U.S. patent application number 10/651534 was filed with the patent office on 2004-03-11 for formed lens and optical pickup device.
This patent application is currently assigned to Konica Corporation. Invention is credited to Atarashi, Yuichi, Hosoe, Shigeru, Mori, Nobuyoshi, Yamashita, Kiyoshi.
Application Number | 20040047048 10/651534 |
Document ID | / |
Family ID | 31996114 |
Filed Date | 2004-03-11 |
United States Patent
Application |
20040047048 |
Kind Code |
A1 |
Hosoe, Shigeru ; et
al. |
March 11, 2004 |
Formed lens and optical pickup device
Abstract
A formed lens produced by pressing an optical material so as to
shape a predetermined configuration, has an optical surface having
a light entrance side, a light exit side and an optical axis. When
a normal line angle is defined as an angle formed between the
optical axis and a normal line at an arbitrary point on the optical
surface, the maximum normal angle is 60.degree. to 90.degree. and
an intersection at which the maximum normal line forming the
maximum normal line angle intersects with the optical axis is
located at the light exit side from the point of the maximum normal
line on the optical surface, and Abbe's number .nu.d of the optical
material is 60 or more.
Inventors: |
Hosoe, Shigeru; (Tokyo,
JP) ; Yamashita, Kiyoshi; (Tokyo, JP) ; Mori,
Nobuyoshi; (Tokyo, JP) ; Atarashi, Yuichi;
(Tokyo, JP) |
Correspondence
Address: |
COHEN, PONTANI, LIEBERMAN & PAVANE
551 FIFTH AVENUE
SUITE 1210
NEW YORK
NY
10176
US
|
Assignee: |
Konica Corporation
Tokyo
JP
|
Family ID: |
31996114 |
Appl. No.: |
10/651534 |
Filed: |
August 29, 2003 |
Current U.S.
Class: |
359/719 |
Current CPC
Class: |
G02B 3/02 20130101 |
Class at
Publication: |
359/719 |
International
Class: |
G02B 003/02; G02B
013/18 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 5, 2002 |
JP |
JP2002-259878 |
Dec 25, 2002 |
JP |
JP2002-373844 |
Claims
What is claimed is:
1. A formed lens produced by pressing an optical material so as to
shape a predetermined configuration, comprising: an optical surface
having a light entrance side, a light exit side and an optical
axis; wherein when a normal line angle is defined as an angle
formed between the optical axis and a normal line at an arbitrary
point on the optical surface, the maximum normal angle is
60.degree. to 90.degree. and an intersection at which the maximum
normal line forming the maximum normal line angle intersects with
the optical axis is located at the light exit side from the point
of the maximum normal line on the optical surface, and wherein
Abbe's number .nu.d of the optical material is 60 or more.
2. The formed lens of claim 1, wherein when a normal line does not
intersect with the optical axis, the normal angle is an angle
formed between the optical axis and a line obtained by projecting
the normal line on a plane which is parallel to the normal line and
includes the optical axis.
3. The formed lens of claim 1, wherein before the optical material
is pressed, the optical material is shaped in a preliminary
form.
4. The formed lens of claim 1, wherein when an equivalent sphere is
defined as a sphere having the same volume of the optical material
shaped in the preliminary form, the surface of the preliminary form
is located in a space between a spherical surface of a half sphere
whose radius is a half of that of the equivalent sphere and a
spherical surface of a double sphere whose radius is a double of
that of the equivalent sphere.
5. The formed lens of claim 1, wherein the optical surface includes
a microscopic structure.
6. The formed lens of claim 5, wherein the microscopic structure is
diffractive grooves.
7. The formed lens of claim 1, wherein the optical material has a
diffractive index not larger than 1.61 for d-line.
8. The formed lens of claim 1, wherein the optical material is a
glass.
9. The formed lens of claim 1, wherein the optical material is a
plastic.
10. The formed lens of claim 1, wherein the formed lens is used in
an optical pickup device.
11. An optical pickup device, comprising: a light source to emit a
light flux having a wavelength of 450 nm or less; a light
converging system having an objective lens being the formed lens
described in claim 1; and a photodetector; wherein the optical
pickup device conducts recording and/or reproducing information by
converging the light flux from the light source on an information
recording plane of an optical information recording medium through
the light converging system.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a formed lens and an
optical pickup device, and in particular, to a formed lens and an
optical pickup device each being formed favorably from a
pre-form.
[0002] When forming a formed lens through hot press-forming, there
is conducted a series of processes wherein a primary processed
product (pre-form or preliminary form) representing an optical
material which is almost spherical is heated, then, the pre-form is
interposed by opposing optical transfer surfaces of a forming die
to be pressed, and surfaces of the pre-form are brought into close
contact with optical transfer surfaces of the forming die by its
pressure so that optical surface shapes may be transferred, and the
pre-form is cooled and solidified, and after that, the opposing
dies are opened and the formed lens is taken out.
[0003] In this case, since there is caused microscopic fluctuation
or polarization by scattering of forming conditions in hot
press-forming, a shape of an optical transfer surface of the
forming die is not always transferred strictly to be an optical
surface shape of a formed lens. Therefore, if the shape of an
optical transfer surface of the forming die is not processed highly
accurately, the optical surface obtained through transfer of that
optical transfer surface has further deviation from the design
value. It is therefore necessary to form a forming die accurately
to the utmost. However, an optical transfer surface of a forming
die that forms a formed lens having a convex optical surface, for
example, is a concave surface, and when manufacturing a forming die
for forming a formed lens having a large maximum normal angle
formed by a normal line at a point on a convex optical surface and
by an optical axis, the concave optical transfer surface is in a
deep shape, which causes a problem that a large tool cannot be used
in processing a forming die because it is difficult for the large
tool to enter, and a small tool needs to be used for processing,
and in that case, premature abrasion of a tool tends to be caused,
changing a tool shape and changing processing power, which makes it
difficult to create the optical surface shape highly accurately.
Therefore, it has been common sense in design for manufacturing
small-sized lenses to make the maximum normal angle (details will
be described later) not to be large in terms of design as far as
possible.
[0004] Baking the maximum normal angle of the optical transfer
surface of a forming die not to be large as far as possible means
that the maximum normal angle of the convex optical surface shape
is made to be small on the optical surface shape of the formed
lens, and it corresponds, in other words, to that the refracting
power of the optical surface is made to be small. In other words,
this means manufacturing of a lens by the use of a design method to
raise a refractive index of a lens material, to lessen a distance
(section thickness) of optical surface of a lens, or to reduce
power load of an optical surface by increasing the number of
lenses. Forming dies for forming a formed lens based on the
conventional design mentioned above are disclosed in the following
technical documents.
[0005] (Document 1)
[0006] TOKKAI No. 2001-341134
[0007] However, when creating a lens having a convex form having a
small maximum normal angle obtained by the conventional design
method, namely, having a gentle gradient (which means that an angle
for the surface that is perpendicular to an optical axis is
smalle), by means of hot press-forming, there exist the following
problems, although processing of an optical transfer surface of a
forming die is relatively easy.
[0008] FIG. 1 is a schematic sectional view of a forming apparatus
in which an optical material is subjected to press-forming to
obtain a conventional lens having a convex form having a gentle
gradient. FIG. 1(a) shows the state wherein upper die 1 facing
lower die 2 descends along die barrel 3, and optical transfer
surface 1a of the upper die 1 comes in contact with heated pre-form
PF which is almost spherical and is placed on optical transfer
surface 2a of the lower die 2. FIG. 1(b) shows the state wherein
the press-forming has been advanced further, and lens L having
thereon the transferred optical transfer surfaces 1a and 2a has
been created.
[0009] A distance from the position of the upper die 1 shown in
FIG. 1(a) to the position of the upper die 1 shown in FIG. 1(b) is
press stroke S for press-forming, and as is apparent from these
figures, when the gradient of the optical transfer surface is
needed to be gentle, the pre-form which is almost spherical is
required to be deformed greatly (to become thinner), and the press
stroke S tends to be large. On the other hand, when the optical
surface of the formed lens needs to be transferred and formed
highly accurately in the hot press-forming, the pre-form PF is
required to be sofented sufficiently first so that its viscosity is
mostly uniform up to its center. In other words, a temperature of
the pre-form PF needs to be uniform accurately from its surface to
its center in the hot press-forming, because viscosity of the
pre-form PF fluctuates sharply depending on temperatures.
[0010] Further, in the case of an optical material, it needs to be
heated for a long time when it is kept totally at a certain
temperature highly accurately, because the thermal conductivity of
the optical material is extremely low, even when it is plastic or
glass. Therefore, if the pre-form PF is heated by the use of an
internal heater after the pre-form PF is put in the forming die, a
forming cycle turns out to be extremely long, namely, the time for
the optical material to occupy the forming die grows longer, and
productivity declines accordingly. Further, the forming die is
exposed to an intense heat for a long time, and its life is
shortened, resulting in an increase of expenses for replacement of
forming dies. Though it is also possible to heat the pre-form PF
before it is put in the forming die, there still is a fear that a
forming system becomes complicated and expensive, resulting in an
increase in a rate of troubles of the system and a decline of
productivity.
[0011] Since the press stroke S is long in the hot press-forming
process as stated above, when conducting highly reproducible press,
an optical material is required to be deformed gradually under the
well-controlled condition in the course of pressing, and the time
required for pressing tends to be long. Therefore, the time for the
optical material to occupy the forming die becomes longer, and
productivity is further declined. Further, the optical material
that is totally of the uniform viscosity means that the optical
material is in the state wherein it is totally close to liquid
uniformly, and therefore, it means that flow deformation is caused
until the moment when the optical material is totally cooled and
solidified. In particular, the tendency that pressure for pressing
against optical transfer surface 1a of upper die 1 is lost by the
flow grows greater, and thereby, the optical material is hardly
pressed against optical transfer surfaces 1a and 2a of the forming
dies under the high pressure, causing the trend that
transferability of the optical surface of lens L is worsened. In
other words, when compared with an occasion where an amount of
press deformation of pre-form PF is small, control of press
conditions is relatively difficult and possibility of an increase
in dispersion for forming is high. In particular, when the forming
dies have on their sides of optical transfer surfaces 1a and 2a the
microscopic forms for the purpose of forming, on an optical surface
of a lens, microscopic structures such as a diffracting groove that
generates diffracted light, its influence is exerted remarkably,
and optical materials are not filled sufficiently in a trough of
the microscopic structure, resulting in a fear of generation of
troubles that an edge portion of the microscopic structure of lens
L formed finally becomes dull. In short, in the conventional hot
press-forming, highly accurate forming is difficult unless press
conditions are established strictly.
[0012] In the cooling process, the forming dies need to be held
even after pressing, because deformation such as shrinkage caused
by cooling is generated until the moment when the optical material
is totally cooled and solidified, and this needs to be controlled,
thus, the cooling time turns out to be long, and the time for the
optical member to occupy the forming dies is further extended, and
productivity is lowered.
[0013] In creating a lens having a shape of convex optical surface
with a gentle gradient as in the past, each of heating process,
pressing process and cooling process has a problem, which makes
highly accurate and highly efficient creation of a lens difficult.
In particular, in an optical pickup device for conducting recording
and reproducing of high density information for the advanced DVD,
it is necessary to use a light source having a shorter wavelength,
and therefore, a lens used in the optical pickup device, an
objective lens, in particular, is desired to be a formed lens which
is more accurate. However, the lens designed by a conventional way
is close to the limit of improvement in accuracy, and there has
become pressing the need to create a lens with a novel concept
which is different from the conventional design method.
SUMMARY OF THE INVENTION
[0014] The present invention has been achieved in view of the
problems caused by the conventional technologies, and its object is
to provide a formed lens capable of obtaining a form of higher
accuracy and realizing high optical performance with low cost and a
highly efficient optical pickup device employing the formed
lens.
[0015] A formed lens described in Item (1) is represented by a
formed lens created by press-forming an optical material wherein
the maximum normal angle among those each being formed by a normal
line at an optional point on at least one optical surface and by an
optical axis is not less than 60.degree. and is not more than
90.degree., and a position where a normal line forming the maximum
normal angle intersects the optical axis is closer to the lens than
at least one optical surface stated above, while, Abbe's number
.nu.d is not less than 60.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a schematic sectional view of a forming apparatus
in which an optical material is subjected to press-forming to
obtain a conventional lens having a convex form having a gentle
gradient.
[0017] FIG. 2 shows a graph wherein a diameter of a spherical
pre-form is represented by the horizontal axis and a forming time
for heating, pressing and cooling in the forming die is represented
by the vertical axis.
[0018] FIG. 3 is a sectional view of a two-element lens used in an
optical pickup device.
[0019] FIG. 4 is a sectional view of a single lens used in an
optical pickup device.
[0020] FIG. 5 is a schematic structure diagram of an optical pickup
device.
[0021] FIG. 6 is a diagram of aberration characteristics for a
two-element objective lens.
[0022] FIG. 7 is a diagram of aberration characteristics for a
single objective lens.
[0023] FIG. 8 is a perspective view showing an example of an
optical element on which the optical surface is not in a shape of
axial symmetry.
[0024] FIG. 9 is a diagram wherein an optical surface of an optical
element in FIG. 8 is projected on a plane that is perpendicular to
the optical axis, and displacement of the optical surface is shown
with contour lines.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0025] In view of the aforementioned problems, the inventors of the
invention examined whether designing the gradient of the convex
optical surface shape to be as gentle as possible only for easiness
of processing by the forming die is really advantageous or not for
realizing production of highly accurate and highly efficient
lenses, and invented a formed lens that can avoid the conventional
weak points radically. Advantages of the invention will be
explained more concretely.
[0026] The convex optical surface shape in axial symmetry where a
maximum normal angle is large (maximum normal angle is not less
than 60.degree. and is not more than 90.degree.) as in the formed
lens described in Item (1) is closer to a hemispherical form than a
conventional lens form, and thereby, it is possible to reduce an
amount of deformation from an optical material such as a primary
processed item (pre-form) in the case of press-forming. Therefore,
it is not necessary to heat uniformly to the center of the optical
material such as a pre-form even in the case of hot press, and it
is possible to shorten the forming cycle extremely because the
press stroke is small. Further, the transferability of an optical
surface shape is excellent, because it is possible to keep the
pressure to be high for pressing an optical material against the
optical surface transferring surface of the forming die to be close
contact, from the early stage in the case of pressing. In short, it
is possible to realize press-forming that is higher in accuracy and
higher in speed than the conventional press-forming.
[0027] However, as s side effect for the large maximum normal
angle, there is caused a problem that a deflection angle grows
greater and a difference of a refraction angle caused by a
wavelength of an incident light flux grows greater, because a
normal angle of its incident optical surface becomes an incident
angle of a light flux, and thereby, an incident angle becomes
greater when a normal angle is greater, and an incident light flux
enters the optical surface, making a small angle with the optical
surface, when the incident light flux is a collimated light that is
in parallel with an optical axis of the formed lens. Namely, if the
dispersion is one for general conventional optical materials,
doubling corresponding to that appears greatly. Therefore, the
maximum normal angle on the optical surface of the lens is made to
be large, an optical material having small dispersion (having large
Abbe's number) is selected. Namely, by selecting an optical
material whose Abbe's number .nu.d is 60 or more, it was possible
to manufacture a highly accurate formed lens having high optical
efficiency in which chromatic aberration is hardly caused.
[0028] In this case, the inventors of the invention first inspected
easiness of manufacture for a highly accurate forming die having a
deep concave optical transfer surface which is needed for forming a
formed lens having a convex optical surface with a large maximum
normal angle like one described in Item (1). With respect to
materials for a forming die, when an optical material is plastic,
electroless-plated nickel is used, and an optical transfer surface
is generally created through cutting by a diamond tool and a super
precision lathe. Accuracy of the point of a blade of a diamond tool
has been improved greatly in recent years, and an R tool having
circularity of 30 nm or less and a cone point tool having a width
of a tip of the point of a blade of 1 .mu.m or less are on the
market to be available easily. Even in the case of a super
precision lathe, a highly precise lathe whose axial resolving power
is 1 nm can be used. In the case of electroless-plated nickel
representing a material for a forming die, high machinability
materials disclosed in TOKKAI No. 2001-353729 have already been
developed by the inventors of the invention, and have been put to
practical use. Accordingly, it has become clear that creating of a
forming die for a formed lens having a convex optical surface on
which a maximum normal angle is large is not difficult, as far as
the plastic lens is concerned, and it is not necessary to design a
lens optical surface shape to have a slight tilt for that
purpose.
[0029] On the other hand, when using glass for the optical
material, processing-resistant materials such as ceramic or carbide
have generally been used as a material for a forming die, because a
temperature for pressing is as high as 500-600.degree. C.
generally. For creating a concave optical transfer surface on this
material for a forming die through processing, it is general to
conduct grinding processing by a diamond grindstone and a super
precision lathe and to conduct polishing processing as a subsequent
processing. In this case, when creating a deep concave optical
transfer surface shape for transferring a convex optical surface
with a large maximum normal angle on a forming die, a dimension of
a grindstone has been required to be small in the past, and there
has been a problem that a decline of grinding ratio accelerated
abrasion of the grindstone or the state of a cutting edge was not
stabilized. Nowadays, however, it is not so difficult to create an
optical transfer surface, compared with the past, owing to the
progress of grinding processing technologies including that a load
on a grindstone was lightened by a method of parallel grinding in
which an axis of grindstone is laid out in a feeding plane, and an
optical transfer surface having excellent surface roughness and
shape accuracy can be created relatively easily, and that a
technology to stabilize a grindstone cutting edge at an excellent
state by using electrolysis like ELID grinding has been put to
practical use. Further, in the field of glass materials, the trend
for low Tg has been advanced, and glass materials having press
temperature of 300-350.degree. C. are also on the market to be
available easily. As the optical material of this kind, K-PG325
introduced by Sumita Kogaku Co. to the market can be used. From the
foregoing, there is an actual condition that necessity of using
processing-resistant materials such as ceramic or carbide for the
material of a forming die has been reduced.
[0030] Further, in the conventional forming method, when forming a
lens having an optical surface on which a maximum normal angle is
large, a radius of a mostly spherical pre-form is sometimes greater
than a central radius of an optical transfer surface of a forming
die, for example, and thereby a phenomenon that gas accumulation is
generated at the central portion of the optical surface to worsen
formed-ability was sometimes caused. At present, however, a method
to make the forming cavity to be vacuum in the course of
press-forming has been put to practical use, thus, it is possible
to realize highly accurate forming without generating gas
accumulation.
[0031] A lens having an optical surface on which a maximum normal
angle is large is in a trend that a tolerance for decentering
(shifting of an optical axis) of an optical surface is generally
lowered, and it is necessary to control decentering between dies
facing each other to be extremely small in the course of forming.
In the structure of a conventional forming machine, it was
difficult to control this decentering. However, it has become
possible to control and adjust the die decentering on an extremely
high accuracy basis by the structures of quite novel forming
machines disclosed in TOKKAI No. 2001-341134 and TOKUGAN Nos.
2002-055241 and 2002-142709 proposed by one of the inventors of the
invention. The aforementioned viewpoint makes it to say that
forming of highly accurate lenses has become easy.
[0032] The actual condition stated above makes it to say that the
conventional technical common sense that "the maximum normal angle
on a lens optical surface should be made small for easiness of die
processing and forming" has now been overthrown by advancement of
processing technologies and development of novel optical materials.
Namely, it has been proved that solving the aforementioned
conventional problems in hot press-forming drastically and ensuring
high productivity of highly accurate and highly efficient lenses
can be attained by making the maximum normal angle positively
against the conventional technical common sense.
[0033] By making the maximum normal angle to be large, a shape of
an optical surface of a formed lens approaches a hemisphere, and
thereby, an amount of press deformation of a pre-form which is
almost spherical, for example, can be reduced and a press stroke
can also be reduced to half. Further, it is not necessary to soften
the whole optical material such as a pre-form because the amount of
press deformation is small, and a portion near the surface of the
optical material such as a pre-form has only to be in viscosity
that allows deformation at the highest temperature, thus, the
processing time can be shortened sharply, and flow deformation in
the course of forming is hardly caused and pressing pressure
becomes high because a portion near the center of the optical
material such as a pre-form can be made to be at high viscosity and
in the state near a solid body in the course of forming, thus the
optical material can be brought into close contact with the optical
transfer surface of the forming die by the great force, which makes
the forming transferability to be extremely excellent. In
particular, when a diffracting groove or the microscopic structure
for prevention of reflection is provided on the surface of an
optical surface, a trough portion of the forming die can also be
filled with optical materials, and an edge portion of the
microscopic structure on the lens optical surface does not become
dull.
[0034] Since it is possible to shorten the forming time including
heating, pressing and cooling, even in the case that the forming
die is exposed to high temperature in a single forming, the time
for that exposure can be shortened, and the time for oxidation of
the optical transfer surface at high temperature and for receiving
damages of the forming die caused by reaction with optical
materials is shortened, resulting in a life of the forming die that
is longer than that in the past. Therefore, it is possible to
reduce the running cost of the forming die and to reduce forming
cost because frequency of interruption of forming caused by
replacement of forming dies resulted from damages of the forming
die is reduced, and output can be kept high by the improved rate of
operation of a forming machine.
[0035] Since it has been impossible to use lenses other than the
lens having a small maximum normal angle, when great power is
needed, the power born by each optical surface has been dispersed
by increasing the number of lenses. In this case, if one lens, for
example, is increased to two lenses, the forming cost is doubled,
and further, lens frame parts for incorporating the two lenses are
needed, thus, incorporating errors are increased, and time and cost
for the incorporating and inspection for the incorporated stated in
addition to inspection for a single lens are necessary, thereby,
the manufacturing cost has been doubled or more, which can be
lessened to half or less by one effort. Namely, the invention makes
it possible to obtain an effect that the cost is half or less of
that in the past when manufacturing lenses while ensuring the
accuracy, efficiency and production yield which are higher than
those in the past.
[0036] Incidentally, with respect to the relationship between a
press stroke and the forming time, there has been published basic
data in S. Hosoe and Y. Masaki "High-speed glass-forming method to
mass-produce precise optics", SPIE Vol. 2576 pp 115-120, 1995
representing a result of the joint study of those including a
person in the inventors of the invention. FIG. 2 shows a graph
which is a part of the results of the study.
[0037] In FIG. 2 wherein a diameter of a spherical pre-form is
represented by the horizontal axis and a forming time including
heating, pressing and cooling in the forming die is represented by
the vertical axis, there are plotted four kinds of graphs depending
on how much percent of the pre-form diameter is a thickness of the
pre-form after pressing. For example, in the case where the
pre-form diameter is 6 mm and a thickness after pressing is 60%,
namely, in the case where the press stroke is 40%, this forming
method requires a forming time of 330 seconds. However, even in the
case of the same pre-form diameter, if the thickness after pressing
is 90%, namely, if the press stroke is 10%, the forming time is
shortened to 180 seconds which is mostly a half. This relates only
to a forming process, and it shows that productivity of a lens in
one kind is mostly doubled.
[0038] If the power is dispersed and the number of lenses is
increased from one to two for reducing the maximum normal angle,
and if the maximum normal angle is reduced from 75.degree. to
45.degree., the press stroke is increased from 10% to 30% of a
diameter of the pre-form which is mostly spherical. If this is
viewed in FIG. 2, when a pre-form diameter is 3 mm, the forming
time for a single lens with maximum normal angle 75.degree. is
about 140 seconds and that for one of two lenses with maximum
normal angle of 45K is about 195 seconds, and the forming time
ratio is 1:1.4. Since the latter is in the case of two lenses, if
the foregoing is taken into consideration, the forming time ratio
is 1:2.8, resulting in a difference from the forming productivity
which is almost three times higher. Moreover, since double forming
dies are needed, a burden for forming die processing is not reduced
even when changing to a two-element lens to make the maximum normal
angle to be small, and there is a possibility to be rather
increased by the aforementioned progress of processing
technologies.
[0039] The inventors of the invention verified as follows regarding
the minimum value of the maximum normal angle which offers the
aforementioned effect remarkably.
[0040] First, with respect to the upper limit of the maximum normal
angle, when it exceeds 90.degree. at which the optical surface is
almost close to a semispherical shape, an undercut is caused on the
forming die, resulting in that a formed lens cannot be removed from
a forming die after forming. It is therefore possible to mention
that "the upper limit of the maximum normal angle is 90.degree. or
less".
[0041] Next, with respect to the lower limit of the maximum normal
angle, when an objective lens for an optical pickup device that is
considered to be a lens with the greatest power is taken as an
example, if a two-element objective lens is structured by using an
optical material having image-side numerical aperture NA of 0.85
and a refractive index of about 1.5, its section shows a sectional
shape indicated in FIG. 3, and its maximum normal angle is
37.degree. on an incidence-side optical surface (third surface when
counted from the light source side) of a second lens (lens on the
right side in FIG. 3). FIG. 4 shows one wherein the specifications
which are mostly the same as the foregoing are designed optically
on a single lens, and its maximum normal angle (.theta. in FIG. 4)
is about 72.degree. on the first surface. Between the both lens
types mentioned above, there is caused a difference in maximum
normal angle of 35.degree. which is shown, together with a pre-form
diameter, a press stroke and others in this case, in Table 1.
1TABLE 1 Refrac- Abbe's Maximum Equivalent Lens Optical tive number
normal Axial preform thickness NA0.85 surface index nd vd angle
thickness Volume diameter ratio Two- First 1.50914 56.5 37.degree.
2.5 mm 20.1 mm.sup.3 3.374 mm 74.0% element surface lens of first
lens Third 1.54351 56.7 37.degree. 1.0 mm 2.6 mm.sup.3 1.706 mm
58.6% surface of second lens Single First 1.50670 70.5 72.degree.
2.2 mm 9.5 mm.sup.3 2.628 mm 83.7% lens surface
[0042] With respect to a press stroke, it is 0.87 mm for the front
lens (first lens) and 0.7 mm for the rear lens (second lens) in the
case of a two-element lens, and it is 0.4 mm in the case of a
single lens. A percentage of a thickness of the formed lens
excluding a press stroke for a diameter of a true sphere imagined
from a pre-form shape is respectively 74%, 59% and 84%. When this
value is greater, it means that an amount of deformation of a
pre-form in forming is less which is advantageous in terms of
uniformity of the pre-form temperature, pressing time and
improvement of transferability by an increase of pressing pressure.
Based on this value, if the forming time is estimated by
interpolating and extrapolating the graph in FIG. 2, it is 190 sec.
for the front lens and 230 sec. for the rear lens in the case of a
two-element lens, and it is 140 sec. in the case of a single lens.
Therefore, the productivity for forming in 140 sec. for a single
lens having the maximum normal angle of 70.degree. is three times
that in 420 sec. of the total forming time in the case of the
two-element lens.
[0043] As a lower limit of the maximum normal angle, therefore, it
is possible to expect clear effects with 70.degree.. However, it is
also possible to expect sufficient effects with an angle of not
less than 60.degree. that is smaller than 70.degree. by 10.degree.,
if the productivity that is about twice in place of three times as
high as another is allowed.
[0044] Incidentally, the normal angle mentioned in the present
specification is an angle formed by an optical axis and a normal
line that is drawn on an optical surface (area through which a
light flux passes) to pass through an optional point on the optical
surface. A value of the greatest normal angle on the optical
surface is called a greatest normal angle (maximum normal angle).
In general formed lenses, a normal angle at a position on the
optical surface is increased monotonously as the aforesaid position
moves outwardly from the center of the optical surface. In the case
of a general optical surface shape, therefore, the position where
the maximum normal angle can be obtained is on the outermost
circumference of the effective optical surface. However,
monotonousness of increase and decrease of normal angles has no
connection with the invention, and the position for the maximum
normal angle is not always made to be at the outermost
circumference of the optical surface, and it may be at an optional
location on the optical surface. Incidentally, in the case of an
area having therein a minute form such as a diffracting structure,
a normal line is to be drawn on a base form such as a base aspheric
surface, not on the actual optical surface.
[0045] Materials mentioned in the present specification represent
all materials including general plastic and general glass which can
be used for optical uses. An optical material to be loaded in a
forming die in the course of press-forming may be either of an
optical material formed to be a pre-form in advance, an optical
material that is dropped in a form of a liquid drop and is loaded
in a forming die, and a liquid that has no outer shape when loaded
in the forming die.
[0046] A formed lens described in Item (2) is represented by a
formed lens created by press-forming an optical material, wherein a
maximum normal angle among normal angles each being formed between
an optical axis and a straight line obtained when a normal line at
an optional point on at least one optical surface is projected on a
plane that includes the optical axis and is in parallel with the
normal line is 60.degree. to 90.degree. at the greatest, and the
position where the aforesaid straight line corresponding to the
normal line that forms the greatest normal angle intersects with
the optical axis is closer to the lens than at least one optical
surface, and Abbe's number .nu.d is not less than 60.
[0047] A formed lens described in Item (1) is one having an optical
surface that is basically symmetric on a rotational basis about an
optical axis, and on the optical surface which is symmetric on
irrotational basis about the optical axis (nonaxisymmetric,
including free curved surface), the normal line on the optical
surface does not intersect usually with an optical axis. A formed
lens described in Item (2) is one having an optical surface that is
symmetric on a irrotational basis, and in this case, a plane which
is in parallel with a normal line among planes including optical
axes is considered, and let it be assumed that an angle formed by a
straight line on the plane obtained by projecting the normal line
on the plane (the normal line is projected on the plane to be in
the direction perpendicular to the plane) and by an optical axis is
called a normal angle. Other points than the foregoing are the same
as those in the formed lens in Item (1) in terms of effects, and
explanation for them will be omitted.
[0048] The invention described in Item (2) will be explained as
follows, more concretely. When the optical surface is not in an
axisymmetric shape, or in the case of a free curved surface, a
normal line on a position on the optical surface sometimes does not
intersect with an optical axis. For example, an optical element (in
exaggeration, bearing a resemblance to a shape of a halved Rugby
ball) shown in FIG. 8 is an example wherein a sectional form of a
meridian plane that is perpendicular to an optical axis is
different from others and an optical surface in a shape of a saddle
is provided. The optical element of this kind can be used as an
objective lens that converges a light flux emitted from a
semiconductor laser, while correcting the astigmatic difference of
the emitting point. In a general optical pickup device, a light
flux emitted from a semiconductor laser is collimated by a
collimator to be a light flux that is mostly parallel to enter an
objective lens. However, at a point of time when it has passed the
collimator, astigmatic difference still remains in the light flux,
and a spread of the light flux in the direction perpendicular to
the optical axis is different, which means that the light flux is
not a perfect parallel light flux. However, when this light flux is
made to enter the formed lens described in Item (2) in phase, the
light flux having a greater angle of a spread can be converged on
the optical surface in the direction of a greater curvature and a
shorter focal length, while, the light flux having a smaller angle
of a spread can be converged on the optical surface in the
direction perpendicular to the light flux of a smaller curvature
and a longer focal length, resulting in that a light flux in any
direction is converged at the same position, and a small spot
diameter can be obtained by a large amount of light.
[0049] The optical surface of the optical element of this kind
having a saddle shape was projected on a plane perpendicular to the
optical axis, and displacement of the optical surface at that time
was shown with contour lines, which is shown in FIG. 9. Normal
lines at positions on the optical surface in direction "a" of a
weakest curvature and in direction b of a strongest curvature
intersect with an optical axis, and the definition described above
can also be used for normal angles. However, when point P on the
optical surface in the direction that is sandwiched between
direction a and direction b, namely point P on the optical surface
in the direction of 45.degree. in FIG. 9 is considered, a contour
line passing through point P is an ellipse, and with respect to its
greatest inclination direction (direction perpendicular to the
contour line), the normal line there does not pass through the
optical axis. In short, the normal line at point P becomes a
position of torsion for the optical axis, and therefore, in Item
(2), if a normal angle is taken to be an angle formed by the normal
line and an optical axis in accordance with a general definition,
the normal angle cannot exist, which is irrational. However, the
purpose is only to prescribe a sharp inclination on an optical axis
of the optical surface, when the normal line of this kind is in the
position (relationship) twisted for the optical axis as in the
optical element shown in FIGS. 8 and 9, the normal angle is defined
as follows, and its angle range is used as the normal angle in Item
(2).
[0050] To be more concrete, for the normal line in the position
twisted on the optical axis, a plane that is in parallel with the
normal line and includes the optical axis is imagined, and an angle
formed by the optical axis and a projected line on the plane that
is formed when the normal line is projected vertically on the plane
is defined as a normal angle. According to this definition, if the
optical surface is steep on the optical axis, the normal angle is
great, while, if the optical surface is gentle on the optical axis,
the normal angle is small, and therefore, the normal angle can be
handled in the same way as in the normal angle based on the
conventional definition.
[0051] Even in the case of the nonaxisymmetric optical surface of
this kind, the maximum normal angle is great, and it raises
pressing pressure so that a die may be pressed by high pressure
when an amount of deformation by forming of optical material is
smaller. Thus, transferability is improved and required optical
functions can be satisfied highly accurately. When using the formed
lens described in Item (2) for correcting the astigmatic difference
of a semiconductor laser, the greatest difference of an optical
surface shape (amount of displacement in the direction of the
optical axis) between the direction where the curvature of the
optical surface is largest and the direction where the curvature of
the optical surface is smallest is only about 150 nm in general. In
spite of such small difference of the optical surface, the optical
surface shape on which the maximum normal angle is 60.degree. or
more is extremely effective for accurate transfer for forming on
the ground of the reason described earlier.
[0052] Incidentally, with respect to unevenness on the optical
surface, when an intersection of the optical axis and the normal
line (or projected straight line) is positioned to be in the lens
material (optical material) for the boundary of the optical surface
on the optical axis, it is defined as a convex surface, and when
the intersection is positioned to be on the air side, it is defined
as a concave surface.
[0053] With respect to the formed lens described in Item (3), it is
preferable that the optical material is formed to be a pre-form
before it is subjected to press-forming. In this case, the press
stroke stated above can be made small, and high productivity can be
attained.
[0054] The pre-form mentioned in the present specification means
one formed before press-forming (a solid body or a liquid body
provided with its outer shape). A solid body which has been
subjected to processing before press-forming (primary processing)
to be formed is naturally included.
[0055] With respect to the formed lens described in Item (4), it is
preferable that the surface of the pre-form is in a shape wherein,
for a sphere having the same volume as that in the pre-form, its
radius with the same center as in the sphere is contained in a
range of a shell between a spherical surface having a half radius
and that having a doubled radius. The optical materials included
within that range are assumed to be called "a sub-spherical shape".
Namely, if the pre-form is in that shape, a formed lens having a
large maximum normal angle satisfying characteristics of Item (1)
or (2) can be formed with a higher precision under the higher
productivity.
[0056] It is preferable that the formed lens described in Item (5)
has a microscopic shape on the face of the optical surface. Namely,
even when a microscopic shape is present on the face of the optical
surface, if a formed lens satisfies characteristics of Item (1) or
(2), the microscopic shape can be transferred from the forming die
onto the formed lens with a high precision, because the
transferability for forming is excellent.
[0057] In this case, the microscopic shape means a shape of
unevenness for giving further optical functions to a base optical
surface of a base aspheric surface for lens design, and it does not
mean more minute shapes resulting from a mere transfer failure or
from surface roughness of the forming die. As a microscopic shape,
there are given, for example, a diffracting groove for giving a
function to generate diffracted light and an antireflection
structure for giving an antireflection function. AS a dimensional
order of the microscopic shape of this kind, there is given an
example of 100 nm-1 mm.
[0058] As a concrete example of those other than a diffracting
groove, there is given one that is called SWS (Sub Wavelength
Structure) and has unevenness smaller than a wavelength of a light
source to be used. This may be of an antireflection structure
called MOTH EYE that reduces a refractive index of the optical
surface equivalently, or of a microscopic structure that conducts
transmission or reflection selectively in accordance with a phase
of light as a polarizing optical surface by means of oriented
grooves, or of a microscopic structure that gives narrow band
filter characteristics that conduct only transmission or reflection
for specific wavelength. With regard to these microscopic
structures, they are already popular as a known technology, and one
of the inventors of the invention discloses them in TOKUGAN No.
2001-299711. Therefore, explanation for them will be omitted here.
In particular, in forming of an optical element having the
microscopic shape on its optical surface, highly accurate transfer
that pushes softened optical materials into the innermost recess of
the microscopic structure of the forming die is necessary, and if
it is difficult, desired optical function cannot be exerted. For
attaining the highly accurate transfer of this kind, if the optical
surface shape becoming its base shape is an optical surface having
a large maximum normal angle as shown in Items (1) and (2) in the
formed lens of the invention, an amount of deformation of optical
materials in press-forming can be small, thus, it is possible to
press the optical material pressure against the forming die while
keeping the optical material pressure to be high, and thereby,
transferability for the microscopic structure can be improved, and
high optical characteristics due to the microscopic functions can
be secured.
[0059] In the formed lens described in Item (6), it is preferable
that the microscopic shape stated above is a diffracting groove.
When a diffracting groove is provided on the optical surface,
utilization efficiency for light (diffraction efficiency) is
extremely high and low cost formed lenses can be obtained.
[0060] In the formed lens described in Item (7), it is preferable
that the refractive index at d line of the optical material
mentioned above is less than 1.61. If the refractive index is
small, a maximum normal angle on the lens optical surface can be
made large in optical design, and formed lenses can be formed with
a high precision and high efficiency.
[0061] In the formed lens described in Item (8), it is preferable
that the aforesaid optical material is glass.
[0062] In the formed lens described in Item (9), it is preferable
that the aforesaid optical material is plastic.
[0063] In the formed lens described in Item (10), it is preferable
that it is used for an optical pickup device.
[0064] In the optical pickup device described in Item (11), it is
preferable that a light source having wavelength .lambda.1
(.lambda.1.ltoreq.450 nm), a light-converging optical system
employing the formed lens described in either one of Items 1-9 as
an objective lens and a photo-detector are provided, and
information is recorded and/or reproduced when a light flux emitted
from the light source is converged on an information recording
surface of an optical information recording medium through the
aforementioned light-converging optical system.
[0065] The embodiment of the invention will be explained as
follows, referring to the drawings.
[0066] FIG. 5 is a schematic structure diagram of an optical pickup
device that conducts recording and reproducing of information for
high density DVD and employs a formed lens relating to the present
embodiment. In FIG. 5, a light flux emitted from semiconductor
laser 111 (wavelength .lambda.1=380 nm-450 nm) representing a light
source is transmitted through 1/4 wavelength plate 113 and beam
splitter 114, then, is stopped down by diaphragm 17 after being
transformed into a parallel light flux by collimator 115
representing a correcting element, and is converged by objective
lens 16 representing a light-converging optical element on
information recording surface 22 through protective layer 21
(thickness t1=0.1-0.7 mm) of optical disk 20.
[0067] Then, the light flux modulated by information bits and
reflected on information recording surface 22 is transmitted again
through objective lens 16 and diaphragm 17, then, passes through
collimator 115, and enters beam splitter 114 and is reflected there
to be given astigmatism by cylindrical lens 117, and it enters
photo-detector 119 through concave lens 118, thus, output signals
therefrom are used to obtain signals to read information recorded
on optical disk 20.
[0068] Further, changes of an amount of light caused by changes in
shape and position of a spot on photo-detector 119 are detected,
and thereby, focusing detection and track detection are conducted.
Based on these detections, a two-dimensional actuator (not shown)
moves objective lens 16 so that a light flux emitted from
semiconductor laser 111 may form an image on recording surface 22
of optical disk 20, and moves objective lens 1616 so that a light
flux emitted from semiconductor laser 111 may form an image on a
prescribed track.
EXAMPLE
[0069] FIG. 4 is a sectional view of an objective lens that
converges a light flux that is emitted from a violet semiconductor
laser and has a wavelength of 405 nm on information recording
surface 22 of optical disk 20 with image-side numerical aperture NA
of 0.85 in an optical pickup device in FIG. 5. FIG. 3 is a
sectional view of a two-element objective lens representing a
comparative example for the objective lens in FIG. 4.
[0070] On the first surface of the objective lens in FIG. 4, there
are provided un-illustrated diffracting grooves, and the
diffracting grooves are in a shape of ring-shaped zones in a shape
of concentric circles when they are viewed in the axial direction
of the lens, and a minimum pitch of the diffracting grooves is 8.8
.mu.m, the number of ring-shaped zones is 30 and its section is in
a shape of serration. For the purpose of forming diffracting
grooves equivalent to the aforementioned diffracting grooves, for
comparison, on the first surface of the front lens that is shown in
FIG. 3 and has maximum normal angle of 37.degree., there were
provided diffracting grooves in a microscopic shape corresponding
to the optical transfer surface of a forming die, and optical
materials each being formed to be a pre-form under the forming
condition considered to be optimum in each case were subjected to
hot press forming, to be compared in terms of transferability. With
respect to the optical materials, optical glass (M-BaCD5 made by
HOYA Co.) having the same specifications such as Abbe's number
.nu.d 61.3 and refractive index nd 1.58913 was used for both
objective lenses shown respectively in FIG. 3 and FIG. 4.
[0071] In the results of the foregoing, satisfactory
transferability was observed in both lenses as far as a trough of
the diffracting groove (convex portion in a forming die) is
concerned, but dullness of about 0.7 .mu.mR was caused on the
objective lens in FIG. 4, and dullness of 3.5 .mu.mR was caused on
the objective lens in FIG. 3 with regard to a crest of the
diffracting groove. This dull portion lowers diffraction efficiency
greatly, and brightness on edge of image field for the objective
lens in FIG. 4 showed a decline of about 8% from the ideal value,
and that for the objective lens in FIG. 3 showed a decline of 46%.
Apparently, there was observed a big difference in transferability
of the diffracting grooves representing a microscopic structure of
an optical surface, and in the peripheral portion where a pitch of
diffracting grooves is small, in particular, a light flux was
scattered and was not converged for the objective lens in FIG. 3,
resulting in a critical decline of an amount of light, although the
objective lens in FIG. 4 was kept within a usable range.
[0072] Further, in the case of the objective lens of an optical
pickup device for conducting recording and/or reproducing for an
optical disk with capacity of high density recording, NA is great,
and in the case of a single lens, a lens shape is close to a
spherical shape and chromatic aberration grows greater. In the
objective lens in FIG. 4, on the other hand, it is possible to
control occurrence of chromatic aberration and to obtain
light-converging characteristic by employing diffracting grooves
having a function to correct chromatic aberration, and it is
further possible to ensure the working distance from the rear
surface (second surface) of the lens to the surface of the optical
disk to be as large as twice that for a two-element lens, by making
the objective lens to be a single lens, thereby, it is possible to
prevent interference between the objective lens and an optical disk
when driving the objective lens to move in the direction of the
optical axis for focus adjustment, which verifies that the
aforementioned objective lens is extremely excellent.
[0073] A precision of the optical surface of the lens in FIG. 4 is
as high as 50 nm or less, and eccentricity sensitivity is as
extremely high as 20 seconds or less in tilt and 1 .mu.m or less in
shift, but the lens can be realized sufficiently by the recent
forming technology stated above through hot press-forming.
[0074] As described earlier, referring to Table 1, in the case of a
two-element lens, the total power as a light-converging lens can be
shared by two lenses, and therefore, power of each individual lens
is not required to be so great, maximum normal angle is 37.degree.
which is not so large and Abbe's number is about 56, which make it
possible to obtain sufficient image forming capacity. In the single
lens, on the contrary, the maximum normal angle is made to be as
large as 72.degree. by using refractive index materials having the
refractive index similar to that of the two-element lens. In this
case, the angle of refraction is increased by an amount equivalent
to the increase of maximum normal angle, and a difference of
deflection angle caused by a difference of a wavelength grows
greater, causing a problem of great chromatic aberration which,
however, is avoided by using optical materials having small
dispersion and by providing diffracting grooves on the optical
surface. Spherical aberration characteristics under this condition
are shown respectively in FIGS. 6 and 7.
[0075] In the figures, spherical aberration for each wavelength is
plotted, under the assumption that a wavelength of the light source
is fluctuated by .+-.5 nm by mode hop and temperature
characteristics from wavelength 405 nm of a violet semiconductor
laser that serves as a center. If the graph does not generate a
great difference from the graph of the central wavelength (405 nm),
the focus movement is considered to be small, and if the graph is
vertical and straight on the horizontal axis from the optical axis
to the peripheral portion, axial chromatic aberration is considered
to be corrected satisfactorily.
[0076] When spherical aberration characteristics of the two-element
lens in FIG. 6 are observed, the spherical aberration
characteristic which is almost vertical on the horizontal axis for
the central wavelength and is extremely flat is observed, and there
is kept the straightforwardness of the graph that only moves in
parallel when a wavelength of a light source is shifted, thus,
axial chromatic aberration is corrected satisfactorily. With regard
to the shift of focus, it is .+-.1.5 .mu.m which is considerably
excellent, and on the wavefront aberration conversion, it is 86
m.lambda. per 1 nm from Table 2, and it is not problematic on
practical use.
[0077] On the other hand, FIG. 7 shows spherical aberration
characteristics of a single lens which are extremely similar to
those of the two-element lens in FIG. 6, which indicates that the
spherical aberration characteristics and wavelength characteristics
which are mostly the same as those in a two-element lens are
realized by a single lens.
[0078] Table 2 mainly shows wavefront aberration characteristics
which depend on wavelength of each of the two-element lens and the
single lens, and the smaller the value is, the more excellent the
aberration characteristic is. The axial wavefront aberration shows
good values if the graph is in a normal line (vertical) on the
optical axis in the spherical aberration diagrams (FIGS. 5 and 6)
as described above. The axial chromatic aberration shows good
values if the graph of spherical aberration is vertical for
wavelength fluctuation and is close to the graph for the central
wavelength as far as possible. The mode hop characteristics shows a
degree of deterioration of aberration caused by shift of focus in
the case of a change in wavelength of a light source when a focus
position is fixed, and it shows good values when the graph for the
central wavelength is close to the graph for each other wavelength
in the spherical aberration diagram. In this case, there is shown
the aberration fluctuation amount for the same focus position in
the case where a wavelength of the semiconductor laser representing
a light source is changed by mode hop by 1 nm, but there is no
problem for practical use if the aberration fluctuation amount is
not more than 100 m.lambda. because a temperature change of about
15.degree. C. is needed for the wavelength to be changed by 1 nm in
the actual mode hop characteristics. The temperature characteristic
shows an amount of aberration fluctuation generated by changes of
the refractive index of optical materials caused by temperature
changes. The wavelength characteristic shows residual aberration in
the occasion where focus taking is made when the wavelength is made
to be the central wavelength plus 5 nm, and it shows good values
when the graph of each wavelength is closer to the normal line in
the spherical aberration diagram.
2TABLE 2 Off-axial wavefront Axial Mode hop Temperature Wavelength
Axial aberration chromatic charac- charac- charac- NA0.85 wavefront
(image aberration teristics teristics teristics 405 nm aberration
height 0.5 mm) (.mu.m/nm) (+1 nm) (+30.degree. C.) (+5 nm)
Two-element 1 m.lambda. 20 m.lambda. 0.26 86 m.lambda. 13 m.lambda.
2 m.lambda. lens Single lens 1 m.lambda. 20 m.lambda. 0.25 81
m.lambda. 13 m.lambda. 5 m.lambda.
[0079] In the optical pickup device, the objective lens having the
values in Table 2 can be put to practical use on the whole, because
allowable values of wavefront aberration are about 30 m.lambda..
Further, the wavelength characteristics are mostly the same for the
two-element lens and for the single lens, and it has become clear
that the wavelength characteristics which are mostly the same as
the two-element lens can be realized in the single lens by using
optical materials having small dispersion and by forming
diffracting grooves, even when the maximum normal angle of the
single lens is made to be great rapidly.
[0080] Namely, it has become clear that, in the single lens, it is
possible to raise stability of forming and to increase speed of
forming to ensure the extremely high productivity by making the
maximum normal angle to be great rapidly, and to control
deterioration of the wavelength characteristics representing a
drawback by using materials having small dispersion optically, and
thereby to realize efficiencies for practical use which are the
same as those in the two-element lens.
[0081] Further, in the single lens, the working distance is just
twice that of the two-element lens as is understood from FIGS. 3
and 4, and thereby, a fear of interference between an objective
lens and an optical disk that is caused when the objective lens is
servo-driven for focus adjustment is drastically reduced, thus, it
was possible to ensure reliability backed by compact structure and
high servo-functions.
[0082] A formed lens that is highly accurate and can realize high
optical efficiencies with low cost and can offer the effects
described concretely in the aforementioned embodiment proved to be
one which can be obtained if Abbe's number of optical material
thereof is 60 or more when creating a lens having a convex optical
surface on which the maximum normal angle is not less than 60 and
is not more than 90 through press-forming.
[0083] As shown in Table 1, the optical design of this kind has
become possible by using the optical material having Abbe's number
of 60 or more despite the use of a refractive index which is mostly
the same, and for expecting the effects of the invention, it is
preferable that Abbe's number is 60 or more as the lower limit of
dispersion.
[0084] Though the invention has been explained, referring to the
embodiment, the invention should not be construed to be limited to
the aforementioned embodiment, appropriate modification and
improvement may naturally be made. The formed lens of the invention
may also be used for a collimator and a cylindrical lens, without
being limited to an objective lens of an optical pickup device.
Further, the optical pickup device may also be one capable of
recording and/or reproducing for information recording media such
as various optical disks without being limited to one capable of
recording and/or reproducing for high density DVD described in the
present embodiment. Further, a use of the formed lens is not
limited to that for an optical pickup device.
[0085] As stated above, the invention makes it possible to provide
a formed lens wherein a higher accurate shape is obtained and high
optical efficiencies can be realized with low cost, from a concept
that is different from a conventional design method, and to provide
a highly capable optical pickup device employing the formed lens
mentioned above.
* * * * *