U.S. patent application number 09/798965 was filed with the patent office on 2001-08-30 for method for recording / reproducing optical information recording medium, optical pickup apparatus, objective lens and design method of objective lens.
This patent application is currently assigned to KONICA CORPORATION.. Invention is credited to Arai, Norikazu, Saito, Shinichiro, Yamazaki, Hiroyuki.
Application Number | 20010017830 09/798965 |
Document ID | / |
Family ID | 27291115 |
Filed Date | 2001-08-30 |
United States Patent
Application |
20010017830 |
Kind Code |
A1 |
Arai, Norikazu ; et
al. |
August 30, 2001 |
Method for recording / reproducing optical information recording
medium, optical pickup apparatus, objective lens and design method
of objective lens
Abstract
An optical pickup apparatus includes a light source for emitting
a light flux; and a converging optical system having a first,
second, and third divided surface, which are divided in the order
named from the vicinity of an optical axis of the converging
optical system, for converging the light flux emitted from the
light source, wherein a beam spot passing through the first and
third divided surfaces is formed onto a first optical information
recording medium having a transparent substrate whose thickness is
t1, and a beam spot passing through the first and second surfaces
is formed onto a second optical information recording medium having
a transparent substrate whose thickness is t2 that is more than t1.
The optical pickup apparatus further includes an image sensor for
receiving a light flux reflected from the first and/or second
optical information recording medium.
Inventors: |
Arai, Norikazu; (Tokyo,
JP) ; Yamazaki, Hiroyuki; (Tokyo, JP) ; Saito,
Shinichiro; (Tokyo, JP) |
Correspondence
Address: |
FINNEGAN, HENDERSON, FARABOW, GARRETT &
DUNNER LLP
1300 I STREET, NW
WASHINGTON
DC
20005
US
|
Assignee: |
KONICA CORPORATION.
|
Family ID: |
27291115 |
Appl. No.: |
09/798965 |
Filed: |
March 6, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09798965 |
Mar 6, 2001 |
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09624342 |
Jul 24, 2000 |
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6243349 |
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09624342 |
Jul 24, 2000 |
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09478514 |
Jan 6, 2000 |
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6118749 |
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09478514 |
Jan 6, 2000 |
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08953683 |
Oct 17, 1997 |
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6061324 |
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Current U.S.
Class: |
369/53.2 ;
369/112.26; G9B/7.102; G9B/7.12 |
Current CPC
Class: |
G11B 7/1275 20130101;
G11B 7/1374 20130101; G11B 2007/0006 20130101; G02B 3/10 20130101;
G11B 7/1367 20130101; G11B 7/1353 20130101; G11B 7/13922 20130101;
G11B 7/1381 20130101; G11B 7/139 20130101; G11B 2007/13727
20130101 |
Class at
Publication: |
369/53.2 ;
369/112.26 |
International
Class: |
G11B 007/135 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 23, 1996 |
JP |
280750/1996 |
Feb 26, 1997 |
JP |
042222/1997 |
Jul 23, 1997 |
JP |
197076/1997 |
Claims
What is claimed is:
1. An optical pickup apparatus comprising: (a) a light source for
emitting a light flux; (b) a converging optical system having a
first, second, and third divided surface, which are divided in the
order named from the vicinity of an optical axis of the converging
optical system, for converging the light flux emitted from the
light source, wherein a beam spot passing through the first and
third divided surfaces is formed onto a first optical information
recording medium having a transparent substrate whose thickness is
t1, and a beam spot passing through the first and second surfaces
is formed onto a second optical information recording medium having
a transparent substrate whose thickness is t2 that is more than t1;
and (c) an image sensor for receiving a light flux reflected from
the first or second optical information recording medium.
2. The optical pickup apparatus of claim 1, wherein an angle
between a normal line at a central position on the second divided
surface and the optical axis is larger than an angle between a
normal line at a central position of a surface interpolated from
the first and third divided surfaces and the optical axis.
3. The optical pickup apparatus of claim 1, wherein a beam spot is
formed on the first optical information recording medium, in a
spherical aberration chart, a spherical aberration curve according
to the second divided surface is positioned under than a spherical
aberration curve according to the first divided surface.
4. The optical pickup apparatus of claim 1, wherein the following
conditional expression is satisfied:
0.60.multidot.NA2<NAL<1.30.mul- tidot.NA2 where NA2
represents a numerical aperture on a side of the second optical
information recording medium of the converging optical system
required for recording or reproducing information on the second
optical information recording medium, and NAL represents a
numerical aperture in a border portion between the first and second
divided surfaces.
5. The optical pickup apparatus of claim 1, wherein the following
conditional expressions are satisfied:
0.60.multidot.NA2<NAL<1.30.m- ultidot.NA2 and
0.01<NAH-NAL<0.12 where NA2 represents a numerical aperture
on a side of the second optical information recording medium of the
converging optical system required for recording or reproducing
information on the second optical information recording medium, NAL
represents a numerical aperture in a border portion between the
first and second divided surfaces, and NAH represents a numerical
aperture in a border portion between the second and third divided
surfaces.
6. The optical pickup apparatus of claim 1, wherein the first,
second and third divided surfaces are formed on concentric
circles.
7. An objective lens for use in an optical pickup apparatus,
comprising: an optical surface which is divided, in the order named
from the vicinity of an optical axis of the objective lens, into a
first, second and third divided surfaces for converging a light
flux emitted from a light source, wherein a beam spot passing
through the first and third divided surfaces is formed onto a first
optical information recording medium having a transparent substrate
whose thickness is t1, and a beam spot passing through the first
and second surfaces is formed onto a second optical information
recording medium having a transparent substrate whose thickness is
t2 that is more than t1.
8. The objective lens of claim 7, wherein an angle between a normal
line at a central position on the second divided surface and the
optical axis is larger than an angle between a normal line at a
central position of a surface interpolated from the first and third
divided surfaces and the optical axis.
9. The objective lens of claim 7, wherein a beam spot is formed on
the first optical information recording medium, in a spherical
aberration chart, a spherical aberration curve according to the
second divided surface is positioned under than a spherical
aberration curve according to the first divided surface.
10. The objective lens of claim 7, wherein the following
conditional expression is satisfied:
0.60.multidot.NA2<NAL<1.30.multidot.NA2 where NA2 represents
a numerical aperture on a side of the second optical information
recording medium of the converging optical system required for
recording or reproducing information on the second optical
information recording medium, and NAL represents a numerical
aperture in a border portion between the first and second divided
surfaces.
11. The objective lens of claim 7, wherein the following
conditional expressions are satisfied:
0.60.multidot.NA2<NAL<1.30.multidot.NA2 and
0.01<NAH-NAL<0.12 where NA2 represents a numerical aperture
on a side of the second optical information recording medium of the
converging optical system required for recording or reproducing
information on the second optical information recording medium, NAL
represents a numerical aperture in a border portion between the
first and second divided surfaces, and NAH represents a numerical
aperture in a border portion between the second and third divided
surfaces.
12. The objective lens of claim 7, wherein the first, second and
third divided surfaces are formed on concentric circles.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a method for
recording/reproducing optical information recording medium, wherein
a light flux emitted from a light source is converged on an
information recording surface by a light-converging optical system,
and information is recorded on an optical information recording
medium to be reproduced, or information on the information
recording surface is reproduced, an optical pickup apparatus, an
objective lens used for the foregoing, and a design method of the
objective lens.
[0002] Recently, in association with practical use of a short
wavelength red semiconductor laser, there has been advanced
development of DVD (digital video disk, or digital versatile disk)
representing an optical information recording medium which is the
same in size as CD (compact disk) that is a conventional optical
information recording medium (optical disk) and has higher capacity
and higher density. In this DVD, numerical aperture NA of the
objective lens on the optical disk side is 0.6 when the short
wavelength semiconductor laser of 635 nm is used. Incidentally, in
the DVD, a track pitch is 0.74 .mu.m and the shortest pit length is
0.4 .mu.m, which is less than a half of track pitch of 1.6 .mu.m
and shortest pit length of 0.83 .mu.m of CD, representing that the
DVD is of higher density. In addition to the CD and DVD mentioned
above, optical disks of various standards, such as, for example,
CD-R (recordable compact disk), LD (laser disk), MD (mini-disk),
and MO (magneto-optical disk) have also been commercialized and
have been spread. Table 1 shows a transparent substrate thickness
and its necessary numerical aperture for each of various optical
disks.
1TABLE 1 Transparent Necessary numerical substrate aperture NA
(light Optical disk thickness (mm) source wavelength .lambda. nm)
CD, CD-R 1.20 0.45(.lambda. = 780) (only for reproducing) CD-R 1.20
0.50(.lambda. = 780) (recording and reproducing) LD 1.25
0.50(.lambda. = 780) MD 1.20 0.45(.lambda. = 780) NO(ISO 3.5 inch
230 MB) 1.20 0.55(.lambda. = 780) MO(ISO 3.5 inch 640 MB) 1.20
0.55(.lambda. = 680) DVD 0.60 0.60(.lambda. = 635)
[0003] Incidentally, for the CD-R, light source wavelength .lambda.
is required to be 780 (.mu.m), but for the other optical disks, it
is possible to use light sources having wavelengths other than
those shown in Table 1, and in this case, necessary numerical
aperture NA can be found in accordance with wavelength .lambda. of
the light source to be used. For example, in the case of CD,
necessary numerical aperture NA is approximated to .lambda.
(.mu.m)/1.73 and in the case of DVD, necessary numerical aperture
NA is approximated to .lambda. (.mu.m)/1.06.
[0004] Now, it is an age where various optical disks having
different sizes, substrate thickness, recording densities, and
wavelengths to be used exist in the market as stated above, and
optical pickup apparatuses capable of handling various optical
disks have been proposed.
[0005] As one of then, there has been proposed an optical pickup
apparatus wherein a light-converging optical system capable of
working with each of different optical disks is provided and the
light-converging optical system is switched depending on an optical
disk to be reproduced. However, in this optical pickup apparatus,
plural light-converging optical systems are needed, resulting in a
cost increase, and a driving mechanism for switching the
light-converging optical system is needed, resulting in a
complicated apparatus, and its switching accuracy is required,
which is not preferable.
[0006] Therefore, there have been proposed various optical pickup
apparatuses each employing a single light-converging optical system
and reproducing a plurality of optical disks.
[0007] As one of them, TOKKAIHEI 7-302437 discloses an optical
pickup apparatus wherein a refraction surface of an objective lens
is divided into plural ring-shaped areas, and each divided area
forms an image on one of optical disks having different thickness
for reproducing.
[0008] In addition, TOKKAIHEI 7-57271 discloses an optical pickup
apparatus wherein an objective lens designed to make wavefront
aberration owned by a converged beam to be 0.07 .lambda. or less is
used in the case of a first optical disk with a transparent
substrate having a thickness of t1, and the objective lens is
defocused slightly in the case of a second optical disk with a
transparent substrate having a thickness of t2, both for forming a
light-converged spot.
[0009] However, in the optical pickup apparatus disclosed in
TOKKAIHEI 7-302437, an incident light amount is divided to two
focal points simultaneously by a single objective lens. It is
therefore necessary to make the laser output to be high, which
results in a cost increase. In the optical pickup apparatus
disclosed in TOKKAIHEI 7-57271, on the other hand, there is caused
an increase of the side lobe jitters when reproducing the second
optical disk. In this case, in particular, the second optical disk
is reproduced forcibly by the objective lens designed to make
wavefront aberration to be 0.07 .lambda. or less in the case of the
first optical disk. Therefore, the numerical aperture which-makes
it possible to reproduce the second optical disk is naturally
limited.
SUMMARY OF THE INVENTION
[0010] An object of the invention, therefore, is to be capable of
recording or reproducing plural optical information recording media
with a single light-converging optical system, realizing at low
cost without being complicated, and working also with optical
information recording medium with high NA.
[0011] Further, the object of the invention is to improve
light-converging characteristics of the optical pickup apparatus
having adjusted spherical aberration which has been proposed by the
inventors of the invention in U.S. application Ser. Nos. 08/761,892
and 08/885,763.
[0012] The objects mentioned above can be attained by the following
structures.
[0013] In optical pickup apparatus having therein:
[0014] a light source;
[0015] a light-converging optical system which converges a light
flux emitted from the light source and has an optical surface that
is divided into a first surface, a second surface and a third
surface so that a light flux passing through the first divided
surface and the third divided surface forms a beam spot on the
first optical information recording medium having a t1-thick
transparent substrate, and a light flux passing through the first
divided surface and the second divided surface forms a beam spot on
the second optical information recording medium having a t2-thick
(t1<t2) transparent substrate; and
[0016] an image sensor that receives a light flux reflected on the
first or the second optical information recording medium.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a schematic structure diagram of an optical pickup
apparatus.
[0018] FIGS. 2 (a) is a sectional view illustratively showing an
objective lens and 2 (b) is its front view viewed from the light
source side.
[0019] FIG. 3 is a sectional view showing on objective lens
illustratively.
[0020] Each of FIGS. 4 (a)-4 (f) represents a diagram wherein a
spherical aberration diagram of an objective lens is shown
illustratively.
[0021] Each of FIGS. 5 (a) and 5 (b) represents a diagram wherein a
wavefront aberration diagram of an objective lens is shown
illustratively.
[0022] FIG. 6 is a schematic structure diagram of an optical pickup
apparatus in the third example.
[0023] FIG. 7 (a) is a sectional view showing illustratively an
objective lens in the fourth example, and FIG. 7 (b) is its front
view viewed from the light source side.
[0024] Each of FIGS. 8 (a) and 8 (b) is an aberration diagram of an
objective lens in the first example.
[0025] Each of FIGS. 9 (a) and 9 (b) is a diagram of wavefront
aberration obtained by viewing in the state wherein the objective
lens in the first example is defocused to the position where the
best wavefront aberration is obtained.
[0026] FIG. 10 is a distribution diagram for relative intensity of
a light-converged spot having the best spot shape obtained in the
course of reproducing a DVD with the objective lens in the first
example.
[0027] FIG. 11 represents a distribution diagram for relative
intensity of a light-converged spot having the best spot shape
obtained in the course of reproducing a CD in the first
example.
[0028] Each of FIGS. 12 (a) and 12 (b) is an aberration diagrams of
an objective lens in the second example.
[0029] Each of FIGS. 13 (a) and 13 (b) is a diagram of wavefront
aberration obtained by viewing in the state wherein the objective
lens in the second example is defocused to the position where the
best wavefront aberration is obtained.
[0030] FIG. 14 is a distribution diagram for relative intensity of
a light-converged spot having the best spot shape obtained in the
course of reproducing a DVD with the objective lens in the second
example.
[0031] FIG. 15 represents a distribution diagram for relative
intensity of a light-converged spot having the best spot shape
obtained in the course of reproducing a CD with the objective in
the second example.
[0032] FIG. 16 represents a distribution diagram for relative
intensity of a light-converged spot having the best spot shape
obtained in the course of reproducing a CD with the objective lens
at the wavelength of 635 nm in the second example.
[0033] Each of FIGS. 17 (a) and 17 (b) is an aberration diagram of
an objective lens in the third example.
[0034] Each of FIGS. 18 (a) and 18 (b) is a diagram of wavefront
aberration obtained by viewing in the state wherein the objective
lens in the third example is defocused to the position where the
best wavefront aberration is obtained.
[0035] FIG. 19 is a distribution diagram for relative intensity of
a light-converged spot having the best spot shape obtained in the
course of reproducing a DVD with the objective lens in the third
example.
[0036] FIG. 20 represents a distribution diagram for relative
intensity of a light-converged spot having the best spot shape
obtained in the course of reproducing a CD with the objective in
the third example.
[0037] Each of FIGS. 21 (a) and 21 (b) is an aberration diagram of
an objective lens in the fourth example.
[0038] Each of FIGS. 22 (a) and 22 (b) is a diagram of wavefront
aberration obtained by viewing in the state wherein the objective
lens in the fourth example is defocused to the position where the
best wavefront aberration is obtained.
[0039] FIG. 23 is a distribution diagram for relative intensity of
a light-converged spot having the best spot shape obtained in the
course of reproducing a DVD with the objective lens in the fourth
example.
[0040] FIG. 24 represents a distribution diagram for relative
intensity of a light-converged spot having the best spot shape
obtained in the course of reproducing a CD with the objective in
the fourth example.
[0041] FIG. 25 represents a distribution diagram for relative
intensity of a light-converged spot having the best spot shape
obtained in the course of reproducing a CD with the objective lens
at the wavelength of 635 nm in the fourth example.
[0042] Each of FIGS. 26 (a) and 26 (b) is an aberration diagram of
an objective lens in the fifth example.
[0043] Each of FIGS. 27 (a) and 27 (b) is a diagram of wavefront
aberration obtained by viewing in the state wherein the objective
lens in the fifth example is defocused to the position where the
best wavefront aberration is obtained.
[0044] FIG. 28 is a distribution diagram for relative intensity of
a light-converged spot having the best spot shape obtained in the
course of reproducing a DVD with the objective lens in the fifth
example.
[0045] FIG. 29 represents a distribution diagram for relative
intensity of a light-converged spot having the best spot shape
obtained in the course of reproducing a CD-R with the objective in
the fifth example.
[0046] FIG. 30 represents a distribution diagram for relative
intensity of a light-converged spot having the best spot shape
obtained in the course of reproducing a CD with the objective lens
at the wavelength of 635 nm in the fifth example.
[0047] Each of FIGS. 31 (a) and 31 (b) is an aberration diagram of
an objective lens in the sixth example.
[0048] Each of FIGS. 32 (a) and 32 (b) is a diagram of wavefront
aberration obtained by viewing in the state wherein the objective
lens in the sixth example is defocused to the position where the
best wavefront aberration is obtained.
[0049] FIG. 33 is a distribution diagram for relative intensity of
a light-converged spot having the best spot shape obtained in the
course of reproducing a DVD with the objective lens in the sixth
example.
[0050] FIG. 34 represents a distribution diagram for relative
intensity of a light-converged spot having the best spot shape
obtained in the course of reproducing a CD-R with the objective in
the sixth example.
[0051] FIG. 35 represents a distribution diagram for relative
intensity of a light-converged spot having the best spot shape
obtained in the course of reproducing a CD with the objective lens
at the wavelength of 635 nm in the sixth example.
[0052] Each of FIGS. 36 (a) and 36 (b) is an aberration diagram of
an objective lens in the seventh example.
[0053] Each of FIGS. 37 (a) and 37 (b) is a diagram of wavefront
aberration obtained by viewing in the state wherein the objective
lens in the seventh example is defocused to the position where the
best wavefront aberration is obtained.
[0054] FIG. 38 is a distribution diagram for relative intensity of
a light-converged spot having the best spot shape obtained in the
course of reproducing a DVD with the objective lens in the seventh
example.
[0055] FIG. 39 represents a distribution diagram for relative
intensity of a light-converged spot having the best spot shape
obtained in the course of reproducing a CD-R with the objective in
the seventh example.
[0056] FIG. 40 represents a distribution diagram for relative
intensity of a light-converged spot having the best spot shape
obtained in the course of reproducing a CD with the objective lens
at the wavelength of 635 nm in the seventh example.
[0057] Each of FIGS. 41 (a) and 41 (b) is an aberration diagram of
an objective lens in the eighth example.
[0058] Each of FIGS. 42 (a) and 42 (b) is a diagram of wavefront
aberration obtained by viewing in the state wherein . the objective
lens in the eighth example is defocused to the position where the
best wavefront aberration is obtained.
[0059] FIG. 43 is a distribution diagram for relative intensity of
a light-converged spot having the best spot shape obtained in the
course of reproducing a DVD with the objective lens in the eighth
example.
[0060] FIG. 44 represents a distribution diagram for relative
intensity of a light-converged spot having the best spot shape
obtained in the course of reproducing a CD with the objective in
the eighth example.
[0061] FIG. 45 represents a distribution diagram for relative
intensity of a light-converged spot having the best spot shape
obtained in the course of reproducing a CD with the objective lens
at the wavelength of 635 nm in the eighth example.
[0062] Each of FIGS. 46 (a) and 46 (b) is an aberration diagram of
an objective lens in the ninth example.
[0063] Each of FIGS. 47 (a) and 47 (b) is a diagram of wavefront
aberration obtained by viewing in the state wherein the objective
lens in the ninth example is defocused to the position where the
best wavefront aberration is obtained.
[0064] FIG. 48 is a distribution diagram for relative intensity of
a light-converged spot having the best spot shape obtained in the
course of reproducing a DVD with the objective lens in the ninth
example.
[0065] FIG. 49 represents a distribution diagram for relative
intensity of a light-converged spot having the best spot shape
obtained in the course of reproducing a CD with the objective in
the ninth example.
[0066] FIG. 50 represents a distribution diagram for relative
intensity of a light-converged spot having the best spot shape
obtained in the course of reproducing a CD with the objective lens
at the wavelength of 635 nm in the ninth example.
[0067] Each of FIGS. 51 (a) and 51 (b) is an aberration diagram of
an objective lens in the tenth example.
[0068] Each of FIGS. 52 (a) and 52 (b) is a diagram of wavefront
aberration obtained by viewing in the state wherein the objective
lens in the tenth example is defocused to the position where the
best wavefront aberration is obtained.
[0069] FIG. 53 is a distribution diagram for relative intensity of
a light-converged spot having the best spot shape obtained in the
course of reproducing a DVD with the objective lens in the tenth
example.
[0070] FIG. 54 represents a distribution diagram for relative
intensity of a light-converged spot having the best spot shape
obtained in the course of reproducing a CD with the objective in
the tenth example.
[0071] FIG. 55 represents a distribution diagram for relative
intensity of a light-converged spot having the best spot shape
obtained in the course of reproducing a CD with the objective lens
at the wavelength of 635 nm in the tenth example.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0072] The basic concept of the invention will be explained as
follows.
[0073] First, an optical pickup apparatus will be explained. FIG. 1
is a schematic structure diagram of an optical pickup apparatus
having a single light source.
[0074] Optical pickup apparatus 10 is composed of semiconductor
laser 11 representing a light source (wavelength .lambda.=610-670
nm), polarized beam splitter 12, collimator lens 13, quarter
wavelength plate 14, aperture-stop 17, objective lens 16,
cylindrical lens 18 representing an astigmatism element which
generates astigmatism, photo-detector 30, and 2-dimensional
actuator 15 used for focus control and tracking control.
[0075] A light flex emitted from semiconductor laser 11 passes
through polarized beam splitter 12, collimator lens 13, and quarter
wavelength plate 14 to become a collimated light flux that is a
circularly polarized light. This light flux is diaphragmed by
aperture-stop 17 and then is converged by objective lens 16 on
information recording plane 22 through transparent substrate 21 of
optical disk 20. Then the light flux modulated by an information
bit and reflected on the information recording plane 22 passes
through again the objective lens 16, the quarter wavelength plate
14 and the collimator lens 13, to enter the polarized beam splitter
12 where the light flux is reflected and is given astigmatism by
cylindrical lens 18, and then enters the photo-detector 30 wherein
signals outputted from the photo-detector 30 are used to obtain
reading (reproducing) signals for information recorded on the
optical disk 20. Further, a change in light amount distribution
caused by a change of the shape of a spot on the photo-detector 30
is detected for in-focus detection and track detection. Namely,
output from the photo-detector 30 is used so that focus error
signals and tracking error signals are generated by a processing
circuit which is not illustrated here. The objective lens 16 is
moved in the direction of the optical axis so that the
2-dimensional actuator (for focus control) 15 may cause light from
the semiconductor laser 11 to form an image on the information
recording plane 22 based on the focus error signals, and the
objective lens 16 is moved in the direction perpendicular to the
optical axis so that the 2-dimensional actuator (for tracking
control) 15 may cause light from the semiconductor laser 11 to form
an image on the prescribed track based on the tracking error
signals.
[0076] In the optical pickup apparatus 10 mentioned above, when
reproducing the first optical disk whose transparent substrate
thickness is t1, such as DVD (t1=0.6 mm), for example, the
objective lens 16 is driven by 2-dimensional actuator 15 so that a
beam spot may form a least circle of confusion (best focus). When
reproducing the second optical disk such as, for example, CD
(t2=1.2 mm) whose transparent substrate thickness is t2 which is
different from t1 (preferably, t2>t1) and whose recording
density is lower than that of the first optical disk, by the use of
the objective lens 16, it is not possible to read (reproduce) bits
(information) in the second optical disk because spherical
aberration is caused for the reason that the transparent substrate
thickness is different (preferably, to be larger) and that the spot
size is larger at the position where the beam spot becomes the
least circle of confusion (the position behind a paraxial focal
point position). However, at the front side position (front focus)
which is closer to the objective lens 16 than the position where
the beam spot becomes the least circle of confusion, there is
formed a core having on its central portion a quantity of light
concentrically and there is formed a flare representing unwanted
light around the core, although a size of the total spot is greater
than the least circle of confusion. This core is used for
reproducing (reading) bits (information) of the second optical
disk, and when reproducing the second optical disk, 2-dimensional
actuator 15 is driven so that the objective lens 16 may be made to
be in its defocused state (front focus).
[0077] Next, there will be explained a first embodiment wherein the
invention is applied to objective lens 16 representing one of a
light-converging optical system of optical pickup apparatus 10 to
reproduce the first optical disk and the second optical disk both
differing each other in terms of transparent substrate thickness as
stated above. In FIGS. 2 (a) and 2 (b), a sectional view showing
objective lens 16 conceptually is shown in FIG. 2 (a), and a front
view thereof viewed from a light source is shown in FIG. 2 (b)
Incidentally, a one-dot chain line shows an optical axis. In the
present embodiment, incidentally, transparent substrate thickness
t1 of the first optical disk is smaller than transparent substrate
thickness t2 of the second optical disk, and optical information is
recorded more densely in the first optical disk than in the second
optical disk.
[0078] Objective lens 16 is a convex lens having positive
refracting power wherein both refracting surface S1 on the light
source side and refracting surface S2 on the optical disk 20 side
are aspherical in shape. The refracting surface S1 of the objective
lens on the light source side is composed of plural (three) divided
surfaces Sd1-Sd3 which are concentric with the optical axis. Steps
are provided on boundaries between the divided surfaces Sd1-Sd3 to
form each of the divided surfaces Sd1-Sd3. In an arrangement in
this objective lens 16, a light flux (first light flux) which
passes through the first divided surface Sd1 including the optical
axis is used to reproduce information recorded in the first optical
disk and to reproduce information recorded in the second
optical-disk, a light flux (second light flux) which passes through
the second divided surface Sd2 which surrounds the first divided
surface Sd1 is mainly used to reproduce information recorded in the
second optical disk, and a light flux (third light flux) which
passes through the third divided surface Sd3 which surrounds the
second divided surface Sd2 is mainly used to reproduce information
recorded in the first optical disk.
[0079] Wording "mainly" in this case means, in the case of a light
flux passing through the second divided surface Sd2, that a ratio
of energy of the core portion at the position where the central
intensity of a beam spot is maximum under the condition that a
light flux passing through the third divided surface Sd3 is
shielded to that of the core portion at the position where the
central intensity of a beam spot is maximum under the condition
that a light flux passing through the third divided surface Sd3 is
not shielded ("light-shielded core energy"/"light-non-shielded core
energy") is within a range of 60-100%. Incidentally, a simple
method for measuring the energy ratio is to measure the peak
intensity Ip at the position where the central intensity of a beam
spot is maximum and beam diameter Dp (to set the position where
intensity is e.sup.-2 for the central intensity) in each occasion,
and to obtain values of Ip.times.Dp to compare them, because the
shape of a core portion is mostly constant.
[0080] By using the light flux emitted from the light source as
stated above in the manner that the first light flux in the
vicinity of an optical axis of a light-converging optical system is
used to reproduce the first and second optical disks, the second
light flux that surrounds the first light flux is mainly used to
reproduce the second optical disk, and the Third light flux
surrounding the second light flux is mainly used to reproduce the
first optical disk, it is possible to reproduce plural optical
disks (two optical disks in the present embodiment) with a single
light-converging optical system, while minimizing a loss of a
quantity of light from a light source. In addition, when
reproducing the second optical disk, the greater part of the third
light flux is an unwanted light, and this unwanted light is not
utilized to reproduce the second optical disk, therefore,
aperture-stop 17 has only to be adjusted, for reproducing, to its
numerical aperture that is necessary to reproduce the first optical
disk, requiring no means to change the numerical aperture of the
aperture-stop 17.
[0081] To say more precisely, when reproducing the first optical
disk (see FIG. 2 (a)), the objective lens 16 causes each of the
first light flux and the third light flux (light fluxes shown with
hatched lines) passing respectively through the first divided
surface Sd1 and the third divided surface Sd3 to form an image on
the first image forming position A which mostly agrees in terms of
position for both light fluxes, and their wavefront aberrations
(wavefront aberrations excluding that for the second light flux)
are not more than 0.5 .lambda.rms.
[0082] In this case, the second light flux (light fluxes shown with
broken lines) passing through the second divided surface Sd2 forms
an image on the second image forming position B which is different
in terms of position from the first image forming position A. When
assuming that the first image forming position A is nearly 0 (zero)
and a distance from it toward the objective lens 16 is negative and
a distance toward the opposite side is positive, this second image
forming position B is made to be away from the first image forming
position A by the distance of -27 .mu.m to -4 .mu.m (the second
image forming position B is made to be closer to the objective lens
than the first image forming position A). Owing to this, the first
optical disk is reproduced mainly by the first light flux and the
third light flux. Incidentally, when the lower limit (-27 .mu.m) is
exceeded, spherical aberration is corrected excessively, resulting
in a poor spot shape for reproduction of the first light flux,
while when the upper limit (-4 Mm) is exceeded, a spot diameter and
a side lobe for reproduction of the second light flux are made
larger. Incidentally, in the present embodiment, the second image
forming position B is made to be away from the first image forming
position A by the distance of -27 .mu.m to -4 .mu.m because of
conditions of t1<t2 and NA1>NA2. However, in the case of
t1>t2 and NA1>NA2, or t1<t2 and NA1<NA2, the second
image forming position B is made to be away from the first image
forming position A by the distance of 4 .mu.m-27 .mu.m. Namely, the
absolute value of the distance between the first image forming
position A and the second image forming position B is made to be
within a range of 4 .mu.m-27 .mu.m.
[0083] When the objective lens 16 mentioned above is used for
reproducing the second optical disk provided therein with a
transparent substrate having the prescribed thickness (t2=1.2 mm)
in the case of the prescribed light flux (collimated light flux)
entering the objective lens 16 as shown in FIG. 3, the second light
flux (shown with lines hatched downward obliquely from left to
right) forms an image on the point F which is located between
position D where an image is formed by light passing the optical
axis and its vicinity among the first light flux (shown with lines
hatched downward obliquely from right to left and position E where
an image is formed by a light flux passing through the periphery of
the first divided area Sd1 (on the second divided area Sd2 side) in
the direction perpendicular to the optical axis. Due to this, the
first light flux and the second light flux are converged on the
vicinity of an information recording plane of the second optical
disk, and the second optical disk is thereby reproduced. In this
case, the third light flux (partly shown with broken lines) is
generated as flare, but the second optical disk can be reproduced
by the core formed by both the first light flux and the second
light flux.
[0084] In other words, in the invention, the first light flux
passing through the optical axis and its vicinity whose numerical
aperture is small is used for reproducing all types of optical
disks which can be reproduced, and a light flux passing through the
outer area of the first divided surface is divided in a way that
each of the divided areas may correspond to each optical disk to be
reproduced, so that each light flux thus divided may be used for
reproducing each optical disk (the first and the second optical
disks). In this case, the light flux used for reproducing the
optical disk (the first optical disk) having the greater numerical
aperture necessary for reproducing optical disk information is
caused to be a light flux (the third light flux) which farther from
the first light flux among the divided light fluxes.
[0085] When the light-converging optical system (objective lens 16)
as that stated above is used, plural optical disks each having a
transparent substrate having a different thickness can be
reproduced by a single light-converging optical system, and
numerical aperture NA2 necessary for reproducing the second optical
disk can be made larger by setting the plane arbitrarily. Further,
by using the light flux near the optical axis (the first light
flux) for reproducing a plurality of optical disks, a loss of a
quantity of light of the light flux from a light source can be made
small. In addition, when reproducing the second optical disk, a
side lobe of a beam spot is reduced, a core having strong beam
intensity is formed, and accurate information can be obtained.
Furthermore, a plurality of optical disks can be reproduced with a
single light-converging optical system without requiring a special
means for changing a numerical aperture of aperture-stop 17.
[0086] Further, when viewed at the central position C (see FIG. 2
(a)) of the second divided surface Sd2 in the direction
perpendicular to the optical axis, an angle formed by a normal line
to the second divided surface Sd2 which is a surface of from
numerical aperture NAL to that NAH and the optical axis is made to
be greater than an angle formed by a normal line to the surface
(aspheric surface where fitting is made through a least square
method by the use of the expression for aspheric surface stated
later) interposed between the first divided surface Sd1 covering
from the optical axis to the numerical aperture NAL and the third
divided surface Sd3 covering from the numerical aperture NAH to the
numerical aperture NA1 and the optical axis. Due to this, both of
the first optical disk and the second optical disk can be
reproduced satisfactorily. In the present embodiment, an angle
formed by a normal line to the second divided surface Sd2 and the
optical axis is made to be greater than an angle formed by a normal
line to the surface interposed between the first divided surface
Sd1 and the third divided surface Sd3 and the optical axis because
of the condition of t2 >t1. In the case of the condition of
t2<t1, an angle formed by a normal line to the second divided
surface Sd2 and the optical axis can be made to be smaller than an
angle formed by a normal line to the surface interposed between the
first divided surface Sd1 and the third divided surface Sd3 and the
optical axis.
[0087] Further, when viewed at the central position C (see FIG. 2
(a)) of the second divided surface Sd2 in the direction
perpendicular to the optical axis, it is preferable that the first
divided surface Sd1-the third divided surface Sd3 are set so that a
difference between an angle formed by a normal line to the second
divided surface Sd2 and the optical axis and an angle formed by a
normal line to the surface (aspheric surface where fitting is made
through a least square method by the use of the expression for
aspheric surface stated later) interposed between the first divided
surface Sd1 and the third divided surface Sd3 and the optical axis
may be within a range of 0.02-1. When this lower limit is exceeded,
a spot shape for reproduction of the second optical disk is
worsened and a side lobe and a spot diameter are made larger, while
when the upper limit is exceeded, aspherical aberration is
corrected excessively and a spot shape for reproduction of the
first optical disk is worsened.
[0088] In the consideration from a different viewpoint, when
assuming that (.DELTA.1L).pi. (rad) represents a phase difference
between light passing through the first divided surface Sd1 from
the second divided surface Sd2 (emitted from a transparent
substrate) and light passing through the second divided surface Sd2
from the position C (see FIG. 2 (a)) that is mostly the center of
the second divided surface Sd2 in the direction perpendicular to
the optical axis (emitted from the transparent substrate), and
(.DELTA.1H).pi. (rad) represents a phase difference between light
passing through the third divided surface Sd3 opposite to the
optical axis side from the second divided surface Sd2 (emitted from
the transparent substrate) and light passing through the second
divided surface Sd2 opposite to the optical axis side from the
aforesaid central position (emitted from a transparent substrate),
in the objective lens 16 having, on at least one side thereof,
plural divided surfaces (three divided surfaces) divided into
plural portions coaxially with an optical axis, the relation of
(.DELTA.1H)>(.DELTA.1L) is satisfied. In this case, with regard
to a sign of the phase difference, a positive sign is for the
direction of light advancement (direction toward an optical disk),
and a phase difference between light passing through the first
divided surface Sd1 or the third divided surface Sd3 (emitted from
the transparent substrate) and light passing through the second
divided surface Sd2 (emitted from the transparent substrate) is
compared. Though (.DELTA.1H) is made to be larger than (.DELTA.1L)
because of the conditions of t1<t2 and NA1>NA2, (.DELTA.1H)
is made to be smaller than (.DELTA.1L) in the case of the
conditions of t1>t2 and NA1>NA2 or of t1<t2 and
NA1<NA2. Namely, (.DELTA.1H) is made not to be equal to
(.DELTA.1L).
[0089] In other words, a step depth measured in the direction from
the third divided surface Sd3 on the boundary between the third
divided surface Sd3 and the second divided surface Sd2 is greater
than a step depth from the first divided surface Sd1 on the
boundary between the first divided surface Sd1 and the second
divided surface Sd2 (a sign for the step depth is positive in the
direction where a surface having smaller refractive index is
changed to a surface having larger refractive index on the border
of the divided surface) Even in this case, as in the foregoing,
when t1 is greater than t2 and NA1 is greater than NA2, or when t1
is smaller than t2 and NA1 is smaller than NA2, the relation
mentioned above is opposite, namely, a step depth of the second
divided surface Sd2 from the third divided surface Sd3 is smaller
than that of the second divided surface Sd2 from the first divided
surface Sd1. Further, it is preferable that a distance from a
position on the surface interpolated between the first divided
surface Sd1 and the third divided surface Sd3 to a position on the
second divided surface Sd2 is asymmetrical about the position that
is mostly the center of the second divided surface Sd2, at the
point that is away from the optical axis by a prescribed length. In
this case, it is preferable that the farther the distance from the
optical axis is, the greater the difference is.
[0090] It has been explained as a standard that divided surfaces
Sd1-Sd3 are provided on refracting surface S1 closer to a light
source of objective lens 16, they may also be provided on a
refracting surface closer to optical disk 20, or this function may
also be provided on one of optical elements (for example,
collimator lens 13) of another light-converging optical system, or
an optical element having this function may also be provided newly
on an optical path. In addition, a function of each of divided
surfaces Sd1-Sd3 may be provided on a different optical
element.
[0091] It has been explained as a standard that the objective lens
16 of an infinite system type employing collimator lens 13 is used,
it is also possible to apply to an objective lens employing no
collimator lens 13 where a divergent light from a light source
enters directly or a divergent light transmitted through a lens
which lowers an extent of divergence enters, or, to an objective
lens employing a coupling lens which changes a light flux from a
light source to a converged light that enters the objective
lens.
[0092] Though there are provided steps on boundaries of the first
divided surface Sd1-the third divided surface Sd3 in the present
embodiment, it is also possible to form divided surfaces
continuously without providing a step on at least one boundary.
With regard to a boundary between divided surfaces, both divided
surfaces may also be connected by prescribed R, without bending the
boundary. This R may be either one provided intentionally or one
which is not provided intentionally (an example of one which is not
provided intentionally is an R on a boundary formed in tooling a
mold which is needed when objective lens 16 is made of plastic)
[0093] It has been explained as a standard that refracting surface
S1 is composed of three divided surfaces Sd1-Sd3 in the present
embodiment, the invention is not limited to this, and the
refracting surface S1 can also be composed of at least three or
more divided surfaces. In this case, it is preferable that the
first divided surface used to reproduce the first optical disk and
the second optical disk is provided in the vicinity of the optical
axis, and a divided surface to be used mainly to reproduce the
second optical disk and a divided surface to be used mainly to
reproduce the first optical disk are provided alternately on a
divided surface outside (in the direction to recede from the
optical axis) the first divided surface. In this case, it is
preferable to provide a divided surface used mainly to reproduce
the second optical disk between numerical aperture NA3 and
numerical aperture NA4 on the optical disk side on objective lens
16 that satisfies conditions of 0.60 (NA2)<NA3<1.3 (NA2) and
0.01<NA4-NA3<0.12. Due to this, it is possible to reproduce
an optical disk having a greater necessary numerical aperture
serving as the second optical disk, without reducing intensity of a
light spot to be converged on the first optical disk. It is further
preferable from the viewpoint of practical use that the upper limit
of NA3 satisfies NA3<1.1 (NA2), the lower limit of NA3 satisfies
0.80 (NA2)<NA3, more preferably 0.85 (NA2)<NA3, and the upper
limit of NA4-NA3 satisfies NA4-NA3<0.1.
[0094] Though a single light source is used for reproducing a
plurality of optical disks, plural light sources may also be used
for each optical disk to be reproduced.
[0095] Though second divided surface Sd2 is provided to be in a
shape of a ring representing a circle concentric with an optical
axis when objective lens 16 is viewed from the light source side,
the invention is not limited to this, and the second divided
surface Sd2 may also be provided to be in a discontinuous ring. The
second divided surface Sd2 may further be composed of a hologram or
a Fresnel lens. When the second divided surface Sd2 is composed of
holograms, one of the light-flux that is divided into zero-th order
diffracted light and first order diffracted light is used to
reproduce the first optical disk, and the other is used to
reproduce the second optical disk. In this case, it is preferable
that a quantity of light of the light flux used for reproduction of
the second optical disk is larger than that of light of the light
flux used for reproduction of the first optical disk.
[0096] It is possible to improve reproduction signals of the second
optical disk, when the best-fit wavefront aberration of a light
flux passing through the first divided surface Sd1 and the third
divided surface Sd3 satisfies 0,05 .lambda. rms (.lambda.(nm) is a
wavelength of light from a light source used for reproducing the
first optical disk) when reproducing the first optical disk
(namely, in the case of passage through a t1-thick transparent
substrate), and the best-fit wavefront aberration of a light flux
passing through the first divided surface Sd1 satisfies 0,07
.lambda. rms representing the diffraction limit (.lambda.(nm) is a
wavelength of light from a light source used for reproducing the
second optical disk) when reproducing the second optical disk
(namely, in the case of passage through a t2-thick transparent
substrate).
[0097] Next, FIGS. 2 (a) and 2 (b) in the case where a single light
source is used will be explained, referring to FIGS. 4 (a)-4 (f)
each representing a diagram wherein spherical aberration of
objective lens 16 is shown typically. In FIGS. 4 (a)-4 (f), FIG. 4
(a) is a diagram of a spherical aberration in the case of
reproduction of the first optical disk, namely, in the case of
passage through a t1-thick transparent substrate, while, FIG. 4 (b)
is a diagram of a spherical aberration in the case of reproduction
of the second optical disk, namely, in the case of passage through
a t2-thick (t2>t1) transparent substrate. Let it be assumed here
that NA1 represents the necessary numerical aperture closer to an
optical disk on a light-converging optical system necessary for
reproducing information on the first optical disk, NA2 represents
the necessary numerical aperture closer to the optical disk on a
light-converging optical system necessary for reproducing
information on the second optical disk (NA2>NA1), NAL represents
a numerical aperture closer to the optical disk on a light flux
passing through the boundary between divided surface Sd1 and
divided surface Sd2 both of the objective lens 16, and NAH
represents a numerical aperture closer to the optical disk on a
light flux passing through the boundary between divided surface Sd2
and divided surface Sd3 both of the objective lens 16.
[0098] The viewpoint which will be explained below shows an
viewpoint in which the objective lens 16 in FIGS. 2(a) and 2(b) is
viewed from another viewpoint (spherical aberration, shape and
wavefront aberration), and items which are not described below are
the same as those in the basic concept explained above.
[0099] With regard to objective lens 16 in FIGS. 2(a) and 2(b), the
first aspheric surface of its first refracting surface S1 and its
second refracting surface S2 (common refracting surface) are first
designed so that the best-fit wavefront aberration of the light
flux converged on the first optical disk having a t1-thick
transparent substrate may be 0.05 .lambda. rms or less. FIG. 4 (c)
shows a diagram of a spherical aberration of the lens obtained
through the design mentioned above. Then, the second aspherical
surface of the first refracting surface is designed, leaving the
second refracting surface S2 (common refracting surface) to be
unchanged so that a spherical aberration may be less in quantity
than the spherical aberration (FIG. 4 (e), t2>t1 in this case)
caused when light is converged on the second optical disk having a
t2-thick (t2.noteq.t1) transparent substrate through a lens having
the first aspheric surface. In this case, it is preferable, for
reproducing satisfactorily the second optical disk under the state
of defocusing, that a paraxial radius of curvature of the second
aspheric surface and that of the first aspheric surface are made to
be the same. A diagram of a spherical aberration caused when light
is converged on the second optical disk by the lens obtained
through this design is shown in FIG. 4 (f), and a diagram of an
aberration caused when light is converged on the first optical disk
by this lens is shown in FIG. 4 (d). The second aspheric surface is
composed in the vicinity of necessary numerical aperture NA2 of the
second optical disk of the first aspheric surface. The vicinity of
necessary numerical aperture NA2 in this case is preferably located
between numerical aperture NA3 and numerical aperture NA4 both
being on the optical disk side on the objective lens 16 satisfying
the condition of 0.60 (NA2)< NA3<1.3 (NA2) (this lower limit
0.60 (NA2) is preferably 0.80 (NA2), more preferably 0.85 (NA2) in
practical use and this upper limit 1.3 (NA2) is preferably 1.1
(NA2) in practical use) and satisfying the condition of
0.01<NA4-NA3<0.12 (preferably 0.1). Let it be assumed that
numerical aperture NAL represents the second aspheric surface
(second divided surface) thus composed which is closer to the
optical axis and numerical aperture NAH (namely, NAL<NAH)
represents that closer to the optical axis.
[0100] Therefore, with regard to the surface shape on refracting
surface S1 of the objective lens 16, the first divided surface Sd1
including the optical axis and the third divided surface Sd3
surrounding the first divided surface Sd1 are of the same shape of
aspheric surface (the first-aspheric surface), and the second
divided surface Sd2 located between the first divided surface Sd1
and the third divided surface Sd3 (in the vicinity of numerical
aperture NA2 necessary to reproduce the second optical disk, namely
NAL-NAH) turns out to be of a shape of the aspheric surface (the
second aspheric surface) which is different from that of the first
divided surface Sd1 and the third divided surface Sd3. A diagram of
spherical aberration caused when light is converged on the first
optical disk through the objective lens 16 is shown in FIG. 4 (a),
and a diagram of spherical aberration caused when light is
converged on the second optical disk through the objective lens 16
is shown in FIG. 4 (b).
[0101] When composing the first aspheric surface and the second
aspheric surface, it is possible to increase a quantity of
converged light in reproduction of the first optical disk by
shifting the second divided surface Sd2 toward the optical axis for
composition of the second divided surface Sd2, and thereby
utilizing a phase difference.
[0102] An expression for an aspheric surface is assumed to be based
on the following expression; 1 X = ( H 2 / r ) / [ 1 + 1 - ( 1 + K
) ( H / r ) 2 ] + j AjH Pj
[0103] wherein, X represents an axis in the direction of an optical
axis, H represents an axis that is perpendicular to an optical
axis, the direction of a forward movement of light takes a positive
sign, r represents a paraxial radius of curvature, K represents a
circular cone coefficient, Aj represents an aspheric surface
coefficient, and Pj represents a value of the power of an aspheric
surface (on condition of Pj.gtoreq.3). Expressions for an aspheric
surface other than the above-mentioned expression may also be used
in the invention. When finding an expression for an aspheric
surface from the shape of the aspheric surface, the aforesaid
expression is used, Pj is made to be natural numbers satisfying
3.ltoreq.Pj.ltoreq.10, and K is made to be 0 for finding the
expression.
[0104] As stated above, objective lens 16 obtained in the present
embodiment is constituted in a way that a spherical aberration
changes discontinuously so that plural optical disks each having a
transparent substrate in different thickness may be reproduced by a
single light-converging optical system at least two aperture
positions (NAL and NAH) in the vicinity of numerical aperture NA2.
Due to such arrangement wherein a spherical aberration changes
discontinuously as stated above, it is possible to arrange freely
light fluxes passing through various numerical apertures (the first
divided surface covering an optical axis to NAL, the second divided
surface covering from NAL to NAH and the third divided surface
covering from NAH to NA1), and thereby it is possible to use the
first light flux for reproduction of all of the plural optical
disks to be reproduced and to use the second light flux and the
third light flux for reproduction of prescribed optical disks among
plural optical disks. Thus, a plurality of optical disks can be
reproduced by a single light-converging optical system (objective
lens 16) which can be realized not to be complicated at the low
cost, and can cope with optical disks with high NA. In addition,
aperture-stop 17 has only to be provided to cope with NA1 that is
of high NA, and even when a numerical aperture (NA1 or NA2) that is
necessary for reproducing an optical disk is changed, it is not
necessary to provide a means to change the aperture-stop 17.
Incidentally, the expression "a spherical aberration changes
discontinuously" in the invention means that a sharp change of
spherical aberration is observed in a diagram of spherical
aberration.
[0105] With regard to the direction in which spherical aberration
changes discontinuously, the spherical aberration is in the
negative direction at numerical aperture NAL and the spherical
aberration is in the positive direction at numerical aperture NAH,
when viewed in the direction from the smaller numerical aperture to
the larger numerical aperture. Due to this, reproduction of an
optical disk having a thin transparent substrate in thickness t1 is
made to be better and reproduction of an optical disk having a
thick transparent substrate in thickness t2 is also made to be
better. Because of NA1>NA2 and t2>t1, a spherical aberration
changes discontinuously in the negative direction at numerical
aperture NAL and in the positive direction at numerical aperture
NAH. In the case of t2<t1 and NA1>NA2, or t2>t1 and
NA1<NA2, however, a spherical aberration changes discontinuously
in the positive direction at numerical aperture NAL and in the
negative direction at numerical aperture NAH.
[0106] When reproducing the second optical disk having a t2-thick
transparent substrate, S-shaped characteristics focus error signal
of optical pickup apparatus 10 are improved when the spherical
aberration (spherical aberration by a light flux passing through
the second divided surface Sd2) within a range from numerical
aperture NAL to numerical aperture NAH is made to be positive.
Though the spherical aberration within a range from numerical
aperture NAL to numerical aperture NAH is made to be positive
because of t2>t1 and NA1>NA2, it can be made to be negative
in the case of t2<t1 and NA1<NA2.
[0107] When wavefront aberration in the case of a light flux
excluding a light flux passing through the range from NAL to NAH
among numerical aperture NA1, namely in the case of a light flux
passing through the range from an optical axis to NAL and the range
from NAH to NA1 is made to be 0.05 .lambda. rms or less (wherein
.lambda. represents a wavelength of a light source) when a t1-thick
transparent substrate exists (see FIG. 4 (a)), the reproduction of
the first optical disk having a t1-thick transparent substrate is
made to be better.
[0108] Under the conditions of t1.times.0.6 mm, t2.times.1.2 mm,
610 nm <.lambda.<670 nm and 0.32<NA2<0.41, it is
preferable to satisfy the condition of 0.60 (NA2)<NAL<1.3
(NA2) (its lower limit 0.60 (NA2) is preferably 0.80 (NA2), more
preferably 0.85 (NA2) in practical use and its upper limit 1.3
(NA2) is preferably 1.1 (NA2) in practical use). When the lower
limit is exceeded, the side lobe turns out to be larger to make
accurate reproduction of information impossible, while when its
upper limit is exceeded, a spot diameter is made smaller than a
diffraction limited spot diameter assumed at wavelength .lambda.
and NA2. NAL mentioned here means NAL on the second divided surface
Sd2.
[0109] It is further preferable to satisfy the condition of
0.01<NAH-NAL<0.12 (the upper limit is preferably 0.1 in
practical use). When this lower limit is exceeded, a spot shape in
the course of reproduction of the second optical disk is worsened
and a side lobe spot diameter is made larger, while when the upper
limit is exceeded, a spot shape in the course of reproduction of
the first optical disk is disturbed and a fall of a quantity of
light is caused. NAL and NAH mentioned here means NAL and NAH on
the second divided surface Sd2.
[0110] To say from another viewpoint (though this is restatement),
NAL and NAH mentioned above are provided (namely, a divided surface
mainly used for reproduction of the second optical disk is
provided) between numerical aperture NA3 and numerical aperture NA4
closer to an optical disk on the objective lens 16 satisfying the
condition of 0.60 (NA2)<NA3<1.3 (NA2) (its lower limit is
preferably 0,85 (NA2), more preferably 0,85 (NA2) in practical use,
and its upper limit 1.3 (NA2) is preferably 1.1 (NA2) in practical
use) and the condition of 0.01<NA4-NA3<0.12 (preferably 0.1)
Due to this, it is possible to reproduce an optical disk having a
larger necessary numerical aperture as the second optical disk,
without lowering intensity of a spot of light converged on the
first optical disk very much.
[0111] When reproducing the second optical disk (when a t2-thick
transparent substrate exists), it is preferable to satisfy the
condition that the spherical aberration between numerical aperture
NAL and numerical aperture NAH is not less than -2
.lambda./(NA2).sup.2 and is not more than 5 .lambda./(NA2).sup.2.
In the case of reproduction, the condition of not more than 3
.lambda./(NA2).sup.2 is preferable, or, when recording is
considered (reproduction is naturally possible), the spherical
aberration greater than 0 (zero) is preferable. When this lower
limit is exceeded, the spherical aberration is corrected
excessively and a spot shape in the course of reproducing the first
optical disk is worsened and a side lobe spot diameter turns out to
be larger. In particular, this condition is preferable when it
satisfies an range of 0-2 .lambda./(NA2).sup.2, and focus error
signals are obtained satisfactorily in this case.
[0112] On the other hand, an angle formed between a normal line to
the second divided surface Sd2 and an optical axis is made to be
greater than that formed between a normal line to a surface
interpolated between the first divided surface Sd1 and the third
divided surface Sd3, when viewed at the central position of the
second divided surface Sd2 in the direction perpendicular to the
optical axis. Due to this, both of the first and second optical
disks can be reproduced satisfactorily. Though an angle formed
between a normal line to the second divided surface Sd2 and an
optical axis is made to be greater than that formed between a
normal line to a surface interpolated between the first divided
surface Sd1 and the third divided surface Sd3 and an optical axis,
because of t2>t1, when t2<t1 and NA1>NA2 or t2>t1 and
NA1<NA2, an angle formed between a normal line to the second
divided surface Sd2 and an optical axis can be made smaller than
that formed between a normal line to a surface interpolated between
the first divided surface Sd1 and the third divided surface
Sd3.
[0113] Further, in the objective lens 16, it is preferable that a
difference between an angle formed between a normal line to the
surface (the second divided surface) from numerical aperture NAL to
numerical aperture NAH and an optical axis and an angle formed
between a normal line to the surface interpolated between the
surface (the first divided surface) from the optical axis to
numerical aperture NAL and the surface (the third divided surface)
from numerical aperture NAH to numerical aperture NA1 and the
optical axis is in a range from 0.02' to 1'. When the lower limit
is exceeded, a spot shape in the course of reproducing the second
optical disk is worsened and a side lobe spot is made larger,
while, when the upper limit is exceeded, the spherical aberration
is corrected excessively and a spot shape in the course of
reproducing the first optical disk is worsened.
[0114] In particular, when viewing in the direction from an optical
axis to the circumference of a circle under the condition of
t2>t1 and NA1>NA2, a point at which a normal line to the
refracting surface and the optical axis intersect changes
discontinuously in the direction to approach the refracting surface
that is closer to the light source, at numerical aperture NAL, and
a point at which a normal line to the refracting surface and the
optical axis intersect changes discontinuously in the direction to
recede from the refracting surface that is closer to the light
source, at numerical aperture NAH. Due to this, the reproduction of
an optical disk having a t1-thick thin transparent substrate is
made to be better, and the reproduction of an optical disk having a
t2-thick thick transparent substrate is made to be better.
[0115] Wavefront aberrations of the objective lens 16 in the
present embodiment are shown in FIGS. 5 (a) and 5 (b). Each of
FIGS. 5 (a) and 5 (b) is a diagram of a wavefront aberration curve
wherein the axis of ordinates represents wavefront aberration
(.lambda.) and the axis of abscissas represents a numerical
aperture. In FIG. 5 (a), a curve of wavefront aberration caused
through a transparent-substrate (thickness of t1) of the first
optical disk is shown with solid lines, wile, in FIG. 5 (b), a
curve of wavefront aberration caused through a transparent
substrate (thickness of t2) of the second optical-disk is shown
with solid lines. The wavefront aberration curve is obtained by
measuring wavefront aberrations by the use of an interferometer
under the condition that the best wavefront aberration is caused
through each transparent substrate.
[0116] As is apparent from each figure, the wavefront aberration
related to the objective lens 16 is discontinuous at two locations
(NAL and NAH to be concrete) in the vicinity of numerical aperture
NA2, when viewed on the wavefront aberration curve. Inclination of
the wavefront aberration on the discontinuous portion (between NAL
and NAH) is different from that of the curve (shown with broken
lines in FIG. 5 (a)) obtained by connecting end portions (an end
closest to NAL and that closest to NAH) of the curves at both sides
of the discontinued portion.
[0117] Next, an optical pickup apparatus having two light sources
will be explained as follows, referring to FIG. 6 which is a
schematic structure diagram of the optical pickup apparatus. Here,
the two light sources 111 and 112 are used in optical pickup
apparatus 100.
[0118] Here, a first semiconductor laser 111 (wavelength
.lambda.1.times.610-670 nm) representing the first light source is
provided for reproducing the first optical disk, and a second
semiconductor laser 112 (wavelength .lambda.2.times.740-870 nm)
representing the second light source is provided for reproducing
the second optical disk. Composition means 119 is a means capable
of composing a light flux emitted from the first semiconductor
laser 111 and a light flux emitted from the second semiconductor
laser 112, and it is a means to make both light fluxes to be in the
same optical path so that both light fluxes may be converged on
optical disk 20 through a single light-converging system.
[0119] When reproducing the first optical disk, a beam is emitted
from the first semiconductor laser 111, and the beam thus emitted
passes through composition means 119, polarized beam splitter 212,
collimator lens 113, and quarter wavelength plate 114 to become a
circularly polarized light flux. This light flux is narrowed by
aperture-stop 117 and converged by objective lens 116 on
information recording plane 22 through transparent substrate 21 of
the first optical disk 20. Then, the light flux modulated by
information bit and reflected on the information recording plane 22
passes again through objective lens 116, quarter wavelength plate
114 and collimator lens 113 to enter polarized beam splitter 212
where the light flux is reflected and given astigmatism by
cylindrical lens 118 to enter optical detector 130 where signals to
read (to reproduce) information recorded on the first optical disk
20 are obtained by the use of signals outputted from the optical
detector 130. Further, a change in distribution of quantity of
light caused by a change in spot shape on the optical detector 130
is detected for the detection of being in focus and detection of
track. The objective lens 116 is moved so that 2-dimensional
actuator 115 may cause light from semiconductor laser 111 to form
an image on information recording plane 22 of the first optical
disk 20, and the objective lens 116 is moved so that light from
semiconductor laser 11 may be caused to form an image on a
prescribed track, based on the detection mentioned above.
[0120] On the other hand, when reproducing the second optical disk,
a beam is emitted from the second semiconductor laser 112, then the
light flux thus emitted is changed in terms of its optical path by
composition means 119, and passes through polarized beam splitter
212, collimator lens 113, quarter wavelength plate 114,
aperture-stop 117 and objective lens 116 to be converged on the
second optical disk 20. Then, the light flux modulated by
information bit and reflected on the information recording plane 22
passes again through objective lens 116, quarter wavelength plate
114, collimator lens 113, polarized beam splitter 212 and
cylindrical lens 118 to enter optical detector 130 where signals to
read (to reproduce) information recorded on the second optical disk
20 are obtained by the use of signals outputted from the optical
detector 130. Further, a change in distribution of quantity of
light caused by a change in spot shape on the optical detector 130
is detected for the detection of being in focus and detection of
track. The objective lens 116 is moved so that 2-dimensional
actuator 115 may cause light from semiconductor laser 111 to form
an image on information recording plane 22 of the second optical
disk 20 under the defocus state, and the objective lens 116 is
moved so that light from semiconductor laser 11 may be caused to
form an image on a prescribed track, based on the detection
mentioned above.
[0121] As objective lens 116 that is one of light-converging
optical systems of the optical pickup apparatus 100, the objective
lens 16 as described above is used. Namely, the objective lens 116
is a convex lens having positive refracting power whose refracting
surface S1 on the light source side and refracting surface S2 on
the optical disk 20 side are of an aspheric shape, and the
refracting surface S1 is composed of plural (three in the present
embodiment) divided surfaces of the first divided surface Sd1-the
third divided surface Sd3 arranged on a coaxial basis with an
optical axis, and a step is given to each boundary between divided
surfaces Sd1-Sd3. The first divided surface Sd1 and the third
divided surface Sd3 are formed by the first aspheric surface which
makes the best-fit wavefront aberration of a light flux emitted
from the first light source 111 and converged on the first optical
disk to be 0.05 .alpha. rms or less, and the second divided surface
is formed by the second aspheric surface which causes spherical
aberration that is less in terms of amount of generation than that
caused when a light flux emitted from the second light source 112
is converged on the second optical disk having a t2-thick
(t2.noteq.t1) transparent substrate through a lens having the first
aspheric surface. In the objective lens, the second aspheric
surface is composed with the first aspheric surface at the location
of its NAL-NAH that is close to necessary numerical aperture NA2 of
the second optical disk.
[0122] The objective lens 116 thus obtained is to have the same
constitution and effect as the objective lens 16 mentioned above
except the following points, and further has the greater degree of
freedom for reproducing plural optical disks because of two light
sources used therefor.
[0123] Since two light sources 111 and 112 are used, the following
preferable range is different from that in the case of employing
the single light source.
[0124] Namely, it is preferable to satisfy the condition of 0.60
(NA2)<NAL<1.1 (NA2) (this lower limit 0.60 (NA2) is
preferably 0.80 (NA2) and more preferably 0.85 (NA2) in practical
use) under the conditions of t1=0.6 mm, t2=1.2 mm, 610
nm<.lambda.1<670 nm, 740 cm<.lambda.2<870 nm and
0.40<NA2 <0.51. When this lower limit is exceeded, a side
lobe is made larger to make accurate reproduction of information
impossible, while, when the upper limit is exceeded, a spot
diameter is made smaller than the diffraction limited spot diameter
assumed at wavelength .lambda.2 and NA2. Incidentally, NAL
mentioned here means NAL on the second divided surface Sd2 in the
case of employment of the second light source 112.
[0125] It is further preferable to satisfy the condition of 0
.01<NPA-NAL<0.12 (this upper limit 0.12 is preferably 0.1 in
practical use). When this lower limit is exceeded, a soot shape in
the course of reproducing the second optical disk is worsened and a
side lobe is made larger, while, when the upper limit is exceeded a
spot shape in the course of reproducing the first optical disk is
disturbed and a fall of quantity of light is caused. Incidentally,
NAL and NAH mentioned here mean NAL and NAH on the second divided
surface Sd2 in the case of employment of the second light source
112.
[0126] It is further preferable to satisfy the condition that the
spherical aberration between numerical aperture NAL and numerical
aperture NAH is within a range of -2 (.lambda.2) /
(NA2).sup.2-(5(.lambda.2)) / (NA2).sup.2, when reproducing the
second optical disk (through a t2-thick transparent substrate).
This condition is preferably not more than 3 (.lambda.2) /
(NA2).sup.2 in the case of reproduction, or it is preferably
greater than 0 (zero) when recording is also considered
(reproduction is naturally possible). Wnen the lower limit is
exceeded, the spherical aberration is corrected excessively and a
spot shape in the course of reproducing the first optical disk is
worsened, while, when the upper limit is exceeded, a spot shape in
the course of reproducing the second optical disk is worsened and a
side lobe spot diameter is made larger. It is especially preferable
that this condition satisfies a range of 0-2 (.lambda.2) /
(NA2).sup.2, and in this case, focus error signals are obtained
satisfactorily.
[0127] To say from another viewpoint, NAL and NAH mentioned above
are provided (namely, a divided surface mainly used for
reproduction of the second optical disk is provided) between
numerical aperture NA3 and numerical aperture NA4 closer to an
optical disk on the objective lens 16 satisfying the condition of
0.60 (NA2)<NA3<1.1 (NA2) (its lower limit is preferably 0,80
(NA2) and more preferably 0,85 (NA2) in practical use) and the
condition of 0.01 <NA4-NA3<0.12 (preferably 0.1) Due to this,
it is possible to reproduce an optical disk having a larger
necessary numerical aperture as the second optical disk, without
lowering intensity of a spot of light converged on the first
optical disk.
[0128] On the other hand, an angle formed between a normal line to
the second divided surface Sd2 and an optical axis is made to be
greater than that formed between a normal line to a surface
interpolated between the first divided surface Sd1 and the third
divided surface Sd3, when viewed at the central position of the
second divided surface Sd2 in the direction perpendicular to the
optical axis. Due to this, both of the first and second optical
disks can be reproduced satisfactorily. Though an angle formed
between a normal line to the second divided surface Sd2 and an
optical axis is made to be greater than that formed between a
normal line to a surface interpolated between the first divided
surface Sd1 and the third divided surface Sd3 and an optical axis,
because of t2>t1 and NA1>NA2. However, when t2 <t1 and
NA1>NA2 or t2 >t1 and NA1<NA2, an angle formed between a
normal line to the second divided surface Sd2 and an optical axis
can be made smaller than that formed between a normal line to a
surface interpolated between the first divided surface Sd1 and the
third divided surface Sd3,
[0129] Further, in the objective lens 116 in the presented
embodiment, it is preferable that an angle formed between a normal
line to the refracting surface and an optical axis is changed to be
not less than 0.05' and to be less than 0.50' at a circular
position of the refracting surface S1 of the objective lens 116
corresponding to at least two aperture positions (NAL and NAH) in
the vicinity of numerical aperture NA2. When the lower limit is
exceeded, a spot shape in the course of reproducing the second
optical disk is worsened and a side lobe spot is made larger,
while, when the upper limit is exceeded, the spherical aberration
is corrected excessively and a spot shape in the course of
reproducing the first optical disk is worsened.
[0130] In particular, when viewing in the direction from an optical
axis to the circumference of a circle under the condition of t2
>t1 and NA1>NA2, a point at which a normal line to the
refracting surface and the optical axis intersect changes
discontinuously in the direction to approach the refracting surface
that is closer to the light source, at numerical aperture NAL, and
a point at which a normal line to the refracting surface and the
optical axis intersect changes discontinuously in the direction to
recede from the refracting surface that is closer to the light
source, at numerical aperture NAH. Due to this, the reproduction of
an optical disk having a t1-thick thin transparent substrate is
made to be better, and the reproduction of an optical disk having a
t2-thick thick transparent substrate is made to be better.
[0131] In consideration from another viewpoint as in the case
employing the objective lens 116 stated above, when assuming that
(.DELTA.1L) .pi. (rad) represents a phase difference between light
passing through the first divided surface Sd1 (emitted from a
transparent substrate) and light passing through the portion on the
second divided surface Sd2 covering from its central position to
the position closest to the optical axis (emitted from the
transparent substrate), and (.DELTA.1H) .pi. (rad) represents a
phase difference between light passing through the third divided
surface Sd3 (emitted from the transparent substrate) and light
passing through the portion on the second divided surface Sd2
covering from its central position to the position farthest from
the optical axis (emitted from the transparent substrate), in
objective lens 116 having on at least one surface thereof a
plurality of divided surfaces (three divided surfaces) which are
divided to be plural on a coaxial basis with the optical axis, the
condition of (.DELTA.1H)>(.DELTA.1L) is satisfied. Even in this
case, as in the foregoing, the condition of
(.DELTA.1H)<(.DELTA.1L) is taken in the case of t1>t2 and
NA1>NA2, or of t1<t2 and NA1<NA2, which results in
(.DELTA.1H) .noteq. (.DELTA.1L) accordingly.
[0132] To say from another viewpoint, a step depth of the second
divided surface Sd2 from the third divided surface Sd3 is greater
than that of the second divided surface Sd2 from the first divided
surface Sd1. Even in this case, a step depth from the third divided
surface Sd3 on a boundary between the third divided surface Sd3 and
the second divided surface Sd2 is smaller than that from the first
divided surface Sd1 on a boundary between the first divided surface
Sd1 and the second divided surface Sd2 in the case of t1>t2 and
NA1>NA2 or t1<t2 and NA1<NA2, as in the foregoing. It is
preferable that a distance between the position of the surface
interpolated between the first divided surface Sd1 and the third
divided surface Sd3 and the position of the second divided surface
Sd2 is asymmetrical about the position which is mostly the center
or the second divided surface Sd2, at the position being away from
the optical axis by a prescribed distance. It is further preferable
in that case that the distance grows greater as it recedes from the
optical axis.
[0133] In the same way as in the aforesaid explanation in the case
employing the objective lens 16, the invention is not limited to
what is described here, such as that divided surfaces Sd1-Sd3 are
provided on the refracting surface S1 of the objective lens 116, an
objective lens of an infinite type is used, steps are provided on
boundaries of divided surfaces, and such as the number of divided
surfaces and a surface shape of the second divided surface.
[0134] Though the first light source 111 and the second light
source 112 are composed by composing means 119, the invention is
not limited to this, and light source 11 can be made to be of a
type wherein it is switched to the first light source 111 and to
the second light source 112.
[0135] It is possible to improve reproduction signals of the second
optical disk, by making the best-fit wavefront aberration of a
light flux passing through the first divided surface Sd1 and the
third divided surface Sd3 to satisfy 0,05 .lambda. rms
(.lambda.(nm) is a wavelength of light from a light source used for
reproducing the first optical disk) when reproducing the first
optical disk (namely when a t1-thick transparent substrate is
passed), and by making the best-fit wavefront aberration of a light
flux passing through the first divided surface Sd1 to satisfy 0,07
.lambda. rms representing the diffraction limit (.lambda. (nm) is a
wavelength of light from a light source used for reproducing, the
second optical disk) when reproducing the second optical disk
(namely when a t2-thick transparent substrate is passed).
[0136] With regard to the objective lens 116 in the present
embodiment, when the inventors of the invention used it by mistake
for the optical pickup apparatus in the first embodiment (or the
second embodiment), it was not only possible to reproduce a DVD as
the first optical disk naturally, but also possible to reproduce a
CD as the second optical disk with a light source having the same
wavelength to their surprise. Namely, the objective lens 116 can
converge light on an information recording plane of each of the
first optical information recording medium having a t1-thick
transparent substrate and the second optical information recording
medium having a t2-thick transparent substrate (t2.noteq.t1), using
a light source having a wavelength of .lambda.1, and it also can
converge light on an information recording plane of the second
optical information recording medium even when a light source
having a wavelength of .lambda.2 is used
(.lambda.2.noteq..lambda.1). Due to this, an objective lens used
for an optical pickup apparatus to reproduce both DVD and CD-R by
the use of two light sources each having a different wavelength
(light source with wavelength of 610-670 nm for DVD, and light
source with wavelength of 780 nm necessary for CD-R) and an
objective lens used for an optical pickup apparatus to reproduce
both DVD and CD by the use of a single light source can be made
common to each other, whereby cost reduction based on mass
production can be realized. In such common objective lens, it is
still necessary to satisfy the conditions of NAL and NAH described
in the first and second embodiments even when the wavelength of the
light source is changed from .lambda.2 to .lambda.1.
[0137] Incidentally, since both the first light source 111 and the
second light source 112 are used on the same magnification here,
only a single light detector can be used, resulting in a simple
structure. Further, two light detectors corresponding to respective
different light sources or light sources having magnification
different from each other may be used.
[0138] Next, an optical pickup apparatus will be explained,
referring FIGS. 7 (a) and 7 (b). FIG. 7 (a) is a sectional view of
objective lens 216, and FIG. 7 (b) is a front view viewed from a
light source. The objective lens 216 is a variation of objective
lens 16 or 116 used in the optical pickup apparatus described
above. The objective lens 216 is one wherein the surface thereof
closer to the light source is composed of five divided refracting
surfaces, which is different from the objective lens 16 whose
surface closer to the light source is composed of three divided
refracting surfaces described above. Incidentally, the present
embodiment employs one divided into five refracting surfaces, and
others are the same as those in the objective lens 16 or 116, and
therefore, the explanation may sometimes be omitted.
[0139] In the present embodiment, objective lens 216 is a convex
lens having positive refracting power wherein refracting surface S1
closer to the light source and refracting surface S2 closer to the
optical disk 20 are of a shape of an aspheric surface. The
refracting surface S1 of the objective lens 216 closer to the light
source is composed of five divided surfaces of the first divided
surface Sd1-fifth divided surface Sd5 which are coaxial with the
optical axis, namely of the first divided surface (Sd1) including
the optical axis (near the optical axis), the second divided
surface Sd2 . . . the (2n+1)th (n is a natural number which is 2 in
the present embodiment) divided surface Sds2n+1. Boundaries of the
divided surfaces Sd1-Sd5 are given steps to form each of the
divided surfaces Sd1-Sd5. In this objective lens 216, a light flux
(the first light flux) passing through the first divided surface
Sd1 including the optical axis is used for reproduction of
information recorded in the first and second optical disks, a light
flux passing through the (2n)th divided surface Sd2n (the second
divided surface Sd2 and the fourth divided surface Sd4 in the
present embodiment) is mainly used for reproduction of information
recorded in the second optical disk, and a light flux passing
through the (2n+1)th divided surface Sd2n+1 (the third divided
surface Sd3 and the fifth divided surface Sd5 in the present
embodiment) is mainly used for reproduction of information recorded
in the first optical disk, As stated above, it is possible to
arrange the (2n)th divided surface to the high NA side by
increasing the number of divided surfaces. Therefore, it is
possible to conduct not only reproduction of the first optical disk
which requires high NA but also reproduction of the second optical
disk having higher NA compared with the first-third embodiments
mentioned above. In addition, a fall of quantity of light in the
course of reproducing the first optical disk caused by the (2n)th
divided surface arranged to the high NA side can be compensated by
the (2n-1)th divided surface (the first divided surface has nothing
to do with this), thus, it is possible to reproduce not only the
first optical disk but also the second optical disk.
[0140] To be concrete, the first aspheric surface of the first
refracting surface S1 and the second refracting surface S2 (common
refracting surface) of the objective lens 216 are designed first so
that the best-fit wavefront aberration of a light flux converged on
the first optical disk having a t1-thick transparent substrate may
be 0.05 .lambda. rms or less. Then, the second aspheric surface of
the first refracting surface is designed with the second refracting
surface S2 (common refracting surface) being left as it is, so that
the spherical aberration relating to the second aspheric surface
may be less in terms of quantity of generation than that generated
through convergence on the second optical disk having a t2-thick
(t2.noteq.t1) transparent substrate through a lens having the first
aspheric surface mentioned above. In this case, it is preferable,
for reproducing the second optical disk under the state of defocus,
to make a paraxial radius of curvature of the second aspheric
surface and that of the first aspheric surface to be the same each
other. The second aspheric surface is composed at two locations of
NAL-NAH in the vicinity of the necessary numerical aperture NA2 of
the second optical disk of the first aspheric surface. The lens
thus obtained is the objective lens 16 in the present
embodiment.
[0141] In the case of composition, it is possible to achieve an
increase in quantity of converged light in the-course of
reproducing the first optical disk by shifting the second divided
surface Sd2 and the fourth divided surface Sd4 toward an optical
axis for composition and, thereby, by utilizing a phase difference.
Though the second divided surface Sd2 and the fourth divided
surface Sd4 are made to be of the same second aspheric surface,
these divided surfaces may also be of a different aspheric surface
and they may further be shifted differently in terms of quantity
toward an optical axis.
[0142] With regard to the vicinity of NA2 for composition of the
second aspheric surface in this case, the condition of 0.60
(NA2)<NA3<1.3 (NA2) (its lower limit 0.60 (NA2) is preferably
0.80 (NA2) and more preferably 0.85 (NA2) in practical use) is
preferable, and it is preferable that the upper limit 1.3 (NA22) is
1.1 (NA2) in practical use. It is further preferable, when a
wavelength of a light source for recording or reproducing the
second optical disk information recording medium is 740-870 nm,
that the upper limit 1.3 (NA2) is located between numerical
aperture NA3 and numerical aperture NA4 on the optical disk side of
the objective lens 16 satisfying 1.1 (NA2) and the condition of
0.01 <NA4-NA3< 0.12 (this upper limit is preferably 0.1 in
practical use).
[0143] In the case of employing the objective lens 16, when
reproducing a DVD having a transparent substrate whose thickness t1
is 0.6mm which is the first optical disk, as mentioned above, light
fluxes passing respectively through the first divided surface Sd1,
the third divided surface Sd3 and the fifth divided surface Sd5
form images on the first image forming positions which are almost
the same position, and their wavefront aberrations (wavefront
aberrations excluding light fluxes passing through the second
divided surface Sd2 and the fourth divided surface Sd4) are 0.05
.lambda. rms or less. The symbol .lambda. in this case represents a
wavelength of a light source.
[0144] In this case, light fluxes passing respectively through the
second divided surface Sd2 and the fourth divided surface Sd4 form
images on the second image forming position that is different from
the first image forming position. The second image forming position
is made to be away from the first image forming position by the
distance ranging from -27 .mu.n to -4 .mu.m when assuming that 0
(zero) represents the first image forming position, and the
direction from that toward the objective lens 16 is negative, and
the direction opposite thereto is positive. In the present
embodiment, the second image forming position is made to be away
from the first image forming position by the distance ranging from
-27 .mu.m to -4 .mu.m because of the conditions of t1<t2 and
NA1>NA2. However, in the case of t1>t2 and NA1>NA2 or
t1<t2 and NA1<NA2, the second image forming position is made
to be away from the first image forming position by the distance
ranging from 4 .mu.m to 27 .mu.m. Namely, an absolute value of the
distance between the first image forming position and the second
image forming position is made to be within a range of 4-27
.mu.m.
[0145] When the objective lens 216 is considered from the viewpoint
of spherical aberration, it is constituted in a manner that the
spherical aberration changes discontinuously at four aperture
positions in the vicinity of numerical aperture NA2 so that a
plurality of optical disks each having a transparent substrate
having a different thickness may be reproduced by a single
light-converging optical system. The spherical aberration changes
discontinuously as stated above (the direction of the change is the
same as that in the first third embodiments mentioned above), and
when it is viewed from the viewpoint of wavefront aberration, the
wavefront aberration is discontinuous at four positions in the
vicinity of numerical aperture NA2, and an inclination of the
wavefront aberration at each point in the discontinuous change is
different from that of the curve obtained by connecting both end
portions of the separated curves at both sides.
[0146] In the objective lens 216 in the present embodiment, it is
preferable to satisfy the condition that spherical aberration
between numerical apertures NAL to NAH is within a range from -2
.lambda./(NA).sup.2 to 5 .lambda./(NA).sup.2 when reproducing the
second optical disk (through a t2-thick transparent substrate)
(wherein, .lambda. represents a wavelength of a light source used
for reproducing the second optical disk). Further, this condition
is preferably 3 .lambda./(NA).sup.2 or less in the case of
reproduction, and it is preferably greater than 0 (zero) when
recording is considered (reproduction is naturally possible).
[0147] On the other hand, when viewed at a central position of the
(2n)th divided surface (the second divided surface Sd2 or the
fourth divided surface) in the direction perpendicular to an
optical axis, an angle formed between a normal line to the (2n)th
divided surface and the optical axis is made to be greater than
that formed between a normal line to the surface interpolated
between the (2n-1)th divided surface (the first divided surface Sd1
or the third divided surface Sd3) and the (2n+1)th divided surface
(the third divided surface Sd3 or the fifth divided surface Sd5)
and the optical axis. Due to this, both the first optical disk and
the second optical disk can be reproduced satisfactorily. Though an
angle formed between a normal line to the (2n)th divided surface
and an optical axis is made to be greater than that formed between
a normal line to a surface interpolated between the (2n-1)th
divided surface and the (2n+1)th divided surface and an optical
axis, because of t2>t1. However, when t2<t1 and NA1>NA2 or
t2>t1 and NA1<NA2, an angle formed between a normal line to
the (2n)th divided surface and an optical axis is made to be
smaller than that formed between a normal line to a surface
interpolated between the (2n-1)th divided surface and the (2n+1)th
divided surface and an optical axis.
[0148] Further, when viewed at the position mostly the center of
the (2n)th divided surface (n is a natural number) representing the
second divided surface Sd2 or the fourth divided surface Sd4 in the
direction perpendicular to an optical axis, it is preferable to
establish the first divided surface Sd1-the (2n+1)th divided
surface so that a difference between an angle formed between a
normal line to the (2n)th divided surface and an optical axis and
angle formed between a normal line to a surface interpolated
between the (2n-1)th divided surface and the (2n+1)th divided
surface and an optical axis is within a range from 0.02' to 1'.
[0149] When considering from another viewpoint as in the case of
employing the objective lens 16, in the objective lens 16 wherein
at least one surface has thereon a plurality of divided surfaces
divided to be on a coaxial basis with an optical axis of the
objective lens, when (.DELTA.nL) .pi. (for example, (.DELTA.1L)
.pi. or .DELTA.2L) .pi.) (rad) is assumed to represent a phase
difference between light passing through the (2n-1)th divided
surface (for example, the first divided surface Sd1 or the third
divided surface Sd3) (emitted from a transparent substrate) and
light passing through the inner portion closer to the optical axis
from the center of the (2n)th divided surface (for example, the
second divided surface Sd2 or the fourth divided surface Sd4)
(emitted from the transparent substrate), and (.DELTA.nH) .pi. (for
example, (.DELTA.1H) .pi. or .DELTA.2H) .pi.) (rad) is assumed to
represent a phase difference between light passing through the
(2n+1)th divided surface (for example, the third divided surface
Sd3 or the fifth divided surface Sd5) (emitted from the transparent
substrate) and light passing through the portion opposite to the
aforesaid inner portion closer to the optical axis from the
aforesaid center of the (2n)th divided surface (for example, the
second divided surface Sd2 or the fourth divided surface Sd4)
(emitted from the transparent substrate) the relation of
(.DELTA.nH)>(.DELTA.nL- ) is satisfied. Even in this case,
(.DELTA.nH) is made to be greater than (.DELTA.nL) in the case of
t1>t2 and NA1>NA2 or t1<t2 and NA1<NAL as in the
foregoing, which means the relation of (.DELTA.nH).noteq.
(.DELTA.nL) accordingly.
[0150] To say this from another viewpoint, a depth of a step from
the (2n+1)th divided surface (for example, the third divided
surface Sd3 or the fifth divided surface Sd5) of the (2n)th divided
surface (for example, the second divided surface Sd2 or the fourth
divided surface Sd4) is greater than a depth of a step from the
(2n-1)th divided surface (for example, the first divided surface
Sd1 or the third divided surface Sd3) of the (2n)th divided surface
(for example, the second divided surface Sd2 or the fourth divided
surface Sd4) Even in this case, a depth of a step from the (2n+1)th
divided surface of the (2n)th divided surface is smaller than a
depth of a step from the (2n-1)th divided surface of the (2n)th
divided surface in the case of t1>t2 and NA1>NA2 or t1<t2
and NA1<NA2 as in the foregoing. Further, at the position that
is away from an optical axis by a prescribed distance, it is
preferable that a difference between a position of the surface
interpolated between the (2n-1)th divided surface and the (2n+1)th
divided surface (for example, the first divided surface Sd1 and the
third divided surface Sd3, or the third divided surface Sd3 and the
fifth divided surface Sd5) and a position of the (2n)th divided
surface (for example, the second divided surface Sd2 or the fourth
divided surface Sd4) is asymmetric about the position that is
mostly the center of the second divided surface (for example, the
second divided surface Sd2 or the fourth divided surface Sd4).
Further, in this case, it is preferable that the difference is made
larger as the distance from the optical axis grows greater.
[0151] Though the refracting surface S1 closer to a light source on
the objective lens 216 is divided into five surfaces, the invention
is not limited to this, and the refracting surfaces may also be
provided on an optical element (for example, a collimator lens) of
another light-converging optical system, or an optical element may
be provided separately.
[0152] Though there are provided steps on boundaries of the first
divided surface Sd1-the fifth divided surface Sd5, it is also
possible to form divided surfaces continuously without providing a
step on at least one boundary. With regard to a boundary between
divided surfaces, both divided surfaces may also be connected by
prescribed R, without bending the boundary. This R may be either
one provided intentionally or one which is not provided
intentionally (an example of one which is not provided
intentionally is an R on a boundary formed in processing a metal
mold which is needed when objective lens 16 is made of
plastic).
[0153] Though second divided surface Sd2 and the fourth divided
surface Sd4 are provided to be in a shape of a ring representing a
circle concentric with an optical axis when objective lens 216 is
viewed from the light source side, the invention is not limited to
this, and they may also be provided to be in a discontinuous ring.
The second divided surface Sd2 and/or the fourth divided surface
Sd4 may further be composed of a hologram or a Fresnel lens. When
the second divided surface Sd2 is composed of holograms, one of the
light flux that is divided into zero-order light and primary light
is used to reproduce the first optical disk, and the other is used
to reproduce the second optical disk. In this case, it is
preferable that a quantity of light of the light flux used for
reproduction of the second optical disk is larger than that of
light of the light flux used for reproduction of the first optical
disk.
[0154] It is possible to improve reproduction signals of the second
optical disk, by making the best-fit wavefront aberration of a
light flux passing through the first divided surface Sd1 and the
third divided surface Sd3 to satisfy 0,05 .lambda. rms or less
(.lambda. (nm) is a wavelength of light from a light source used
for reproducing the first optical disk) when reproducing the first
optical disk (namely when a t1-thick transparent substrate is
passed), and by making the best-fit wavefront aberration of a light
flux passing through the first divided surface Sd1 to satisfy 0,07
.lambda. rms (.lambda. (nm) is a wavelength of light from a light
source used for reproducing the second optical disk) representing
the diffraction limit when reproducing the second optical disk
(namely when a t2-thick transparent substrate is passed).
[0155] In the case of employing the objective lenses 16 and 116
stated in detail above, the first divided surface is made to be the
surface including an optical axis. However, the surface covering an
extremely narrow area around the optical axis which has no
influence on the light convergence may also be flat, convex or
concave because such surface covering an extremely narrow area
around the optical axis hardly affects the light convergence. In
short, a divided surface used for reproducing the second optical
disk has only to be provided in the vicinity of NA2 and the first
divided surface has only to be inside the divided surface used for
reproducing the second optical disk toward the optical axis.
[0156] In the above-mentioned statement, the explanation is only
for reproduction of information recorded on an optical disk, which,
however, is the same even in the case of recording information on
the optical disk on the ground that a light spot obtained by
converging light by a light-converging optical system (objective
lens) is important, thus can naturally be used effectively also for
recording.
[0157] In addition, in the case of employing the objective lenses
16, 116 and 216 stated above, there is an effect that S-shaped
characteristics of focus error signals are improved.
[0158] In the following examples, let it be assumed that a DVD
(transparent substrate thickness t1=0.6 mm, necessary numerical
aperture NA1=0.60 (.lambda.=635 nm))is used as a first optical
disk, and a CD (transparent substrate thickness t2=1.2 mm,
necessary numerical aperture NA2=0.45 (.lambda.=780 nm)) or a CD-R
(transparent substrate thickness t2=1.2 mm, necessary numerical
aperture NA2=0.50 (.lambda.=780 rm) (however, NA2=0.45
(.lambda.=780 nm) in the case of only reproduction)) is used as a
second optical disk. In the following examples of the objective
lens 16, there is shown an arrangement relating to entry and
thereafter of a light flux, on the assumption that collimator lens
13 capable of collimating into a collimated light flux that is
mostly free from aberration is used, because the collimator lens
13, when its design is optimum, can cause a collimated light flux
being almost free from aberration to enter the objective lens 16.
With an aperture-stop arranged on the light source side on the
objective lens 16 serving as the first plane, a radius of curvature
on the lens plane that is i-th from the first plane is represented
by ri, a distance between the i-th plane and the (i+1)th plane in
the case of reproducing a DVD is represented by di (in the case of
reproducing a CD, when a numerical value is described on di', that
value is used, and when no numerical value is described, the value
is the same as, di), and a refractive index for the distance at a
wavelength of a light flux of a laser light source is represented
by ni. When an aspheric surface is used for the optical plane, the
expression of the aspheric surface mentioned above serves as the
base.
[0159] Descriptions in Tables 4, 7, 8, 11, 14, 15, 18, 19, 22, 23,
26, 27, 30, 31, 34, 35, 38 and 39 are conducted as follows. A
numeral in parentheses following NAL or NAH represents the number
of order in divided surfaces (for example, NAL (2) shows a value of
NAL on the second divided surface).
[0160] H2n mid represents a height from an optical axis to the
central position of the second divided surface in the direction
perpendicular to the optical axis.
[0161] (Q2n-1, 2n+1, mid) represents an angle formed between a
normal line to the surface interpolated between the (2n-1)th
divided surface and the (2n+1)th divided surface at height H2n mid
and an optical axis.
[0162] (Q2n, mid) represents an angle formed between a normal line
to the second divided surface at height H2n mid and an optical
axis.
[0163] The symbol (.DELTA.Q2n, mid) shows a difference between
(Q2n, mid) and (Q2n-1, 2n+1, mid). In this case, n represents a
natural number.
[0164] An angle formed between a normal line to the surface
interpolated between the (2n-1)th divided surface and the (2n+1)th
divided surface at its central position and an optical axis is an
average angle of an angle formed between a normal line to the
imagined surface where the (2n-1)th divided surface is extended
toward the second divided surface at height H2n mid from the
optical axis and the optical axis and an angle formed between a
normal line to the imagined surface where the (2n+1)th divided
surface is extended toward the second divided surface at height H2n
mid from the optical axis and the optical axis.
[0165] In this case, when imagining the surface concretely,
Expression 1 of an aspherical surface may be referred to. "Defocus"
described on the lower portion of each of FIGS. 9 (a), 9 (b), 13
(a), 13 (b), 18 (a), 18 (b), 22 (a), 22 (b), 27 (a), 27 (b), 32
(a), 32 (b), 37 (a), 37 (b), 42 (a), 42 (b), 47 (a), 47 (b) and 52
(a), 52 (b) represents an amount by which the objective lens 16 is
moved in the direction of an optical axis for obtaining the
best-fit wavefront aberration under the condition that the
advancing direction of a light flux from a light source is
positive, from the position of the objective lens 16 agreeing with
a geometric focus position on an information recording plane of an
optical disk (through a transparent substrate having a prescribed
thickness and refractive index).
EXAMPLE 1
[0166] Example 1 represents an example wherein the invention is
applied to objective lens 16 which is to be mounted on optical
pickup apparatus 10 stated above and is provided with steps on
boundaries of the first divided surface Sd1-the third divided
surface Sd3 of the objective lens.
[0167] Optical data of the objective lens are shown in Tables 2 and
3.
2TABLE 2 Wavelength .lambda. 635 nm Focal length 3.36 mm
Aperture-stop diameter .phi.4.04 mm Lateral magnification 0 of
objective lens i ri di di' ni 1 .infin. 0.000 1.0 2 2.114 2.200
1.5383 3 -7.96 1.757 1.377 1.0 4 .infin. 0.600 1.200 1.58
[0168]
3TABLE 3 Aspheric surface data Second First 0 < H < 1.212
(First divided surface) surface aspheric 1.347 .ltoreq. H (Third
divided surface) (refracting surface .kappa. = -0.88658 surface) A1
= 0.51091 .times. 10.sup.-2 P1 = 4.0 A2 = 0.27414 .times. 10.sup.-3
P2 = 6.0 A3 = 0.11020 .times. 10.sup.-4 P3 = 8.0 A4 = -0.72311
.times. 10.sup.-5 P4 = 10.0 Second 1.212 .ltoreq. H < 1.347
aspheric (Second divided surface) surface .kappa. = -0.94120 A1 =
0.61109 .times. 10.sup.-2 P1 = 4.0 A2 = 0.30854 .times. 10.sup.-3
P2 = 6.0 A3 = 0.20160 .times. 10.sup.-4 P3 = 8.0 A4 = -0.81949
.times. 10.sup.-5 P4 = 10.0 Third surface .kappa. = -0.24879
.times. 10.sup.2 (refracting surface) A1 = 0.94269 .times.
10.sup.-2 P1 = 4.0 A2 = -0.32152 .times. 10.sup.-2 P2 = 6.0 A3 =
0.53282 .times. 10.sup.-3 P3 = 8.0 A4 = -0.37853 .times. 10.sup.-4
P4 = 10.0
[0169] In the objective lens of the present example, a position
where the first aspheric surface intersects with an optical axis is
the same as that where the second aspheric surface intersects with
an optical axis.
[0170] FIG. 8 (a) shows a diagram of spherical aberration in the
case of transmission through a t1-thick transparent substrate
(hereinafter referred to as in the case of reproduction of a DVD),
while, FIG. 8 (b) shows a diagram of spherical aberration in the
case of transmission through a t2-thick (=1.2 mm) transparent
substrate (hereinafter referred to as in the case of reproduction
of a CD). FIG. 9 (a) shows a diagram of wavefront aberration viewed
under the state of defocusing at the position where the best-fit
wavefront aberration is obtained in the case of reproduction of a
DVD, while, FIG. 9 (b) shows a diagram of wavefront aberration
viewed under the state of defocusing at the position where the
best-fit wavefront aberration is obtained in the case of
reproduction of a CD. Table 4 shows numerical apertures for NAL and
NAH, quantities of spherical aberration caused, angles each being
formed between a normal line and an optical axis, normal lines and
each condition.
4 TABLE 4 Spherical aberration (mm) Numerical In DVD In CD Height H
aperture reproduction reproduction 1.212 NAL(1) = 0.3606 -0.15363
.times. 10.sup.-4 0.15933 .times. 10.sup.-1 NAL(2) = 0.3617
-0.10720 .times. 10.sup.-1 0.53341 .times. 10.sup.-2 1.374 NAH(2) =
0.4024 -0.13510 .times. 10.sup.-1 0.67338 .times. 10.sup.-2 NAH(3)
= 0.4008 -0.16412 .times. 10.sup.-4 0.20059 .times. 10.sup.-1
0.60NA2 = 0.60 .times. 0.366 = 0.220 1.3NA2 = 1.3 .times. 0.366 =
0.476 NAH-NAL = 0.4024 - 0.3617 = 0.0407 -2.lambda./(NA2).sup.2 =
-2 .times. 635 nm/(0.366).sup.2 = -9.48 .mu.m 5.lambda./(NA2).sup.2
= 5 .times. 635 nm/(0.366).sup.2 = 23.7 .mu.m H2mid = (1.212 +
1.374)/2 = 1.280 .theta.1, 3 , mid = 33.69622 .theta.2 , mid =
33.81796 .DELTA..theta.2 mid = 33.81796 - 33.69622 = 0.12174
[0171] FIG. 10 shows a diagram of relative intensity distribution
of a light-converged spot in the case where the best spot shape is
obtained in reproduction of a DVD, while, FIG. 11 shows a diagram
of relative intensity distribution of a light-converged spot in the
case where the best spot shape is obtained in reproduction of a
CD.
EXAMPLE 2
[0172] Example 2 represents an example wherein the invention is
applied to objective lens 116 which is to be mounted on optical
pickup apparatus 100 (wavelength .lambda.1 of the first light
source=635 nm, and wavelength .lambda.2 of the second light source
=780 nm) and is provided with steps on boundaries of the first
divided surface Sd1-the third divided surface Sd3 of the objective
lens.
[0173] Optical data of the objective lens are shown in Tables 5 and
6.
5TABLE 5 Wavelength .lambda. 635 nm 780 nm Focal length 3.36 mm
3.39 mm Aperture-stop diameter .phi.4.04 mm Lateral magnification 0
of objective lens i ri 4di di' ni ni' 1 .infin. 0.000 1.0 1.0 2
2.114 2.200 1.5383 1.5337 3 -7.963 1.757 1.401 1.0 1.0 4 .infin.
0.600 1.200 1.58 1.58
[0174]
6TABLE 6 Aspheric surface data Second First 0 < H < 1.414
(First divided surface) surface aspheric 1.549 .ltoreq. H (Third
divided surface) (refracting surface .kappa. = -0.9770 surface) A1
= 0.63761 .times. 10.sup.-3 P1 = 3.0 A2 = 0.36688 .times. 10.sup.-3
P2 = 4.0 A3 = 0.83511 .times. 10.sup.-2 P3 = 5.0 A4 = -0.37296
.times. 10.sup.-2 P4 = 6.0 A5 = 0.46548 .times. 10.sup.-3 P5 = 8.0
A6 = -0.43124 .times. 10.sup.-4 P6 = 10.0 Second 1.414 .ltoreq. H
< 1.549 aspheric (Second divided surface) surface .kappa. =
-0.12982 .times. 10 A1 = 0.79671 .times. 10.sup.-2 P1 = 3.0 A2 =
-0.13978 .times. 10.sup.-1 P2 = 4.0 A3 = 0.26968 .times. 10.sup.-1
P3 = 5.0 A4 = -0.11073 .times. 10.sup.-1 P4 = 6.0 A5 = 0.10432
.times. 10.sup.-2 P3 = 8.0 A6 = -0.74338 .times. 10.sup.-4 P4 =
10.0 Third surface .kappa. = -0.24914 .times. 10.sup.2 (refracting
surface) A1 = 0.13775 .times. 10.sup.-2 P1 = 3.0 A2 = -0.41269
.times. 10.sup.-2 P2 = 4.0 A3 = 0.21236 .times. 10.sup.-1 P3 = 5.0
A4 = -0.13895 .times. 10.sup.-1 P4 = 6.0 A5 = 0.16631 .times.
10.sup.-2 P5 = 8.0 A6 = -0.12138 .times. 10.sup.-3 P6 = 10.0
[0175] In the objective lens of the present example, a position
where the first aspheric surface intersects with an optical axis is
the same as that where the second aspheric surface intersects with
an optical axis. The symbol ni' in Table 5 represents a refractive
index in the second light source (.lambda.2=780 nm).
[0176] FIG. 12 (a) shows a diagram of spherical aberration in the
case of reproduction of a DVD, while, FIG. 12 (b) shows a diagram
of spherical aberration in the case of reproduction of a CD. FIG.
13 (a) shows a diagram of wavefront aberration viewed under the
state of defocusing at the position where the best-fit wavefront
aberration is obtained in the case of reproduction of a DVD, while,
FIG. 13 (b) shows a diagram of wavefront aberration viewed under
the state of defocusing at the position where the best-fit
wavefront aberration is obtained in the case of reproduction of a
CD. Table 7 shows numerical apertures for NAL and NAH, quantities
of spherical abberation caused, angles each being formed between a
normal line and an optical axis, normal lines and each
condition.
7 TABLE 7 In DVD reproduction Spherical In CD reproduction (mm)
Height Numerical aberration Numerical Spherical H aperture (mm)
aperture aberration 1.414 NAL(1) = 0.4207 0.24061 .times. 10.sup.-3
NAL(1) = 0.4172 0.2393 .times. 10.sup.-1 NAL(2) = 0.4232 -0.20032
.times. 10.sup.-1 NAL(2) = 0.4197 0.37703 .times. 10.sup.-2 1.549
NAH(2) = 0.4642 -0.24054 .times. 10.sup.-1 NAH(2) = 0.4604 0.52181
.times. 10.sup.-2 NAH(3) = 0.4608 0.60913 .times. 10.sup.-3 NAH(3)
= 0.4571 0.2965 .times. 10.sup.-1 0.60NA2 = 0.60 .times. 0.45 =
0.270 1.1NA2 = 1.1 .times. 0.45 = 0.495 NAH-NAL = 0.4604 - 0.4197 =
0.0407 -2(.lambda.2)/(NA2).sup.2 = 2 .times. 780 nm/(0.45).sup.2 =
-7.70 .mu.m 5(.lambda.2)/(NA2).sup.2 = 5 .times. 780
nm/(0.45).sup.2 = 19.26 .mu.m H2mid = (1.414 + 1.549)/2 = 1.482
.theta.1, 3 , mid = 33.62261 .theta.2 , mid = 38.87220
.DELTA..theta.2, mid = 38.87220 - 38.62261 = 0.24959
[0177] FIG. 14 shows a diagram of relative intensity distribution
of a light-converged spot in the case where the best spot shape is
obtained in reproduction of a DVD, while, FIG. 15 shows a diagram
of relative intensity distribution of a light-converged spot in the
case where the best spot shape is obtained in reproduction of a
CD.
[0178] Even when the objective lens in the present embodiment is
mounted on optical pickup apparatus 10 employing a single light
source (wavelength .lambda.1 of the light source=635 nm),
reproduction was possible not only for DVD but also for CD. FIG. 16
shows a diagram of relative intensity distribution of a
light-converged spot in the case where the best spot shape is
obtained in reproduction of a CD. Numerical apertures for NAL and
NAH, quantities of spherical aberration caused, angles each being
formed between a normal line and an optical axis, normal lines and
each condition in this case are shown in Table 8.
8TABLE 8 .lambda. = 635 nm In DVD reproduction In CD reproduction
Spherical Spherical Height Numerical aberration Numerical
aberration H aperture (mm) aperture (mm) 1.414 NAL(1) = 0.4207
0.24061 .times. 10.sup.-3 NAL(1) = 0.4207 0.22575 .times. 10.sup.-1
NAL(2) = 0.4232 -0.20032 .times. 10.sup.-1 NAL(2) = 0.4232 0.25983
.times. 10.sup.-2 1.549 NAH(2) = 0.4642 -0.24054 .times. 10.sup.-1
NAH(2) = 0.4642 0.38067 .times. 10.sup.-2 NAH(3) = 0.4608 0.60913
.times. 10.sup.-3 NAH(3) = 0.4608 0.28016 .times. 10.sup.-1 0.60NA2
= 0.60 .times. 0.366 = 0.220 1.3NA2 = 1.3 .times. 0.366 = 0.476
NAH-NAL = 0.4642 - 0.4232 = 0.0410 -2(.lambda.)/(NA2).sup.2 = -2
.times. 635 nm/(0.366).sup.2 = -9.48 .mu.m 5(.lambda.)/(NA2).sup.2
= 5 .times. 635 nm/(0.366).sup.2 = 23.7 .mu.m H2mid = (1.414 +
1.549)/2 = 1.482 .theta.1, 3 , mid = 38.62261 .theta.2, mid =
38.87220 .DELTA..theta.2, mid = 38.87220 - 38.62261 = 0.24959
EXAMPLE 3
[0179] Example 3 represents an example wherein the invention is
applied to objective lens 16 which is to be mounted on optical
pickup apparatus 10 with a single light source and is provided with
a step on a boundary between the second divided surface Sd2 and the
third divided surface Sd3 and is provided with no step on a
boundary between the first divided surface Sd1 and the second
divided surface Sd3 of the objective lens.
[0180] Optical data of the objective lens are shown in Tables 9 and
10.
9TABLE 9 Wavelength .lambda. 635 nm Focal length 3.36 mm
Aperture-stop diameter .phi.4.04 mm Lateral magnification 0 of
objective lens i ri di di' ni 1 .infin. 0.000 1.0 2 2.114 2.2000
1.5383 3 -7.963 1.757 1.377 1.0 4 .infin. 0.600 1.200 1.58
[0181]
10TABLE 10 Aspheric surface data Second First 0 < H < 1.212
(First divided surface) surface aspheric 1.347 .ltoreq. H (Third
divided surface) (refracting surface .kappa. = -0.88658 surface) A1
= 0.51091 .times. 10.sup.-2 P1 = 4.0 A2 = 0.27414 .times. 10.sup.-3
P2 = 6.0 A3 = 0.11020 .times. 10.sup.-4 P3 = 8.0 A4 = -0.72311
.times. 10.sup.-5 P4 = 10.0 Second 1.212 .ltoreq. H < 1.347
aspheric (Second divided surface) surface d2 = 2.200702 .kappa. =
-0.94120 A1 = 0.61109 .times. 10.sup.-2 P1 = 4.0 A2 = 0.30854
.times. 10.sup.-3 P2 = 6.0 A3 = 0.20160 .times. 10.sup.-4 P3 = 8.0
A4 = -0.81949 .times. 10.sup.-5 P4 = 10.0 Third surface .kappa. =
-0.24879 .times. 10.sup.2 (refracting surface) A1 = 0.94269 .times.
10.sup.-2 P1 = 4.0 A2 = -0.32152 .times. 10.sup.-2 P2 = 6.0 A3 =
0.53282 .times. 10.sup.-3 P3 = 8.0 A4 = -0.37853 .times. 10.sup.-4
P4 = 4.0
[0182] The expression of "d2=2.200702" in the column of "Second
aspheric surface" in Table 9 represents a distance on an optical
axis between an intersecting point where the optical axis
intersects with the second aspheric surface (second divided
surface) when it is extended, following the aspherical shape
thereof, and the third surface. Namely, owing to this value; the
first divided surface is connected with the second divided surface
continuously (without having any steps).
[0183] FIG. 17 (a) shows a diagram of spherical aberration in the
case of reproduction of a DVD, while, FIG. 17 (b) shows a diagram
of spherical aberration in the case of reproduction of a CD. FIG.
18 (a) shows a diagram of wavefront aberration viewed under the
state of defocusing at the position where the best-fit wavefront
aberration is obtained in the case of reproduction of a DVD, while,
FIG. 18 (b) shows a diagram of wavefront aberration viewed under
the state of defocusing at the position where the best-fit
wavefront aberration is obtained in the case of reproduction of a
CD. Table 11 shows numerical apertures for NAL and NAH, quantities
of spherical aberration caused, angles each being formed between a
normal line and an optical axis, normal lines and each
condition.
11 TABLE 11 Spherical aberration (mm) Numerical In DVD In CD Height
H aperture reproduction reproduction 1.212 NAL(1) = 0.3606 -0.15363
.times. 10.sup.-1 0.15933 .times. 10.sup.-1 NAL(2) = 0.3617
-0.11068 .times. 10.sup.-1 0.49864 .times. 10.sup.-2 1.374 NAH(2) =
0.4024 -0.13857 .times. 10.sup.-1 0.63914 .times. 10.sup.-2 NAH(3)
= 0.4008 -0.16412 .times. 10.sup.-1 0.20059 .times. 10.sup.-1 0.60
NA2 = 0.60 .times. 0.366 = 0.220 1.3 NA2 = 1.3 .times. 0.366 =
0.476 NAH-NAL = 0.4024 - 0.3617 = 0.0407 -2.lambda./(NA2).sup.2 =
-2 .times. 635 nm/(0.366).sup.2 = -9.48 .mu.m 5.lambda./(NA2).sup.2
= 5 .times. 635 nm/(0.366).sup.2 = 23.7 .mu.m H2mid = (1.212 +
1.374)/2 = 1.280 .theta.1,3,mid = 33.69622 .theta.2,mid = 33.81796
.DELTA..theta.2 mid = 33.81796 - 33.69622 = 0.12174
[0184] FIG. 19 shows a diagram of relative intensity distribution
of a light-converged spot in the case where the best spot shape is
obtained in reproduction of a DVD, while, FIG. 20 shows a diagram
of relative intensity distribution of a light-converged spot in the
case where the best spot shape is obtained in reproduction of a
CD.
EXAMPLE 4
[0185] Example 4 represents an example wherein the invention is
applied to objective lens 116 which is to be mounted on optical
pickup-apparatus 100 (wavelength .lambda.1 of the first light
source=635 nm, and wavelength .lambda.2 of the second light source
=780 nm) and is provided with steps on boundaries of the first
divided surface Sd1-the third divided surface Sd3 of the objective
lens 116.
[0186] Optical data of the objective lens are shown in Tables 12
and 13.
12 TABLE 12 Wavelength .lambda. 635 nm 780 nm Focal length 3.36 mm
3.39 mm Aperture-stop diameter .phi.4.04 mm Lateral magnification
of 0 objective lens i ri di di' ni ni' 1 .infin. 0.000 1.0 1.0 2
2.114 2.200 1.5383 1.5337 3 -7.963 1.757 1.401 1.0 1.0 4 .infin.
0.600 1.200 1.58 1.58 5 .infin.
[0187]
13TABLE 13 Aspheric surface data Second First 0 .ltoreq. H <
1.397 (First divided surface) surface aspheric 1.532 .ltoreq. H
(Third divided surface) (refracting surface .kappa. = -0.97700
surface) A1 = 0.63761 .times. 10.sup.-3 P1 = 3.0 A2 = 0.36688
.times. 10.sup.-3 P1 = 4.0 A3 = 0.83511 .times. 10.sup.-2 P1 = 5.0
A4 = -0.37296 .times. 10.sup.-2 P1 = 6.0 A5 = 0.46548 .times.
10.sup.-3 P1 = 8.0 A6 = -0.43124 .times. 10.sup.-4 P1 = 10.0 Second
1.397 .ltoreq. H < 1.532 aspheric (Second divided surface)
surface d2 = 2.1996 .kappa. = -0.11481 .times. 10.sup.-1 A1 =
0.70764 .times. 10.sup.-2 P1 = 3.0 A2 = 0.13388 .times. 10.sup.-1
P1 = 4.0 A3 = 0.24084 .times. 10.sup.-1 P1 = 5.0 A4 = -0.97636
.times. 10.sup.-2 P1 = 6.0 A5 = 0.93136 .times. 10.sup.-3 P1 = 8.0
A6 = -0.68008 .times. 10.sup.-4 P1 = 10.0 Third surface .kappa. =
-0.24914 .times. 10.sup.-2 (refracting surface) A1 = 0.13775
.times. 10.sup.-2 P1 = 3.0 A2 = -0.41269 .times. 10.sup.-2 P1 = 4.0
A3 = 0.21236 .times. 10.sup.-1 P1 = 5.0 A4 = -0.13895 .times.
10.sup.-1 P1 = 6.0 A5 = 0.16631 .times. 10.sup.-2 P1 = 8.0 A6 =
-0.12138 .times. 10.sup.-3 P1 = 10.0
[0188] The expression of "d2=2.1996" in the column of "Second
aspheric surface" in Table 13 represents a distance on an optical
axis between an intersecting point where the optical axis
intersects with the second aspheric surface (second divided
surface) when it is extended, following the aspherical shape
thereof, and the third surface. This is to increase a quantity of
converged light (peak intensity) by shifting the second divided
surface toward the optical axis by d2 and thereby by providing a
phase difference. The symbol ni' in Table 12 represents a
refractive index in the second light source (.lambda.2=780 nm).
[0189] FIG. 21 (a) shows a diagram of spherical aberration in the
case of reproduction of a DVD, while, FIG. 21 (b) shows a diagram
of spherical aberration in the case of reproduction of a CD. FIG.
22 (a) shows a diagram of wavefront aberration viewed under the
state of defocusing at the position where the best-fit wavefront
aberration is obtained in the case of reproduction of a DVD, while,
FIG. 22 (b) shows a diagram of wavefront aberration viewed under
the state of defocusing at the position where the best-fit
wavefront aberration is obtained in the case of reproduction of a
CD. Table 14 shows numerical apertures for NAL and NAH, quantities
of spherical aberration caused, angles each being formed between a
normal line and an optical axis, normal lines and each
condition.
14 TABLE 14 In DVD reproduction In CD reproduction Spherical
Spherical Height Numerical aberration Numerical aberration H
aperture (mm) aperture (mm) 1.397 NAL(1) = 0.4156 0.16787 .times.
10.sup.-3 NAL(1) = 0.4122 0.23237 .times. 10.sup.-1 NAL(2) = 0.4176
-0.15961 .times. 10.sup.-1 NAL(2) = 0.4142 0.71899 .times.
10.sup.-2 1.532 NAH(2) = 0.4584 -0.19079 .times. 10.sup.-1 NAH(2) =
0.4547 0.94214 .times. 10.sup.-2 NAH(3) = 0.4558 0.59045 .times.
10.sup.-3 NAH(3) = 0.4521 0.28918 .times. 10.sup.-1 0.60 NA2 = 0.60
.times. 0.45 = 0.270 1.1 NA2 = 1.1 .times. 0.45 = 0.495 NAH-NAL =
0.4547 - 0.4142 = 0.0405 -2(.lambda.2)/(NA2).sup.2 = -2 .times. 780
nm/(0.45).sup.2 = -7.70 .mu.m 5(.lambda.2)/(NA2).sup.2 = 5 .times.
780 nm/(0.45).sup.2 = 19.26 .mu.m H2mid = (1.397 + 1.532)/2 = 1.465
.theta.1,3,mid = 38.21395 .theta.2,mid = 38.41159
.DELTA..theta.2,mid = 38.41159 - 38.21395 = 0.19764
[0190] FIG. 23 shows a diagram of relative intensity distribution
of a light-converged spot in the case where the best spot shape is
obtained in reproduction of a DVD, while, FIG. 24 shows a diagram
of relative intensity distribution of a light-converged spot in the
case where the best spot shape is obtained in reproduction of a
CD.
[0191] Even when the objective lens in the present embodiment is
mounted on optical pickup apparatus 10 employing a single light
source (wavelength .lambda.1 of the light source=635 nm)
reproduction was possible not only for DVD but also for CD. FIG. 25
shows a diagram of relative intensity distribution of a
light-converged spot in the case where the best spot shape is
obtained in reproduction of a CD. Numerical apertures for NAL and
NAH, quantities of spherical aberration caused, angles each being
formed between a normal line and an optical axis, normal lines and
each condition in this case are shown in Table 15.
15 TABLE 15 In DVD reproduction In CD reproduction Spherical
Spherical Height Numerical aberration Numerical aberration H
aperture (mm) aperture (mm) 1.397 NAL(1) = 0.4156 0.16787 .times.
10.sup.-3 NAL(1) = 0.4156 0.21913 .times. 10.sup.-1 NAL(2) = 0.4176
-0.15961 .times. 10.sup.-1 NAL(2) = 0.4176 0.60126 .times.
10.sup.-2 1.532 NAH(2) = 0.4584 -0.19079 .times. 10.sup.-1 NAH(2) =
0.4584 0.80011 .times. 10.sup.-2 NAH(3) = 0.4558 0.59045 .times.
10.sup.-3 NAH(3) = 0.4558 0.27319 .times. 10.sup.-1 0.60 NA2 = 0.60
.times. 0.366 = 0.220 1.3 NA2 = 1.3 .times. 0.366 = 0.476 NAH-NAL =
0.4584 - 0.4176 = 0.0408 -2(.lambda.2)/(NA2).sup.2 = -2 .times. 635
nm/(0.366).sup.2 = -9.48 .mu.m 5(.lambda.2)/(NA2).sup.2 = 5 .times.
635 nm/(0.366).sup.2 = 23.7 .mu.m H2mid = (1.397 + 1.532)/2 = 1.465
.theta.1,3,mid = 38.21395 .theta.2,mid = 38.41159
.DELTA..theta.2,mid = 38.41159 - 38.21395 = 0.19764
EXAMPLE 5
[0192] Example 5 represents an example wherein the invention is
applied to objective lens 116 which is to be mounted on optical
pickup apparatus 100 (wavelength .lambda.1 of the first light
source=635 nm, and wavelength .lambda.X2 of the second light source
=780 nm) and is provided with steps on boundaries of the first
divided surface Sd1-the third divided surface Sd3 of the objective
lens 116. In the present example, a CD-R is assumed as the second
optical disk, which indicates the relation of NA2=0.5 accordingly.
Optical data of the objective lens are shown in Tables 16 and
17.
16 TABLE 16 Wavelength .lambda. 635 nm 780 nm Focal length 3.36 mm
3.39 mm Aperture-stop diameter .phi.4.04 mm Lateral magnification
of 0 objective lens i ri di di' ni ni' 1 .infin. 0.000 1.0 1.0 2
2.114 2.200 1.5383 1.5337 3 -7.963 1.757 1.401 1.0 1.0 4 .infin.
0.600 1.200 1.58 1.58 5 .infin.
[0193]
17TABLE 17 Aspheric surface data Second First 0 .ltoreq. H <
1.515 (First divided surface) surface aspheric 1.751 .ltoreq. H
(Third divided surface) (refracting surface .kappa. = -0.97700
surface) A1 = 0.63761 .times. 10.sup.-3 P1 = 3.0 A2 = 0.36688
.times. 10.sup.-3 P1 = 4.0 A3 = 0.83511 .times. 10.sup.-2 P1 = 5.0
A4 = -0.37296 .times. 10.sup.-2 P1 = 6.0 A5 = 0.46548 .times.
10.sup.-3 P1 = 8.0 A6 = -0.43124 .times. 10.sup.-4 P1 = 10.0 Second
1.515 .ltoreq. H < 1.751 aspheric (Second divided surface)
surface .kappa. = -0.11481 .times. 10.sup.-1 A1 = 0.70764 .times.
10.sup.-2 P1 = 3.0 A2 = 0.13388 .times. 10.sup.-1 P1 = 4.0 A3 =
0.24084 .times. 10.sup.-1 P1 = 5.0 A4 = -0.97636 .times. 10.sup.-2
P1 = 6.0 A5 = 0.93136 .times. 10.sup.-3 P1 = 8.0 A6 = -0.68008
.times. 10.sup.-4 P1 = 10.0 Third surface .kappa. = -0.24914
.times. 10.sup.-2 (refracting surface) A1 = 0.13775 .times.
10.sup.-2 P1 = 3.0 A2 = -0.41269 .times. 10.sup.-2 P1 = 4.0 A3 =
0.21236 .times. 10.sup.-1 P1 = 5.0 A4 = -0.13895 .times. 10.sup.-1
P1 = 6.0 A5 = 0.16631 .times. 10.sup.-2 P1 = 8.0 A6 = -0.12138
.times. 10.sup.-3 P1 = 10.0
[0194] In the objective lens of the present example, a position
where the first aspheric surface intersects with an optical axis is
the same as that where the second aspheric surface intersects with
an optical axis. The symbol ni' in Table 16 represents a refractive
index in the second light source (.lambda.2=780 nm).
[0195] FIG. 26 (a) shows a diagram of spherical aberration in case
of reproduction of a DVD, while, FIG. 26 (b) shows a diagram of
spherical aberration in the case of reproduction of a CD-R. FIG. 27
(a) shows a diagram of wavefront aberration viewed under the state
of defocusing at the position where the best-fit wavefront
aberration is obtained in the case of reproduction of a DVD, while,
FIG. 27 (b) shows a diagram of wavefront aberration viewed under
the state of defocusing at the position where the best-fit
wavefront aberration is obtained in the case of reproduction of a
CD-R. Table 18 shows numerical apertures for NAL and NAH,
quantities of spherical aberration caused, angles each being formed
between a normal line and an optical axis, normal lines and each
condition.
18 TABLE 18 In DVD reproduction In CD-R reproduction Spherical
Spherical Height Numerical aberration Numerical aberration H
aperture (mm) aperture (mm) 1.515 NAL(1) = 0.4507 0.56250 .times.
10.sup.-3 NAL(1) = 0.4470 0.28187 .times. 10.sup.-1 NAL(2) = 0.4532
-0.18638 .times. 10.sup.-1 NAL(2) = 0.4496 0.91439 .times.
10.sup.-2 1.751 NAH(2) = 0.5253 -0.26720 .times. 10.sup.-1 NAH(2) =
0.5211 0.12335 .times. 10.sup.-1 NAH(3) = 0.5212 0.22836 .times.
10.sup.-3 NAH(3) = 0.5170 0.38838 .times. 10.sup.-1 0.60 NA2 = 0.60
.times. 0.50 = 0.300 1.1 NA2 = 1.1 .times. 0.50 = 0.550 NAH-NAL =
0.5211 - 0.4496 = 0.0715 -2(.lambda.2)/(NA2).sup.2 = -2 .times. 780
nm/(0.50).sup.2 = -6.24 .mu.m 5(.lambda.2)/(NA2).sup.2 = 5 .times.
780 nm/(0.50).sup.2 = 15.6 .mu.m H2mid = (1.515 + 1.751)/2 = 1.633
.theta.1,3,mid = 42.17430 .theta.2,mid = 42.44207
.DELTA..theta.2,mid = 42.44207 - 42.17430 = 0.26777
[0196] FIG. 28 shows a diagram of relative intensity distribution
of a light-converged spot in the case where the best spot shape is
obtained in reproduction of a DVD, while, FIG. 29 shows a diagram
of relative intensity distribution of a light-converged spot in the
case where the best spot shape is obtained in reproduction of a
CD-R.
[0197] Even when the objective lens in the present embodiment is
mounted on optical pickup apparatus 10 employing a single light
source (wavelength .lambda.1 of the light source=635 nm),
reproduction was possible not only for DVD but also for CD. FIG. 30
shows a diagram of relative intensity distribution of a
light-converged spot in the case where the best spot shape is
obtained in reproduction of a CD. Numerical apertures for NAL and
NAH, quantities of spherical aberration caused, angles each being
formed between a normal line and an optical axis, normal lines and
each condition in this case are shown in Table 19.
19 TABLE 19 In DVD reproduction In CD reproduction Spherical
Spherical Height Numerical aberration Numerical aberration H
aperture (mm) aperture (mm) 1.515 NAL(1) = 0.4507 0.56250 .times.
10.sup.-3 NAL(1) = 0.4507 0.26624 .times. 10.sup.-1 NAL(2) = 0.4532
-0.18638 .times. 10.sup.-1 NAL(2) = 0.4532 0.77566 .times.
10.sup.-2 1.751 NAH(2) = 0.5253 -0.26720 .times. 10.sup.-1 NAH(2) =
0.5253 0.10403 .times. 10.sup.-1 NAH(3) = 0.5212 0.22836 .times.
10.sup.-3 NAH(3) = 0.5212 0.36667 .times. 10.sup.-1 0.60 NA2 = 0.60
.times. 0.366 = 0.220 1.3 NA2 = 1.3 .times. 0.366 = 0.476 NAH-NAL =
0.5253 - 0.4532 = 0.0721 -2(.lambda.2)/(NA2).sup.2 = -2 .times. 635
nm/(0.366).sup.2 = -9.48 .mu.m 5(.lambda.2)/(NA2).sup.2 = 5 .times.
635 nm/(0.366).sup.2 = 23.7 .mu.m H2mid = (1.515 + 1.751)/2 = 1.633
.theta.1,3,mid = 42.17430 .theta.2,mid = 42.44207
.DELTA..theta.2,mid = 42.44207 - 42.17430 = 0.26777
EXAMPLE 6
[0198] Example 6 represents an example wherein objective lens 216
which is to be mounted on optical pickup apparatus 100 (wavelength
.lambda.1 of the first light source=635 nm, and wavelength
.lambda.2 of the second light source=780 nm) and the objective lens
216 wherein steps are provided on boundaries of the first divided
surface Sd1-the fifth divided surface Sd5 of the objective lens 216
is mounted. In the present example, a CD-R is assumed as the second
optical disk, which indicates the relation of NA2=0.5 accordingly.
h. Spherical aberration
[0199] Optical data of the objective lens are shown in Tables 20
and 21.
20 TABLE 20 Wavelength .lambda. 635 nm 780 nm Focal length 3.36 mm
3.39 mm Aperture-stop diameter .phi.4.04 mm Lateral magnification
of 0 objective lens i ri di di' ni ni' 1 .infin. 0.000 1.0 1.0 2
2.114 2.200 1.5383 1.5337 3 -7.963 1.757 1.401 1.0 1.0 4 .infin.
0.600 1.200 1.58 1.58 5 .infin.
[0200]
21TABLE 21 Aspheric surface data Second First 0 .ltoreq. H <
1.481 (First divided surface) surface aspheric 1.549 .ltoreq. H
< 1.700 (Third divided surface) (refracting surface 1.784
.ltoreq. H (Fifth divided surface) surface) .kappa. = -0.97700 A1 =
0.63761 .times. 10.sup.-3 P1 = 3.0 A2 = 0.36688 .times. 10.sup.-3
P1 = 4.0 A3 = 0.83511 .times. 10.sup.-2 P1 = 5.0 A4 = -0.37296
.times. 10.sup.-2 P1 = 6.0 A5 = 0.46548 .times. 10.sup.-3 P1 = 8.0
A6 = -0.43124 .times. 10.sup.-4 P1 = 10.0 Second 1.481 .ltoreq. H
< 1.549 (Second divided surface) aspheric 1.700 .ltoreq. H <
1.784 (Fourth divided surface) surface .kappa. = -0.11481 .times.
10.sup.-1 A1 = 0.70764 .times. 10.sup.-2 P1 = 3.0 A2 = 0.13388
.times. 10.sup.-1 P1 = 4.0 A3 = 0.24084 .times. 10.sup.-1 P1 = 5.0
A4 = -0.97636 .times. 10.sup.-2 P1 = 6.0 A5 = 0.93136 .times.
10.sup.-3 P1 = 8.0 A6 = -0.68008 .times. 10.sup.-4 P1 = 10.0 Third
surface .kappa. = -0.24914 .times. 10.sup.-2 (refracting surface)
A1 = 0.13775 .times. 10.sup.-2 P1 = 3.0 A2 = -0.41269 .times.
10.sup.-2 P1 = 4.0 A3 = 0.21236 .times. 10.sup.-1 P1 = 5.0 A4 =
-0.13895 .times. 10.sup.-1 P1 = 6.0 A5 = 0.16631 .times. 10.sup.-2
P1 = 8.0 A6 = -0.12138 .times. 10.sup.-3 P1 = 10.0
[0201] In the objective lens of the present example, a point where
the first aspheric surface (surfaces of the first, the third and
the fifth divided surfaces (or their extended surfaces) intersects
with an optical axis and a point where a surface formed by
extending each of the second divided surface Sd2 and the fourth
divided surface Sd4 (both composing the second aspheric surface)
intersects with an optical axis are on the same position. The
symbol ni' in Table 22 represents a refractive index in the second
light source (.lambda.2=780 nm).
[0202] FIG. 31 (a) shows a diagram of spherical aberration in the
case of reproduction of a DVD, while, FIG. 31 (b) shows a diagram
of spherical aberration in the case of reproduction of a CD-R. FIG.
32 (a) shows a diagram of wavefront aberration viewed under the
state of defocusing at the position where the best-fit wavefront
aberration is obtained in the case of reproduction of a DVD, while,
FIG. 32 (b) shows a diagram of wavefront aberration viewed under
the state of defocusing at the position where the best-fit
wavefront aberration is obtained in the case of reproduction of a
CD-R. Table 22 shows numerical apertures for NAL and NAH,
quantities of spherical aberration caused, angles each being formed
between a normal line and an optical axis, normal lines and each
condition.
22 TABLE 22 In DVD reproduction In CD-R reproduction Spherical
Spherical Height Numerical aberration Numerical aberration H
aperture (mm) aperture (mm) 1.481 NAL (1) = 0.4406 0.48121 .times.
10.sup.-3 NAL (1) = 0.4370 0.26737 .times. 10.sup.-1 NAL (2) =
0.4430 -0.17798 .times. 10.sup.-1 NAL (2) = 0.4393 0.85891 .times.
10.sup.-2 1.549 NAH (2) = 0.4636 -0.19553 .times. 10.sup.-1 NAH (2)
= 0.4598 0.96765 .times. 10.sup.-2 NAH (3) = 0.4608 0.60932 .times.
10.sup.-3 NAH (3) = 0.4571 0.29652 .times. 10.sup.-1 1.700 NAL (3)
= 0.5059 0.39402 .times. 10.sup.-3 NAL (3) = 0.5018 0.36389 .times.
10.sup.-1 NAL (4) = 0.5096 -0.24649 .times. 10.sup.-1 NAL (4) =
0.5055 0.11709 .times. 10.sup.-1 1.784 NAH (4) = 0.5354 -0.28119
.times. 10.sup.-1 NAH (4) = 0.5312 0.12767 .times. 10.sup.-1 NAH
(5) = 0.5310 0.13148 .times. 10.sup.-3 NAH (5) = 0.5268 0.40512
.times. 10.sup.-1 0.60NA2 = 0.60 .times. 0.50 = 0.300 1.1NA2 = 1.1
.times. 0.50 = 0.550 NAH (4) - NAL (2) = 0.5312 - 0.4393 = 0.0919
-2(.lambda.2)/(NA2).sup.2 = -2 .times. 780 nm/(0.50).sup.2 = -6.24
.mu.m 5(.lambda.2)/(NA2).sup.2 = 5 .times. 780 nm/(0.50).sup.2 =
15.6 .mu.m H2mid = (1.481 + 1.549)/2 = 1.515 .theta.1,3,mid =
39.41130.degree. .theta.2,mid = 39.52307.degree.
.DELTA..theta.2,mid = .theta.1,3,mid - .theta.2mid = 39.62807 -
39.41130 = 0.21677.degree. H4mid = (1.700 + 1.784)/2 = 1.742
.theta.3,5,mid = 44.62556.degree. .theta.4,mid = 44.94902.degree.
.DELTA..theta.4,mid = .theta.3,5,mid - .theta.4mid = 44.94902 -
44.62556 = 0.32346.degree.
[0203] FIG. 33 shows a diagram of relative intensity distribution
of a light-converged spot in the case where the best spot shape is
obtained in reproduction of a DVD, while, FIG. 34 shows a diagram
of relative intensity distribution of a light-converged spot in the
case where the best spot shape is obtained in reproduction of a
CD-R.
[0204] Even when the objective lens in the present embodiment is
mounted on optical pickup apparatus 10 employing a single light
source (wavelength .lambda.1 of the light source=635 nm),
reproduction was possible not only for DVD but also for CD. FIG. 35
shows a diagram of relative intensity distribution of a
light-converged spot in the case where the best spot shape is
obtained in reproduction of a CD. Numerical apertures for NAL and
NAH, quantities of spherical aberration caused, angles each being
formed between a normal line and an optical axis, normal lines and
each condition in this case are shown in Table 23.
23 TABLE 23 In DVD reproduction In CD reproduction Spherical
Spherical Height Numerical aberration Numerical aberration H
aperture (mm) aperture (mm) 1.481 NAL (1) = 0.4406 0.48121 .times.
10.sup.-3 NAL (1) = 0.4406 0.25244 .times. 10.sup.-1 NAL (2) =
0.4430 -0.17798 .times. 10.sup.-1 NAL (2) = 0.4430 0.72646 .times.
10.sup.-2 1.549 NAH (2) = 0.4636 -0.19553 .times. 10.sup.-1 NAH (2)
= 0.4636 0.82240 .times. 10.sup.-2 NAH (3) = 0.4608 0.60932 .times.
10.sup.-3 NAH (3) = 0.4608 0.28016 .times. 10.sup.-1 1.700 NAL (3)
= 0.5059 0.39402 .times. 10.sup.-3 NAL (3) = 0.5059 0.34375 .times.
10.sup.-1 NAL (4) = 0.5096 -0.24649 .times. 10.sup.-1 NAL (4) =
0.5096 0.99199 .times. 10.sup.-2 1.784 NAH (4) = 0.5354 -0.28119
.times. 10.sup.-1 NAH (4) = 0.5354 0.10732 .times. 10.sup.-1 NAH
(5) = 0.5310 0.13146 .times. 10.sup.-3 NAH (5) = 0.5310 0.38227
.times. 10.sup.-1 0.60NA2 = 0.60 .times. 0.366 = 0.220 1.3NA2 = 1.3
.times. 0.366 = 0.476 NAH (4) - NAL (2) = 0.5354 - 0.4430 = 0.0924
-2 (.lambda.2)/(NA2).sup.2 = -2 .times. 635 nm/(0.366).sup.2 =
-9.48 .mu.m 5(.lambda.2)/(NA2).sup.2 = 5 .times. 635
nm/(0.366).sup.2 = 23.7 .mu.m H2mid = (1.481 + 1.549)/2 = 1.515
.theta.1,3,mid = 39.41130.degree. .theta.2,mid = 39.62807.degree.
.DELTA..theta.2,mid = .theta.1,3,mid - .theta.2mid = 39.62807 -
39.41130 = 0.21677.degree. H4mid = (1.700 + 1.784)/2 = 1.742
.theta.3,5,mid = 44.62556.degree. .theta.4,mid = 44.94902.degree.
.DELTA..theta.4,mid = .theta.3,5,mid - .theta.4mid = 44.94902 -
44.62556 = 0.32346.degree.
EXAMPLE 7
[0205] Example 7 represents an example wherein objective lens 216
which is to be mounted on optical pickup apparatus 100 (wavelength
.lambda.1 of the first light source=635 nm, and wavelength
.lambda.2 of the second light source=780 nm), and the objective
lens 216 wherein steps are provided on boundaries of the first
divided surface Sd1-the fifth divided surface Sd5 of the objective
lens 216 is mounted. In the present example, a CD-R is assumed as
the second optical disk, which indicates the relation of NA2=0.5
accordingly.
[0206] Optical data of the objective lens are shown in Tables 24
and 25.
24 TABLE 24 Wavelength .lambda. 635 nm 780 nm Focal length 3.36 mm
3.39 mm Aperture-stop diameter .phi.4.04 mm Lateral magnification
of 0 objective lens i ri di di' ni ni' 1 .infin. 0.000 1.0 1.0 2
2.114 2.200 1.5383 1.5337 3 -7.963 1.757 1.401 1.0 1.0 4 .infin.
0.600 1.200 1.58 1.58 5 .infin.
[0207]
25TABLE 25 Aspheric surface data Second First 0 .ltoreq. H <
1.481 (First divided surface) surface aspheric 1.549 .ltoreq. H
< 1.700 (Third divided surface) (refracting surface 1.784
.ltoreq. H (Fifth divided surface) surface) .kappa. = -0.97700 A1 =
0.63761 .times. 10.sup.-3 P1 = 3.0 A2 = 0.36688 .times. 10.sup.-3
P1 = 4.0 A3 = 0.83511 .times. 10.sup.-2 P1 = 5.0 A4 = -0.37296
.times. 10.sup.-2 P1 = 6.0 A5 = 0.46548 .times. 10.sup.-3 P1 = 8.0
A6 = -0.43124 .times. 10.sup.-4 P1 = 10.0 Second 1.481 .ltoreq. H
< 1.549 (Second divided surface) aspheric 1.700 .ltoreq. H <
1.734 (Fourth divided surface) surface d2 = 2.1996 d4 = 2.2003
.kappa. = -0.11481 .times. 10.sup.-1 A1 = 0.70764 .times. 10.sup.-2
P1 = 3.0 A2 = 0.13388 .times. 10.sup.-1 P1 = 4.0 A3 = 0.24084
.times. 10.sup.-1 P1 = 5.0 A4 = -0.97636 .times. 10.sup.-2 P1 = 6.0
A5 = 0.93136 .times. 10.sup.-3 P1 = 8.0 A6 = -0.68008 .times.
10.sup.-4 P1 = 10.0 Third surface .kappa. = -0.24914 .times.
10.sup.-2 (refracting surface) A1 = 0.13775 .times. 10.sup.-2 P1 =
3.0 A2 = -0.41269 .times. 10.sup.-2 P1 = 4.0 A3 = 0.21236 .times.
10.sup.-1 P1 = 5.0 A4 = -0.13895 .times. 10.sup.-1 P1 = 6.0 A5 =
0.16631 .times. 10.sup.-2 P1 = 8.0 A6 = -0.12138 .times. 10.sup.-3
P1 = 10.0
[0208] Descriptions of "d2=2.1996" and "d4=2.2003" in the column of
"Second aspheric surface" in Table 25 respectively represent a
distance on the optical axis between the third surface and a point
where the optical axis intersects with the second divided surface
(on the second aspheric surface) extended to the optical axis
according to the expression of an aspheric surface shape, and a
distance on the optical axis between the third surface and a point
where the optical axis intersects with the fourth divided surface
(on the second aspheric surface) extended to the optical axis
according to the expression of an aspheric surface shape. This
means that the second divided surface is shifted toward the optical
axis by d2 and the fourth divided surface is shifted toward the
optical axis by d4 to provide a phase difference between them so
that a quantity of converged light (peak intensity) may be
increased. The symbol ni' in Table 24 represents a refractive index
in the second light source (.lambda.2=780 nm) FIG. 36 (a) shows a
diagram of spherical aberration in the case of reproduction of a
DVD, while, FIG. 36 (b) shows a diagram of spherical aberration in
the case of reproduction of a CD-R. FIG. 37 (a) shows a diagram of
wavefront aberration viewed under the state of defocusing at the
position where the best-fit wavefront aberration is obtained in the
case of reproduction of a DVD, while, FIG. 37 (b) shows a diagram
of wavefront aberration viewed under the state of defocusing at the
position where the best-fit wavefront aberration is obtained in the
case of reproduction of a CD-R. Table 26 shows numerical apertures
for NAL and NAH, quantities of spherical aberration caused, angles
each being formed between a normal line and an optical axis, normal
lines and each condition.
26 TABLE 26 In DVD reproduction In CD-R reproduction Spherical
Spherical Height Numerical aberration Numerical aberration H
aperture (mm) aperture (mm) 1.481 NAL (1) = 0.4406 0.48121 .times.
10.sup.-3 NAL (1) = 0.4370 0.26737 .times. 10.sup.-1 NAL (2) =
0.4430 -0.17794 .times. 10.sup.-1 NAL (2) = 0.4393 0.85936 .times.
10.sup.-2 1.549 NAH (2) = 0.4636 -0.19550 .times. 10.sup.-1 NAH (2)
= 0.4598 0.96802 .times. 10.sup.-2 NAH (3) = 0.4608 0.60932 .times.
10.sup.-3 NAH (3) = 0.4571 0.29652 .times. 10.sup.-1 1.700 NAL (3)
= 0.5059 0.39402 .times. 10.sup.-3 NAL (3) = 0.5018 0.36389 .times.
10.sup.-1 NAL (4) = 0.5096 -0.24648 .times. 10.sup.-1 NAL (4) =
0.5055 0.11708 .times. 10.sup.-1 1.784 NAH (4) = 0.5354 -0.28114
.times. 10.sup.-1 NAH (4) = 0.5312 0.12771 .times. 10.sup.-1 NAH
(5) = 0.5310 0.13146 .times. 10.sup.-3 NAH (5) = 0.5268 0.40512
.times. 10.sup.-1 0.60NA2 = 0.60 .times. 0.50 = 0.300 1.1NA2 = 1.1
.times. 0.50 = 0.550 NAH (4) - NAL (2) = 0.5312 - 0.4393 = 0.0919
-2 (.lambda.2)/(NA2).sup.2 = -2 .times. 780 nm/(0.50).sup.2 = -6.24
.mu.m 5(.lambda.2)/(NA2).sup.2 = 5 .times. 780 nm/(0.50).sup.2 =
15.6 .mu.m H2mid = (1.481 + 1.549)/2 = 1.515 .theta.1,3,mid =
39.41130.degree. .theta.2,mid = 39.62807.degree.
.DELTA..theta.2,mid = .theta.1,3,mid - .theta.2mid = 39.62807 -
39.41130 = 0.21677.degree. H4mid = (1.700 + 1.784)/2 = 1.742
.theta.3,5,mid = 44.62556.degree. .theta.4,mid = 44.94902.degree.
.DELTA..theta.4,mid = .theta.3,5,mid - .theta.4mid = 44.94902 -
44.62556 = 0.32346.degree.
[0209] FIG. 38 shows a diagram of relative intensity distribution
of a light-converged spot in the case where the best spot shape is
obtained in reproduction of a DVD, while, FIG. 39 shows a diagram
of relative intensity distribution of a light-converged spot in the
case where the best spot shape is obtained in reproduction of a
CD-R.
[0210] Even when the objective lens in the present embodiment is
mounted on optical pickup apparatus 10 employing a single light
source (wavelength .lambda.1 of the light source=635 nm),
reproduction was possible not only for DVD but also for CD. FIG. 40
shows a diagram of relative intensity distribution of a
light-converged spot in the case where the best spot shape is
obtained in reproduction of a CD. Numerical apertures for NAL and
NAH, quantities of spherical aberration caused, angles each being
formed between a normal line and an optical axis, normal lines and
each condition in this case are shown in Table 27.
27 TABLE 27 In DVD reproduction In CD reproduction Spherical
Spherical Height Numerical aberration Numerical aberration H
aperture (mm) aperture (mm) 1.481 NAL (1) = 0.4406 0.48121 .times.
10.sup.-3 NAL (1) = 0.4406 0.25244 .times. 10.sup.-1 NAL (2) =
0.4430 -0.17794 .times. 10.sup.-1 NAL (2) = 0.4430 0.72688 .times.
10.sup.-2 1.549 NAH (2) = 0.4636 -0.19550 .times. 10.sup.-1 NAH (2)
= 0.4636 0.82274 .times. 10.sup.-2 NAH (3) = 0.4608 0.60932 .times.
10.sup.-3 NAH (3) = 0.4608 0.28016 .times. 10.sup.-1 1.700 NAL (3)
= 0.5059 0.39402 .times. 10.sup.-3 NAL (3) = 0.5059 0.34375 .times.
10.sup.-1 NAL (4) = 0.5096 -0.24648 .times. 10.sup.-1 NAL (4) =
0.5096 0.99201 .times. 10.sup.-2 1.784 NAH (4) = 0.5354 -0.28114
.times. 10.sup.-1 NAH (4) = 0.5354 0.10737 .times. 10.sup.-1 NAH
(5) = 0.5310 0.13146 .times. 10.sup.-3 NAH (5) = 0.5310 0.38227
.times. 10.sup.-1 0.60NA2 = 0.60 .times. 0.366 = 0.220 1.3NA2 = 1.3
.times. 0.366 = 0.476 NAH (4) - NAL (2) = 0.5354 - 0.4430 = 0.0924
-2(.lambda.2)/(NA2).sup.2 = -2 .times. 635 nm/(0.366).sup.2 = -9.48
.mu.m 5(.lambda.2)/(NA2).sup.2 = 5 .times. 635 nm/(0.366).sup.2 =
23.7 .mu.m H2mid = (1.481 + 1.549)/2 = 1.515 .theta.1,3, mid =
39.41130.degree. .theta.2,mid = 39.62807.degree.
.DELTA..theta.2,mid = .theta.1,3,mid - .theta.2mid = 39.62807 -
39.41130 = 0.21677.degree. H4mid = (1.700 + 1.784)/2 = 1.742
.theta.3,5,mid = 44.62556.degree. .theta.4,mid = 44.94902.degree.
.DELTA..theta.4,mid = .theta.3,5,mid - .theta.4mid = 44.94902 -
44.62556 = 0.32346.degree.
EXAMPLE 8
[0211] Example 8 represents an example wherein the invention is
applied to objective lens 116 which is to be mounted on optical
pickup apparatus 100 (wavelength .lambda.1 of the first light
source=635 nm, and wavelength .lambda.2 of the second light
source=780 nm), and in which the steps are provided on boundaries
of the first divided surface Sd1 the third divided surface Sd3.
[0212] Optical data of the objective lens are shown in Tables
29.
28 TABLE 28 Wavelength .lambda. 635 nm 780 nm Focal length 3.36 mm
3.39 mm Aperture-stop diameter .phi.4.04 mm Lateral magnicfication
of 0 objective lens i ri di di' ni ni' 1 .infin. 0.000 1.0 1.0 2
2.114 2.200 1.5383 1.5337 3 -7.963 1.757 1.401 1.0 1.0 4 .infin.
0.600 1.200 1.58 1.58 5 .infin.
[0213]
29TABLE 29 Aspheric surface data Second First 0 .ltoreq. H <
1.279 (First divided surface) surface aspheric 1.532 .ltoreq. H
(Third divided surface) (refracting surface .kappa. = -0.97700
surface) A1 = 0.63761 .times. 10.sup.-3 P1 = 3.0 A2 = 0.36688
.times. 10.sup.-3 P1 = 4.0 A3 = 0.83511 .times. 10.sup.-2 P1 = 5.0
A4 = -0.37296 .times. 10.sup.-2 P1 = 6.0 A5 = 0.46548 .times.
10.sup.-3 P1 = 8.0 A6 = -0.43124 .times. 10.sup.-4 P1 = 10.0 Second
1.279 .ltoreq. H < 1.532 (Second divided surface) aspheric d2 =
2.1995 surface .kappa. = 0.11481 .times. 10.sup.-1 A1 = 0.70764
.times. 10.sup.-2 P1 = 3.0 A2 = 0.13338 .times. 10.sup.-1 P1 = 4.0
A3 = 0.24084 .times. 10.sup.-1 P1 = 5.0 A4 = -0.97636 .times.
10.sup.-2 P1 = 6.0 A5 = 0.93136 .times. 10.sup.-3 P1 = 8.0 A6 =
-0.68008 .times. 10.sup.-4 P1 = 10.0 Third surface .kappa. =
-0.24914 .times. 10.sup.-2 (refracting surface) A1 = 0.13775
.times. 10.sup.-2 P1 = 3.0 A2 = -0.41259 .times. 10.sup.-2 P1 = 4.0
A3 = 0.21236 .times. 10.sup.-1 P1 = 5.0 A4 = -0.13895 .times.
10.sup.-1 P1 = 6.0 A5 = 0.16631 .times. 10.sup.-2 P1 = 8.0 A6 =
0.12138 .times. 10.sup.-3 P1 = 10.0
[0214] Descriptions of "d2=2.1996" in the column of "Second
aspheric surface" in Table 29 represents a distance on the optical
axis between the third surface and a point where the optical axis
intersects with the second divided surface (on the second aspheric
surface) extended to the optical axis according to the expression
of an aspheric surface shape.
[0215] In the objective lens of the present example, a position
where the first aspheric surface intersects with an optical axis is
the same as that where the second aspheric surface intersects with
an optical axis. The symbol ni' in Table 28 represents a refractive
index in the second light source (.lambda.2=780 nm).
[0216] FIG. 41 (a) shows a diagram of spherical aberration in the
case of reproduction of a DVD, while, FIG. 41 (b) shows a diagram
of spherical aberration in the case of reproduction of a CD. FIG.
42 (a) shows a diagram of wavefront aberration viewed under the
state of defocusing at the position where the best-fit wavefront
aberration is obtained in the case of reproduction of a DVD, while,
FIG. 42 (b) shows a diagram of wavefront aberration viewed under
the state of defocusing at the position where the best-fit
wavefront aberration is obtained in the case, of reproduction of a
CD-R. Table 30 shows numerical apertures for NAL and NAH,
quantities of spherical aberration caused, angles each being formed
between a normal line and an optical axis, normal lines and each
condition.
30 TABLE 30 In DVD reproduction In CD-R reproduction Spherical
Spherical Height Numerical aberration Numerical aberration H
aperture (mm) aperture (mm) 1.279 NAL (1) = 0.3806 -0.35533 .times.
10.sup.-3 NAL (1) = 0.3775 0.18675 .times. 10.sup.-1 NAL (2) =
0.3821 -0.13685 .times. 10.sup.-1 NAL (2) = 0.3790 0.53763 .times.
10.sup.-2 1.532 NAH (2) = 0.4584 -0.19077 .times. 10.sup.-1 NAH (2)
= 0.4547 0.94234 .times. 10.sup.-2 NAH (3) = 0.4558 0.59045 .times.
10.sup.-3 NAH (3) = 0.4521 0.28918 .times. 10.sup.-1 0.60NA2 = 0.60
.times. 0.45 = 0.270 1.1NA2 = 1.1 .times. 0.45 = 0.495 NAH - NAL =
0.4547 - 0.3790 = 0.0757 -2 (.lambda. 2)/(NA2).sup.2 - 2 .times.
780 nm/(0.45).sup.2 = -7.70 .mu.m 5(.lambda.2)/(NA2).sup.2 = 5
.times. 780 nm/(0.45).sup.2 = 19.26 .mu.m H2mid = (1.279 + 1.532)/2
= 1.406 .theta.1,3,mid = 36.78417.degree. .theta.2,mid =
36.96074.degree. .DELTA..theta.mid = 36.96074- 36.78417 =
0.17657.degree.
[0217] FIG. 43 shows a diagram of relative intensity distribution
of a light-converged spot in the case where the best spot shape is
obtained in reproduction of a DVD, while, FIG. 44 shows a diagram
of relative intensity distribution of a light-converged spot in the
case where the best spot shape is obtained in reproduction of a
CD.
[0218] Even when the objective lens in the present embodiment is
mounted on optical pickup apparatus 10 employing a single light
source (wavelength .lambda.1 of the light source=635 nm),
reproduction was possible not only for DVD but also for CD. FIG. 45
shows a diagram of relative intensity distribution of a
light-converged spot in the case where the best spot shape is
obtained in reproduction of a CD. Numerical apertures for NAL and
NAH, quantities of spherical aberration caused, angles each being
formed between a normal line and an optical axis, normal lines and
each condition in this case are shown in Table 31.
31 TABLE 31 In DVD reproduction In CD reproduction Spherical
Spherical Height Numerical aberration Numerical aberration H
aperture (mm) aperture (mm) 1.279 NAL (1) = -0.35533 .times.
10.sup.-3 NAL (1) = 0.17571 .times. 10.sup.-1 0.3806 0.3806 NAL (2)
= -0.13685 .times. 10.sup.-1 NAL (2) = 0.43934 .times. 10.sup.-2
0.3321 0.3820 1.532 NAH (2) = -0.19077 .times. 10.sup.-1 NAH (2) =
0.80030 .times. 10.sup.-2 0.4584 0.4584 NAH (3) = 0.59045 .times.
10.sup.-3 NAH (3) = 0.27319 .times. 10.sup.-1 0.4558 0.4558 0.60
NA2 = 0.60 .times. 0.366 = 0.220 1.3 NA2 = 1.3 .times. 0.366 =
0.476 NAH - NAL = 0.4584 - 0.3820 = 0.0764
-2(.lambda.2)/(NA2).sup.2 = -2 .times. 635 nm/(0.366).sup.2 = -9.48
.mu.m 5(.lambda.2)/(NA2).sup.2 = 5 .times. 635 nm/(0.366).sup.2 =
23.7 .mu.m H2mid = (1.279 + 1.532)/2 = 1.406 .theta.1, 3, mid =
36.78417.degree. .theta.2, mid = 36.96074.degree. .DELTA..theta.mid
= 36.96074 - 36.78417 = 0.17657.degree.
(Example 9)
[0219] Example 9 represents an example wherein the invention is
applied to objective lens 116 which is to be mounted on optical
pickup apparatus 100 (wavelength .lambda.1 of the first light
sources=635 nm, and wavelength .lambda.2 of the second light
source=780 nm), and in which the steps are provided on boundaries
first divided surface Sd1-the third divided surface Sd3.
[0220] Optical data of the objective lens are shown in Tables 32
and 33.
32 TABLE 32 Wavelength .lambda. 635 nm 780 nm Focal length 3.36 mm
3.39 mm Aperture-stop diameter .phi.404 mm Lateral magnification of
0 objective lens i ri di di' ni ni' 1 .infin. 0.0000 1.0 1.0 2
2.117 2.2000 1.5383 1.5337 3 -7.903 1.7580 1.3390 1.0 1.0 4 .infin.
0.6000 1.2000 1.58 1.58 5 .infin.
[0221]
33TABLE 33 Aspheric surface data Second First 0 .ltoreq. H
<1.270 (First divided surface) surface aspheric 1.520 .ltoreq. H
(Third divided surface) (refracting surface .kappa. = -0.97770
surface) A1 = -0.36792 .times. 10.sup.-2 P1 = 3.0 A2 = 0.21127
.times. 10.sup.-1 P2 = 4.0 A3 = -0.24914 .times. 10.sup.-1 P3 = 5.0
A4 = 0.23908 .times. 10.sup.-1 P4 = 6.0 A5 = -0.12789 .times.
10.sup.-1 P5 = 7.0 A6 = 0.32635 .times. 10.sup.-2 P6 = 8.0 A7 =
-0.11776 .times. 10.sup.-3 P7 = 10.0 Second 1.270 .ltoreq. H <
1.520 (Second divided surface) aspheric d2 = 2.200 surface .kappa.
= -0.96728 .times. 10.sup.-0 A1 = -0.44081 .times. 10.sup.-2 P1 =
3.0 A2 = 0.21265 .times. 10.sup.-1 P2 = 4.0 A3 = -0.24757 .times.
10.sup.-1 P3 = 5.0 A4 = 0.24042 .times. 10.sup.-1 P4 = 6.0 A5 =
-0.12826 .times. 10.sup.-1 P5 = 7.0 A6 = 0.32570 .times. 10.sup.-2
P6 = 8.0 A7 = -0.11713 .times. 10.sup.-3 P7 = 10.0 Third surface
.kappa. = -0.19532 .times. 10.sup.-2 (refracting surface) A1 =
0.25586 .times. 10.sup.-4 P1 = 3.0 A2 = 0.22177 .times. 10.sup.-1
P2 = 4.0 A3 = -0.32988 .times. 10.sup.-1 P3 = 5.0 A4 = 0.32771
.times. 10.sup.-1 P4 = 6.0 A5 = -0.17803 .times. 10.sup.-1 P5 = 7.0
A6 = 0.40149 .times. 10.sup.-2 P6 = 8.0 A7 = -0.92804 .times.
10.sup.-4 P7 = 10.0
[0222] Descriptions of "d2=2.200" in the column of "Second aspheric
surface" in Table 33 represents a distance on the optical axis
between the third surface and a point where the optical axis
intersects with the second divided surface (on the second aspheric
surface) extended to the optical axis according to the expression
of an aspheric surface shape.
[0223] In the objective lens of the present example, a position
where the first aspheric surface intersects with an optical axis is
the same as that where the second aspheric surface intersects with
an optical axis. The symbol ni' in Table 32 represents a refractive
index in the second light source (.lambda.2= 780 nm).
[0224] FIG. 46(a) shows a diagram of spherical aberration in the
case of reproduction of a DVD, while, FIG. 46(b) shows a diagram of
spherical aberration in the case of reproduction of a CD. FIG.
47(a) shows a diagram of wavefront aberration viewed under the
state of defocusing at the position where the best-fit wavefront
aberration is obtained in the case of reproduction of a DVD, while,
FIG. 47(b) shows a diagram of wavefront aberration viewed under the
state of defocusing at the position where the best-fit wavefront
aberration is obtained in the case of reproduction of a CD-R. Table
34 shows numerical apertures for NAL and NAH, quantities of
spherical aberration caused, angles each being formed between a
normal line and an optical axis, normal lines and each
condition.
34 TABLE 34 In DVD reproduction In CD-R reproduction Spherical
Spherical Height Numerical aberration Numerical aberration H
aperture (mm) aperture (mm) 1.270 NAL (1) = -0.29200 .times.
10.sup.-3 NAL (1) = 0.15633 .times. 10.sup.-1 0.3780 0.3748 NAL (2)
= -0.11676 .times. 10.sup.-1 NAL (2) = 0.68900 .times. 10.sup.-2
0.3789 0.3758 1.520 NAH (2) = -0.20034 .times. 10.sup.-1 NAH (2) =
0.77675 .times. 10.sup.-2 0.4546 0.4508 NAH (3) = 0.24165 .times.
10.sup.-2 NAH (3) = 0.25251 .times. 10.sup.-1 0.4523 0.4485 0.60
NA2 = 0.60 .times. 0.45 = 0.270 1.3 NA2 = 1.1 .times. 0.45 = 0.495
NAH - NAL = 0.4508 - 0.3758 = 0.0750 -2(.lambda.2)/(NA2).sup.2 = -2
.times. 780 nm/(0.45).sup.2 = -7.70 .mu.m 5(.lambda.2)/(NA2).sup.2
= 5 .times. 780 nm/(0.45).sup.2 = 19.26 .mu.m H2mid = (1.270 +
1.520)/2 = 1.395 .theta.1, 3, mid = 36.54832 .theta.2, mid =
36.68357.degree. .DELTA..theta.mid = 36.68357 - 36.54832 =
0.13525.degree.
[0225] FIG. 48 shows a diagram of relative intensity distribution
of a light-converged spot in the case where the best spot shape is
obtained in reproduction of a DVD, while, FIG. 49 shows a diagram
of relative intensity distribution of a light-converged spot in the
case where the best spot shape is obtained in reproduction of a
CD.
[0226] Even when the objective lens in the present embodiment is
mounted on optical pickup apparatus 10 employing a single light
source (wavelength .lambda.1 of the light source=635 nm),
reproduction was possible not only for DVD but also for CD. FIG. 50
shows a diagram of relative intensity distribution of a
light-converged spot in the case where the best spot shape is
obtained in reproduction of a CD. Numerical apertures for NAL and
NAH, quantities of spherical aberration caused, angles each being
formed between a normal line and an optical axis, normal lines and
each condition in this case are shown in Table 35.
35 TABLE 35 In CD (.kappa.635 nm) In DVD reproduction reproduction
Spherical Spherical Height Numerical aberration Numerical
aberration H aperture (mm) aperture (mm) 1.270 NAL (1) = -0.29200
.times. 10.sup.-3 NAL (1) = 0.14740 .times. 10.sup. 1 0.3780 0.3780
NAL (2) = -0.11676 .times. 10.sup.-1 NAL (2) = 0.60778 .times.
10.sup.-2 0.3789 0.3789 1.520 NAH (2) = -0.20034 .times. 10.sup.-1
NAH (2) = 0.65378 .times. 10.sup.-2 0.4546 0.4546 NAH (3) = 0.24165
.times. 10.sup.-2 NAH (3) = 0.23856 .times. 10.sup.-1 0.4523 0.4523
0.60 NA2 = 0.60 .times. 0.366 = 0.220 1.3 NA2 = 1.3 .times. 0.366 =
0.476 NAH - NAL = 0.4546 - 0.3789 = 0.0757
-2(.lambda.2)/(NA2).sup.2 = -2 .times. 635 nm/(0.366).sup.2 = -9.48
.mu.m 5(.lambda.2)/(NA2).sup.2 = 5 .times. 635 nm/(0.366).sup.2 =
23.7 .mu.m H2mid = (1.270 + 1.520)/2 = 1.395 .theta.1, 3, mid =
36.54832.degree. .theta.2, mid = 36.6835.degree. .DELTA..theta.mid
= 36.68357 - 36.54832 = 0.1352.degree.
EXAMPLE 10
[0227] Example 10 represents an example wherein the invention is
applied to objective lens 116 which is to be mounted on the optical
pickup apparatus 100 (wavelength .lambda.1 of the first light
source=635 nm, and wavelength .lambda.2 of the second light source
=780 nm), and in which the steps are provided on boundaries of the
first divided surface Sd1-the third divided surface Sd3.
[0228] Optical data of the objective lens are shown in Tables 36
and 37.
36 TABLE 36 Wavelength .lambda. 635 nm 780 nm Focal length 3.36 mm
3.39 mm Aperture-stop diameter .phi.404 mm Lateral magnification of
0 objective lens i ri di di' ni ni' 1 .infin. 0.0000 1.0 1.0 2
2.114 2.2000 1.5383 1.5337 3 -7.963 1.757 1.401 1.0 1.0 4 .infin.
0.6000 1.2000 1.58 1.58 5 .infin.
[0229]
37TABLE 37 Aspheric surface data Second First 0 .ltoreq. H
<1.111 (First divided surface) surface aspheric 1.481 .ltoreq. H
(Third divided surface) (refracting surface .kappa. = -0.97770
surface) A1 = 0.63761 .times. 10.sup.-3 P1 = 3.0 A2 = 0.36688
.times. 10.sup.-4 P2 = 4.0 A3 = 0.83511 .times. 10.sup.-2 P3 = 5.0
A4 = -0.37296 .times. 10.sup.-2 P4 = 6.0 A5 = 0.46548 .times.
10.sup.-3 P5 = 8.0 A6 = -0.43124 .times. 10.sup.-4 P6 = 10.0 Second
1.111 .ltoreq. H < 1.481 (Second divided surface) aspheric d2 =
2.1995 surface .kappa. = -0.11481 .times. 10.sup.-1 A1 = 0.70764
.times. 10.sup.-2 P1 = 3.0 A2 = -0.13388 .times. 10.sup.-1 P2 = 4.0
A3 = 0.24084 .times. 10.sup.-1 P3 = 5.0 A4 = -0.97636 .times.
10.sup.-2 P4 = 6.0 A5 = 0.93136 .times. 10.sup.-3 P5 = 8.0 A6 =
-0.68008 .times. 10.sup.-4 P6 = 10.0 Third surface .kappa. =
-0.24914 .times. 10.sup.-2 (refracting surface) A1 = 0.13775
.times. 10.sup.-2 P1 = 3.0 A2 = -0.41269 .times. 10.sup.-2 P2 = 4.0
A3 = 0.21236 .times. 10.sup.-1 P3 = 5.0 A4 = -0.13895 .times.
10.sup.-1 P4 = 6.0 A5 = 0.16631 .times. 10.sup.-2 P5 = 8.0 A6 =
-0.12138 .times. 10.sup.-3 P6 = 10.0
[0230] The value of "d2=2.1995" of the second aspheric surface in
Table 37 represents a distance from an intersecting point between
the second aspheric surface (the second divided surface) extended
to an optical axis in accordance with the expression for an
aspheric surface shape and the optical axis to the third surface.
The symbol ni' in Table 32 represents a refractive index in the
second light source (.lambda.2=780 nm).
[0231] FIG. 51 (a) shows a diagram of spherical aberration in the
case of reproduction of a DVD, while, FIG. 51 (b) shows a diagram
of spherical aberration in the case of reproduction of a CD. FIG.
52 (a) shows a diagram of wavefront aberration viewed under the
state of defocusing at the position where the best-fit wavefront
aberration is obtained in the case of reproduction of a DVD, while,
FIG. 52 (b) shows a diagram of wavefront aberration viewed under
the state of defocusing at the position where the best-fit
wavefront aberration is obtained in the case of reproduction of a
CD. Table 38 shows numerical apertures for NAL and NAH, quantities
of spherical aberration caused, angles each being formed between a
normal line and an optical axis, normal lines and each
condition.
38 TABLE 38 In CD (.kappa.780 nm) In DVD reproduction reproduction
Spherical Spherical Height Numerical aberration Numerical
aberration H aperture (mm) aperture (mm) 1.111 NAL (1) = -0.65069
.times. 10.sup.-3 NAL (1) = 0.13417 .times. 10.sup. 1 0.3307 0.3280
NAL (2) = -0.10281 .times. 10.sup.-1 NAL (2) = 0.37802 .times.
10.sup.-2 0.3317 0.3289 1.481 NAH (2) = -0.17788 .times. 10.sup.-1
NAH (2) = 0.86005 .times. 10.sup.-2 0.4430 0.4393 NAH (3) = 0.48121
.times. 10.sup.-3 NAH (3) = 0.26737 .times. 10.sup.-1 0.4406 0.4370
0.60 NA2 = 0.60 .times. 0.45 = 0.270 1.1 NA2 = 1.1 .times. 0.45 =
0.495 NAH - NAL = 0.4393 - 0.3289 = 0.1104
-2(.lambda.2)/(NA2).sup.2 = -2 .times. 780 nm/(0.45).sup.2 = -7.70
.mu.m 5(.lambda.2)/(NA2).sup.2 = 5 .times. 780 nm/(0.45).sup.2 =
19.26 .mu.m H2mid = (1.111 + 1.481)/2 = 1.296 .theta.1, 3, mid =
34.07635.degree. .theta.2, mid = 34.21711.degree. .DELTA..theta.mid
= 34.21711 - 34.07635 = 0.14076.degree.
[0232] FIG. 53 shows a diagram, of relative intensity distribution
of a light-converged spot in the case where the best spot shape is
obtained in reproduction of a DVD, while, FIG. 54 shows a diagram
of relative intensity distribution of a light-converged spot in the
case where the best spot shape is obtained in reproduction of a
CD.
[0233] Even when the objective lens in the present embodiment is
mounted on optical pickup apparatus 10 employing a single light
source (wavelength .lambda.1 of the light source=635 nm),
reproduction was possible not only for DVD but also for CD. FIG. 55
shows a diagram of relative intensity distribution of a
light-converged spot in the case where the best spot shape is
obtained in reproduction of a CD. Numerical apertures for NAL and
NAH, quantities of spherical aberration caused, angles each being
formed between a normal line and an optical axis, normal lines and
each condition in this case are shown in Table 39.
39 TABLE 39 In CD (.kappa.635 nm) In DVD reproduction reproduction
Spherical Spherical Height Numerical aberration Numerical
aberration H aperture (mm) aperture (mm) 1.111 NAL (1) = -0.65069
.times. 10.sup.-3 NAL (1) = 0.12601 .times. 10.sup. 1 0.3307 0.3307
NAL (2) = -0.10281 .times. 10.sup.-1 NAL (2) = 0.30498 .times.
10.sup.-2 0.3317 0.3317 1.481 NAH (2) = -0.17788 .times. 10.sup.-1
NAH (2) = 0.72752 .times. 10.sup.-2 0.4430 0.4430 NAH (3) = 0.48121
.times. 10.sup.-3 NAH (3) = 0.25244 .times. 10.sup.-1 0.4406 0.4406
0.60 NA2 = 0.60 .times. 0.45 = 0.220 1.1 NA2 = 1.1 .times. 0.45 =
0.476 NAH - NAL = 0.4430 - 0.3317 = 0.1113
-2(.lambda.2)/(NA2).sup.2 = -2 .times. 635 nm/(0.366).sup.2 = -9.48
.mu.m 5(.lambda.2)/(NA2).sup.2 = 5 .times. 635 nm/(0.366).sup.2 =
23.7 .mu.m H2mid = (1.111 + 1.481)/2 = 1.296 .theta.1, 3, mid =
34.07635.degree. .theta.2, mid = 34.21711.degree. .DELTA..theta.mid
= 34.21711 - 34.07635 = 0.14076.degree.
[0234] Examples 1-10 stated above show that two optical disks each
being different in terms of a thickness of a transparent substrate
were reproduced satisfactorily by a single light-converging optical
system (a single objective lens in the system). There was not
problem even in the case of recording. In Examples 2 and 4 through
10, in particular, it was possible to reproduce a DVD representing
the first optical disk and a CD-R representing the second optical
disk (requiring the wavelength of a light source of 780 nm), by
using two light sources. Further, in these Examples 2 and 4 through
10, it was possible to reproduce a DVD and a CD satisfactorily by
the use of a single light source. In addition, Examples 5-7 were
capable of handling the second optical disk with necessary
numerical aperture NA that is as high as 0.5, and of being used for
recording a CD-R.
[0235] In Examples 1, 3, and 8-10 among Examples 1-10, the
reproduction signals of the second optical disk having a 1.2
mm-thick transparent substrate were excellent. The reason for the
foregoing is that the best-fit wavefront aberration of the light
flux passing through the first divided surface (that is called an
amount of wavefront aberration in the first divided surface)
satisfies 0.07 .lambda. which is the diffraction limited
performance as shown in Table 40.
40TABLE 40 Thickness of a transparent substrate of an optical
information recording medium 1.2 (mm) Amount of wavefront
aberration in Example No. the first divided surface Light source
wavelength .lambda. = 635 (nm) 1. 0.063 (.lambda.rms) 2. 0.097
(.lambda.rms) 3. 0.063 (.lambda.rms) 4. 0.090 (.lambda.rms) 5.
0.143 (.lambda.rms) 6. 0.126 (.lambda.rms) 7. 0.126 (.lambda.rms)
8. 0.054 (.lambda.rms) 9. 0.047 (.lambda.rms) 10. 0.025
(.lambda.rms) Light source wavelength .lambda. = 780 (nm) 2. 0.083
(.lambda.rms) 4. 0.078 (.lambda.rms) 5. 0.123 (.lambda.rms) 6.
0.108 (.lambda.rms) 7. 0.108 (.lambda.rms) 8. 0.047 (.lambda.rms)
9. 0.040 (.lambda.rms) 10. 0.022 (.lambda.rms)
[0236] In Table 36, an amount of wavefront aberration in the first
divided surface in the case of reproducing the second optical disk
having a 1.2 mm-thick transparent substrate under the light source
wavelength .lambda. of 635 mm is shown on the upper portion of the
table, while in Examples 2 and 4 through 9, an amount of wavefront
aberration in the first divided surface in the case of reproducing
the second optical disk having a 1.2 mm-thick transparent substrate
under the light source wavelength .lambda. of 780 nm is shown on
the lower portion of the table, because two light sources are
used.
[0237] When assuming that "n" represents a natural number in
Example 1-10 stated above, Table 41 shows a value of (.DELTA.nL)
.pi. (e.g., (.DELTA.1L) .pi. or (.DELTA.2L) .pi.) (rad) which is a
phase difference between light passing through the (2n-1 )th
divided surface (e.g., the first divided surface Sd1 or the third
divided surface Sd3) and (emitted from the transparent substrate)
and light-passing through the almost center and the portion closer
to the optical axis than the center on the (2n)th divided surface
(e.g., the second divided surface Sd2 or the fourth divided surface
Sd4 and that of (.DELTA.nH) .pi. (e.g., (.DELTA.1H) .pi. or
(.DELTA.2H) .pi.) (rad) which is a phase difference between light
passing through the (2n+1)th divided surface (e.g., the third
divided surface Sd3 or the fifth divided surface Sd5) and (emitted
from the transparent substrate) and light passing through the
portion farther from the optical axis than the center on the (2n)th
divided surface (e.g., the second divided surface Sd2 or the fourth
divided surface Sd4) and (emitted from the transparent substrate).
In this case, with regard to the sign of the phase difference, the
direction of light advancement (the direction toward the optical
disk) is positive, and a phase difference between light passing
through the (2n-1)th divided surface or the (2n+1)th divided
surface and (emitted from the transparent substrate) and light
passing through the (2n)th divided surface and (emitted from the
transparent substrate) is compared.
41TABLE 37 Thickness of a transparent substrate of an optical
information recording medium 0.6 (mm) Light source wavelength
.lambda. = 635 (nm) Example No. (.DELTA.1H).pi.(rad)
(.DELTA.1L).pi.(rad) (.DELTA.2H).pi.(rad) (.DELTA.2L).pi.(rad) 1.
1.64.pi. 1.19.pi. -- -- 2. 4.67.pi. 3.36.pi. -- -- 3. 0.53.pi.
0.00.pi. -- -- 4. 4.24.pi. 3.46.pi. -- -- 5. 6.27.pi. 3.93.pi. --
-- 6. 3.93.pi. 3.35.pi. 6.76.pi. 5.80.pi. 7. 4.23.pi. 3.65.pi.
6.33.pi. 5.36.pi. 8. 4.39.pi. 2.83.pi. -- -- 9. 1.50.pi. 0.33.pi.
-- -- 10. 3.86.pi. 2.17.pi. -- --
[0238] As is apparent from the table above, the condition of
(.DELTA.nH)>(.DELTA.nL) is satisfied in all of the Examples
1-10. Each value in Table 41 shows a phase difference of a light
flux entering each divided surface on each of boundaries of divided
surfaces Sd1-Sd3 (or Sd5)
[0239] As stated above, recording and reproduction of plural
optical information recording media can be conducted by a single
light-converging optical system in the invention, which therefore
realizes low cost without complicating a matter, and makes it
possible to handle optical information recording media each having
a high NA. In addition, in the invention, generation of spherical
aberration is utilized positively, and recording and reproduction
of plural optical information recording media can therefore be
conducted by a single light-converging optical system.
* * * * *