U.S. patent application number 14/273545 was filed with the patent office on 2014-09-04 for objective lens element.
This patent application is currently assigned to Panasonic Corporation. The applicant listed for this patent is Panasonic Corporation. Invention is credited to Katsuhiko HAYASHI, Kazuhiko ISHIMARU, Yoshiaki KOMMA, Yasuhiro TANAKA, Michihiro YAMAGATA.
Application Number | 20140247493 14/273545 |
Document ID | / |
Family ID | 43758403 |
Filed Date | 2014-09-04 |
United States Patent
Application |
20140247493 |
Kind Code |
A1 |
HAYASHI; Katsuhiko ; et
al. |
September 4, 2014 |
OBJECTIVE LENS ELEMENT
Abstract
An objective lens element which has excellent compatibility with
optical discs having different base material thicknesses is
provided. An objective lens element 141 has optically functional
surfaces on an incident side and an exit side. Either one of the
optically functional surfaces is divided into an inner part 131B
including a rotational symmetry axis and an outer part 131F which
is a ring-shaped region surrounding the inner part 131B. On the
inner part 131B, a plurality of discontinuous steps are provided.
The plurality of steps change in height in the same direction from
the optical axis toward the outer part, and each of the steps
causes a constant optical path difference longer than the
wavelength .lamda..sub.1 to the first incident light beam 61 and
causes a constant optical path difference shorter than the
wavelength .lamda..sub.2 to the second incident light beam 62.
Inventors: |
HAYASHI; Katsuhiko; (Nara,
JP) ; TANAKA; Yasuhiro; (Hyogo, JP) ;
YAMAGATA; Michihiro; (Osaka, JP) ; KOMMA;
Yoshiaki; (Osaka, JP) ; ISHIMARU; Kazuhiko;
(Osaka, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Panasonic Corporation |
Osaka |
|
JP |
|
|
Assignee: |
Panasonic Corporation
Osaka
JP
|
Family ID: |
43758403 |
Appl. No.: |
14/273545 |
Filed: |
May 9, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13414724 |
Mar 8, 2012 |
8760989 |
|
|
14273545 |
|
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|
|
PCT/JP2010/005694 |
Sep 17, 2010 |
|
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13414724 |
|
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Current U.S.
Class: |
359/571 ;
359/719; 359/742 |
Current CPC
Class: |
G02B 3/08 20130101; G11B
7/1374 20130101; G11B 7/1353 20130101; G11B 7/13922 20130101; G11B
7/1367 20130101; G11B 2007/0006 20130101; G02B 5/1876 20130101;
G02B 2005/1804 20130101; G11B 7/139 20130101 |
Class at
Publication: |
359/571 ;
359/742; 359/719 |
International
Class: |
G02B 3/08 20060101
G02B003/08; G02B 5/18 20060101 G02B005/18 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 17, 2009 |
JP |
2009-216223 |
Claims
1. An objective lens element which has optically functional
surfaces on an incident side and an exit side, which converges a
first incident light beam of a wavelength .lamda..sub.1 through a
base plate having a thickness t.sub.1 to form a spot, and which
converges a second incident light beam of a wavelength
.lamda..sub.2 longer than the wavelength .lamda..sub.1 through a
base plate having a thickness t.sub.2 larger than the thickness
t.sub.1 to form a spot, wherein at least either one of the optical
function surfaces is a refractive surface which deflects the first
and second incident light beams by refractive power all over the
surface, the at least either one of the optical function surfaces
being divided into an inner part which includes a rotational
symmetry axis and through which the first and second incident light
beams that substantially contribute to spot formation pass, and an
outer part which is a ring-shaped region surrounding the inner part
and through which only the first incident light beam that
substantially contributes to spot formation passes, and the at
least either one of the optical function surfaces having a
plurality of discontinuous steps on the inner part, and the
plurality of steps change in height in the same direction from the
optical axis toward the outer part, and each of the steps causes a
constant optical path difference longer than the wavelength
.lamda..sub.1 to the first incident light beam and causes a
constant optical path difference shorter than the wavelength
.lamda..sub.2 to the second incident light beam.
2. The objective lens element according to claim 1, wherein each of
the steps provides an optical path difference of about 1.25
wavelengths to light of the wavelength .lamda..sub.1 and provides
an optical path difference of about 0.75 wavelength to light of the
wavelength .lamda..sub.2.
3. The objective lens element according to claim 1, wherein the
outer part has an aspherical shape in which no steps are
formed.
4. The objective lens element according to claim 1, wherein a
diffraction grating having a sawtooth shape is formed on the outer
part, and the sawtooth shape has such a height that an optical path
difference of about 1 wavelength is provided to light of the
wavelength .lamda..sub.1.
5. The objective lens element according to claim 1, wherein steps
each having such a height that an optical path difference which is
substantially an integral multiple of wavelength .lamda..sub.1 is
provided to light of the wavelength .lamda..sub.1 are formed on the
outer part.
6. The objective lens element according to claim 5, wherein each
step on the outermost portion of the outer part has such a height
that an optical path difference which is substantially N times (N
is an integer other than multiples of 5) of wavelength
.lamda..sub.1 is provided to the light of the wavelength
.lamda..sub.1.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a U.S. continuation application filed
under 35 USC 111A claiming benefit under 35 USC 120 and 365C of PCT
application JP2010/005694, filed Sep. 17, 2010, which claims
priority to Japanese Patent Application No. 2009-216223, filed on
Sep. 17, 2009. The foregoing applications are hereby incorporated
herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to an objective lens element
for use in an optical head device that performs at least one of
recording, reproducing, and erasing of information on an optical
information storage medium.
[0004] 2. Description of the Background Art
[0005] As media that record a large amount of information with a
high density, there are optical information storage media such as
optical discs. Optical discs record information as pit-shaped
patterns thereon, and are widely used for the purposes of recording
digital audio files, video files, document files, and data files.
Examples of functions required for performing recording,
reproducing, and erasing of information on an optical disc with
high reliability by using a light beam converged on a micro spot
are a converging function to form a diffraction-limited micro spot,
focus control (focus servo) of an optical system, tracking control,
and pit signal (information signal) detection.
[0006] In recent years, due to advancement of optical system design
technology and shortening of the wavelengths of semiconductor
lasers which are light sources, development has progressed
concerning optical discs that have a higher-density storage
capacity further than ever. One approach to density increase is to
increase the optical disc-side numerical aperture (NA) of a
light-converging optical system which converges a light beam to
form a micro spot on the optical disc. However, when the NA of the
light-converging optical system is increased, an amount of a
generated aberration increases with respect to a certain amount of
tilt of the optical axis. In order to prevent this problem, it is
necessary to decrease the thickness of a layer (hereinafter,
referred to as "base material thickness") provided on a recording
surface of the optical disc. In the present specification, the
"base material thickness" means a thickness from a light beam
incident surface to an information recording surface of an optical
disc.
[0007] For compact discs (CD) which are first generation optical
discs, infrared light (a wavelength .lamda..sub.3: 780 to 820 nm)
and an objective lens having an NA of 0.45 are used. The base
material thickness of CD is 1.2 mm. For DVD which is second
generation, red light (a wavelength .lamda..sub.2: 630 to 680 nm)
and an objective lens having an NA of 0.6 are used. The base
material thickness of DVD is 0.6 mm. For third generation optical
discs, blue light (a wavelength .lamda..sub.1: 390 to 415 nm) and
an objective lens having an NA of 0.85 are used. The base material
thickness of third generation optical discs is 0.1 mm. As described
above, as the recording density increases, the base material
thickness of the optical disc decreases.
[0008] In view of economical efficiency and space occupied by an
apparatus, an optical information recording/reproducing apparatus
is desired which can perform recording and reproducing on optical
discs having different base material thicknesses and recording
densities. For this, a light-converging optical system which can
converge a light beam to a diffraction limit on a recording surface
of each of optical discs having different base material
thicknesses, and an optical head device including this
light-converging optical system, are necessary. In addition, when
recording and reproducing are performed on an optical disc having a
large base material thickness, it is necessary to converge a light
beam on a recording surface located deeper than a beam incident
surface of the optical disc, and thus the focal length has to be
increased.
[0009] Prior art documents disclose configurations intended for
compatible reproducing and compatible recording on an optical disc
having a base material thickness of 0.6 mm and compatible with the
wavelength .lamda..sub.2 (red light) and on an optical disc having
a base material thickness of 0.1 mm and compatible with the
wavelength .lamda..sub.1 (blue light).
[0010] A first prior art example is a configuration in which a
wavelength-selective phase plate is combined with an objective
lens. This is disclosed in Japanese Laid-Open Patent Publication
No. 10-334504 and the Proceedings of ISOM2001 (Session We-C-05),
P30. The configuration disclosed in the Proceedings of ISOM2001
(Session We-C-05), P30 will be described with reference to FIGS. 21
and 22. FIG. 21 illustrates a schematic configuration of an optical
head device. Parallel light emitted from a blue optical system 51
including a blue light source of a wavelength .lamda..sub.1 (405
nm) passes through a beam splitter 161 and a wavelength selection
phase plate 205 and is converged by an objective lens 50 on an
information recording surface of an optical disc 10 (third
generation optical disc) having a base material thickness of 0.1
mm. The light reflected by the optical disc 10 travels along the
reverse path and is detected with a detector of the blue optical
system 51. Diverging light emitted from a red optical system 52
including a red light source of a wavelength .lamda..sub.2 (650 nm)
is reflected by the beam splitter 161, passes through the
wavelength selection phase plate 205, and is converged by the
objective lens 50 on an information recording surface of an optical
disc 10 (second generation optical disc: DVD) having a base
material thickness of 0.6 mm. The light reflected by the optical
disc 10 travels along the reverse path and is detected with a
detector of the red optical system 52.
[0011] The objective lens 50 is designed such that when the
parallel light of the wavelength .lamda..sub.1 is incident thereon,
a convergence spot is formed at a position where the light has
passed through a protective layer having a base material thickness
of 0.1 mm. When recording and reproducing are performed on DVD, a
spherical aberration occurs due to a difference in base material
thickness. In order to compensate the spherical aberration, the
light beam emitted from the red optical system 52 is adjusted to be
diverging light, and the wavelength selection phase plate 205 is
used. When light incident on the objective lens is adjusted to be
diverging light, a new spherical aberration occurs. Thus, the
spherical aberration occurring due to the difference in base
material thickness can be cancelled by this new spherical
aberration. Further, the wavefront is corrected also by the
wavelength selection phase plate 205.
[0012] FIGS. 22A and 22B are a plan view and a cross-sectional view
of the wavelength selection phase plate 205. The phase plate 205
has phase steps 205a of heights h and 3h. Here, the refractive
index with respect to the wavelength .lamda..sub.1 is n.sub.1, and
h=.lamda..sub.1/(n.sub.1-1). When the light of the wavelength
.lamda..sub.1 is used, an optical path difference caused by the
phase step of the height h is the used wavelength .lamda..sub.1 and
corresponds to a phase difference of 2.pi.. Thus, the optical path
difference is the same as a phase difference of 0. Therefore, the
phase steps 205a do not influence the phase distribution of the
light of the wavelength .lamda..sub.1 and hence do not influence
recording and reproducing on the optical disc 10. Meanwhile, when
the light of the wavelength .lamda..sub.2 is used, designing is
performed such that an optical path difference (h.times.(n.sub.2-1)
which is provided by the step to the light of the wavelength
.lamda..sub.2 has a value other than an integral multiple of the
wavelength (e.g., 0.6 times). By utilizing the phase difference
caused by the optical path difference, the aberration compensation
described above is performed.
[0013] As a second prior art example, a configuration in which a
refraction type objective lens and a diffraction element are
combined is disclosed. In Japanese Laid-Open Patent Publication No.
2004-071134, in an optical head device which performs recording or
reproducing on a high-density optical disc by using an objective
lens having a high NA, a sawtooth-like diffraction element is used
in order to be able to also perform recording or reproducing on
conventional optical discs such as DVD. The sawtooth height is set
such that when blue light is used, the length of the optical path
becomes 2.lamda., and 2nd order diffracted light is used. The
sawtooth-like diffraction element emits 1st order diffracted light
when red light is incident thereon. The blaze direction is as in a
convex lens type, and chromatic aberration compensation of the
refractive lens is performed. The diffraction order when red light
is used is lower than the diffraction order when blue light is
used. Thus, the sawtooth-like diffraction element serves as a
concave lens for red light, thereby providing an effect that the
working distance can be increased.
[0014] Further, Japanese Laid-Open Patent Publication No.
2004-071134 also discloses a diffraction element shown in FIG. 23A.
The diffraction element shown in FIG. 23A has a stair-like
cross-sectional shape and is composed of consecutive unit steps
whose number is an integer number. Each unit step provides an
optical path difference of about 1.25 wavelengths to a light beam
of the wavelength .lamda..sub.1. Specifically, the wavelength
.lamda..sub.1 is 390 to 415 nm, and a stair shape is provided in
which one cycle consists of steps which are 0 times, 1 times, 2
times, and 3 times that of the unit step in height from the outer
side of the diffraction element toward the optical axis. With
respect to blue light, as shown in FIG. 23B, the phase changes in
the same direction as that of the stair shape and a convex lens
effect is exerted. With respect to red light, as shown in FIG. 23C,
the phase changes in the direction opposite to that of the stair
shape and a concave lens effect is exerted. Thus, when blue light
is used, a chromatic aberration compensation effect of the
refractive lens is obtained. In addition, when red light is used,
an effect that the working distance (the interval between the
objective lens surface and the surface of an optical disc) can be
increased is obtained due to the concave lens effect.
[0015] As a third conventional art example, a configuration in
which a relay lens is inserted between an infrared light source and
an objective lens, thereby also realizing compatibility with a
first generation optical disc having a base material thickness of
1.2 mm, is disclosed in Japanese Laid-Open Patent Publication No.
2004-281034.
[0016] Japanese Laid-Open Patent Publication Nos. 10-334504 and
2004-071134 merely disclose the method for compatibility with the
above second generation optical discs and the above third
generation optical discs. In addition, Japanese Laid-Open Patent
Publication No. 2004-281034 discloses the method for compatibility
with the above first generation optical discs, but requires a relay
lens.
[0017] Further, it is desired that an element that realizes
compatibility is integrally formed on the objective lens surface,
in view of cost reduction by decrease in number of parts. However,
in the conventional art described above, only the exemplary
configuration, in which the phase plate or the diffraction element
is provided independently of the refraction type objective lens, is
disclosed, and there is no description about integrally forming an
element, which realizes compatibility, on the objective lens
surface.
[0018] Moreover, in order to produce objective lenses at low cost
and in large quantities, the material of the objective lenses is
preferably resin rather than glass. In general, the material cost
of resin is low, and it is also possible to mold resin at a lower
temperature than to mold glass. Thus, the mold can be used long and
the molding time can be shortened. Therefore, by molding resin to
produce objective lenses, the manufacturing cost can be reduced.
However, the refractive index of a high-NA objective lens made of
resin changes due to temperature change. The refractive index
change causes the refractive power of the lens surface to shift
from a designed value, whereby a spherical aberration occurs. A
lower-order aberration greatly deteriorates the quality of an
information reproduction signal, and thus a 3rd order spherical
aberration is problematic.
SUMMARY OF THE INVENTION
[0019] It is an object of the present invention to provide an
objective lens element having excellent compatibility with optical
discs having different base material thicknesses. In addition, it
is another object of the present invention to provide an objective
lens element whose change in spherical aberration amount is small
even when the environmental temperature changes.
[0020] The present invention is directed to an objective lens
element which has optically functional surfaces on an incident side
and an exit side, which converges a first incident light beam of a
wavelength .lamda..sub.1 through a base plate having a thickness
t.sub.1 to form a spot, and which converges a second incident light
beam of a wavelength .lamda..sub.2 longer than the wavelength
.lamda..sub.1 through a base plate having a thickness t.sub.2
larger than the thickness t.sub.1 to form a spot.
[0021] In the objective lens element, at least either one of the
optical function surfaces is a refractive surface which deflects
the first and second incident light beams by refractive power all
over the surface, is divided into an inner part which includes a
rotational symmetry axis and through which the first and second
incident light beams that substantially contribute to spot
formation pass, and an outer part which is a ring-shaped region
surrounding the inner part and through which only the first
incident light beam that substantially contributes to spot
formation passes, and has a plurality of discontinuous steps on the
inner part. The plurality of steps change in height in the same
direction from the optical axis toward the outer part, and each of
the steps causes a constant optical path difference longer than the
wavelength .lamda..sub.1 to the first incident light beam and
causes a constant optical path difference shorter than the
wavelength .lamda..sub.2 to the second incident light beam.
[0022] Alternatively, in the objective lens element, at least
either one of the optical function surfaces is a refractive surface
which deflects the first and second incident light beams by
refractive power all over the surface, is divided into an inner
part which includes a rotational symmetry axis and through which
the first and second incident light beams that substantially
contribute to spot formation pass, and an outer part which is a
ring-shaped region surrounding the inner part and through which
only the first incident light beam that substantially contributes
to spot formation passes, and has a plurality of discontinuous
steps. Each of the plurality of steps causes an optical path
difference which is an integral multiple of the wavelength
.lamda..sub.1, to the first incident light beam at a predetermined
environmental temperature. When an environmental temperature
changes, tendency of spherical aberration change caused by change
in shape of the refractive surface and tendency of spherical
aberration change caused by change in optical path difference
provided by the steps which is caused by change in refractive index
are opposite to each other.
[0023] According to the present invention, by the steps provided on
the optically functional surface, converging light of the
wavelength .lamda..sub.1 is further converged and converging light
of the wavelength .lamda..sub.2 is diffused. Thus, the focal point
of the objective lens element can be changed so as to correspond to
each of the base material thickness t.sub.1 and t.sub.2. In
addition, an objective lens element whose change in spherical
aberration amount is small even when the environmental temperature
changes while light of the wavelength .lamda..sub.1 is used can be
realized.
[0024] These and other objects, features, aspects and advantages of
the present invention will become more apparent from the following
detailed description of the present invention when taken in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1A is a diagram illustrating steps formed on a surface
of an objective lens element according to Embodiment 1;
[0026] FIG. 1B is a diagram illustrating steps formed on a surface
of an objective lens element according to Embodiment 1;
[0027] FIG. 1C is a diagram illustrating steps formed on a surface
of an objective lens element according to Embodiment 1;
[0028] FIG. 2 is a diagram illustrating the objective lens element
according to Embodiment 1;
[0029] FIG. 3 is a diagram illustrating a mold used for
manufacturing the objective lens element shown in FIG. 2 and a
method for manufacturing the mold;
[0030] FIG. 4 is a diagram illustrating an example of a step
structure formed on an outer part;
[0031] FIG. 5A is a diagram illustrating an example of a
diffraction grating formed on the outer part;
[0032] FIG. 5B is a diagram illustrating an example of a
diffraction grating formed on the outer part;
[0033] FIG. 5C is a diagram illustrating an example of a
diffraction grating formed on the outer part;
[0034] FIG. 6 is a diagram illustrating steps formed on a surface
of an objective lens element according to Embodiment 2;
[0035] FIG. 7 is a diagram illustrating steps formed on a surface
of an objective lens element according to Embodiment 3;
[0036] FIG. 8 is a diagram illustrating the objective lens element
according to Embodiment 3;
[0037] FIG. 9A is a diagram illustrating steps formed on a surface
of an objective lens element according to Embodiment 4;
[0038] FIG. 9B is a diagram illustrating steps formed on a surface
of an objective lens element according to Embodiment 4;
[0039] FIG. 9C is a diagram illustrating steps formed on a surface
of an objective lens element according to Embodiment 4;
[0040] FIG. 9D is a diagram illustrating steps formed on a surface
of an objective lens element according to Embodiment 4;
[0041] FIG. 10 is a diagram illustrating the objective lens element
according to Embodiment 4;
[0042] FIG. 11 is a diagram illustrating steps formed on a surface
of an objective lens element according to Embodiment 5;
[0043] FIG. 12 is a diagram illustrating the objective lens element
according to Embodiment 5;
[0044] FIG. 13 is a diagram when the objective lens shown in FIG.
12 is viewed from below;
[0045] FIG. 14 is a diagram illustrating an optical head device
according to Embodiment 6;
[0046] FIG. 15 is a diagram illustrating an optical information
apparatus according to Embodiment 7;
[0047] FIG. 16 is a diagram illustrating a computer according to
Embodiment 8;
[0048] FIG. 17 is a diagram illustrating an optical disc player
according to Embodiment 9;
[0049] FIG. 18 is a diagram illustrating an optical disc recorder
according to Embodiment 10;
[0050] FIG. 19 is a diagram illustrating an information server
according to Embodiment 11;
[0051] FIG. 20 is a diagram illustrating a vehicle according to
Embodiment 12;
[0052] FIG. 21 is a diagram illustrating a schematic configuration
of a conventional optical head device;
[0053] FIG. 22A is a plan view of an optical element shown in FIG.
21;
[0054] FIG. 22B is a cross-sectional view of the optical element
shown in FIG. 21;
[0055] FIG. 23A is a diagram illustrating a step structure provided
on a surface of an objective lens element;
[0056] FIG. 23B is a diagram illustrating a step structure provided
on a surface of an objective lens element; and
[0057] FIG. 23C is a diagram illustrating a step structure provided
on a surface of an objective lens element.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Embodiment 1
Compatibility with BD and DVD
[0058] FIG. 1 is a diagram illustrating steps formed on a surface
of an objective lens element according to Embodiment 1. FIG. 1A is
a diagram illustrating the physical shape of the steps. It should
be noted that in reality, the steps are formed on a base surface
which is a refractive surface (aspherical shape) in the surface of
the objective lens element. FIG. 1B shows an amount of phase change
provided by each step shown in FIG. 1A to light of a wavelength
.lamda..sub.1, and FIG. 1C shows an amount of phase change provided
by each step shown in FIG. 1A to light of a wavelength
.lamda..sub.2 (.lamda..sub.2>.lamda..sub.1). As an example, the
light of the wavelength .lamda..sub.1 is blue light (390 to 415 nm)
for BD, and the light of the wavelength .lamda..sub.2 is red light
(630 to 680 nm) for DVD.
[0059] In FIG. 1A, the vertical direction indicates the thickness
of the lens material in the optical axis direction, or the height
thereof. For example, when a polyolefin resin is used as the
material of the objective lens element, an element material having
a refractive index n.sub.b of about 1.522 with respect to first
incident light can be used. A step d.sub.1 (hereinafter, also
referred to as "unit step") is dimensioned so as to cause an
optical path difference of about 1.25 wavelengths, or a phase
difference of about (2.pi.+.pi./2), to the light of the wavelength
.lamda..sub.1. The unit step d.sub.1 can be represented by the
following equation.
d.sub.1=.lamda..sub.1/(n.sub.b-1).times.1.25
[0060] It should be noted that the "optical path difference" means
the difference between the length of an optical path in the case
where the steps are present (the medium of the step portion is the
lens material) and the length of an optical path in the case where
no steps are present (the medium of the step portion is air).
[0061] Where the unit step is d.sub.1, the height (level) from the
base surface is an integral multiple of d.sub.1. Thus, an amount of
phase change provided by the step shape to the light of the
wavelength .lamda..sub.1 is an integral multiple of (2.pi.+.pi./2).
This substantially means that the amount of phase change changes by
.pi./2 each time the height from the base surface increases by one
step (d.sub.1) as shown in FIG. 1B.
[0062] Among lens materials composed of a polyolefin resin, a lens
material having a refractive index n.sub.r of about 1.505 with
respect to the light of the wavelength .lamda..sub.2 is available.
The optical path difference provided by the step d.sub.1 to the
light of the wavelength .lamda..sub.2 can be represented by
d.sub.1.times.(n.sub.r-1). Where .lamda..sub.1 is 405 nm,
.lamda..sub.2 is 650 nm, n.sub.b is 1.522, and n.sub.r is 1.505,
the optical path difference corresponds to about 0.75 wavelength,
and this means that the amount of phase change changes by -.pi./2
each time the height from the base surface increases by one step
(d.sub.1).
[0063] When the height of each step structure from the base surface
is an integral multiple of d.sub.1 and a stair-like cross-sectional
shape is provided as shown in FIG. 1A, the amount of phase change
provided to the light of the wavelength .lamda..sub.1 changes by
.pi./2 per step as shown in FIG. 1B. In other words, the optical
path difference changes toward the optical axis of the objective
lens element in steps of +1/4 wavelength.
[0064] Meanwhile, as shown in FIG. 1C, the amount of phase change
provided to the light of the wavelength .lamda..sub.2 changes by
-.pi./2 per step. In other words, the optical path difference
changes toward the optical axis of the objective lens element in
steps of -1/4 wavelength. The amount of phase change is positive
when the light of the wavelength .lamda..sub.1 is used, while the
amount of phase change is negative when the light of the wavelength
.lamda..sub.2 is used. This means that the light of the wavelength
.lamda..sub.1 and the light of the wavelength .lamda..sub.2 are
subjected to the opposite actions from the steps.
[0065] Here, when the steps are formed such that the height from
the base surface increases toward the inner side of the objective
lens element as shown in FIG. 1A, the optical path of the light of
the wavelength .lamda..sub.1 is lengthened toward the inner side.
On the other hand, the optical path of the light of the wavelength
.lamda..sub.2 is shortened toward the inner side. In other words,
when the intervals at which the phase steps are formed are
appropriately set and the objective lens element is configured to
exert a convex lens effect on the first incident light, a concave
lens effect is exerted on second incident light. Then, the focal
point of the first incident light gets close to the objective lens
element, while the focal point of the second incident light moves
away from the objective lens element. Thus, an effect is obtained
that the second incident light can be converged on an information
recording surface through a thicker base material.
[0066] In the present embodiment, the steps shown in FIG. 1A are
formed in a stair shape in which the height monotonically increases
or decreases. Thus, the step structure in the present embodiment
does not serve as a diffraction element having a periodic structure
in which the height increases and decreases every certain cycle
(see FIG. 23A), but serves as an element which changes a phase in a
stair-like manner.
[0067] FIG. 2 is a diagram illustrating the objective lens element
according to Embodiment 1.
[0068] On an optically functional surface of the objective lens
element 141 on an incident side, an inner part 131B including a
rotational symmetry axis and a ring-shaped outer part 131F
surrounding the inner part 131B are provided. When the light of the
wavelength .lamda..sub.1 is used, the objective lens element 141
converges first incident light 61 incident on both the inner part
131B and the outer part 131F, and forms a spot on an information
recording surface 91 of an optical disc 9 through a base material
having a thickness t.sub.1. In addition, when the light of the
wavelength .lamda..sub.2 is used, the objective lens element 141
converges second incident light 62 incident on the inner part 131B,
and forms a spot on an information recording surface 101 of an
optical disc 10 through a base material having a thickness t.sub.2.
The outer part 131F is a region which substantially does not
contribute to spot formation when the light of the wavelength
.lamda..sub.2 is used. Where the numerical aperture defined when
the first incident light 61 is converged on the information
recording surface 91 of the optical disc 9 is NA.sub.1 and the
numerical aperture defined when the second incident light 62 is
converged on the information recording surface 101 of the optical
disc 10 is NA.sub.2, NA.sub.1 is equal to or higher than 0.85 and
NA.sub.2 is equal to or higher than 0.6.
[0069] In other words, the inner part 131B is a region shared by
the light of the wavelength .lamda..sub.1 and the light of the
wavelength .lamda..sub.2, and the outer part 131F is a region
dedicated for the light of the wavelength .lamda..sub.1. On the
inner part 131B, the stair-like step structure shown in FIG. 1A is
formed. The outer part 131F can be configured as in any of (1) to
(4) below.
[0070] (1) On the outer part 131F as well, the same step structure
as that formed on the inner part 131B is formed. In this case, the
outer part 131F suffices to be designed such that when the light of
the wavelength .lamda..sub.1 is used, the light incident on the
outer part 131F is converged on the information recording surface
101 through the base material having the thickness t.sub.1, and
when the light of the wavelength .lamda..sub.2 is used, the light
incident on the outer part 131F generates a great aberration at a
position where the light has passed through the base material
having the thickness t.sub.2 and is substantially not converged on
the information recording surface 91.
[0071] (2) Alternatively, no steps may be formed on the outer part
131F, and the outer part 131F may be designed such that when the
light of the wavelength .lamda..sub.1 is used, the light incident
on the outer part 131F is converged on the information recording
surface 101 through the base material having the thickness t.sub.1,
and when the light of the wavelength .lamda..sub.2 is used, the
light incident on the outer part 131F generates a great aberration
at a position where the light has passed through the base material
having the thickness t.sub.2 and is substantially not converged on
the information recording surface 91.
[0072] (3) Still alternatively, steps each of which causes an
optical path difference of 1 wavelength to the first incident light
of the wavelength .lamda..sub.1 may be formed on the outer part
131F. In this case, a phase difference substantially does not occur
in the light of the wavelength .lamda..sub.1 having passed through
the outer part 131F, and thus the light of the wavelength
.lamda..sub.1 incident on the outer part 131F is converged on the
information recording surface 101 through the base material having
the thickness t.sub.1. Meanwhile, each of these steps provides a
phase difference which is about 0.6 times of the wavelength
.lamda..sub.2, to the light of the wavelength .lamda..sub.2. Thus,
a great aberration occurs and the light of the wavelength
.lamda..sub.2 incident on the outer part 131F is substantially not
converged on the information recording surface 91.
[0073] (4) Still alternatively, a sawtooth-like diffraction grating
may be formed on the outer part 131F, and designing may be
performed such that the light of the wavelength .lamda..sub.1
incident on the outer part 131F is converged on the information
recording surface 101 through the base material having the
thickness t.sub.1 and the light of the wavelength .lamda..sub.2
incident on the outer part 131F is substantially not converged on
the information recording surface 91. In this case, the sawtooth
height is preferably set to such a height that an optical path
difference of 1 wavelength is caused to the light of the wavelength
.lamda..sub.1 as shown in FIG. 5A. When the sawtooth height is set
to a height equal to 1 wavelength of the light of .lamda..sub.1 as
shown in FIG. 5B, the diffraction efficiency of the 1st order
diffracted light increases. Thus, the light of the wavelength
.lamda..sub.1 incident on the outer part 131F can be converged
through the base material having the thickness t.sub.1. Meanwhile,
with respect to the light of the wavelength .lamda..sub.2, the
sawtooth height is only about 0.6 times of wavelength as shown in
FIG. 5C. Thus, the light of the wavelength .lamda..sub.2 incident
on the outer part 131F is diffracted into 1st order diffracted
light and zero order diffracted light and is not converged on a
point, and the outer part 131F exerts an aperture limiting
function.
[0074] As in the present embodiment, a monotonically-increasing
type or monotonically decreasing type stair-like step structure is
suitable to be integrally formed directly on the surface of the
objective lens element 141. The reason will be described with
reference to FIG. 3.
[0075] FIG. 3 is a diagram illustrating a mold used for
manufacturing the objective lens element 141 shown in FIG. 2 and a
method for manufacturing the mold.
[0076] A concavity of the mold 1401 shown in FIG. 3 is a surface
for shaping the incident-side surface of the objective lens element
141 shown in FIG. 2. In order to integrally form the above steps on
the surface of the objective lens element 141, ring-shaped recessed
and raised portions corresponding to the shape of the steps are
formed on the concavity of the mold 1401. When the height of the
steps increases toward the inner side as shown in FIG. 1A, the
ring-shaped recessed portions formed on the concavity of the mold
1401 suffices to be deeper on the inner side. For this mold shape,
machining is possible by cutting with a cutter 1402. Cutting is
performed with rotation of the mold 1401 about a central axis 1401C
of the mold 1401, to form concentric ring zones on the concavity of
the mold 1401.
[0077] The cutter 1402 has a surface 1402A and an inclined surface
1402B which makes a predetermined angle with respect to the surface
1402A, and its thickness decreases toward its pointed end. During
cutting, the surface 1402A is located so as to extend upwardly and
substantially perpendicularly from a portion where the cutter 1402
contacts the mold 1401, and the steep step shape on the outer side
is cut. The inclined surface 1402B is located so as to be inclined
toward the inner side. In order to prevent the cutter 1402 from
being chipped or broken, the cutter 1402 has to have a certain
level of strength. For this, the inclined surface 1402B is
desirably provided such that the thickness increases with
increasing distance from the pointed end. In the example of FIG. 3,
the inclined surface 1402B is inclined toward the inner side, and
no problem occurs in terms of cutting, since the ring-shaped
recessed portions formed on the concavity of the mold 1401 are
monotonically deepened toward the inner side. Meanwhile, when the
step structure shown in FIG. 23A is formed, it is necessary to
form, on the concavity of the mold, ring-shaped recessed portions
whose depth periodically increases and decreases. Thus, cutting
with a tool as shown in FIG. 3 is difficult. On the other hand, the
thickness monotonically changes in the stair-like structure in the
present embodiment, and thus machining can be realized as
designed.
Embodiment 2
Temperature Compensation of Resin Objective Lens
[0078] As a material forming an objective lens element, glass and
resin (plastic) are often used. Glass changes less in properties
with respect to temperature change, and thus the performance
thereof can stably be obtained. Meanwhile, resin can be changed in
shape even at a low temperature as compared to glass, and thus the
manufacturing cost can be reduced. In addition, resin has an
advantage of being lightweight. However, resin has a drawback in
that its change in refractive index with temperature change is
greater by 10 times than that of glass. Particularly, when an
objective lens having a high numerical aperture is formed from
resin, the refractive index changes with temperature change, and
thus the spherical aberration changes, which is a problem.
[0079] In a system for optical disc called Blue-Ray Disc (BD)
employing blue light, the numerical aperture (NA) is set so as to
be equal to or higher than 0.85, and information reproducing is
realized with a high recording density. When an objective lens for
BD having a numerical aperture of 0.85 or higher is formed by a
resin lens, it is desirable to reduce change in spherical
aberration at temperature change. Particularly, it is desirable to
separate a spherical aberration into each order and to reduce a 3rd
order spherical aberration. The lowest order spherical aberration
is caused by a power component, namely, a lens effect, and is
reduced by a focus control mechanism of an optical pickup in a
short time of about 1 ms. The optical pickup for BD includes means
for compensating a 3rd order spherical aberration but often does
not include means for directly detecting a 3rd order spherical
aberration. This is because in order to obtain an amount of change
in 3rd order spherical aberration as a spherical aberration
detection signal detected within 1 ms, a detection optical system
of the optical pickup including a photodetector is complicated,
leading to size increase and cost increase. In general, a 3rd order
spherical aberration is often compensated by driving the spherical
aberration compensating means in accordance with output of a
temperature sensor provided in the optical pickup. In other words,
since a spherical aberration is not directly detected and open loop
control is performed in accordance with output of the temperature
sensor, real-time compensation is not performed within 1 ms but
low-accurate compensation is performed for a time period longer
than 1 ms. Thus, it is desirable to reduce an amount of change in
3rd order spherical aberration with temperature change.
Specifically, the amount of change in 3rd order spherical
aberration per temperature change of 30.degree. C. is desirably
equal to or less than 70 m.lamda.. 70 m.lamda. or .lamda./14
corresponds to so-called Marechal Criterion. The amount of change
in 3rd order spherical aberration per temperature change of
30.degree. C. is more preferably equal to or less than 60 m.lamda..
In addition, the amount of change in 5th order spherical aberration
per temperature change of 30.degree. C. is preferably equal to or
less than 30 m.lamda..
[0080] FIG. 6 is a diagram illustrating steps formed on a surface
of an objective lens element according to Embodiment 2.
[0081] As an example, an objective lens having a focal length of
1.3 mm and a numerical aperture of 0.86 was produced from a
polyolefin resin. At that time, when the temperature changes by
30.degree. C., a 3rd order spherical aberration change of about 130
m.lamda. occurred. Thus, as shown in FIG. 6, steps each of which
causes an optical path difference which is an integral multiple of
the wavelength .lamda..sub.1 at a specific environmental
temperature, are provided on a surface of an objective lens
element. FIG. 6 illustrates the case where the optical path
difference caused by each step is 5 times of the wavelength
.lamda..sub.1, but the present invention is not particularly
limited to 5 times. Being an integral multiple is a condition, and
numerical values other than 5 times, such as 6 times and 4 times,
are also possible depending on the designing conditions.
[0082] Where the physical height of each step is d.sub.5, the
optical path difference caused by the step is represented by
d.sub.5.times.(n.sub.5-1). Here, n.sub.5 is the refractive index of
the lens material. When the refractive index of the lens material
changes due to temperature change, the optical path difference
caused by the step also changes. This change is utilized to reduce
a spherical aberration caused by change in angle of refraction at a
refractive surface, particularly, a 3rd order spherical aberration.
Specifically, the steps are designed such that when the
environmental temperature changes, tendency of change in spherical
aberration caused by change in refractive surface shape is opposite
to tendency of change in spherical aberration caused by change in
optical path difference provided by the steps which is caused by
change in refractive index. As a result, when the environmental
temperature changes, a spherical aberration caused by change in
refractive surface shape and a spherical aberration caused by
change in optical path difference provided by the steps which is
caused by change in refractive index cancel each other, whereby an
aberration is reduced.
[0083] In order to reduce a spherical aberration caused by
temperature change when the light of the wavelength .lamda..sub.1
is used, the thickness of the steps shown in FIG. 6 desirably
monotonically increases or monotonically decreases in the same
direction. When being configured as described above, a mold for
molding the objective lens element can easily be produced as
described in Embodiment 1.
Embodiment 3
Temperature Compensation of Resin Objective Lens and Compatibility
with BD and DVD
[0084] The present embodiment realizes compatibility with BD and
DVD as described in Embodiment 1 and suppression of a spherical
aberration in the resin objective lens at temperature change as
described in Embodiment 2, at the same time. Here, compatibility
means that a single objective lens element can form a spot on a
recording surface through the base material having the thickness
t.sub.1 when the light of the wavelength .lamda..sub.1 is used, and
can form a spot on a recording surface through the base material
having the thickness t.sub.2 when the light of the wavelength
.lamda..sub.2 is used.
[0085] FIG. 7 is a diagram illustrating steps formed on a surface
of an objective lens element according to Embodiment 3. FIG. 7(a)
is a diagram illustrating the physical shape of steps which realize
the compatibility described in Embodiment 1. FIG. 7(b) is a diagram
illustrating the physical shape of steps for realizing the
spherical aberration suppression described in Embodiment 2. Each of
the steps shown in FIG. 7(b) causes an optical path difference
which is an integral multiple of the wavelength .lamda..sub.1, at a
specific temperature as described in Embodiment 2. FIG. 7(c) is a
diagram illustrating a shape obtained by combining the step shape
of FIG. 7(a) and the step shape of FIG. 7(b). In Embodiment 3, the
step shape of FIG. 7(c) is formed on a base surface which is a
refractive surface (aspherical shape) in the surface of the
objective lens element.
[0086] It should be noted that the steps of FIG. 7(b) which are
components of the steps of FIG. 7(c) have differences from those
according to Embodiment 2 (FIG. 6). The optical path difference
caused by each step of FIG. 6 to the light of the wavelength
.lamda..sub.1 is an integral multiple of the wavelength
.lamda..sub.1, while the optical path difference caused by each
step of FIG. 7(b) to the light of the wavelength .lamda..sub.1 is
5N times (N is an integer other than 0) of the wavelength
.lamda..sub.1. At that time, each step of FIG. 7(b) causes an
optical path difference which is 3N times of the wavelength
.lamda..sub.2, to the light of the wavelength .lamda..sub.2, and a
great aberration does not occur. Compatibility with the wavelength
.lamda..sub.1 and the wavelength .lamda..sub.2 is realized by the
step shape of FIG. 7(a), and the steps of FIG. 7(b) are intended to
suppress a spherical aberration caused by temperature change when
the light of the wavelength .lamda..sub.1 is used. Thus, it is
desirable that each step of FIG. 7(b) does not cause a great
aberration to the light of the wavelength .lamda..sub.2. Due to
this reason, the optical path difference caused by each step of
FIG. 7(b) to the light of the wavelength .lamda..sub.1 is desirably
5N times (N is an integer other than 0) of the wavelength
.lamda..sub.1.
[0087] Each of the step structure of the FIG. 7(a) and the step
structure of FIG. 7(b) is desirably a stair-like structure in which
the thickness monotonically increases or monotonically decreases in
the same direction. When being configured as described above, the
step structure of FIG. 7(c) is also a stair-like structure in which
the thickness monotonically increases or monotonically decreases.
Thus, a mold for molding the objective lens element can easily be
produced.
[0088] FIG. 8 is a diagram illustrating the objective lens element
according to Embodiment 3.
[0089] On at least an inner part 132B, the stair-like structure
shown in FIG. 7(c) is formed. The reference characters 132p and
132q assigned to the inner part 132B of FIG. 8 correspond to the
steps shown in FIG. 7(b). When the light of the wavelength
.lamda..sub.1 is used, the objective lens element 142 converges the
first incident light 61 incident on both the inner part 132B and an
outer part 132F, and forms a spot on the information recording
surface 91 of the optical disc 9 through the base material having
the thickness t.sub.1. In addition, when the light of the
wavelength .lamda..sub.2 is used, the objective lens element 142
converges the second incident light 62 incident on the inner part
132B, and forms a spot on the information recording surface 101 of
the optical disc 10 through the base material having the thickness
t.sub.2. Where the numerical aperture defined when the first
incident light 61 is converged on the information recording surface
91 of the optical disc 9 is NA.sub.1 and the numerical aperture
defined when the second incident light 62 is converged on the
information recording surface 101 of the optical disc 10 is
NA.sub.2, NA.sub.1 is equal to or higher than 0.85 and NA.sub.2 is
equal to or higher than 0.6.
[0090] In other words, the inner part 132B is a region shared by
the light of the wavelength .lamda..sub.1 and the light of the
wavelength .lamda..sub.2, and the outer part 132F is a region
dedicated for the light of the wavelength .lamda..sub.1. On the
inner part 132B, the stair-like step structure shown in FIG. 1A is
formed. The outer part 132F can be configured as in (5) to (8)
below.
[0091] (5) A step structure which is the same as that provided on
the inner part 132B is formed on the outer part 132F. In this case,
the outer part 132F suffices to be designed such that the light of
the wavelength .lamda..sub.1 forms a spot on the information
recording surface 91 through the base material having the thickness
t.sub.1 but the light of the wavelength .lamda..sub.2 causes a
great aberration and is not converged on a position through the
base material having the thickness t.sub.2.
[0092] (6) Alternatively, the steps of FIG. 7(a) may not be formed
on the outer part 132F. In this case, the outer part 132F suffices
to be designed such that the light of the wavelength .lamda..sub.1
forms a spot on the information recording surface 91 through the
base material having the thickness t.sub.1 but the light of the
wavelength .lamda..sub.2 causes a great aberration and is not
converged through the base material having the thickness
t.sub.2.
[0093] (7) Still alternatively, the steps shown in FIG. 4 each of
which causes an optical path difference of 1 wavelength to the
light of the wavelength .lamda..sub.1 may be formed on the outer
part 132F. In this case, with the optical path difference of 1
wavelength of the wavelength .lamda..sub.1, a phase difference
substantially does not occur. Thus, similarly to the case where no
steps are formed, the light of the wavelength .lamda..sub.1 is
converged through the base material having the thickness t.sub.1,
but a phase difference of about 0.6 wavelength occurs in the light
of the wavelength .lamda..sub.2 and hence the light of the
wavelength .lamda..sub.2 is not converged through the base material
having the thickness t.sub.2.
[0094] (8) Still alternatively, a sawtooth-like diffraction grating
may be formed on the outer part 132F. In this case, the outer part
132F suffices to be designed such that the light of the wavelength
.lamda..sub.1 forms a spot on the information recording surface 91
through the base material having the thickness t.sub.1 but the
light of the wavelength .lamda..sub.2 is not converged through the
base material having the thickness t.sub.2. Further, the sawtooth
height is preferably set to such a height that an optical path
difference of 1 wavelength is caused to the light of the wavelength
.lamda..sub.1 as described with reference to FIG. 5A. When the
sawtooth height is equal to 1 wavelength of the wavelength
.lamda..sub.1 as shown in FIG. 5B, the diffraction efficiency of
the 1st order diffracted light increases. Meanwhile, with respect
to the light of wavelength .lamda..sub.2, the sawtooth height is
only about 0.6 times of wavelength as shown in FIG. 5C. Thus, light
is diffracted into 1st order diffracted light and zero order
diffracted light and is not converged on a point. Therefore, an
aperture limiting effect on the light of the wavelength
.lamda./incident on the outer part 132F can be obtained.
[0095] (9) In order to suppress a spherical aberration caused by
temperature change when the light of wavelength .lamda..sub.1 is
used, the steps shown in FIG. 6 may be provided on the outer part
132F. In addition, the steps shown in FIG. 6 may be formed on any
of the above shapes provided on the outer part 132F. In this case,
in order to facilitate production of a mold for the objective lens
element 142, it is desirable to monotonically increase or
monotonically decrease the thickness of the steps.
Embodiment 4
Compatibility with BD, DVD, and CD
[0096] An objective lens element according to the present
embodiment converges light of a wavelength .lamda..sub.3 through a
base material having a thickness t.sub.3, in addition to the
compatibility with the wavelengths .lamda..sub.1 and .lamda..sub.2
as described in Embodiment 1.
Here, t.sub.1<t.sub.2<t.sub.3, and
NA.sub.1>NA.sub.2>NA.sub.3.
NA.sub.3 is the numerical aperture defined when the light of the
wavelength .lamda..sub.3 is converged. The light of the wavelength
.lamda..sub.3 is, for example, infrared light of 780 to 820 nm.
[0097] Since the numerical aperture NA.sub.3 is the lowest, the
innermost portion of the objective lens element is used for
conversing the light of the wavelength .lamda..sub.3 on an
information recording surface. Thus, the innermost portion of the
objective lens element has to be formed so as to be able to
converge light of three kinds of wavelengths through corresponding
base materials having thicknesses. For that reason, in the present
embodiment, a stair shape shown in FIG. 9A is formed on the
innermost portion of the objective lens element.
[0098] FIG. 9 is a diagram illustrating steps formed on a surface
of the objective lens element according to Embodiment 4. FIG. 9A is
a diagram illustrating the physical shape of the steps formed on
the surface of the objective lens element according to Embodiment
4. The shape of FIG. 9A is formed on a base surface which is a
refractive surface (aspherical shape) in the surface of the
objective lens element. FIG. 9B shows an amount of phase change
provided by each step shown in FIG. 9A to the light of the
wavelength .lamda..sub.1. FIG. 9C shows an amount of phase change
provided by each step shown in FIG. 9A to the light of the
wavelength .lamda..sub.2. FIG. 9D shows an amount of phase change
provided by each step shown in FIG. 9A to the light of the
wavelength .lamda..sub.3.
[0099] In FIG. 9A, the vertical direction indicates the thickness
of the lens material in the optical axis direction, or the height
thereof. Each step is designed to have such a height that an
optical path difference of about 1.25 wavelengths is provided to
the light of the wavelength .lamda..sub.1. The optical path
difference of about 1.25 wavelengths corresponds to a phase
difference of about (2.pi.+.pi./2). Due to the steps, an amount of
phase change changes by .pi./2 per step as shown in FIG. 9B.
[0100] Further, due to the reason described in Embodiment 1, with
respect to the light of the wavelength .lamda..sub.2, an amount of
phase change changes by about -.pi./2 per step (see FIG. 9C).
[0101] Moreover, with respect to the light of the wavelength
.lamda..sub.3, each step provides an optical path difference of
about -0.375 wavelength. Due to the steps, an amount of phase
change changes by about -0.75.pi. per step as shown in FIG. 9D.
[0102] The amount of phase change provided to the light of the
wavelength .lamda..sub.1 is positive, while the amounts of phase
change provided to the light of the wavelengths .lamda..sub.2 and
.lamda..sub.3 are negative. This indicates that the steps shown in
FIG. 9A exert an effect which is opposite to that on the light of
the wavelength .lamda..sub.1, on the light of the wavelengths
.lamda..sub.2 and .lamda..sub.3.
[0103] When the thickness of the steps increase toward the inner
side of the objective lens element as shown in FIG. 9A, the optical
path difference of the light of the wavelength .lamda..sub.1 is
lengthened toward the inner side. Meanwhile, the optical path
differences of the light of the wavelengths .lamda..sub.2 and
.lamda..sub.3 are shortened toward the inner side. When the
intervals at which the phase steps are formed are designed as
appropriate and the phase steps are configured to exert a convex
lens effect on the light of the wavelength .lamda..sub.1, a concave
lens effect is exerted on red light and infrared light. Then, the
focal point of the light of the wavelength .lamda..sub.1 gets close
to the objective lens element, while the focal points of the light
of the wavelengths .lamda..sub.2 and .lamda..sub.3 move away from
the objective lens element. Thus, the light of the wavelengths
.lamda..sub.2 and .lamda..sub.3 can be converged on information
recording surfaces through base materials thicker than the
thickness t.sub.1.
[0104] In the present embodiment, the steps shown in FIG. 9A are
formed in a stair shape in which the height monotonically increases
or decreases. Thus, the step structure in the present embodiment
does not serve as a diffraction element having a periodic structure
in which the height increases and decreases every certain cycle
(see FIG. 23A), but serves as an element which changes a phase in a
stair-like manner. In this case, a mold for molding the objective
lens element can easily be produced.
[0105] FIG. 10 is a diagram illustrating the objective lens element
according to Embodiment 4.
[0106] On at least an inner part 133C, the stair shape shown in
FIG. 9A is formed. When the light of the wavelength .lamda..sub.1
is used, the objective lens element 143 converges the first
incident light 61 incident on the inner part 133C, an intermediate
part 133B, and an outer part 133F, and forms a spot on the
information recording surface 91 of the optical disc 9 through the
base material having the thickness t.sub.1. When the light of the
wavelength .lamda..sub.2 is used, the objective lens element 143
converges the second incident light 62 incident on the inner part
133C and the intermediate part 133B, and forms a spot on the
information recording surface 101 of the optical disc 10 through
the base material having the thickness t.sub.2. When the light of
the wavelength .lamda..sub.3 is used, the objective lens element
143 converges third incident light 63 incident on the inner part
133C, and forms a spot on an information recording surface 111 of
an optical disc 11 through the base material having the thickness
t.sub.3. Here, NA.sub.1 is equal to or higher than 0.85, NA.sub.2
is equal to or higher than 0.6, and NA.sub.3 is equal to or higher
than 0.45.
[0107] In other words, the inner part 133C is a region shared by
the light of the wavelengths .lamda..sub.1, .lamda..sub.2, and
.lamda..sub.3, the intermediate part 133B is a region shared by the
light of the wavelengths .lamda..sub.1 and .lamda..sub.2, and the
outer part 133F is a region dedicated for the light of the
wavelength .lamda..sub.1. A region which is a combination of the
inner part 133C and the intermediate part 133B corresponds to the
inner part 131B shown in FIG. 1. The outer part 133F corresponds to
the outer part 131F shown in FIG. 1. Any of the above-described
exemplary configurations of the outer part can be applied to the
outer part 133F.
Embodiment 5
Temperature Compensation of Resin Objective Lens Element and
Compatibility with BD, DVD, and CD
[0108] The present embodiment realizes compatibility with the three
wavelengths as described in Embodiment 4 and suppression of a
spherical aberration caused by temperature change as described in
Embodiment 2, at the same time.
[0109] FIG. 11 is a diagram illustrating steps formed on a surface
of an objective lens element according to Embodiment 5.
Specifically, FIG. 11(a) is a diagram illustrating the physical
shape of steps which realize compatibility with the wavelengths
.lamda..sub.1, .lamda..sub.2, and .lamda..sub.3. FIG. 11(b) is a
diagram illustrating the physical shape of steps for suppressing a
spherical aberration caused by temperature change of the resin
objective lens. Each of the steps shown in FIG. 11(b) causes an
optical path difference which is an integral multiple of the
wavelength .lamda..sub.1, at a specific environmental temperature.
FIG. 11(c) is a diagram illustrating steps obtained by combining
the steps of FIG. 11(a) and the steps of FIG. 11(b). The
cross-sectional shape of FIG. 11(c) is formed on a base surface
which is a refractive surface (aspherical shape) in the surface of
the objective lens element.
[0110] In FIG. 11(b), there are differences from the steps shown in
FIG. 6 (Embodiment 2) and FIG. 7(b) (Embodiment 3). The optical
path difference caused by each step is an integral multiple of the
wavelength .lamda..sub.1 in the example of FIG. 6, and is 5N times
(N is an integer other than 0) of the wavelength .lamda..sub.1 in
the example of FIG. 7. In FIG. 11(b), the optical path difference
caused by each step is set so as to be 10M times (M is an integer
other than 0) of the wavelength .lamda..sub.1 as a condition. The
condition is equal to limiting N to an even number, namely, N=2M,
in Embodiment 3. At this time, each step of FIG. 11(b) causes an
optical path difference of 6M times of wavelength to the light of
the wavelength .lamda..sub.2, and thus does not cause a great
aberration. Further, each step of FIG. 11(b) causes an optical path
difference of 5M times of wavelength to the light of the wavelength
.lamda..sub.3, and thus does not cause a great aberration. The
compatibility with the three wavelengths is realized with the step
shape shown in FIG. 11(a), and the steps shown in FIG. 11(b) are
intended to suppress a spherical aberration caused by environmental
temperature change when the light of the wavelength .lamda..sub.1
is used. Thus, it is desirable that the steps shown in FIG. 11(b)
do not cause a great aberration to the light of the wavelengths
.lamda..sub.2 and .lamda..sub.3. Therefore, the optical path
difference caused by each step of FIG. 11(b) is desirably 10M times
(M is an integer other than 0) of the wavelength .lamda..sub.1.
[0111] The steps shown in FIG. 11(c) preferably have a stair shape
in which the height monotonically increases or decreases. In this
case, a mold for molding the objective lens element can easily be
produced.
[0112] FIG. 12 is a diagram illustrating the objective lens element
according to Embodiment 5. Steps 134p and 134q shown in FIG. 12 are
the steps shown in FIG. 11(b). FIG. 13 is a schematic plan view
when FIG. 12 is viewed from below. A region having an effective
range (or an effective diameter) 134D is divided into three
concentric regions, namely, an outer part 134F, an intermediate
part 134B, and an inner part 134C. It should be noted that
boundaries 134A, 134D, and 134E shown by broken lines in FIG. 13
are imaginary boundaries, and it is unnecessary to actually provide
boundary lines thereon.
[0113] On at least the inner part 133C, the stair shape shown in
FIG. 11(c) is formed. When the light of the wavelength
.lamda..sub.1 is used, the objective lens element 144 converges the
first incident light 61 incident on the inner part 134C, the
intermediate part 134B, and the outer part 134F, and forms a spot
on the information recording surface 91 of the optical disc 9
through the base material having the thickness t.sub.1. When the
light of the wavelength .lamda..sub.2 is used, the objective lens
element 144 converges the second incident light 62 incident on the
inner part 134C and the intermediate part 134B, and forms a spot on
the information recording surface 101 of the optical disc 10
through the base material having the thickness t.sub.2. When the
light of the wavelength .lamda..sub.3 is used, the objective lens
element 144 converges the third incident light 63 incident on the
inner part 133C, and forms a spot on the information recording
surface 111 of the optical disc 11 through the base material having
the thickness t.sub.3. Here, NA.sub.1 is equal to or higher than
0.85, NA.sub.2 is equal to or higher than 0.6, and NA.sub.3 is
equal to or higher than 0.45.
[0114] In other words, the inner part 134C is a region shared by
the light of the wavelengths .lamda..sub.1, .lamda..sub.2, and
.lamda..sub.3, the intermediate part 134B is a region shared by the
light of the wavelengths .lamda..sub.1 and .lamda..sub.2, and the
outer part 134F is a region dedicated for the light of the
wavelength .lamda..sub.1. A region which is a combination of the
inner part 134C and the intermediate part 134B corresponds to the
inner part 132B shown in FIG. 8. The outer part 134F corresponds to
the outer part 132F shown in FIG. 8. Any of the above-described
exemplary configurations of the outer part can be applied to the
outer part 134F.
[0115] In the objective lens elements according to Embodiments 1
and 3 to 5 described above, the outer part is a region dedicated
for the light of the wavelength .lamda..sub.1, but may have an
aspherical shape in which no steps are formed. Alternatively, a
sawtooth-like diffraction grating may be formed on the outer part,
and the sawtooth height may be set to such a height that an optical
path difference of about 1 wavelength is provided to the light of
the wavelength .lamda..sub.1. Still alternatively, steps each
having such a height that an optical path difference which is about
an integral multiple of wavelength is provided to the light of the
wavelength .lamda..sub.1 may be formed on the outer part. Still
alternatively, the height of each step formed on the outer part may
be such a height that an optical path difference which is about N
times (N is an integer) of wavelength is provided to the light of
the wavelength .lamda..sub.1. In this case, N may be an integer
other than multiples of 5. When N is an integer other than
multiples of 5, the wavefront of the light of the wavelength
.lamda..sub.2 is disturbed, and an aperture limiting effect can be
obtained. When N is an odd number, the wavefront of the light of
the wavelength .lamda..sub.3 is disturbed, and an aperture limiting
effect can be obtained. Due to such a configuration, aperture
limitation can be performed such that the light of the wavelengths
.lamda..sub.2 and .lamda..sub.3 incident on the outer part
substantially does not contribute to spot formation on an
information recording surface, and the numerical aperture can be
set to a desired value.
Embodiment 6
[0116] FIG. 14 is a diagram illustrating an optical head device
according to Embodiment 6. The optical head device includes a laser
beam source 1 which emits blue light of the wavelength
.lamda..sub.1 (390 nm to 415 nm: normally 408 nm), a laser beam
source 20 which selectively emits red light of the wavelength
.lamda..sub.2 (630 nm to 680 nm: normally 660 nm) and infrared
light of the wavelength .lamda..sub.3 (770 nm to 810 nm: normally
780 nm), a collimating lens (first convex lens) 8, an upward
reflection mirror 12 which bends an optical axis, and the objective
lens element 143. The optical disc 9 is a third generation optical
disc (e.g., BD) which has a base material having a thickness
t.sub.1 (about 0.1 mm) and on which at least one of recording,
reproducing, and erasing is performed with a light beam of the
wavelength .lamda..sub.1. The optical disc 10 is a second
generation optical disc (e.g., DVD) which has a base material
having a thickness t.sub.2 (about 0.6 mm) and on which at least one
of recording, reproducing, and erasing is performed with a light
beam of the wavelength .lamda..sub.2. The optical disc 11 is a
first generation optical disc (e.g., CD) which has a base material
having a thickness t.sub.3 (about 1.2 mm) and on which at least one
of recording, reproducing, and erasing is performed with a light
beam of the wavelength .lamda..sub.3. In FIG. 14, only the base
materials (portions from light incident surfaces to recording
surfaces) of the optical discs 9 and 10 are shown. In reality, each
of the optical discs 9 and 10 is attached to a protective material
in order to reinforce mechanical strength and to have a thickness
of 1.2 mm which is the same as the thickness of CD. The optical
disc 10 is attached to a protective material having a thickness of
0.6 mm, and the optical disc 9 is attached to a protective material
having a thickness of 1.1 mm. The optical disc 11 also has a thin
protective material, but illustration of the protective materials
is omitted for simplification.
[0117] In the present embodiment, the configuration employing the
two-wavelength laser beam source 20 which emits the light of the
wavelengths .lamda..sub.2 and .lamda..sub.3 is shown. However, a
configuration in which different light sources are employed for
these wavelengths, respectively and optical paths are combined by
using a dichroic mirror is also possible.
[0118] The laser beam sources 1 and 20 are preferably semiconductor
laser sources. When semiconductor laser sources are used, the
optical head device and an optical disc recording/reproducing
apparatus employing this optical head device can be decreased in
size, weight, and power consumption.
[0119] When any of recording, reproducing, and erasing is performed
on the highest-recording density optical disc 9, a light beam 61 of
the wavelength .lamda..sub.1 emitted from the laser beam source 1
is reflected by a beam splitter 4, is converted by the collimating
lens 8 into substantially parallel light, and is further converted
by a quarter wavelength plate 5 into polarized light. The quarter
wavelength plate 5 is designed to serve as a quarter wavelength
plate for the light of both wavelengths .lamda..sub.1 and
.lamda..sub.2. The light emitted from the quarter wavelength plate
5 is bent by the upward reflection mirror 12, and is converged by
the objective lens element 143 on the information recording surface
91 through the base material of the optical disc 9. Here, for
convenience of illustration, the upward reflection mirror 12 is
shown to bend the light beam in the upward direction of the
drawing. However, in reality, the upward reflection mirror 12 is
disposed so as to bend the light beam optical axis in a direction
perpendicular to the sheet.
[0120] The light beam reflected by the information recording
surface travels along the optical path in the reverse direction, is
converted by the quarter wavelength plate 5 into linearly polarized
light having a polarization plane different from an initial
polarization plane, almost totally passes through the beam splitter
4, and is totally reflected by a beam splitter 16. The light beam
reflected by the beam splitter 16 is diffracted by a detection
diffraction element 31, then is converged by a detection lens 32,
and is incident on a photodetector 33. Output of the photodetector
33 is subjected to arithmetic processing to obtain a servo signal
and an information signal which are used for focus control and
tracking control. The beam splitter 4 includes a polarization
splitting film which totally reflects linearly polarized light of
the wavelength .lamda..sub.1 having a certain direction and totally
passes linearly polarized light of the wavelength .lamda..sub.1
having a direction orthogonal to the certain direction. The
polarization splitting film totally passes light beams of the
wavelengths .lamda..sub.2, and .lamda..sub.3. As described above,
the beam splitter 4 is an optical path branching element which has
polarization properties as well as wavelength selectivity. When the
beam splitter 4 has no polarization dependency, it is possible to
omit the quarter wavelength plate 5.
[0121] Next, when any of recording, reproducing, and erasing is
performed on the optical disc 10, a light beam 62 of the wavelength
.lamda..sub.2 which is emitted from the laser beam source 20 and is
substantially linearly polarized light passes through the beam
splitter 16 and the beam splitter 4 and is converted by the
collimating lens 8 into substantially parallel light. The light
beam emitted from the collimating lens 8 is bent by the upward
reflection mirror 12 and is converged by the objective lens element
143 on the information recording surface 101 of the optical disc
10.
[0122] The light beam reflected by the information recording
surface travels along the optical path in the reverse direction,
almost totally passes through the beam splitter 4, and is totally
reflected by the beam splitter 16. The light beam reflected by the
beam splitter 16 is diffracted by the detection diffraction element
31, then is converged by the detection lens 32, and is incident on
the photodetector 33. Output of the photodetector 33 is subjected
to arithmetic processing to obtain a servo signal and an
information signal which are used for focus control and tracking
control. In order to obtain a servo signal for the optical disc 9
and a servo signal for the optical disc 10 by using the
photodetector 33 shared by the wavelengths .lamda..sub.1 and
.lamda..sub.2, the light-emitting point of the laser beam source 1
and the light-emitting point (the light of the wavelength
.lamda..sub.2) of the laser beam source 20 are located in a
construct-image relation with respect to a common position on the
objective lens element 143 side. When such a configuration is
provided, the number of detectors and the number of wires can be
reduced.
[0123] The beam splitter 16 includes a polarization splitting film
which totally passes linearly polarized light of the wavelength
.lamda..sub.2 having a certain direction and totally reflects
linearly polarized light having a direction orthogonal to the
certain direction. The polarization splitting film totally passes a
light beam of the wavelength .lamda..sub.1. As described above, the
beam splitter 16 is also an optical path branching element which
has polarization properties as well as wavelength selectivity. When
the beam splitter 16 has no polarization dependency, it is possible
to omit the quarter wavelength plate 5.
[0124] The operation of each component and the optical path when
any of recording, reproducing, and erasing is performed on the
optical disc 11 are the same as those in the case of the optical
disc 10, except that the wavelength of the light emitted from the
laser beam source 20 is different. Thus, the repeated description
is omitted.
[0125] In the example of FIG. 14, in the objective optical system,
the objective lens element 143 described in Embodiment 4 is used,
but the objective lens element 144 according to Embodiment 5 may be
used.
[0126] In the objective optical system, the objective lens element
141 according to the Embodiment 1 or the objective lens element 142
according to Embodiment 3 may be used. In this case, a light source
which emits only light of the wavelength .lamda..sub.2 can be used
instead of the laser beam source 20, and an optical head device
which enables compatibility with the optical disc 9 and the optical
disc 10 can be configured.
[0127] Further, in the objective optical system, the objective lens
element according to Embodiment 2 may be used. In this case, an
optical head device dedicated for the optical disc 9 can be
configured without providing the laser beam source 20.
Alternatively, an optical head device may be configured by
combining the objective lens element according to Embodiment 2 and
an objective lens element which enables compatibility with light of
the wavelengths .lamda..sub.2 and .lamda..sub.3.
[0128] Additional components of the optical head device will be
described below.
[0129] A three-beam grating (diffraction element) 3 may be disposed
between the laser beam source 1 and the beam splitter 4. Due to
this configuration, it is possible to detect a tracking error
signal of the optical disc 9 by the well-known differential
push-pull (DPP) method.
[0130] A relay lens 2 may be disposed between the laser beam source
1 and the beam splitter 4. Due to this configuration, the numerical
aperture on the collimating lens 8 side of the light beam 61 of the
wavelength .lamda..sub.1 can be set to an appropriate value.
[0131] A three-beam grating (diffraction element) 22 may be
disposed between the laser beam source 20 and the beam splitter 16.
Due to this configuration, it is possible to detect a tracking
error signal of the optical disc 10 by the well-known differential
push-pull (DPP) method.
[0132] It is effective to change the parallelism of the light beam
by moving the collimating lens 8 along the optical axis direction
(the right-left direction in FIG. 14). There are errors in the
thicknesses of the base materials. In addition, when the optical
disc 9 is a multilayer disc, a base material having a predetermined
thickness is provided between recording layers. A spherical
aberration occurs due to an error of the base material thickness
and the base material between the recording layers, but can be
compensated by moving the collimating lens 8 along the optical axis
direction.
[0133] Spherical aberration compensation performed by moving the
collimating lens 8 is possible with about several hundreds m.lamda.
when the NA is 0.85, and a spherical aberration corresponding to
increase/decrease of .+-.30 .mu.m in base material thickness can
also be compensated. In addition, when recording, reproducing, or
erasing is performed on the optical disc 11 by using the light beam
of the wavelength .lamda..sub.3, the collimating lens 8 may be
moved toward the left side of FIG. 14, namely, toward the laser
beam source 20. In this case, the collimating lens 8 converts a
light beam travelling toward the objective lens element 143, into
diverging light, can move a convergence spot on the optical disc 11
further away from the objective lens element 143, and can
compensate a part of an aberration caused by the base material
thickness. When the part of the aberration is compensated by the
collimating lens 8, an aberration compensation amount required for
the objective lens element 143 can be reduced. In addition, the
number of formed steps can be reduced and hence it can be made easy
to produce the objective lens element 143.
[0134] The beam splitter 4 may pass a portion (e.g., about 10%) of
the linearly polarized light of the wavelength .lamda..sub.1
emitted from the laser beam source 1. The passed portion of the
light beam is guided by a converging lens 6 to a photodetector 7. A
signal obtained from the photodetector 7 is used to monitor change
in amount of the light emitted by the laser beam source 1, and the
change in amount of the light is fed back, whereby control can be
performed to keep the amount of the light emitted by the laser beam
source 1, constant.
[0135] The beam splitter 4 may reflect a portion (e.g., about 10%)
of the linearly polarized light emitted from the laser beam source
20. The reflected light beam is guided by the converging lens 6 to
the photodetector 7. A signal obtained from the photodetector 7 is
used to monitor change in amount of the light emitted by the laser
beam source 20, and the change in amount of the light is fed back,
whereby control can be performed to keep the amount of the light
emitted by the laser beam source 20, constant.
Embodiment 7
[0136] FIG. 15 is a diagram illustrating an optical information
apparatus according to Embodiment 7.
[0137] The optical disc 9 is retained on a turntable 182 by a
clamper 163 and is rotated by a motor 164 (the same applies to the
optical discs 10 and 11). An optical head device 155 is the optical
head device described in Embodiment 6 and includes any of the
objective lens elements described in Embodiments 1 to 5. The
optical head device 155 is driven by a drive unit 151 to a position
which faces a desired track on the optical disc 9 (10, 11).
[0138] The optical head device 155 detects a positional
relationship with the optical disc 9 and outputs a focus error
signal or a tracking error signal to an electric circuit 153. In
response to the signal, the electric circuit 153 sends a signal for
finely adjusting the position of the objective lens element. On the
basis of the output signal from the electric circuit 153, the
optical head device 155 performs focus control or tracking control
and performs recording, reproducing, or erasing of information.
[0139] The optical information apparatus 167 according to the
present embodiment employs the optical head device 155 described in
Embodiment 6, and thus is compatible with a plurality of optical
discs having different recording densities. Therefore, the optical
information apparatus 167 can be configured to be small in size and
weight at low cost.
Embodiment 8
[0140] FIG. 16 is a diagram illustrating a computer according to
Embodiment 8.
[0141] The computer 300 includes the optical information apparatus
167 according to Embodiment 7, an arithmetical unit 364 such as a
central processing unit (CPU). The computer 300 is connected to an
input device 365 for performing input of information, such as a
keyboard, a mouse, and a touch panel, and an output device 181 such
as a printer and a display device displaying information. The
computer 300 performs an operation on the basis of information
inputted through the input device 365, information read out from an
optical disc through the optical information apparatus 167, or the
like, and outputs the result of the operation to the output device
181.
Embodiment 9
[0142] FIG. 17 is a diagram illustrating an optical disc player
according to Embodiment 9.
[0143] The optical disc player 321 includes the optical information
apparatus 167 according to Embodiment 7, a decoder 366 which
converts an information signal obtained from the optical
information apparatus 167 into image information, and a liquid
crystal monitor 320. The optical disc player 321 can also be used
as a car navigation system. Alternatively, an optical disc player
which does not include a liquid crystal monitor may be
configured.
Embodiment 10
[0144] FIG. 18 is a diagram illustrating an optical disc recorder
according to Embodiment 10.
[0145] The optical disc recorder 110 includes the optical
information apparatus 167 according to Embodiment 7 and an encoder
368 which converts image information into information for
recording. The optical disc recorder 110 preferably further
includes the decoder 366 which converts an information signal
obtained from the optical information apparatus 167 into image
information. By providing the decoder 366 and the encoder 368, the
optical disc recorder 110 can be configured to be able to perform
both recording and reproducing. In FIG. 18, the optical disc
recorder 110 is connected to an output device 361 such as a display
device and a printer, but may be integrated with the output device
361.
Embodiment 11
[0146] FIG. 19 is a diagram illustrating an information server
according to Embodiment 11.
[0147] The information server 336 includes the optical information
apparatus 167 according to Embodiment 7, a changer 334 which
accommodates a plurality of optical discs and inserts an optical
disc into the optical information apparatus 167 or takes out an
optical disc from the optical information apparatus 167, and an
input/output terminal 369. The input/output terminal 369 is a wired
or wireless interface used for introducing information to be
recorded on an optical disc and outputting information read from an
optical disc to the outside. The information server 336 is
connected to a network or another apparatus (e.g., a computer, a
phone, or a television tuner) via the input/output terminal 369,
and can exchange information therewith. The information server 336
may be connected to the output device 361 such as a display device
and a printer. Alternatively, the information server 336 may not
necessarily include the changer.
Embodiment 12
[0148] FIG. 20 is a diagram illustrating a vehicle according to
Embodiment 12.
[0149] The vehicle shown in FIG. 20 includes a body 233, the
optical information apparatus 167 according to Embodiment 7, an
accommodation section 239 which accommodates a plurality of optical
discs, and a changer 238 which inserts an optical disc into the
optical information apparatus 167 or takes out an optical disc from
the optical information apparatus 167. On the body 233, a power
generation section 234 which generates power, a fuel storage
section which stores a fuel supplied to the power generation
section 234, and a power source 236 are mounted. In the case of a
train or an automobile, the vehicle further includes wheels 235 for
running. In addition, when the vehicle is an automobile, the
vehicle includes a steering wheel 230.
[0150] Since the vehicle according to the present embodiment
includes the changer 238 and the optical disc accommodation section
239, many optical discs can readily be used. When an arithmetical
unit 364 which processes information obtained from an optical disc
to convert the information into an image, a semiconductor memory
237 which temporarily stores information, and a display device 242
are mounted on the body 233, it is possible to reproduce video
information recorded on an optical disc. In addition, when an
amplifier 240 and a speaker 241 are provided in the body 233, it is
possible to reproduce sound or music recorded on an optical disc.
When a position sensor such as a GPS 232 is provided, a current
position or a travelling direction can be displayed as an image on
the display device 242 in combination with map information
reproduced from an optical disc, or can be outputted as sound from
the speaker 241. When a wireless communication section 250 is
provided, information can be obtained from the outside and used
with information of an optical disc in a complementary manner.
[0151] The present invention can be used in a BD/DVD-compatible or
BD/DVD/CD-compatible optical head device.
[0152] While the invention has been described in detail, the
foregoing description is in all aspects illustrative and not
restrictive. It will be understood that numerous other
modifications and variations can be devised without departing from
the scope of the invention.
* * * * *