. This optical pick-up device 
includes a variable diaphragm la for varying the aperture diameter, so 
that data can be recorded onto a longer wavelength disc as well as a 
shorter wavelength disc, but with the discs having the same thickness, and 
information can be reproduced therefrom. The variable diaphragm la is 
installed between the objective lens 2 and a collimating lens 5. The 
variable diaphragm 1a controls a beam 4 emitted from a light source 9 and 
transmitted through a beam splitter 6, by appropriately adjusting the area 
of the beam passing region, i.e., the numerical aperture (NA). The 
diametral aperture of the variable diaphragm la is adjusted in accordance 
with the focused spot size on the disc being employed and always passes 
the light beam 4a of the central region but selectively passes or blocks 
the beam 4b of the peripheral region. In FIG. 3, a reference numeral 3 
denotes a disc, a reference numeral 7 denotes a focusing lens and a 
reference numeral 8 denotes a photodetector. 
In the optical device having the above configuration, if the variable 
diaphragm is formed by a mechanical diaphragm, its structural resonance 
characteristics change depending on the effective aperture of the 
diaphragm. The installation of the diaphragm onto an actuator for driving 
the objective lens becomes difficult in practice. To solve this problem, 
liquid crystals may be used for forming the diaphragm. This, however, 
greatly impedes the miniaturization of the system, deteriorates 
heat-resistance and endurance and increases manufacturing costs. 
Alternatively, a separate objective lens for each disc may be provided so 
that a specific objective lens is used for a specific disc. In this case, 
however, since a driving apparatus is needed for changing lenses, the 
configuration becomes complex and the manufacturing cost increases 
accordingly. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an optical pickup 
device which is inexpensive and easily fabricated. 
It is another object of the present invention to provide an optical pickup 
device whose light utilizing efficiency is enhanced and which can form 
aberration-reduced spots. 
To accomplish the above object, there is provided an optical pickup device 
according to the present invention comprising: a light source; an 
objective lens provided along a light path of the light source, facing the 
plane of a disc and having a predetermined effective diameter; a light 
splitter provided between the objective lens and the light source; a 
photodetector for detecting the light reflected from the disc and split by 
the beam splitter; and light controlling means provided in the light path, 
facing the photodetector lens for controlling the light of the 
intermediate region between near- and far axis regions of an reflected 
light beam. 
In the optical pickup device according to the present invention, a general 
spherical lens or Fresnel lens may be used for the objective lens. The 
light controlling means may be separately provided in a separate member. 
Otherwise, the light controlling means may be provided in any optical 
elements positioned along the light path facing the photodetector or 
photodetector itself. 
The light controlling means may be provided as a type of a coating film for 
blocking, absorbing, scattering or reflecting the light. Also, the light 
controlling means may be provided as an independent transparent member 
positioned along the light passing region or a light controlling groove 
for scattering or reflecting the light between near- and far axis regions 
positioned in any existing optical elements. 
Also, the light controlling film and light controlling groove as the light 
controlling means may have an annular or perimetrical polygonal (e.g., a 
square) shape. Also, it is preferred that the light controlling groove is 
formed to be sloped by a predetermined angle or to be spherical, the 
bottom plane thereof not being perpendicular with respect to the light 
path, so that the incident light can be reflected in a direction not 
parallel with the light path. 
In the optical pickup device according to the present invention, it is 
preferred that the photodetector has a detection region of a size 
corresponding to the near-axis region light reflected from the thick disc 
and to the near- and far axis region light reflected from the thin disc, 
so that only the near-axis light is detected for the thick disc and both 
the near- and far-axis light is detected for the thin disc. 
Further, it is preferred that the photodetector has a first detection 
region divided into multiple sub-regions and a second detection region 
divided into multiple sub-regions surrounding the first detection region. 
At this time, the first detection region has a detection region of a size 
corresponding to the near-axis region light reflected from the thick disc 
and to the near- and far axis region light reflected from the thin disc, 
so that only the near-axis light is detected for the thick disc and both 
the near- and far-axis light is detected for the thin disc. Also, it is 
preferred that the second detection region receives the light outside the 
near axis, i.e., the far axis region light, depending on the distance 
between the disc and objective lens. 
Particularly, it is preferred that the first detection region of the 
photodetector and the second detection region surrounding the first 
detection region are both vertically and horizontally symmetrical in terms 
of overall structure. It is most preferred that the respective regions are 
divided into four parts to be thus symmetrical.

DETAILED DESCRIPTION OF THE INVENTION 
In the present invention, the light of the intermediate region between 
near- and far-axis regions having much of the components of spherical 
aberration is blocked, shielded or scattered so that the light having less 
components of spherical aberration reaches the photodetector, thereby 
stabilizing a focus signal. Thus, a disc drive which can be used 
compatibly for different kinds of discs having different thicknesses, 
e.g., 1.2 mm compact disc and 0.6 mm digital video disc, can be easily 
manufactured at a low cost. The near-axis region represents the region 
around the central axis of the lens (defined as an optical axis in the 
optics) having a substantially negligible aberration, the far axis region 
represents the region which is farther from the optical axis than the 
near-axis region, and the intermediate region is the region between the 
near-axis region and the far axis region. 
FIG. 4 is a schematic diagram of an optical pickup device according to the 
present invention, in which the light focusing states of a thin disc and a 
thick disc are compared. 
In FIG. 4, reference numerals 300a and 300b represent a thin disc (e.g., 
0.6 mm digital video disc) and a thick disc (e.g., 1.2 mm compact disc), 
respectively. 
A general objective lens 200 is positioned in front of the digital video 
disc 300a or compact disc 300b. The objective lens 200 having a 
predetermined effective diameter focuses an incident light 400 from a 
light source 900 onto the disc 300a or 300b, or receives the light 
reflected from the disc 300a or 300b. A quarter wavelength plate 500 is 
provided in the rear of the objective lens 200. A beam splitter 700 is 
positioned between the quarter wavelength plate 500 and a collimating lens 
600 adjacent the light source 900. 
A focusing lens 800, a light controlling member 810 as a feature of the 
present invention and a photodetector 820 are positioned along the light 
path of the reflected from the beam splitter 700. The photodetector 820 is 
electrically connected to a magneto-optical disc determiner 830. The 
photo-magnetic disc determiner 830 is connected to a differential 
amplifier 841 and an adder 842 for obtaining a photo-magnetic signal. 
In the optical pickup device according to the present invention having the 
aforementioned configuration, the light controlling member 810 is made of 
a transparent material and has on its surface a light controlling film 811 
for absorbing, scattering or reflecting the light of the intermediate 
region between near- and far axis region having many components of 
spherical aberration, among light beams passing through the same and 
travelling toward the photodetector 820. 
In other words, as shown in FIG. 5, the light controlling film 811 controls 
(e.g., blocks, absorbs, scatters, diffracts, refracts or reflects) the 
light of the intermediate region between near- and far axis region having 
many components of spherical aberration. Therefore, only the light beams 
of the near- and far axis regions reach the photodetector 820. The light 
controlling film 811 for blocking the light of the intermediate region may 
have various shapes such as an annular ring or perimetrical polygon (e.g., 
square or pentagon). Also, in view of types, the light controlling film 
811 may be provided as a coating film or a physical structure for blocking 
the light travelling path. The light controlling film 811 to be described 
hereinafter is the one having the broadest meaning. 
The photodetector 820 has the following structural characteristics. 
The photodetector 820 is square in terms of its overall structure. A first 
detection region 821 divided into four parts is positioned in the center 
and a second detection region 822 divided into four parts is provided 
around the first detection region 821. The first detection region 821 
includes four square light detecting elements A1, B1, C1 and D1 and the 
second light detecting region 822 includes four L-shaped light detection 
elements A2, B2, C2 and D2. 
The first detection region 821 is as large as an outer square drawn 
tangentially with respect to the light distributed region produced by the 
light passing through the inner side of the light controlling film 811 
when the objective lens 200 is in an in-focus state with respect to the 
digital video disc 300a. In this case, the second detection region 822 is 
large enough to encompass all light beams incident beyond the light 
controlling film 811. This will be described with reference to FIGS. 7-12 
for better understanding. 
FIG. 7 shows the light distribution when the objective lens 200 is in an 
in-focus state with respect to the digital video disc 300a. The light 
distributed region of the near axis light passing through the inner side 
of the light controlling film 811 is internally tangent to the first 
detection region 821. The light passing through the outer side of the 
light controlling film 811 is distributed narrowly only in the second 
detection region 822. 
FIG. 8 shows the light distribution when the objective lens 200 is in a 
far-focus state with respect to the digital video disc 300a. In this case, 
as shown in FIG. 8, the light distribution lies horizontally, that is, 
throughout the horizontal light receiving elements B2, B1, D1 and D2. 
FIG. 9 shows the light distribution when the objective lens 200 is in a 
near-focus state with respect to the digital video disc 300a. In this 
case, as shown in FIG. 9, the light distributed region is vertically 
elongated, that is, throughout the vertical light receiving elements A2, 
A1, C1 and C2. 
FIG. 10 shows the light distribution when the objective lens 200 is in an 
in-focus state with respect to the compact disc 300b. The light 
distributed region for the near axis light passing through the inner side 
of the light controlling film 811 is internally tangent to the first 
detection region 821, as in the case of the digital video disc 300b. The 
light passing through the outer side of the light controlling film 811 is 
distributed widely in the second detection region 822 so that the second 
detection region 822 is internally tangent to the light distributed region 
for the light passing through the outer side of the light controlling film 
811. 
FIG. 11 shows the light distribution when the objective lens 200 is in a 
far-focus state with respect to the compact disc 300b. In this case, as 
shown in FIG. 11, the light distribution is horizontally elongated, that 
is, throughout the horizontal light receiving elements B2, B1, D1 and D2. 
In this case, the light passing through the inner side of the light 
controlling film 811 is also distributed in the first detection region 821 
and the light passing through the outer side of the light controlling film 
811 is distributed to be horizontally elongated in the second detection 
region 822. 
FIG. 12 shows the light distribution when the objective lens 200 is in a 
near-focus state with respect to the compact disc 300b. In this case, 
unlike in FIG. 11, the light distributed region is vertically elongated, 
that is, throughout the vertical light receiving elements A2, A1, C1 and 
C2. However, in this case, the light passing through the inner side of the 
light controlling film 811 is also distributed in the first detection 
region 821. 
As described above, according to the present invention, only the light 
passing through the inner side of the light controlling film 811, i.e., 
the near-axis light having small spherical aberration, reaches the first 
detection region 821. 
In driving the optical pickup device according to the present invention, 
when information is reproduced or recorded from a thin disc (digital video 
disc) 300a, signals generated from/onto both the first and second 
detection regions 821 and 822 are used. When information is reproduced or 
recorded from/onto a thick disc (compact disc) 300b, a signal only from 
the first detection region 821 is used. 
FIG. 13 is a curve for comparing a focus signal S1 obtained from the signal 
generated only from the first detection region 821 in a state where the 
light controlling film 811 as a feature of the present invention is used 
for the thick disc (compact disc) with a focus signal S2 obtained from all 
the signals generated from the first and second detection regions 821 and 
822 in a state where the light controlling film 811 as a feature of the 
present invention is not used for the thick disc (compact disc). The width 
of the first detection region 821 for the photodetector used herein is set 
to 90 .mu.m and that of the second detection region 822 enclosing the 
first detection region 821 is set to 160 .mu.m. Accordingly, as shown in 
FIG. 13, in case of using the compact disc 300b, if the light controlling 
film 811 is adopted and the first detection region 821 is used, a more 
stable focus signal S1 can be obtained, compared to the case when the 
first and second detection regions 821 and 822 are used. Also, when the 
compact disc is used, the far-axis light which exhibits a high degree of 
spherical aberration is made to be widely distributed in the second 
detection region 822. Thus, the focus signal S2 is increased and the 
symmetry for the focusing direction can be maintained. 
As described above, according to the optical pickup device of the present 
invention, in order to read information from at least two kinds of discs 
having different thicknesses, i.e., a compact disc and a digital video 
disc, a light controlling film and an eight-segment photodetector are 
adopted so that only the near-axis light is received in the photodetector 
when the information is read from the compact disc and the near- and 
far-axis light is received in the photodetector when the information is 
read from the digital video disc. Therefore, when the thick disc is used, 
a signal corresponding to the near axis region is obtained. When the thin 
disc is used, a relatively stable signal corresponding to both regions, 
i.e., the near and far axes, is obtained. 
As described above, compared with the conventional optical pickup device, 
the optical pickup device according to the present invention adopts a 
light blocking or scattering means which is simple and easy to fabricate, 
e.g., a light controlling film formed on a transparent member or a light 
blocking or scattering groove formed on the objective lens; whereas the 
conventional optical pickup device adopts a complex and expensive hologram 
lens. Also, since the light is used without being separated by a hologram 
lens, the optical pickup device according to the present invention 
exhibits an improved light utilizing efficiency. Also, since a signal 
which can discriminate the disc type is obtained, a separate element is 
not required for discriminating the disc type. The present invention has 
been described by way of exemplary embodiment to which it is not limited. 
Variations and modifications will occur to those skilled in the art 
without departing from the present invention, the scope of which is to be 
determined from the claims appended hereto.