U.S. patent application number 09/788519 was filed with the patent office on 2001-10-11 for objective lens, optical pickup device and optical disk device.
Invention is credited to Asoma, Yoshito.
Application Number | 20010028514 09/788519 |
Document ID | / |
Family ID | 18573755 |
Filed Date | 2001-10-11 |
United States Patent
Application |
20010028514 |
Kind Code |
A1 |
Asoma, Yoshito |
October 11, 2001 |
Objective lens, optical pickup device and optical disk device
Abstract
An objective lens comprises a single objective lens adapted to a
finite system having first and second aspherical surfaces and a
hologram is formed on at least one of said aspherical surfaces. The
light of the positive 1st order or that of the negative 1st order
of the hologram is subjected to optimal correction of spherical
aberration under actual operating conditions. When a semiconductor
laser is used as light source, the change in the spherical
aberration caused by the change in the refractive index arising as
a result of the change in the environmental temperature of the
medium between the first surface and the second surface is
substantially offset by the change in the spherical aberration of
the hologram attributable to the change in the oscillation
wavelength of the semiconductor laser of the light source caused by
the change in the environmental temperature so that, if the
objective lens has a numerical aperture greater than 0.5, the
astigmatism is favorably corrected regardless of the change in the
environment particularly in terms of temperature. Such an objective
lens can make it easy to downsize an optical pickup device
comprising it.
Inventors: |
Asoma, Yoshito; (Saitama,
JP) |
Correspondence
Address: |
RADER, FISHMAN & GRAUER, P. L. L. C.
Suite 501
1233 20th Street, NW
Washington
DC
20036
US
|
Family ID: |
18573755 |
Appl. No.: |
09/788519 |
Filed: |
February 21, 2001 |
Current U.S.
Class: |
359/719 ;
359/642; 369/112.23; G9B/7.102; G9B/7.113; G9B/7.12 |
Current CPC
Class: |
G11B 7/1374 20130101;
G11B 7/1353 20130101; G11B 7/13922 20130101 |
Class at
Publication: |
359/719 ;
359/642; 369/112.23 |
International
Class: |
G02B 003/02; G02B
013/18; G11B 007/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 23, 2000 |
JP |
P2000-052197 |
Claims
What is claimed is:
1. An objective lens having a numerical aperture greater than 0.5
and comprising a single objective lens adapted to a finite system,
said objective lens having first and second aspherical surfaces; a
hologram being formed on at least one of said aspherical surfaces;
when using a semiconductor laser as light source, the change in the
spherical aberration caused by the change in the refractive index
arising as a result of the change in the environmental temperature
of the medium between the first surface and the second surface
being substantially offset by the change in the spherical
aberration of the hologram attributable to the change in the
oscillation wavelength of the semiconductor laser of the light
source caused by the change in the environmental temperature.
2. The objective lens according to claim 1, wherein, if the radius
of curvature of the first surface and that of the second surface
are respectively r1 and r2 and the focal length is f, while the
cone constants of the first and second surfaces are respectively k1
and k2 and the distance between the two surfaces is D, they satisfy
the requirements of the formulas (1) through (5) listed below.
0.65<r1/f<0.75 (1) 1.2<r2/f<1.4 (2) -0.85<k1<-0.7
(3) 0.2<k2<0.8 (4) 0.8<D/f<1.2 (5)
3. The objective lens according to claim 1, wherein said objective
lens is made of a plastic material.
4. The objective lens according to claim 1, wherein the light of
the positive 1st order or that of the negative 1st order of the
hologram is subjected to optimal correction of spherical aberration
under actual operating conditions.
5. An optical pickup device comprising: a semiconductor laser
operating as light source; an objective lens for converging the
flux of light emitted from the semiconductor laser onto the signal
recording surface of an optical recording medium; a photodetector
for detecting the flux of light converged onto the signal recording
surface by the objective lens and reflected from said signal
recording surface; said objective lens having a numerical aperture
greater than 0.5 and comprising a single objective lens adapted to
a finite system, said objective lens having first and second
aspherical surfaces; a hologram being formed on at least one of
said aspherical surfaces; and when using a semiconductor laser as
light source, the change in the spherical aberration caused by the
change in the refractive index arising as a result of the change in
the environmental temperature of the medium between the first
surface and the second surface being substantially offset by the
change in the spherical aberration of the hologram attributable to
the change in the oscillation wavelength of the semiconductor laser
of the light source caused by the change in the environmental
temperature.
6. The optical pickup device according to claim 5, wherein, if the
radius of curvature of the first surface and that of the second
surface of the objective lens are respectively r1 and r2 and the
focal length of the objective lens is f, while the cone constants
of the first and second surfaces of the objective lens are
respectively k1 and k2 and the distance between the two surfaces of
the objective lens is D, they satisfy the requirements of the
formulas (1) through (5) listed below. 0.65<r1/f<0.75 (1)
1.2<r2/f<1.4 (2) -0.85<k1<-0.7 (3) 0.2<k2<0.8 (4)
0.8<D/f<1.2 (5)
7. The optical pickup device according to claim 5, wherein said
objective lens is made of a plastic material.
8. The optical pickup device according to claim 5, wherein the
light of the positive 1st order or that of the negative 1st order
of the hologram is subjected to optimal correction of spherical
aberration under actual operating conditions.
9. An optical disk device comprising: a rotary drive mechanism for
holding an optical disk and driving it to rotate; and an optical
pickup device for recording information signals on or reproducing
information signals from the optical disk driven to rotate by said
rotary drive mechanism; said optical pickup device including: a
semiconductor laser operating as light source; an objective lens
for converging the flux of light emitted from the semiconductor
laser onto the signal recording surface of an optical recording
medium; a photodetector for detecting the flux of light converged
onto the signal recording surface by the objective lens and
reflected from said signal recording surface; said objective lens
having a numerical aperture greater than 0.5 and comprising a
single objective lens adapted to a finite system, said objective
lens having first and second aspherical surfaces; a hologram being
formed on at least one of said aspherical surfaces, the light of
the positive 1st order or that of the negative 1st order of the
hologram being subjected to optimal correction of spherical
aberration under actual operating conditions; and when using a
semiconductor laser as light source, the change in the spherical
aberration caused by the change in the refractive index arising as
a result of the change in the environmental temperature of the
medium between the first surface and the second surface being
substantially offset by the change in the spherical aberration of
the hologram attributable to the change in the oscillation
wavelength of the semiconductor laser of the light source caused by
the change in the environmental temperature.
10. The optical disk device according to claim 9, wherein, if the
radius of curvature of the first surface and that of the second
surface of the objective lens are respectively r1 and r2 and the
focal length of the objective lens is f, while the cone constants
of the first and second surfaces of the objective lens are
respectively k1 and k2 and the distance between the two surfaces of
the objective lens is D, they satisfy the requirements of the
formulas (1) through (5) listed below. 0.65<r1/f<0.75 (1)
1.2<r2/f<1.4 (2) -0.85<k1<-0.7 (3) 0.2<k2<0.8 (4)
0.8<D/f<1.2 (5)
11. The optical disk device according to claim 9, wherein said
objective lens is made of a plastic material.
12. The optical disk device according to claim 9, wherein the light
of the positive 1st order or that of the negative 1st order of the
hologram is subjected to optimal correction of spherical aberration
under actual operating conditions.
13. An objective comprising a single objective lens adapted to a
finite system and made of a plastic material, said objective lens
having: first and second aspherical surfaces; a hologram formed on
at least one of said aspherical surfaces; and the change in the
spherical aberration of the hologram attributable to the change in
the oscillation wavelength of the semiconductor laser of the light
source caused by the change in the environmental temperature
arising to reduce the change in the spherical aberration caused by
the change in the refractive index arising as a result of the
change in the environmental temperature of the medium between the
first surface and the second surface.
14. The objective lens according to claim 13, wherein, if the
radius of curvature of the first surface and that of the second
surface are respectively r1 and r2 and the focal length is f, while
the cone constants of the first and second surfaces are
respectively k1 and k2 and the distance between the two surfaces is
D, they satisfy the requirements of the formulas (1) through (5)
listed below. 0.65<r1/f<0.75 (1) 1.2<r2/f<1.4 (2)
-0.85<k1<-0.7 (3) 0.2<k2<0.8 (4) 0.8<D/f<1.2
(5)
15. An objective lens according to claim 13, wherein the light of
the positive 1st order or that of the negative 1st order of the
hologram is subjected to optimal correction of spherical aberration
under actual operating conditions.
16. The objective lens according to claim 13, wherein said
objective lens has a numerical aperture greater than 0.5.
17. An optical pickup device comprising: a semiconductor laser
operating as light source; an objective lens for converging the
flux of light emitted from the semiconductor laser onto the signal
recording surface of an optical recording medium; and a
photodetector for detecting the flux of light converged onto the
signal recording surface by the objective lens and reflected from
said signal recording surface; said objective lens being a single
objective lens adapted to a finite system and made of a plastic
material, said objective lens having: first and second aspherical
surfaces; and a hologram formed on at least one of said aspherical
surfaces; the change in the spherical aberration of the hologram
attributable to the change in the oscillation wavelength of the
semiconductor laser of the light source caused by the change in the
environmental temperature arising to reduce the change in the
spherical aberration caused by the change in the refractive index
arising as a result of the change in the environmental temperature
of the medium between the first surface and the second surface.
18. The optical pickup device according to claim 17, wherein, if
the radius of curvature of the first surface and that of the second
surface of the objective lens are respectively r1 and r2 and the
focal length of the objective lens is f, while the cone constants
of the first and second surfaces of the objective lens are
respectively k1 and k2 and the distance between the two surfaces of
the objective lens is D, they satisfy the requirements of the
formulas (1) through (5) listed below. 0.65<r1/f<0.75 (1)
1.2<r2/f<1.4 (2) -0.85<k1<-0.7 (3) 0.2<k2<0.8 (4)
0.8<D/f<1.2 (5)
19. The optical pickup device according to claim 17, wherein the
light of the positive 1st order or that of the negative 1st order
of the hologram is subjected to optimal correction of spherical
aberration under actual operating conditions.
20. The optical pickup device according to claim 17, wherein said
objective lens has a numerical aperture greater than 0.5.
21. An optical disk device comprising: a rotary drive mechanism for
holding an optical disk and driving it to rotate; and an optical
pickup device for recording information signals on or reproducing
information signals from the optical disk driven to rotate by said
rotary drive mechanism; said optical pickup device including: a
semiconductor laser operating as light source; an objective lens
for converging the flux of light emitted from the semiconductor
laser onto the signal recording surface of an optical recording
medium; and a photodetector for detecting the flux of light
converged onto the signal recording surface by the objective lens
and reflected from said signal recording surface; said objective
lens being a single objective lens adapted to a finite system and
made of a plastic material, said objective lens having: first and
second aspherical surfaces; and a hologram formed on at least one
of said aspherical surfaces; the change in the spherical aberration
of the hologram attributable to the change in the oscillation
wavelength of the semiconductor laser of the light source caused by
the change in the environmental temperature arising to reduce the
change in the spherical aberration caused by the change in the
refractive index arising as a result of the change in the
environmental temperature of the medium between the first surface
and the second surface.
22. The optical disk device according to claim 21, wherein, if the
radius of curvature of the first surface and that of the second
surface of the objective lens are respectively r1 and r2 and the
focal length of the objective lens is f, while the cone constants
of the first and second surfaces of the objective lens are
respectively k1 and k2 and the distance between the two surfaces of
the objective lens is D, they satisfy the requirements of the
formulas (1) through (5) listed below. 0.65<r1/f<0.75 (1)
1.2<r2/f<1.4 (2) -0.85<k1<-0.7 (3) 0.2<k2<0.8 (4)
0.8<D/f<1.2 (5)
23. The optical disk device according to claim 21, wherein the
light of the positive 1st order or that of the negative 1st order
of the hologram is subjected to optimal correction of spherical
aberration under actual operating conditions.
24. The optical disk device according to claim 21, wherein said
objective lens has a numerical aperture greater than 0.5.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to an objective lens, to an optical
pickup device comprising such an objective lens and also to an
optical disk device comprising such an optical pickup device.
[0003] 2. Related Background Art
[0004] A number of different types of optical recording medium such
as optical disks have been proposed along with optical pickup
devices adapted to write information signals to and read
information signals from such an optical recording medium. There
have also been proposed a number of different optical disk devices
comprising an optical pickup device and adapted to record
information signals to and reproduce information signals from an
optical disk that is used as optical recording medium.
[0005] An optical pickup device is provided with a semiconductor
laser operating as light source. The flux of light emitted from the
semiconductor laser is converged onto the signal recording surface
of the optical recording medium by means of an objective lens.
Then, the optical pickup device detects the flux of light reflected
by the signal recording surface of the optical recording medium by
means of its optical detector.
[0006] Therefore, the optical pickup device can read any of the
information signals recorded on the optical recording medium on the
basis of the output of the optical detector and write information
signals on the optical recording medium by irradiating the optical
recording medium with a flux of light.
[0007] Meanwhile, efforts have been paid to reduce the diameter of
the light spot formed on the optical recording medium as a result
of the convergence of the flux of light irradiating the optical
recording medium by increasing the numerical aperture (NA) of the
objective lens in order to raise the density of recording
information signals on the optical recording medium.
[0008] However, as the numerical aperture of the objective lens is
increased, the power of the lens is also increased to make it
difficult to design the imaging optical system of the optical
pickup device so as to favouably correct the aberration of the
optical system and maintain the corrected spherical aberration in
the environment that is changing incessantly particularly in terms
of temperature.
[0009] Thus, it has been highly difficult to achieve a numerical
aperture greater than 0.5 in the case of the objective lens of a
finite system. The term of "the objective lens of a finite system"
as used herein refers to an objective lens adapted to receive a
divergent flux of light emitted from a spot light source separated
from it by a finite distance and converge the flux of light onto
the signal recording surface of an optical recording medium.
[0010] In the case of the objective lens of an infinite system
where the objective lens is adapted to receive a parallel flux of
light and converge it onto the signal recording surface of an
optical recording medium, there have been known those having a
numerical aperture greater than 0.5. However, when using the
objective lens of an infinite system, it is difficult to downsize
the optical pickup device because a collimator lens has to be
arranged between the objective lens and the light source in order
to transform the divergent flux of light emitted from the light
source into a parallel flux of light.
BRIEF SUMMARY OF THE INVENTION
[0011] In view of the above identified circumstances, it is
therefore an object of the present invention to provide an
objective lens having a numerical aperture greater than 0.5 while
maintaining the favorably corrected spherical aberration in any
environment that is changing incessantly particularly in terms of
temperature and adapted to downsizing the optical pickup device
comprising it. Another object of the present invention is to
provide an optical pickup device comprising such an objective lens
and an optical disk device comprising such an optical pickup
device.
[0012] According to the invention, the above objects are achieved
by providing an objective lens having a numerical aperture greater
than 0.5 and comprising a single objective lens adapted to a finite
system, said objective lens having first and second aspherical
surfaces;
[0013] a hologram being formed on at least one of said aspherical
surfaces, the light of the positive 1st order or that of the
negative 1st order of the hologram being subjected to optimal
correction of spherical aberration under actual operating
conditions;
[0014] when using a semiconductor laser as light source, the change
in the spherical aberration caused by the change in the refractive
index arising as a result of the change in the environmental
temperature of the medium between the first surface and the second
surface being substantially offset by the change in the spherical
aberration of the hologram attributable to the change in the
oscillation wavelength of the semiconductor laser of the light
source caused by the change in the environmental temperature.
[0015] In another aspect of the invention, there is provided an
optical pickup device comprising:
[0016] a semiconductor laser operating as light source;
[0017] an objective lens for converging the flux of light emitted
from the semiconductor laser onto the signal recording surface of
an optical recording medium; and
[0018] a photodetector for detecting the flux of light converged
onto the signal recording surface by the objective lens and
reflected from said signal recording surface;
[0019] said objective lens having a numerical aperture greater than
0.5 and comprising a single objective lens adapted to a finite
system, said objective lens having first and second aspherical
surfaces;
[0020] a hologram being formed on at least one of said aspherical
surfaces, the light of the positive 1st order or that of the
negative 1st order of the hologram being subjected to optimal
correction of spherical aberration under actual operating
conditions;
[0021] when using a semiconductor laser as light source, the change
in the spherical aberration caused by the change in the refractive
index arising as a result of the change in the environmental
temperature of the medium between the first surface and the second
surface being substantially offset by the change in the spherical
aberration of the hologram attributable to the change in the
oscillation wavelength of the semiconductor laser of the light
source caused by the change in the environmental temperature.
[0022] In a further aspect of the invention, there is provided an
optical disk device comprising:
[0023] a rotary drive mechanism for holding an optical disk and
driving it to rotate; and
[0024] an optical pickup device for recording information signals
on or reproducing information signals from the optical disk driven
to rotate by said rotary drive mechanism; said optical pickup
device including:
[0025] a semiconductor laser operating as light source;
[0026] an objective lens for converging the flux of light emitted
from the semiconductor laser onto the signal recording surface of
an optical recording medium; and
[0027] a photodetector for detecting the flux of light converged
onto the signal recording surface by the objective lens and
reflected from said signal recording surface;
[0028] said objective lens having a numerical aperture greater than
0.5 and comprising a single objective lens adapted to a finite
system, said objective lens having first and second aspherical
surfaces;
[0029] a hologram being formed on at least one of said aspherical
surfaces, the light of the positive 1st order or that of the
negative 1st order of the hologram being subjected to optimal
correction of spherical aberration under actual operating
conditions;
[0030] when using a semiconductor laser as light source, the change
in the spherical aberration caused by the change in the refractive
index arising as a result of the change in the environmental
temperature of the medium between the first surface and the second
surface being substantially offset by the change in the spherical
aberration of the hologram attributable to the change in the
oscillation wavelength of the semiconductor laser of the light
source caused by the change in the environmental temperature.
[0031] As described above, an objective lens according to the
invention comprises a single objective lens adapted to a finite
system, said objective lens having first and second aspherical
surfaces, a hologram being formed on at least one of said
aspherical surfaces, the light of the positive 1st order or that of
the negative 1st order of the hologram being subjected to optimal
correction of spherical aberration under actual operating
conditions;
[0032] When using a semiconductor laser as light source, the change
in the spherical aberration caused by the change in the refractive
index arising as a result of the change in the environmental
temperature of the medium between the first surface and the second
surface is substantially offset by the change in the spherical
aberration of the hologram attributable to the change in the
oscillation wavelength of the semiconductor laser of the light
source caused by the change in the environmental temperature.
[0033] Thus, the present invention provides an objective lens
having a numerical aperture greater than 0.5 while maintaining the
favorably corrected spherical aberration in any environment that is
changing incessantly particularly in terms of temperature and
adapted to downsizing the optical pickup device comprising it.
According to the invention, there are also provided an optical
pickup device comprising such an objective lens and an optical disk
device comprising such an optical pickup device.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
[0034] FIG. 1 is a schematic cross sectional view of an objective
lens according to the invention, illustrating its
configuration;
[0035] FIG. 2 is a schematic lateral view of an optical pickup
device according to the invention and comprising an objective lens
as illustrated in FIG. 1;
[0036] FIG. 3 is a schematic block diagram of an optical disk
device according to the invention and comprising an optical pickup
device as illustrated in FIG. 2;
[0037] FIG. 4 shows graphs illustrating the spherical aberration
and the astigmatism of light of the 0-th order of the hologram of
Example 1 of the present invention;
[0038] FIG. 5 shows graphs illustrating the spherical aberration
and the astigmatism of light of the 1st order of the hologram of
Example 1 of the present invention when the wavelength of the flux
of incident light is equal to the reference wavelength;
[0039] FIG. 6 shows graphs illustrating the spherical aberration
and the astigmatism of light of the hologram of Example 1 of the
present invention when the oscillation wavelength of the
semiconductor laser is changed along with the refractive index of
the medium by the change in the environmental temperature;
[0040] FIG. 7 shows graphs illustrating the spherical aberration
and the astigmatism of light of the 0-th order of the hologram of
Example 2 of the present invention;
[0041] FIG. 8 shows graphs illustrating the spherical aberration
and the astigmatism of light of the 1st order of the hologram of
Example 2 of the present invention when the wavelength of the flux
of incident light is equal to the reference wavelength; and
[0042] FIG. 9 shows graphs illustrating the spherical aberration
and the astigmatism of light of the hologram of Example 2 of the
present invention when the oscillation wavelength of the
semiconductor laser is changed along with the refractive index of
the medium by the change in the environmental temperature.
DETAILED DESCRIPTION OF THE INVENTION
[0043] Now, the present invention will be described by referring to
the accompanying drawing that illustrate preferred embodiments of
the invention.
[0044] Referring to FIG. 1, an objective lens 7 according to the
invention has a numerical aperture greater than 0.5 and comprises a
single objective lens adapted to a finite system. Both the first
surface 1 and the second surface 2 of the objective lens are
aspherical. A transparent and uniform medium 3 typically made of a
synthetic resin material is arranged between the first surface 1
and the second surface 2.
[0045] Then, a hologram (HOE) 4 is formed on at least either the
first surface 1 or the second surface 2. The light of the positive
1st order or that of the negative 1st order of the hologram 4 is
subjected to optimal correction of spherical aberration under
actual operating conditions. In other words, when there is no
hologram 4, neither the first surface 1 nor the second surface 2
are subjected to optimal correction of spherical aberration under
actual operating conditions.
[0046] When a semiconductor laser is used as light source for
emitting a flux of light that is transmitted through the objective
lens 7, it is so arranged that the change in the spherical
aberration caused by the change in the refractive index arising as
a result of the change in the environmental temperature of the
medium 4 between the first surface 1 and the second surface 2 is
substantially offset by the change in the spherical aberration of
the hologram 4 attributable to the change in the oscillation
wavelength of the semiconductor laser of the light source caused by
the change in the environmental temperature.
[0047] Referring to FIG. 1, if the radius of curvature of the first
surface 1 and that of the second surface 2 of the objective lens 7
are respectively r1 and r2 and the focal length of the objective
lens 7 is f, while the cone constants of the first and second
surfaces of the objective lens are respectively k1 and k2 and the
distance between the two surfaces is D, they satisfy the
requirements of the formulas (1) through (5) listed below.
0.65<r1/f<0.75 (1)
1.2<r2/f<1.4 (2)
-0.85<k1<-0.7 (3)
0.2<k2<0.8 (4)
0.8<D/f<1.2 (5)
[0048] The change in the spherical aberration caused by the change
in the refractive index arising as a result of the change in the
environmental temperature of the medium 4 between the first surface
1 and the second surface 2 is substantially offset by the change in
the spherical aberration of the hologram 4 attributable to the
change in the oscillation wavelength of the semiconductor laser of
the light source caused by the change in the environmental
temperature when the requirements of the formulas (1) through (5)
above are satisfied.
[0049] An optical pickup device according to the invention and
comprising an objective lens according to the invention that has a
configuration as described above is provided with a semiconductor
laser 5 operating as light source as shown in FIG. 2. The divergent
flux of light emitted from the semiconductor laser 5 is reflected
by a beam splitter 6 before entering the objective lens 7. The beam
splitter 6 is a plate having a pair of major planes that are
parallel to each other and inclined by 45.degree. relative to the
optical axis of the flux of light emitted from the semiconductor
laser 5 such that the optical path of the flux of light is
deflected by 90.degree. as the flux of light is reflected by the
corresponding one of the surfaces of the beam splitter 6.
[0050] The flux of light that enters the objective lens 7 is then
converged on the signal recording surface of optical disk 10 that
operates as optical recording medium. The flux of light converged
by the objective lens 7 is then reflected by the signal recording
surface and once again enters the objective lens 7 as returning
flux of light so that it is converged by the objective lens 7. The
returning flux of light is then transmitted through the beam
splitter 6 and received by photodetector 9 by way of detection lens
8.
[0051] Since the returning flux of light gives rise to astigmatism
when transmitted through the beam splitter 6, it is possible to
detect the focussing error signal indicating the distance between
the focal point of the objective lens 7 and the signal recording
surface of the optical disk 10 by detecting the direction and the
extent of the astigmatism.
[0052] With the above optical pickup device, it is possible to read
any of the information signals recorded on the optical disk 10 on
the basis of the optical detection output of the photodetector 9
and write information signals on the optical disk 10 by irradiating
the optical disk 10 with the flux of light emitted from the light
source.
[0053] Now, referring to FIG. 3, an optical disk device according
to the invention and comprising an optical pickup device according
to the invention and having the above described configuration is
provided with a rotary drive mechanism 11 adapted to hold an
optical disk 10 at the center thereof and drive it to rotate. In
the optical disk device, the optical pickup device 12 is supported
by a feed mechanism 13 in a state where the objective lens of the
optical pickup device 12 is arranged vis-a-vis the signal recording
surface of the optical disk 10 that is driven to rotate by the
rotary drive mechanism 11. The optical disk 10 can be radially
moved by the feed mechanism.
[0054] The output signal of the photodetector of the optical pickup
device 12 is transmitted to and demodulated by signal processing
device 14. Additionally, in the optical disk device, the
semiconductor laser of the optical pickup device 12, the rotary
drive mechanism 11 and the feed mechanism 13 are controlled by the
signal processing device 14 such that information signals are
recorded on and reproduced from the optical disk 10 by the optical
disk device.
[0055] Now, the present invention will be described further by way
of examples of objective lens 7.
[0056] In the following examples, a plastic material was used for
the medium of the objective lens 7, which was designed so as to be
used in an optical pickup device for so-called "DVDs (Digital
Versatile Discs" (trademark). The refractive index n of the medium
was 1.539397 when the wavelength .lambda. of the incident flux of
light was equal to the reference wavelength, or 655 nm.
EXAMPLE 1
[0057] An objective lens was prepared with a focal length f of 3.6
mm and a numerical aperture NA of 0.60.
[0058] As for the first surface, the values listed below were
selected respectively for the radius of curvature R1, the cone
constant k1, the aspheric factors A1, B1, C1 and D1.
[0059] R1=2.4613 mm
[0060] k1=-7.98135.times.10.sup.-1
[0061] A1=-2.2088.times.10.sup.-4
[0062] B1=4.3924.times.10.sup.-5
[0063] C1=-5.3094.times.10.sup.-6
[0064] D1=-1.8085.times.10.sup.-6
[0065] As for the second surface, the values listed below were
selected respectively for the radius of curvature R2, the cone
constant k2, the aspheric factors A2, B2, C2 and D2.
[0066] R2=-4.9441 mm
[0067] k2=5.96593.times.10.sup.-1
[0068] A2=1.73154.times.10.sup.-2
[0069] B2=-2.86095.times.10.sup.-3
[0070] C2=3.20654.times.10.sup.-4
[0071] D2=-1.64321.times.10.sup.-5
[0072] The distance between the two surfaces was defined as
follows. D=3.2 mm
[0073] The above values satisfy the requirements of the formulas
(1) through (5).
[0074] FIG. 4 shows graphs illustrating the spherical aberration
and the astigmatism of light of the 0-th order of the hologram 4 of
Example 1 as aberration curves of the objective lens 7. In other
words, the aberration curves represents the spherical aberration
and the astigmatism of the objective lens 7 when no hologram 4 is
provided. When no hologram 4 is provided, the first and second
surfaces are not subjected to optimal correction of spherical
aberration in actual operating conditions.
[0075] FIG. 5 shows graphs illustrating the spherical aberration
and the astigmatism of light of the 1st order of the hologram 4 of
Example 1 when the wavelength of the flux of incident light is
equal to the reference wavelength. In other words, the curves
represents the spherical aberration and the astigmatism of the
objective lens 7 in actual operating conditions. As shown in FIG.
5, the first and second surfaces are subjected to optimal
correction of spherical aberration in actual operating conditions
for the light of the positive 1st order (or the light of the
negative 1st order) of the hologram 4.
[0076] FIG. 6 shows graphs illustrating the spherical aberration
and the astigmatism of light of the hologram of Example 1 when the
oscillation wavelength of the semiconductor laser is changed by 8
nm along with the refractive index of the medium by the change in
the environmental temperature. As shown in FIG. 6, when a
semiconductor laser is used for the light source, the change in the
spherical aberration caused by the change in the refractive index
arising as a result of the change in the environmental temperature
of the medium 4 between the first surface 1 and the second surface
2 is substantially offset by the change in the spherical aberration
of the hologram 4 attributable to the change in the oscillation
wavelength of the semiconductor laser of the light source caused by
the change in the environmental temperature.
EXAMPLE 2
[0077] An objective lens was prepared with a focal length f of 3.2
mm and a numerical aperture NA of 0.60.
[0078] As for the first surface, the values listed below were
selected respectively for the radius of curvature R1, the cone
constant k1, the aspheric factors A1, B1, C1 and D1.
[0079] R1=2.2104 mm
[0080] k1=-7.3971.times.10.sup.-1
[0081] A1=-2.0437.times.10.sup.-3
[0082] B1=2.8339.times.10.sup.-5
[0083] C1=2.0377.times.10.sup.-5
[0084] D1=-9.3615.times.10.sup.-6
[0085] As for the second surface, the values listed below were
selected respectively for the radius of curvature R2, the cone
constant k2, the aspheric factors A2, B2, C2 and D2.
[0086] R2=-4.1313 mm
[0087] k2=3.97415.times.10.sup.-1
[0088] A2=2.77642.times.10.sup.-2
[0089] B2=-6.05964.times.10.sup.-3
[0090] C2=8.92811.times.10.sup.-4
[0091] D2=-6.18937.times.10.sup.5
[0092] The distance between the two surfaces was defined as
follows. D=3 mm.
[0093] The above values satisfy the requirements of the formulas
(1) through (5).
[0094] FIG. 7 shows graphs illustrating the spherical aberration
and the astigmatism of light of the 0-th order of the hologram 4 of
Example 2 as aberration curves of the objective lens 7. In other
words, the aberration curves represents the spherical aberration
and the astigmatism of the objective lens 7 when no hologram 4 is
provided. When no hologram 4 is provided, the first and second
surfaces are not subjected to optimal correction of spherical
aberration in actual operating conditions.
[0095] FIG. 8 shows graphs illustrating the spherical aberration
and the astigmatism of light of the 1st order of the hologram 4 of
Example 2 when the wavelength of the flux of incident light is
equal to the reference wavelength. In other words, the curves
represents the spherical aberration and the astigmatism of the
objective lens 7 in actual operating conditions. As shown in FIG.
8, the first and second surfaces are subjected to optimal
correction of spherical aberration in actual operating conditions
for the light of the positive 1st order (or the light of the
negative 1st order) of the hologram 4.
[0096] FIG. 9 shows graphs illustrating the spherical aberration
and the astigmatism of light of the hologram of Example 2 when the
oscillation wavelength of the semiconductor laser is changed by 8
nm along with the refractive index of the medium by the change in
the environmental temperature. As shown in FIG. 9, when a
semiconductor laser is used for the light source, the change in the
spherical aberration caused by the change in the refractive index
arising as a result of the change in the environmental temperature
of the medium 4 between the first surface 1 and the second surface
2 is substantially offset by the change in the spherical aberration
of the hologram 4 attributable to the change in the oscillation
wavelength of the semiconductor laser of the light source caused by
the change in the environmental temperature.
* * * * *