U.S. patent application number 11/126168 was filed with the patent office on 2005-09-15 for object lens producing device and producing method.
This patent application is currently assigned to Sony Corporation. Invention is credited to Maeda, Fumisada, Nagashima, Shinichi.
Application Number | 20050201221 11/126168 |
Document ID | / |
Family ID | 19087605 |
Filed Date | 2005-09-15 |
United States Patent
Application |
20050201221 |
Kind Code |
A1 |
Maeda, Fumisada ; et
al. |
September 15, 2005 |
Object lens producing device and producing method
Abstract
An objective lens unit manufacturing apparatus for manufacturing
an objective lens unit, with a numerical aperture not less than
0.7, made up by a plurality of lenses, includes a positioning
mechanism (31) for positioning one lens (2) in a
cylindrically-shaped lens holder (3) of a synthetic resin material,
using another lens (1), already mounted and secured to the lens
holder, as a reference, and for securing the one lens to the lens
holder, for setting relative positions of the one and the other
lenses.
Inventors: |
Maeda, Fumisada; (Tokyo,
JP) ; Nagashima, Shinichi; (Tokyo, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
Sony Corporation
Tokyo
JP
|
Family ID: |
19087605 |
Appl. No.: |
11/126168 |
Filed: |
May 11, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11126168 |
May 11, 2005 |
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10399867 |
Apr 29, 2003 |
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10399867 |
Apr 29, 2003 |
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PCT/JP02/08748 |
Aug 29, 2002 |
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Current U.S.
Class: |
369/44.23 ;
369/112.23; 369/112.24 |
Current CPC
Class: |
G02B 7/021 20130101;
G11B 7/121 20130101; G11B 7/13922 20130101; G02B 27/62 20130101;
G11B 7/22 20130101; G11B 7/1374 20130101; G11B 2007/13727
20130101 |
Class at
Publication: |
369/044.23 ;
369/112.24; 369/112.23 |
International
Class: |
G11B 007/135 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 29, 2001 |
JP |
2001-260394 |
Claims
1-2. (canceled)
3. An objective lens unit manufacturing apparatus for manufacturing
an objective lens unit with a numerical aperture not less than 0.7,
made up by a plurality of lenses, comprising: a positioning
mechanism for positioning one lens in a cylindrically-shaped lens
holder of a synthetic resin material, using another lens, already
mounted and secured to said lens holder, as a reference, and for
securing said one lens to said lens holder, for setting relative
positions of said one and the other lens: a mechanism for measuring
the state of offset between the lenses using the laser light; a
light source for causing a light beam to fall on the respective
lenses in the lens holder; a reflecting member or reflecting said
light beam converged by said respective lenses; and a focusing
error signal detecting mechanism for receiving the light beam
reflected by said reflecting member to fall again on said
respective lenses to detect the focusing error signals with respect
to said reflecting member; wherein the amount of spherical
aberration in the light beam transmitted through said respective
lenses is measured, based on changes in said focusing error signals
when said reflecting member is moved along the optical axes of the
respective lenses, to adjust the distance between the respective
lenses based on said amount of spherical aberration.
4. The objective lens unit manufacturing apparatus according to
claim 3 wherein said reflecting member is vibrated along the
optical axes of the respective lenses.
5. The objective lens unit manufacturing apparatus according to
claim 3 wherein the lens holder includes lens inserting sections
into which the outer rims of the respective lenses are inserted to
control the offset of said respective lenses; said respective
lenses being inserted into said lens inserting sections to control
the offset of the respective lenses; said respective lenses being
secured to said lens holder with an adhesive as the offset of the
respective lenses is controlled by said lens inserting
sections.
6. The objective lens unit manufacturing apparatus according to
claim 5 further comprising: a light source for causing a light beam
to fall on the respective lenses in the lens holder; a reflecting
member for reflecting said light beam converged by said respective
lenses; and a focusing error signal detecting mechanism for
receiving the light beam reflected by said reflecting member to
fall again on said respective lenses to detect the focusing error
signals with respect to said reflecting member; wherein the amount
of spherical aberration in the light beam transmitted through said
respective lenses is measured, based on changes in said focusing
error signals when said reflecting member is moved along the
optical axes of the respective lenses, to adjust the distance
between the respective lenses based on said amount of spherical
aberration.
7. The objective lens unit manufacturing apparatus for
manufacturing an objective lens unit used as an objective lens unit
of an optical pickup device for writing or reading out information
signals on or from an optical recording medium, according to claim
5, further comprising: a mechanism for determining the distance
between the lens surface closest to the optical recording medium
and an end face of a lens holder lying around said lens surface
along the optical axis.
8. The objective lens unit manufacturing apparatus for
manufacturing an objective lens unit used as an objective lens unit
of an optical pickup device for writing or reading out information
signals on or from an optical recording medium, according to claim
3, wherein the other lens used as a positioning reference for said
one lens is the lens remotest from the optical recording medium,
with a surface of the outer rim of said one lens remote from the
optical recording medium being used as a reference surface for
positioning said one lens; the surface of the outer rim of said
other lens close to said optical recording medium being used as
said positioning reference for said one lens.
9. The object lens unit manufacturing apparatus for manufacturing
an objective lens unit as claimed in claim 8 wherein, in securing
said one lens to said lens holder, said one lens is held, the
distance of said one lens to said other lens is determined in a
state not position-controlled by said lens holder, insofar as the
direction of the optical axis is concerned, said one lens being
secured to said lens holder by an adhesive.
10. The objective lens unit manufacturing apparatus for
manufacturing an objective lens unit as claimed in claim 8 wherein,
in securing said one lens to said lens holder, said one lens is
held, the parallelism of said one lens to said other lens is
determined in a state not position-controlled by said lens holder,
insofar as the tilt with respect to the optical axis is concerned,
said one lens being secured to said lens holder by an adhesive.
11. The objective lens unit manufacturing apparatus for
manufacturing an objective lens unit as claimed in claim 8 further
comprising: a positioning member for positioning a reference
surface of a lens; and a mechanism for measuring the parallelism
between said reference surface and said positioning member, using a
laser light beam.
12. The objective lens unit manufacturing apparatus for
manufacturing an objective lens unit as claimed in claim 8 further
comprising: a positioning member for positioning a reference
surface of a lens; and a mechanism for measuring the parallelism
between said reference surface and said positioning member in a
state in which said reference surface is abutted against said
positioning member.
13. The objective lens unit manufacturing apparatus for
manufacturing an objective lens unit as claimed in claim 8 further
comprising: a positioning member for positioning a reference
surface of a lens; and a mechanism for abutting said reference
surface against said positioning member by attracting said lens by
a pneumatic pressure differential.
14. The objective lens unit manufacturing apparatus for
manufacturing an objective lens unit as claimed in claim 8 wherein
the parallelism between the surface of the outer rim of said other
lens remote from the optical recording medium and the surface of
the outer rim of said one lens close to said optical recording
medium is measured by detecting only the reflected light from the
surface of said outer rim of said other lens remote from the
optical recording medium, using a photodetector, in a state in
which the light is incident on the outer rim of said other lens and
in which the light transmitted through said outer rim and reflected
by the surface of said outer rim close to said optical recording
medium is not referred to said photodetector.
15-22. (canceled)
Description
TECHNICAL FIELD
[0001] This invention relates to a method and apparatus for
producing an objective lens unit made up by a plural number of
objective lenses and which may be used with advantage for an
optical pickup used in turn for writing information signals on a
optical recording medium and for reading out information signals
recorded thereon.
BACKGROUND ART
[0002] Up to now, an optical recording medium, exemplified by an
optical disc, has been used as a recording medium for information
signals. An optical pickup device is used for writing or reading
out information signals on or from an optical recording medium. The
optical pickup device includes a semiconductor laser, as a light
source for radiating a light beam to be illuminated on the optical
recording medium, and an objective lens unit for condensing the
light beam radiated from the semiconductor laser for illuminating
the light beam to a signal recording surface of the optical
recording medium.
[0003] In the optical pickup device, the spot diameter of the light
beam illuminated on the signal recording surface of the optical
recording medium may be reduced to realize high recording density
of the information signals recorded on the optical recording medium
to enable readout of the information signals recorded to high
density.
[0004] For reducing the spot diameter of the light beam illuminated
on the signal recording surface of the optical recording medium, it
is effective to shorten the wavelength of the light beam radiated
from the light source and to enlarge the numerical aperture (NA) of
the objective lens condensing the light beam.
[0005] The present Assignee has proposed an objective lens unit of
a larger numerical aperture (NA) in JP Laying-Open Patent
Publication H-8-315404 and JP Laying-Open Patent Publication
H-10-123410. The objective lens unit disclosed in this patent
publication is composed of a double-lens set made up of two lenses,
and has a numerical aperture not less than 0.7.
[0006] Up to now, a lens composed of one lens set made up by a
single lens, or so-called a "single lens", has been used
extensively as an objective lens unit used in an optical pickup
device. The single lens can be prepared by so-called glass mold
forming. A lens of high performance can be formed with high
reproducibility by fabricating the metal die to high precision and
by high precision temperature management during casting. If the
lens is to have a larger value of the numerical aperture (NA) of
for example 0.7 or larger, a larger refractive power is required of
the lens, such that the first surface of the light beam incident
side of the lens needs to be a non-spherical surface with a larger
curvature. In light of for example mold release properties, it is
extremely difficult to form the objective lens unit having a
non-spherical surface of a large curvature using a metal die.
Moreover, with an objective lens unit having a non-spherical
surface of a larger curvature and a larger numerical aperture (NA),
the light beam radiated from the light source cannot be condensed
accurately on the signal recording surface even on occurrence of
perturbations resulting from the slightest tilt relative to the
optical axis.
[0007] With the objective lens unit comprised of a double-lens set
composed of two lenses, as disclosed in the above Publications, the
refractive power can be dispersed to two lenses to moderate the
curvature of the respective lens surfaces as well as to decrease
the non-spherical surface coefficients. Consequently, the objective
lens unit can be formed to a desired machining accuracy, using a
metal die, so that it becomes possible to suppress deterioration of
the optical performance caused by for example the tilt of the lens
relative to the optical axis.
[0008] With the objective lens unit of a double-lens set composed
of two lenses, the respective lenses can be molded with a metal die
to prevent its optical characteristics from being deteriorated.
However, the respective lenses need to be registered to each other
highly accurately, i.e., it is necessary to get the optical axes of
the respective lenses of the objective lens unit registered with
each other high accurately without producing eccentricity in the
respective lenses and to maintain the distance and parallelism
between the respective lenses highly accurately.
[0009] For producing an objective lens unit of a double-lens set
composed of two lenses, there are such a method consisting in
causing the laser light to fall on the objective lenses, put
together, and in forming an interferometer by the respective lenses
to adjust the relative position thereof, and such a method
consisting in causing the laser light to be transmitted through the
objective lenses put together and in observing the near-field
pattern of the laser light to make the adjustment. With these
methods, the phenomena observed are not changed independently for
respective adjustment parameters, such that adjustment is extremely
time-consuming due to many looped procedures required for achieving
the final performance.
[0010] In assembling, there is such a method which consists in
providing a gap between the lens holder 3 and the lens and in
adjusting the lens position within the gap range. With this method,
an adhesive, such as a UV curable resin, needs to be charged into
the gap following the adjustment and cured in situ to secure the
lens to the lens holder. The lens secured in position in this
manner in the lens holder with an adhesive is likely to undergo
misregistration due to environmental changes, such as increasing
temperature or humidity.
[0011] In order to overcome the problems caused by an adhesive, it
has been proposed to set the tilt of the lens and its location
along the direction of the optical axis depending on the machining
accuracy of the lens holder. That is, a step is formed within the
lens holder and the outer rim of the lens is abutted against the
step to set the tilt of the lens and its location along the optical
axis. If, in this structure, the step is formed high accurately,
the lens position can be set similarly accurately.
[0012] In the case of, for example, an objective lens unit composed
of a double-lens set made up by two lenses, with an effective
diameter of 3 mm, it is necessary to maintain parallelism between
the two lenses on the order of 0.1 deg. For maintaining this
accuracy, it is necessary to maintain the error along the optical
axis of the surface of the outer lens rim carried by the step
within the lens holder to a value on the order of 1 .mu.m. It is
however extremely difficult to have the two lenses mounted within
the lens holder as this high degree of accuracy is maintained.
Additionally, depending on the mounting environment, fine dust and
dirt on the order of 1 .mu.m tend to be intruded into a space
between the step within the lens holder and the lens to render it
difficult to maintain parallelism between the two lenses.
[0013] In the case of an objective lens set, comprised of a double
lens set composed of two lenses, the distance between the lens
surfaces, comprised of curved surfaces of the lenses, may be kept
constant to suppress the generation of spherical aberration
ascribable to e.g., errors in lens thicknesses, as disclosed in
Japanese Laying-Open Patent Publication H-10-255303.
[0014] In the lenses with severe conditions of the relevant
standard, the assembling accuracy is optimized by adjustment, as
the amount of spherical aberration is detected during assembling,
to take up an error caused by variations in accuracy of a metal die
used for molding a lens, or in the molding conditions. Although the
spherical aberration of the lenses being adjusted may be measured
using an interferometer, a complicated apparatus is needed, while
the production cost is increased.
DISCLOSURE OF THE INVENTION
[0015] It is therefore an object of the present invention to
provide a method and apparatus for producing a novel objective lens
unit, whereby the deficiencies inherent to the aforementioned
conventional technique may be overcome.
[0016] It is another object of the present invention to provide a
method and apparatus for producing a novel objective lens unit,
whereby an objective lens unit, with a numerical aperture not less
than 0.7, which is made up by plural objective lenses, in which the
relative positions of the lenses may be adjusted to high accuracy
and in which it is possible to suppress the generation of the
spherical aberration, may be produced readily.
[0017] For accomplishing these objects, the present invention
provides an objective lens unit manufacturing apparatus for
manufacturing an objective lens unit, with a numerical aperture not
less than 0.7, made up by a plurality of lenses, comprising a
positioning mechanism for positioning one lens in a
cylindrically-shaped lens holder of a synthetic resin material,
using another lens, already mounted and secured to the lens holder,
as a reference, and for securing the one lens to the lens holder,
for setting relative positions of the one and the other lenses.
[0018] The present invention also provides an objective lens unit
manufacturing method for manufacturing an objective lens unit, with
a numerical aperture not less than 0.7, made up by a plurality of
lenses, by positioning a lens in a cylindrically-shaped lens holder
of a synthetic resin material, using another lens, already mounted
and secured to the lens holder, as a reference, and for securing
the one lens to the lens holder, for setting relative positions of
the one and the other lenses, in which the method comprises causing
a light beam to be incident on the respective lenses in the lens
holder, reflecting the light beam, converged by the respective
lenses, by a reflecting member, causing the light beam reflected by
the reflecting member to be re-incident on the respective lenses to
detect focusing error signals with respect to the reflecting
member, based on a light beam transmitted through the respective
lenses, and determining the amount of the spherical aberration in
the light beam transmitted through the respective lenses, based on
changes in the focusing error signals when the reflecting member is
moved along the optical axes of the respective lenses, to adjust
the distance between the respective lenses based on the amount of
the spherical aberration.
[0019] In a manufacturing method for an objective lens unit,
according to the present invention, a lens holder is provided lens
inserting sections into which the outer rims of a plurality of
lenses are inserted to control the offset of the respective
lenses.
[0020] Another manufacturing method for an objective lens unit,
according to the present invention, is a method for manufacturing
an objective lens unit used as an objective lens unit of an optical
pickup device for writing or reading out information signals on or
from an optical recording medium, wherein the objective lens unit
manufacturing apparatus determines the distance between the lens
surface closest to the optical recording medium and an end face of
a lens holder lying around the lens surface along the optical
axis.
[0021] In a further manufacturing method for an objective lens
unit, according to the present invention, the other lens, used as a
positioning reference for the one lens, is the lens remote from the
optical recording medium, with the surface of an outer rim of the
other lens remote from the optical recording medium being used as a
reference surface for positioning the one lens. The surface of an
outer rim of the one lens close to the optical recording medium
being used as a positioning reference surface. In positioning the
lens reference surface, using a positioning member, the parallelism
between the reference surface and the positioning member is
measured using laser light.
[0022] In an objective lens unit manufacturing method for
manufacturing an objective lens unit, in positioning the lens
reference surface using a positioning member, the parallelism
between the reference surface and the positioning member is
measured with the reference surface abutting against the
positioning member.
[0023] In an objective lens unit manufacturing method for
manufacturing an objective lens unit, in positioning the lens
reference surface, using a positioning member, the lenses are
attracted under a pneumatic pressure differential for abutting the
reference surface against the positioning member.
[0024] In an objective lens unit manufacturing method for
manufacturing an objective lens unit, the parallelism between the
surface of the outer rim of the other lens remote from the optical
recording medium and the surface of the outer rim of the one lens
close to the optical recording medium is measured by detecting only
the reflected light from the surface of the outer rim of the one
lens remote from the optical recording medium, using a
photodetector, in a state in which the light is incident on the
outer rim of the other lens and in which the light transmitted
through the outer rim and reflected by the surface of the outer rim
close to the optical recording medium is not returned to the
photodetector.
[0025] Other objects, features and advantages of the present
invention will become more apparent from reading the embodiments of
the present invention as shown in the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] FIG. 1 is a longitudinal cross-sectional view of an
objective lens unit according to the present invention.
[0027] FIG. 2 is a longitudinal cross-sectional view of the
objective lens unit built into a lens holder.
[0028] FIG. 3 is a perspective view looking from the first
lens.
[0029] FIG. 4 is a perspective view looking from the second
lens.
[0030] FIG. 5 is a perspective view of a lens holder forming the
objective lens unit looking from a first lens fitting portion.
[0031] FIG. 6 is a perspective view of the lens holder forming the
objective lens unit looking from a second lens fitting portion.
[0032] FIG. 7 is a plan view showing a lens holder forming the
objective lens unit of the present invention.
[0033] FIG. 8 is a cross-sectional view taken along line VIII-VIII
of FIG. 7.
[0034] FIG. 9 is a bottom plan view showing the lens holder forming
the objective lens unit of the present invention.
[0035] FIG. 10 is a cross-sectional view taken along line X-X of
FIG. 9.
[0036] FIG. 11 is a graph showing the relation between the
eccentricity between the lenses and the value of the wavefront
aberration.
[0037] FIG. 12 is a longitudinal cross-sectional view showing a
metal die for forming the lens holder.
[0038] FIG. 13 is a graph showing the parallelism between the
lenses and the value of the wavefront aberration.
[0039] FIG. 14 is a graph showing the distance between the lenses
and the value of the wavefront aberration.
[0040] FIG. 15 is a longitudinal cross-sectional view showing the
process of assembling the objective lens unit.
[0041] FIG. 16 is a schematic longitudinal cross-sectional view
showing the state in which the second lens of the objective lens
unit of the present invention has been introduced into the lens
holder.
[0042] FIG. 17 is a longitudinal cross-sectional view showing an
objective lens unit mounted on an objective lens manufacturing
device according to the present invention.
[0043] FIG. 18 is a longitudinal cross-sectional view showing
another objective lens unit mounted on an objective lens
manufacturing device according to the present invention.
[0044] FIG. 19 is a longitudinal cross-sectional view showing still
another objective lens unit mounted on an objective lens
manufacturing device according to the present invention.
[0045] FIG. 20 is a longitudinal cross-sectional view showing a
conventional objective lens unit mounted on an objective lens
manufacturing device according to the present invention.
[0046] FIG. 21 is a longitudinal cross-sectional view showing an
objective lens unit in which a lens holder is provided with a
protector.
[0047] FIG. 22 is a plan view thereof.
[0048] FIG. 23 is a side view showing a structure of an objective
lens manufacturing device according to the present invention, with
a portion thereof being broken away.
[0049] FIG. 24A is a plan view showing the shape of a device
reference plane for positioning the first lens in the objective
lens manufacturing device according to the present invention, FIG.
24B is a longitudinal cross-sectional view thereof and FIG. 24C is
a plan view showing a reference plane of the first lens.
[0050] FIG. 25 is a flowchart showing the sequence of operations of
an objective lens unit producing method employing the objective
lens manufacturing device according to the present invention.
[0051] FIG. 26 is a flowchart showing the sequence of operations
next following the sequence of operations of FIG. 25.
[0052] FIG. 27 is a side view showing an objective lens unit
driving device performing first adjustment in an objective lens
unit manufacturing device according to the present invention, with
a portion thereof being broken away.
[0053] FIG. 28 is a side view showing an objective lens unit
driving device performing second adjustment in an objective lens
unit manufacturing device according to the present invention, with
a portion thereof being broken away.
[0054] FIG. 29 is a side view showing an objective lens unit
driving device performing the mounting of a first lens in an
objective lens unit manufacturing method according to the present
invention, with a portion thereof being broken away.
[0055] FIG. 30 is a side view showing an objective lens unit
driving device performing the mounting of a second lens in an
objective lens unit manufacturing method according to the present
invention, with a portion thereof being broken away.
[0056] FIG. 31 is a side view of the objective lens unit driving
device executing the positioning to the lens holder of the second
lens in the objective lens unit manufacturing method according to
the present invention, with a portion thereof being broken
away.
[0057] FIG. 32 is a side view of the objective lens unit driving
device executing the confirmation of the offset between the lenses
in the objective lens unit manufacturing method according to the
present invention, with a portion thereof being broken away.
[0058] FIG. 33 is a flowchart showing the sequence of operations
for manufacturing an objective lens unit using another objective
lens unit manufacturing device according to the present
invention.
[0059] FIG. 34 is a flowchart showing the sequence of operations
next following the sequence of operations of FIG. 33.
[0060] FIG. 35 is a side view showing a first device of another
objective lens unit manufacturing device according to the present
invention.
[0061] FIG. 36 is a side view showing the state in which the second
lens is being positioned to the lens holder in the objective lens
unit manufacturing device shown in FIG. 35, with a portion thereof
being broken away.
[0062] FIG. 37 is a longitudinal cross-sectional view showing the
second lens positioned in the objective lens unit manufacturing
device shown in FIG. 35 and the lens holder 3.
[0063] FIG. 38 is a side view showing a second device of the other
objective lens unit manufacturing device according to the present
invention.
[0064] FIG. 39 is a longitudinal cross-sectional view showing
essential portions of the second device of the other objective lens
unit manufacturing device according to the present invention.
[0065] FIG. 40 is a plan view showing essential portions of the
second device of the other objective lens unit manufacturing device
according to the present invention, with a portion thereof being
broken away.
[0066] FIGS. 41A to 41C are graphs showing the relationship between
the spherical aberration in the objective lens unit and focusing
error signals.
[0067] FIG. 42 is a graph showing the relationship between the
spherical aberration in the objective lens unit and the central
value of the focusing error signals.
[0068] FIG. 43 is a plan view showing the structure of an objective
lens driving device employing an objective lens unit manufactured
using the method or the device according to the present
invention.
[0069] FIG. 44 is a side view showing the structure of an objective
lens driving device employing the objective lens unit shown in FIG.
43.
[0070] FIG. 45 is a side view showing the optical pickup device and
a recording and/or reproducing apparatus having an objective lens
unit driving device employing an objective lens unit manufactured
using the method or the device according to the present
invention.
BEST MODE FOR CARRYING OUT THE INVENTION
[0071] Referring to the drawings, preferred embodiments of the
present invention will be explained in detail.
[0072] An objective lens unit of the present invention is made up
by plural lenses each having a numerical aperture (NA) of 0.7 or
larger. Specifically, the objective lens unit is made up by a
double-lens set of two lenses 1, 2, and has a numerical aperture
(NA) of 0.85, as shown in FIG. 1.
[0073] The objective lens unit of the present invention is built
into an optical pickup device having a light source radiating the
light beam with a center wavelength of 405 nm. That is, the
objective lens unit according to the present invention is mainly
used for condensing the light beam having a center wavelength of
405 nm.
[0074] In the following explanation, it is assumed that the
objective lens unit has an effective diameter of 3 mm.
Specifically, the manufacturing method for the objective lens unit
of the present invention is the method for assembling an objective
lens unit. An objective lens unit according to the present
invention is made up by first and second lenses 1, 2 and a lens
holder 3 for holding the lenses 1, 2, as shown in FIGS. 2, 3 and
4.
[0075] The first and second lenses 1, 2 are formed of a vitreous
material, and are prepared by so-called glass mold casting of
forming a vitreous material using a metal die. The shape of the
lens surfaces of the lenses 1, 2, formed as non-spherical or
spherical surfaces, position relationships between the lens
surfaces and outer rims 1a, 2a and so forth depend on the machining
accuracy of the metal dies for molding, and on the casting
conditions.
[0076] The lens holder 3 is formed to approximately a cylindrical
shape by injection molding of epoxy resin, using silica (silicon
dioxide) as a filler, as shown in FIGS. 5 to 10. To this lens
holder 3 are fitted the first and second lenses 1, 2. The first and
second lenses 1, 2, inserted in position in the lens holder 3, are
secured with an adhesive, such as UV curable resin.
[0077] In this objective lens unit, the eccentricity of the lenses
1, 2 relative to the optical axis P.sub.1, that is positions of the
lenses in bi-axial directions, corresponding to directions within
the planar surface perpendicular to the optical axis P.sub.1, among
the relative positions between the lenses 1, 2, are set by the
outer diameters of the outer rims 1a, 2a of the lenses 1, 2, and by
the inner diameter of the lens holder 3. That is, first and second
lens fitting portions 4, 5 in which to insert the lenses 1, 2 and
which control the eccentricities thereof are formed in inner sides
of the lens holder 3, as shown in FIGS. 2 and 5.
[0078] If, in the objective lens unit formed by combining the first
and second lenses 1, 2, relative eccentricities are produced
between the lens surfaces of the first and second lenses 1, 2, the
aberration is increased to deteriorate the optical performance, as
shown in FIG. 11.
[0079] It is noted that the relative eccentricities between the
lens surfaces of the first and second lenses 1, 2 mean position
offset in the planar direction perpendicular to the common optical
axis of the lenses 1, 2, centered about this optical axis.
[0080] If, in an objective lens unit with an effective diameter of
3 mm and the numerical aperture NA of 0.85, the relative
eccentricities between the lens surfaces of the first and second
lenses 1, 2 exceed .+-.30 .mu.m, the RMS value of the aberration
exceeds the Marshall criteria threshold (wavefront aberration of
0.07 .lambda.rms). That is, if, in an objective lens unit, formed
by combining the first and second lenses 1, 2, the effective
diameter is 3 mm, the numerical aperture NA is 0.85 and the working
center wavelength is 405 nm, the lenses 1, 2 need to be secured to
the lens holder 3 so that an error range of the coaxial degree of
the lens surfaces of the respective lenses 1, 2 relative to the
outer diameter of the lens bolder 3 will be within 30 .mu.m. The
following factors may be surmised to be responsible for these
relative eccentricities between the lens surfaces of the first and
second lenses 1 and 2:
[0081] (1) The error in the coaxial degree between the outer
peripheral surfaces of the outer rims 1a, 2a of the lenses 1, 2 and
the lens surfaces;
[0082] (2) the error in the coaxial degree between the lens fitting
portions 4, 5 of the lens holder 3; and
[0083] (3) the clearance between the lens fitting portions 4, 5 and
the outer rims 1a, 2a of the lenses 1, 2.
[0084] Since the accumulation of these three factors determines the
amount of the eccentricities of the lens surfaces of the respective
lenses 1, 2, it is essential that at least the coaxial degree of
the lens surfaces of the respective lenses 1, 2 relative to the
outer diameter of the lens holder 3 be within 30 .mu.m.
[0085] That is, since the outer peripheral surfaces 1b, 2b of the
outer rims 1a, 2a of the lenses 1, 2 are formed as one with the
lens surfaces by glass mold casting employing a metal die, the
coaxial degree between the outer peripheral surfaces 1b, 2b of the
outer rims 1a, 2a and the lens surfaces depends on the machining
accuracy of the metal die for casting and on the casting
conditions. When the effective diameter of the objective lens is 3
mm, the outer peripheral surfaces 1b, 2b of the outer rims 1a, 2a
and the lens surfaces are formed so as to have a coaxial degree
with an error range within 30 .mu.m.
[0086] A metal die for forming the lens holder 3 is comprised of a
portion 101 for casting the first lens fitting portion 4 and a
portion 102 for forming the second lens fitting portion 5, these
portions 4, 5 being formed on the same convex die 103, as shown in
FIG. 12. The metal die for forming the lens holder 3 is composed of
the convex die 103 and a concave die 104 into which is inserted the
convex die 103. In this metal die, as the convex die 103 is
introduced into the concave die 104, the space defined between the
convex die 103 and the concave die 104 serves as a cavity 105 into
which the molten resin is introduced. The lens holder 3 is formed
by the molten resin being charged into the cavity 105.
[0087] The portions 101, 102 for casting the first lens fitting
portion 4 and the second lens fitting portion 5, respectively,
provided to the convex die 103 used for forming the lens holder 3,
are formed by concurrent machining on a lathe, that is by machining
by the same chucking operation, so that the coaxial degree is
maintained to high accuracy. In case the effective diameter of the
objective lens unit is 3 mm and a light beam having a working
center wavelength of 405 nm is to be condensed, the first lens
fitting portion 4 of the lens holder 3 formed is formed so as to
have a high coaxial degree, with an error range less than 30 .mu.m,
relative to the second lens fitting portion 5.
[0088] The first lens fitting portion 4, provided to the lens
holder 3, is formed to have an inner diameter R.sub.1 approximately
equal to the outer diameter R.sub.2 of the outer rim 1a of the
first lens 1, as shown in FIG. 2. In case the effective diameter of
the objective lens is 3 mm and the light beam with a working center
wavelength of 405 nm is to be condensed, the inner diameter R.sub.1
of the first lens fitting portion 4 is designed to suffer an error
less than 30 .mu.m with respect to the outer diameter R.sub.2 of
the outer rim 1a of the first lens 1. An inner diameter R.sub.3 of
the second lens fitting portion 5 is approximately equal to an
outer diameter R.sub.4 of the outer rim 2a of the second lens 2. It
is noted that, in case the effective diameter of the objective lens
unit is 3 mm and the light beam with a working center wavelength of
405 nm is to be condensed, the inner diameter R.sub.3 of the second
lens fitting portion 5 is designed to suffer an error less than 30
.mu.m with respect to the outer diameter R.sub.4 of the outer rim
2a of the first lens 2.
[0089] If, in the objective lens unit according to the present
invention, the parallelism between the two lenses 1 and 2 is
deteriorated, the aberration is increased, thus lowering the
optical properties, as shown in FIG. 14. If, in an objective lens
unit with an effective diameter of 3 mm and with the numerical
aperture NA of 0.85, the light beam with a working center
wavelength of 405 nm is to be condensed, the RMS value of the
aberration exceeds the Marshall criteria threshold (wavefront
aberration of 0.07 .lambda.rms) in case the parallelism between the
first and second lenses 1, 2 exceeds .+-.0.1 degree.
[0090] If, with the outer diameter of the lens of 2 mm, the
parallelism between the first and second lenses 1, 2 is to be
comprised in a range of .+-.0.1 degree, it is necessary to support
the first and second lenses 1, 2 by the lens bolder 3 within an
error range of .+-.3.5 .mu.m in accordance with the following
equation:
2 [mm].times.tan(.+-.0.1[deg])=.+-.3.5 .mu.m.
[0091] It is extremely difficult to form the lens holder 3, molded
using the metal die, and which hold the first and second lenses 1,
2 with the parallelism within the above-mentioned error range to
high reproducibility. Even if the lens holder 3 capable of holding
the first and second lenses 1, 2 with such high accuracy could be
formed, it is extremely difficult to put the first and second
lenses 1, 2 and the lens holder 3 together without interposition of
fine dust and dirt in-between under a working environment of
assembling the objective lens unit. As a result, it is extremely
difficult to mount the first and second lenses 1, 2 on the lens
holder 3 with parallelism within the above-defined error range.
[0092] With the objective lens unit of the present invention,
unless the distance between the first and second lenses 1, 2, put
together as the optical axes P.sub.1 of the lenses are coincident
and as the parallelism within a preset error range is maintained,
is kept within a preset range, the aberration is increased, while
the optical characteristics are deteriorated, as shown in FIG. 14.
If, with the effective diameter and the numerical aperture of the
objective lens unit of 3 mm and 0.85, respectively, and with the
light beam to be condensed thereby having a working center
wavelength of 405 nm, an error in the distance between the first
and second lenses 1, 2 exceeds .+-.13 .mu.m, the RMS value of the
aberration exceeds the limit of the Marshall criteria (wavefront
aberration: 0.07 Arms).
[0093] It is extremely difficult to form the lens holder 3, capable
of holding the first and second lenses 1, 2, forming the objective
lens, with the distance therebetween of the above-defined error
range, to high reproducibility, using a metal die.
[0094] It is extremely difficult to form the lens holder 3, capable
of holding the objective lens unit, based solely on the casting
accuracy in using a metal die device, as the optical axes P.sub.1
of the first and second lenses 1, 2 are registered highly
accurately, high degree of parallelism is maintained between the
lenses 1, 2, and as the error in the distance between the lenses 1,
2 is maintained to be within a preset range.
[0095] With the objective lens unit of the present invention, the
precision in the parallelism and the distance between the first and
second lenses 1, 2 are guaranteed, using an assembling jig capable
of adjusting the assembling accuracy, without being dependent on
the precision in casting the lens holder 3.
[0096] That is, in setting the relative positions of the first and
second lenses 1, 2 forming the objective lens unit of the present
invention, the first lens 1 is inserted into the first lens fitting
portion 4 provided within the lens holder 3 and is immobilized
using a UV curable resin. At this time, the first lens 1 is
introduced into the first lens fitting portion 4 so that its first
surface S.sub.1 being convex with a large radius of curvature will
be protruded from the lens holder 3, as shown in FIGS. 2 and 15.
The first surface S.sub.1 of the first lens 1 operates as an
incident side for the light beam radiated from a light source when
the objective lens unit is mounted on an optical pickup device.
[0097] The first lens 1, secured to the lens holder 3, is supported
on a base block 30 of the jig. At this time, the first lens 1 is
carried as its surface 1c towards the first surface of the outer
rim 1a is placed on a supporting surface provided on the base block
30 of the jig.
[0098] The second lens 2, combined with the first lens 1 to form
the objective lens unit of the present invention, is held by a
holding jig 31, the relative position of which with respect to the
base block 30 supporting the first lens 1 along with the lens
holder 3 is maintained to high accuracy. The second lens 2 is
introduced into the second lens fitting portion 5 of the lens
holder 3, carried by the base block 30, as this holding jig 31 is
moved towards the base block 30.
[0099] Meanwhile, the holding jig 31 is connected to an air suction
device so as to hold the second lens 2 at its distal end by air
suction in a direction indicated by arrow E in FIG. 15.
[0100] The second lens 2 is introduced into the second lens fitting
portion 5 so that the lens surface being convex to a larger radius
of curvature faces a second surface S.sub.2 of the first lens 1
secured to the lens holder 3.
[0101] The convex surface of the second lens 2, facing the second
surface S.sub.2 of the first lens 1, forms a third surface S.sub.3
of the objective lens unit.
[0102] The second lens 2, introduced into the second lens fitting
portion 5, is positioned, with the first lens 1 secured to the
first lens fitting portion 4 of the lens holder 3 as a reference,
and is bonded to the lens holder 3, using an adhesive, such as a UV
curable resin.
[0103] When the first lens 1 is secured to the lens holder 3, the
position of the first lens 1 along its optical axis with respect to
the lens holder 3 and the tilt of the first lens 1 with respect to
the center axis of the lens holder 3 are controlled by an abutting
surface 1d towards the second surface S.sub.2 of the outer rim la
compressing against a step 3a provided to the first lens fitting
portion 4 formed on the lens holder 3.
[0104] An adhesive film with thickness on the order of 10 .mu.m is
interposed between the abutting surface 1d of the outer rim 1a of
the first lens 1 and the step 3a within the lens holder 3, for
securing the first lens 1 to the lens holder 3. As the adhesive, a
UV curable resin, curable by illumination by the UV light, is
used.
[0105] The objective lens unit according to the present invention
is used for an optical pickup device adapted for writing or reading
out information signals on or from an optical recording medium.
When used in an optical pickup device, the objective lens unit of
the present invention is arranged so that the first lens 1 and the
second lens 2 will be located towards the light source radiating
the light beam and towards an optical recording medium 110,
respectively, as shown in FIGS. 1 and 2. Since the objective lens
unit used in the optical pickup device is used for converging the
light beam radiated from the light source on a signal recording
surface 111 of the optical recording medium 110, the first and
second lenses 1, 2 are combined so that the first surface S.sub.1
and the third surface S.sub.3, being convex to a larger radius of
curvature, will be located towards the light source radiating the
light beam.
[0106] In the objective lens unit of the present invention, the
mounting position of the second lens 2 is set with the mounting
position of the first lens 1 as reference. Thus, with the objective
lens unit of the present invention, the second lens 2 is positioned
with the surface 1c of the outer rim 1a of the first lens 1, on
which falls a light beam L.sub.1, as a reference surface, as shown
in FIG. 2.
[0107] The second lens 2, secured to the lens holder 3 with the
mounting position of the first lens 1, secured in position to the
lens holder 3, as reference, is introduced into and carried by the
second lens fitting portion 5 of the lens holder 3 and hence is
such a state in which the eccentricity of the first lens 1 relative
to the optical axis of the first lens is suppressed, that is in
which the offset of the first lens 1 in a planar direction
perpendicular to the optical axis is suppressed. When introduced
into the second lens fitting portion 5, the second lens 2 is in
such a state which, as the eccentricity thereof relative to the
optical axis of the first lens 1 is suppressed, enables adjustment
of the parallelism of the second lens 2 relative to the first lens
1, that is the tilt and the distance with respect to the optical
axis.
[0108] That is, when forming the second lens 2, the metal die is
pressured in a direction along the thickness of the second lens 2,
that is in a direction parallel to its optical axis. With the
second lens 2, formed through this molding process, corner portions
2c, 2c on the outer peripheral side of the outer rim 2a, which the
vitreous material reaches last during the casting process, present
curved surfaces, whereas the portion in an outer peripheral surface
2b of the outer rim 2a which becomes a cylindrical surface parallel
to the optical axis of the second lens 2 is only a portion with a
width W.sub.1 of approximately 100 .mu.m at a mid portion along the
direction of thickness, as shown in FIG. 16. The contact surface of
the outer peripheral surface 2b of the outer rim 2a of the so
formed second lens 2 with the inner peripheral surface of the
second lens fitting portion 5 is only the ring-shaped portion of a
width W.sub.1 of the order of 100 .mu.m. As a result, the second
lens 2, inserted into the second lens fitting portion 5, can be
rotated in a direction perpendicular to the optical axis about a
ring-like portion of the width W.sub.1 of the order of 100 .mu.m,
contacted with the inner peripheral surface of the second lens
fitting portion 5, as the center of rotation. Additionally, the
second lens 2 can be moved in a direction along the optical axis
within the second lens fitting portion 5.
[0109] In an objective lens unit manufacturing device according to
the present invention, as later explained, detection of the tilt of
an optical axis of the first lens 1 relative to the reference
surface of the device is by illuminating the laser light to a
reference surface 1c of an outer rim 1a of the first objective lens
1, on which is incident the light beam L.sub.1, by reflecting the
light reflected back from the reference surface 1c of the first
lens 1 by a mirror 42, so that the light will be incident on
detection means, such as a first CCD (imaging device) 32, and by
inputting an output detected by the CCd 32 to the first monitor 33,
as shown in FIG. 17.
[0110] The surface 1e on the outer rim 1a of the first lens 1,
positioned towards the optical recording medium 110, is formed as a
surface inclined towards the outer rim, as shown in FIG. 18. That
is, since the surface 1e of the outer rim 1a of the first lens 1
positioned towards the optical recording medium 110 is formed as an
inclined surface, the outer rim of which is proximate to the
reference surface 1c, a portion of the laser light illuminated on
the reference surface 1c and transmitted through this reference
surface 1c proceeds as indicated by a broke line in FIG. 18,
without reaching the first CCD 32, even if the light is reflected
by the surface 1e lying towards the optical recording medium 110,
thus assuring optimum detection of the tilt of the first lens
1.
[0111] That is, if the surface 1e of the outer rim 1a of the first
lens 1, lying towards the optical recording medium 110, is not
inclined, as shown in FIG. 20, but is formed as a surface parallel
to the reference surface 1b, a portion of the laser light
illuminated on the reference surface 1c may be transmitted through
the reference surface 1c to reach the surface 1e parallel to the
reference surface 1c so as to be then reflected by this surface 1e
to proceed as indicated by a broken line in FIG. 20 to get to the
first CCD 32 after reflection by the mirror 42. If the light
reflected by the reference surface 1c and the light reflected by
the surface 1e towards the optical recording medium 110 reach the
first CCD 32 in a mixed state, the two light beams undergo
interference to prove to be a noise against detection of tilt of
the first lens 1. There is also the risk that multiple reflection
be produced between the reference surface 1c and the surface 1e
towards the optical recording medium 110 to render it impossible to
detect the tilt of the first lens 1 satisfactorily.
[0112] On the other hand, the surface 1e of the outer rim 1a of the
first lens 1 towards the optical recording medium 110 may be formed
as an inclined surface recessed from the outer rim side of the
outer rim 1a towards the center of the lens 1, as shown in FIG. 18.
In this case, the light reflected by the surface 1e towards the
optical recording medium 110 facing the reference surface 1c is
reflected from the center towards the outer rim of the first lens
1, as indicated by a broken line in FIG. 18, thus reliably
prohibiting the light reflected back from the surface 1e from
falling on the first CCD 32.
[0113] The surface 1e of the outer rim 1a of the first lens 1 may
also be formed as an inclined surface which is recessed from the
outer rim side of the outer rim 1a towards the center of the lens
1, and which also smoothly merges to the second surface S.sub.2 of
the first lens 1, as shown in FIG. 19.
[0114] Moreover, for preventing the light reflected back form the
surface towards the optical recording medium 110 of the outer rim
1a of the first lens 1 from falling on the first CCD 32, an AR coat
(anti-reflection film) may be provided on this surface 1e to
inhibit reflection of the laser light incident on ths surface
1e.
[0115] When the objective lens of the present invention is used in
an optical pickup device for writing or reading out information
signals for an optical recording medium, such as an optical disc,
it is necessary for the normal operation to be guaranteed in a
range of a temperature environment from sub-freezing point to
60.degree. C. or higher. If the temperature environment is changed
throughout this range, the air present in the space formed between
the first and second lenses 1, 2, inserted into and carried by the
lens holder 3, is changed appreciably in density.
[0116] The air present in the hermetically sealed space between the
first and second lenses 1, 2 is changed appreciably in pneumatic
pressure due to changes in the temperature environment. That is, if
the temperature rises, the pneumatic pressure between the lenses 1
and 2 is larger than the atmospheric pressure, thus producing the
pressure which tends to separate the lenses 1 and 2 away from each
other. If conversely the temperature falls, the pneumatic pressure
between the lenses 1 and 2 is smaller than the atmospheric
pressure, thus producing a pressure which tends to cause the lenses
1 and 2 to approach to each other.
[0117] Meanwhile, the first lens 1 has an abutment surface 1d,
lying towards the second surface S.sub.2 of the outer rim 1a,
bonded to the step 3a of the first lens fitting portion 4 with a UV
curable resin. The second lens 2, on the other hand, is bonded in
position with an adhesive, charged into three recessed adhesive
coating portions 3b, formed by cutting out portions of the inner
rim of the second lens fitting portion 5, as shown in FIGS. 4 and
7.
[0118] Since the first and second lenses 1, 2 are secured to the
lens holder 3 with an adhesive, as described above, the adhesive
securing the first and second lenses 1, 2 to the lens holder 3, in
particular the adhesive securing the second lens 2 to the lens
holder 3, is subjected to a stress, due to repeated changes in
temperature, representing a so-called heat cycle, thus producing
irreversible variations in the distance and tilt of the lens to
give rise to deteriorated optical aberration characteristics.
[0119] Assuming that the space between the first and second lenses
1, 2, carried by the lens holder 3, is a hermetically sealed space,
the air present in the space between the first and second lenses 1,
2 is pressurized, when the second lens 2 is inserted into the lens
holder 3 after mounting the first lens 1 on the lens holder 3, with
the result that the pressure acting to separate the lenses 1, 2
away from each other is present at all times between the lenses 1
and 2.
[0120] Thus, with the objective lens unit of the present invention,
an air vent 3c is provided in the lens holder 3, as shown in FIGS.
3 and 5, such that the space between the first and second lenses 1,
2 is a space communicating with an outer side of the lens holder 3.
This air vent 3c is formed by forming a recess 3d in a portion of
the step 3a, compressing against the abutment surface 1d of the
outer rim 1a of the first lens 1, so that the recess 3d is not
contacted with the abutment surface 1d of the outer rim 1a, and by
forming a cut-out 3e for establishing communication between the
recess 3d with the outer peripheral surface of the lens holder 3,
as shown in FIG. 5.
[0121] By forming the air vent 3c in the lens holder 3, the space
between the first and second lenses 1, 2 does not prove a
hermetically sealed space, while air in this space is not subjected
to changes in the pneumatic pressure, even if the density is
changed with changes in temperature, so that no pressure is applied
to the lenses 1, 2, and hence no deterioration in optical
aberration characteristics is produced despite occurrence of
repeated changes in temperature.
[0122] Since the space between the first and second lenses 1, 2
does not prove to be a hermetically sealed space, there is no risk
of air present in the space between the first and second lenses 1,
2 becoming compressed and pressurized on insertion of the second
lens 2 into the lens holder 3 after mounting the first lens 1 to
the lens holder 3.
[0123] For providing the recess 3d in a portion of the step 3a of
the lens holder 3, the portion of the convex die 103 for forming
the lens holder 3, which is destined to form the recess 3d, is
removed by cutting to produce the step 3a.
[0124] On the other hand, a gate G, provided for charging molten
resin into a metal die in injection molding the lens holder 3, is
provided in a groove 3f formed in the outer peripheral surface of
the lens holder 3 in register with a cut-out 3e forming the air
vent 3c, as shown in FIG. 5.
[0125] When the objective lens of the present invention is used in
an optical pickup device for writing or reading out information
signals for an optical recording medium, it is necessary for the
normal operation to be guaranteed in a range of a temperature
environment from sub-freezing point to 60.degree. C. or higher. If
the temperature environment is changed throughout this range, the
distance between the first and second lenses 1, 2 is changed due to
thermal expansion or contraction of the lens holder 3 itself.
[0126] If, with the effective diameter of 3 mm, the working center
frequency of 405 nm and the numerical aperture of 0.85, the
distance between the first and second lenses 1, 2 is subjected to
an error within 13 .mu.m, and the linear expansion coefficient of
the material forming the lens holder 3 is .alpha., the following
relationship:
.alpha..times..DELTA.t.times.L<13.times.10.sup.-3 (mm)
[0127] is derived.
[0128] Here, .DELTA.t (temperature change) and L (length of the
lens holder) are set as follows:
.DELTA.t=60/2=30(.degree. C.) and
L=2 (mm).
[0129] From these conditions, the following condition for the
linear expansion coefficient .alpha. of the material making up the
lens holder 3:
.alpha.<2.times.10.sup.-4
[0130] is derived.
[0131] Meanwhile, if an epoxy resin, having silica (silicon
dioxide) as a filler, is selected as the material forming the lens
holder 3, the linear expansion coefficient can be suppressed to a
value on the order of 1.times.10.sup.-5.
[0132] The objective lens unit of a double-lens set, made up by two
lenses, with the numerical aperture (NA) of not less than 0.7, has
a working distance (the physical distance between the surface of
the optical recording medium and the end face of the objective
lens) smaller than that of a single objective lens used in a
conventional optical pickup device. In the optical pickup device,
the objective lens position is controlled by focusing servo in such
a manner that the distance between the surface of the optical
recording medium and the end face of the objective lens will at all
times be a preset working distance. The focusing servo, in turn,
controls the light beam, converged by the objective lens, so that
the light beam, converged by the objective lens, will be focussed
on the signal recording surface of the optical recording medium.
For example, if disturbances, such as external shock, is applied to
the optical pickup device, the objective lens unit may become
offset from its design position. If, in this case, the working
distance is long, the probability of collision between the optical
recording medium and the objective lens unit is low. However, if
the working distance is short, the probability of collision between
the rim and the objective lens unit becomes higher.
[0133] If, when the collision occurs between the optical recording
medium and the objective lens unit, the surface of the optical
recording medium is directly contacted with the lens surface of the
objective lens unit, these surfaces are damaged to deteriorate
recording and/or reproducing characteristics.
[0134] With the objective lens unit of the present invention, a
protector 6 is mounted on one end face of the lens holder 3 for
encircling the outer periphery of the second lens 2, in order not
to damage the surface of the optical recording medium or the lens
surface of the objective lens unit, as shown in FIGS. 20 and 21.
The protector 6 is formed by for example a film of fluorine resin
exhibiting elasticity and is provided for being protruded closer to
the optical recording medium 110 than the lens surface of the
second lens 2 lying closest to the optical recording medium, as
shown in FIG. 20. The protector 6 helps prevent direct contact of
the surface of the optical recording medium 110 with the lens
surface of the objective lens unit, while buffering the force of
impact caused by collision between the optical recording medium 110
and the objective lens unit. Moreover, since the protector 6 has an
only small frictional coefficient against the surface of the
optical recording medium, it is also possible to avoid
scorching.
[0135] If this protector 6 is provided as described above and
should collide against the surface of the optical recording medium
110, the protector 6 operates effectively to prevent the surface of
the optical recording medium 110 from being damaged. However, the
fluorine resin or the like material, forming the protector 6, is
exfoliated. Such debris 6a from the protector 6 becomes accumulated
on the lens surface of the second lens 2. If the debris 6a is
accumulated within an area of the lens surface of the second lens 2
traversed by the light beam, the light path of the light beam is
interrupted to lower the transmission ratio, while affecting
optical characteristics, such as optical spatial frequency.
[0136] Meanwhile, the debris 6a, produced on exfoliation from the
protector 6, is migrated along the direction of relative movement
of the optical recording medium 110 with respect to the objective
lens unit, that is along the tangential direction of the optical
recording medium 110, as indicated by arrow X in FIG. 22.
[0137] Thus, in the objective lens unit of the present invention,
the protector 6 is formed as one with a cut-out 7 larger in breadth
than the diameter R.sub.5 of the light beam L1 on the lens surface
of the second lens 2, as shown in FIG. 21. In use, this objective
lens unit is mounted so that the cut-out 7 is on the path of
relative movement of the optical recording medium 110 with respect
to the objective lens unit.
[0138] Referring to the drawings, an objective lens unit
manufacturing device, according to the present invention, used for
manufacturing the above-described objective lens unit, is now
explained.
[0139] Referring to FIG. 23. The objective lens unit manufacturing
device of the present invention includes an assembling unit for
positioning and assembling first and second lenses 1, 2 to a lens
holder 3, and a detection unit for monitoring the parallelism of
the lens surfaces of the first and second lenses 1 and 2 and the
outer rims 1a and 2a.
[0140] The assembling unit includes a base unit 30, having a device
reference surface 30a, on which to set the first lens 1, and a
holding unit 31, having a device reference surface 31a on which to
set the second lens 2 in position. The device reference surface
30a, as an upper surface of the base unit 30, is adjusted so as to
be precisely normal to the optical axis of the first lens 1 placed
thereon.
[0141] This base unit 30 is substantially cylindrically-shaped,
with an inner spacing, such that, by placing the outer rim 1a of
the first lens 1 on the device reference surface 30a and by
extracting the inner air via suction bore 30b to outside, the first
lens 1 may be held in position by the air pressure differential
across its inner and outer sides. At this time, the upper end of
the base unit 30 is closed by the first lens 1. A glass cover for
hermetically sealing the inner spacing is mounted to the upper end
of the base unit 30. The glass cover 34 is inclined with respect to
the device reference surface 30a on the upper end to reflect e.g.,
the laser light illuminated on the outer rim 1a of the first lens 1
so as not to produce stray light.
[0142] The holding unit 31 is substantially cylindrically-shaped to
delimit an inner spacing, and is adapted for holding the second
lens 2 under an air pressure differential across its inner side and
the outer side produced on evacuating the inner area through a
suction port 31b to outside with the outer rim 2a of the second
lens 2 abutting against the device reference surface 31a at the
lower end of the unit. At this time, the lower end of the holding
unit 31 is closed by the second lens unit 2. The upper end of the
holding unit 31 is fitted with a glass cover 35 for hermetically
sealing an inner spacing. The glass cover 35 is inclined with
respect to the device reference surface 31a at the lower end of the
unit so as not to produce the stray light caused by reflection of
for example the laser light illuminated on the second lens 2.
[0143] The holding unit 31 is supported by a so-called cross roll
bearing, that is a uni-axial movement stage 36, and is movable
along the optical axes of the first and second lenses 1 and 2. The
amount of movement of this holding unit 31 may be detected by for
example a magnetic length measurement device 37. The holding unit
31 is moved by a driving power source 38, such as a pneumatic
cylinder, a linear motor or a stepping motor.
[0144] The parallelism of the outer rim 1a of the first lens 1, set
on the device reference surface 30a of the base unit 30, with
respect to the device reference surface 30a, may be detected by a
detection system provided with a first laser light source 39. As
the first laser light source 39, any suitable monochromatic light
source, such as a gas laser or a solid laser, may be used in
addition to a semiconductor laser.
[0145] The light beam radiated from the first laser light source 39
is turned into collimated light, with an enlarged beam diameter, by
a collimator lens and a beam expander 40. The light beam, now
turned into the collimated light, is transmitted through a beam
splitter 41, a mirror 42 and a glass cover 34 at the lower end of
the base unit 30 so as to fall on the upper end of the base unit
30. If the first lens 1 is set on the device reference surface 30a
at the upper end of the base unit 30, this collimated light is
reflected by the reference surface 1b of the outer rim 1a of the
first lens 1 and thence returned through the glass cover 34 and the
mirror 42 to the beam splitter 41.
[0146] Meanwhile, the device reference surface 30a at the upper end
of the base unit 30 does not cover the entire surface of the
reference surface 1b of the outer rim 1a of the first lens 1 but
compresses against only a portion of the reference surface 1b at
for example three places to cover only the portions in question, as
shown in FIGS. 24A to 24C. Thus, the laser light L.sub.2, oncoming
from the lower end of the base unit 30, is incident on and
reflected from the portion of the reference surface 1b of the outer
rim 1a of the first lens 1 which is not covered by the device
reference surface 30a at the upper end of the base unit 30, as
shown in FIG. 24C.
[0147] The return light from the reference surface 1b of the outer
rim 1a of the first lens 1 is reflected back from the reflecting
surface of the beam splitter 41 and branched from the return
optical path to the first laser light source 39 to fall through
mirror 43 on the first CCD 32 operating as detection means. The
image picked up by the first CCD 32 is displayed on the first
monitor 33. The collimating of the light beam from the first laser
light source 39 is adjusted on the imaging surface of the first CCD
32 so that the beam diameter will be minimized.
[0148] The parallelism of the outer rim 2a of the second lens 2,
held by suction by the device reference surface 31a of the holding
unit 31, is detected by a detection system having a second laser
light source 44. As the second laser light source 44, any suitable
monochromatic light source, such as a gas laser or a solid laser,
may be used in addition to a semiconductor laser.
[0149] The light beam radiated from the second laser light source
44 is turned into collimated light, with an enlarged beam diameter,
by a collimator lens and a beam expander 45. The light beam, now
turned into the collimated light, is transmitted through a beam
splitter 46, a mirror 47 and the glass cover 35 at the upper end of
the holding unit 31 so as to fall on the lower end of the holding
unit 31. If the first lens is held on the device reference surface
31a at the lower end of the holding unit 31, this collimated light
is reflected by the reference surface 1b of the outer rim 1a of the
first lens 1 and thence returned through the glass cover 3 and the
mirror 47 to the beam splitter 46. The return light is reflected
back from the reflecting surface of the beam splitter 46 and
branched from the return optical path to the second laser light
source 44 to fall through mirror 48 on the second CCD 49 operating
as detection means. The image picked up by the second CCD 49 is
displayed on the second monitor 50. The collimating of the light
beam from the second laser light source 44 is adjusted on the
imaging surface of the second CCD 49 so that the beam diameter will
be minimized.
[0150] The sequence of operations for assembling the objective lens
unit, described above, is now explained by referring to the
flowchart shown in FIGS. 25 and 26. The objective lens unit is
assumed to be used for an optical pickup device, with the numerical
aperture (NA) of 0.85, a center working wavelength (.lambda.) of
405 nm and an effective diameter of 3 mm. It should be noted that,
in the completed objective lens, the major surface of the outer rim
2a of the second lens 2 closer to the optical recording medium and
the major surface of the outer rim 1a of the first lens 1 remote
from the optical recording medium need to be within a range of tilt
of roughly 0.2 mrad to 0.3 mrad. Thus, with the objective lens unit
manufacturing device, the surface used for positioning the outer
rim 1a of the first lens 1 and the surface used for positioning the
outer rim 2a of the second lens 2 need to be adjusted at the outset
to a precision higher than that needed for these lenses 1 and
2.
[0151] This adjustment is by the laser light radiated from the
first laser light source 39, as shown in FIG. 27. That is, in a
step st1 shown in FIG. 25, the lenses 1 and 2 are dismounted from
the manufacturing device, and the laser light radiated from the
first laser light source 39 and reflected back from the device
reference surface 31a of the holding unit 31 is caused to fall on
the first CCD 32. In a step st2, the position of the first CCD 32
is adjusted, as the first monitor 33 is viewed, so that the light
receiving position is the center position.
[0152] In the first monitor 33, since the device reference surface
31a of the holding unit 31 is toroidally-shaped (doughnut-shaped),
a concentric diffraction pattern may be observed, as shown in FIG.
27. In adjusting the position of the first CCD 32, adjustment is
made so that the center of this concentric pattern is in register
with the center of the image format of the first monitor 33.
[0153] In the next step st3 of FIG. 25, a planar reflective mirror
51 is placed on the device reference surface 30a of the base unit
30, as shown in FIG. 28. In the next step st4 of FIG. 25, the
device reference surface 30a is adjusted for tilt so that, when the
reflected light of the light beam from the first laser light source
39 by the reflective mirror 51 is incident on the first CCD 32, the
light beam projected on the first monitor 33 will be at the center
of the image format.
[0154] Up to this step st4, the device reference surface 31a of the
holding unit 31 is rendered parallel to the device reference
surface 30a of the base unit 30.
[0155] Similarly, in a step st5, the position of a second CCD 49 is
adjusted using a second laser light source 44 and a second monitor
50.
[0156] Then, processing transfers to a step st8 where the first
lens 1, already introduced into and bonded to the lens holder 3 in
the steps st6 and st7, is mounted on the device reference surface
30a of the base unit 30, as shown in FIG. 29. At this time, air in
the base unit 30 is drawn to outside to lower the inner pressure to
attract the first lens 1 to the device reference surface 30a.
[0157] It is noted that the reference surface 1b of the outer rim
1a of the first lens 1 needs to be abutted accurately against the
device reference surface 30a of the base unit 30. The reason is
that, if dust and dirt, for example, are interposed between these
reference surfaces 1b and 30a, the lenses 1 and 2 undergo tilting
relative to each other, after assembling, thus worsening the
optical performance, such as aberration. Thus, even when the first
lens 1 is placed on the device reference surface 30a of the base
unit 30, it is necessary for the parallelism between the reference
surface 1b of the first lens 1 and the device reference surface 30a
to be able to be observed.
[0158] Referring to FIG. 24B, a cut-out is provided in the device
reference surface 30a, as shown in FIG. 24B, so that, as this
device reference surface 30a is maintained, the light reflected on
the reference surface 1b of the first lens 1 may be returned to the
first CCD 32.
[0159] Since the reference surface 1b of the first lens 1 is
toroidally-shaped, the reflected light on this reference surface 1b
may be observed, in a step st9 of FIG. 25, as being a concentric
diffraction pattern. If the reference surface 1b of the first lens
1 is completely in tight contact with the device reference surface
30a of the base unit 30, the concentric diffraction pattern,
displayed on the first monitor 33, is at the center of the image
format, because the reference surfaces 1b and 30a are completely
parallel to each other.
[0160] If it has been verified in a step st10 from the display
surface of the first monitor 33 that, due to dust and dirt becoming
trapped between the reference surfaces 1b, 30a, the parallelism
between the reference surfaces 1b and 30a is not within the
reference value, that is if the concentric diffraction pattern is
not at the center of the image format of the first monitor 33,
processing reverts to the step st8 to set the first lens 1 again or
to sweep the reference surfaces 1b and 30a.
[0161] Up to this step, the verticality or parallelism of the outer
rim 1a of the first lens 1 and the optical axis or the device
reference surface 30a of the base unit 30 may be confirmed. The
malfunctioning state of the first lens 1 may be detected before
proceeding to the next step.
[0162] Then, processing transfers to a step st11 where the second
lens 2 is held by the device reference surface 31a of the holding
unit 31 by sucking air inside the holding unit 31, as shown in FIG.
26.
[0163] Then, processing transfers to a step st12 of FIG. 25 where
it is confirmed whether or not the light reflected from the second
lens 2 is incident on the center of the second CCD 49, in the same
way as above. If, in a step st13, the offset of the center of the
reflected light from the image center is not comprised within the
reference value, processing reverts to the step st11 to mount the
second lens 2 again or to sweep the device reference surface
31a.
[0164] Up to this step, the parallelism between the second lens 2
and the first lens 1 is comprised within a preset range. In this
state, the second lens 2 is caused to descend from above, in a step
st14 of FIG. 26, to the lens holder 3, as shown in FIG. 31.
[0165] When the first and second lenses 1, 2 are attracted to the
device reference surfaces 30a, 31a, the parallelism between these
lenses 1 and 2 is kept. However, these lenses 1 and 2 are not
positioned with respect to offset.
[0166] The lens holder 3 is molded from a thermosetting resin, as
described above, and has the function of suppressing the offset
between the first and second lenses 1 and 2 to less than several
.mu.m. That is, the offset between the first and second lenses 1
and 2 may be removed by moving the second lens 2 along two axes
perpendicular to the optical axis, so as to be positioned along the
inner radius of the lens holder 3, at the same time as the second
lens 2 is introduced into the lens holder 3.
[0167] After the offset between the first and second lenses 1, 2
has been removed by the lens holder 3, the holding unit 31 is
caused to descend, in a step st15 in FIG. 26, to a preset position
at which the second lens 2 is to be positioned ultimately. In order
for the device reference surface 31a to be halted at this position,
there is provided a stopper, for example. Meanwhile, the length
measurement device 37 may be used to monitor whether or not the
holding unit 31 has been lowered to the correct position.
[0168] In a step st16, the parallelism between the reference
surface 1b and 2a of the lenses 1 and 2 is checked. If the
parallelism is unsatisfactory, processing reverts to the step st18
and, if it is satisfactory, processing transfers to the next step
st17. Thus, in the present manufacturing device, it is possible to
check, before positioning and bonding the second lens 2 to the lens
holder 3, it is possible to check whether or not the parallelism
and the distance between the lenses 1 and 2 are comprised within
reference values.
[0169] In the next step st17, the second lens 2 is bonded to the
lens holder 3, using a UV light curable adhesive, as the second
lens 2 has been positioned to the lens holder 3 within a reference
range.
[0170] With the manufacturing device according to the present
invention, it can be checked, in a step st18, by observing the beam
position in the second CCD 49, even after curing of the UV light
curable resin, whether or not the parallelism between the first and
second lenses 1 and 2 is within a reference level. A lens unit is
determined to be a reject if the parallelism between the two lenses
1 and 2 is not confined within a reference value.
[0171] In a step st19, when the adhesive is cured completely, the
attraction of the second lens 2 with respect to the device
reference surface 31a of the holding unit 31 is canceled, when the
adhesive is cured completely. A lens assy composed of the lenses 1
and 2 and the lens holder 3 is spaced apart from the device
reference surface 31a.
[0172] In the next step st20, the holding unit 31 is retreated
towards above and an offset confirming mirror 52 is inserted to a
space above the lens assy, as shown in FIG. 32. The laser light
radiated from the first laser light source 39 is incident on and
transmitted through the respective lenses 1 and 2 of the lens assy
so as to be received through an offset check mirror 52 by a third
CCD 53. The diffraction pattern, imaged by this third CCD 53, is
displayed on a third monitor 54.
[0173] In a step st21, shown in FIG. 26, it is checked whether or
not the offset between the first and second lenses 1 and 2 is
comprised within a prescribed range. Turning to the detection of
the offset between the two lenses, the offset of the second lens 2
relative to the first lens 1 may be detected by transmitting the
laser light through the first and second lenses 1 and 2 and by
remote-detecting the center position of the diffraction pattern of
the transmitted light, as disclosed in Japanese Laying-Open Patent
Publication H-10-255304. A lens unit is determined to be a reject
if the parallelism between the two lenses 1 and 2 is not confined
within a reference value.
[0174] In a step st22, the aberration of the objective lens unit,
composed of the first and second lenses 1 and 2, is measured. With
the aberration within the reference range, processing transfers to
a step st23 to complete the objective lens unit. With the
aberration outside the reference range, the objective lens unit is
determined to be a reject.
[0175] A device for introducing the first lens 1 in the lens holder
3, into which the second lens 2 has already been introduced and
bonded in position, is now explained.
[0176] The process for manufacturing the objective lens unit, using
the manufacturing device, hereinafter explained, is now explained
with reference to a flowchart shown in FIGS. 33 and 34.
[0177] The objective lens unit manufacturing device is made up by a
device section for introducing the second lens 2 into the lens
holder 3, as shown in FIG. 35, and a device section for introducing
the first lens 1 into the lens holder 3, into which the second lens
2 has been introduced, as shown in FIG. 38.
[0178] The device section for introducing the second lens 2 into
the lens holder 3 includes a holding unit 31 and a base unit 30, as
in the case of the above-described manufacturing device, as shown
in FIG. 35. This device section introduces the second lens 2 in
position in the lens holder 3.
[0179] The holding unit 31 is substantially cylindrically-shaped,
with an inner spacing, as in the above-described manufacturing
device, and houses the second lens 2 under an air pressure
differential produced across the inner and outer sides on
evacuating the inner spacing by drawing air to outside through a
suction through-hole 31b with the outer rim 2a of the second lens 2
abutting on the lower device reference surface 31a. The lower end
of the holding unit 31 is kept closed at this time by the second
lens 2. The upper end of this holding unit 31 is fitted with a
glass cover 35 for hermetically sealing the inner spacing.
[0180] The holding unit 31 is supported by a so-called cross roll
bearing, that is a uni-axial movement stage 36, and is movable
along the optical axis of the second lens 2. The amount of movement
of this holding unit 31 may be detected by for example a magnetic
length measurement device 37. The holding unit 31 is moved by a
driving power source 38, such as a pneumatic cylinder, a linear
motor or a stepping motor.
[0181] The base unit 30 is substantially columnar-shaped and has
its upper surface as a device reference surface 30a. This device
reference surface 30a in the previous embodiment is shaped to
receive the outer rim 1a of the first lens 1. In the present
embodiment, the device reference surface 30a is shaped to set the
lens holder 3 directly thereon.
[0182] In the present manufacturing device, the absolute distances
of the device reference surfaces 30a, 31a along the optical axes
are detectable by the length measurement device 37.
[0183] First, in a step st31, shown in FIG. 33, the parallelism
between the device reference surface 31a of the holding unit 31 and
the device reference surface 30a of the base unit 30 is adjusted
accurately at the outset. If, in a step st32, the parallelism
between the device reference surfaces 31a, 30a is comprised within
a preset value, processing transfers to a step st33 and, if
otherwise, processing reverts to the step st31.
[0184] In the next step st33, the lens holder 3 is set on the
device reference surface 30a of the base unit 30, as shown in FIG.
35. The outer diameter of the device reference surface 30a is set
so as to be equal to the outer diameter of the first lens 1, with
the step 3a as the reference surface of the lens holder 3 abutting
against the device reference surface 30a.
[0185] In a step st34, shown in FIG. 33, the reference surface 2b
of the second lens 2 is abutted against the device reference
surface 31a of the holding unit 31, and air within this holding
unit 31 is drawn by suction to attract the second lens 2 to the
holding unit 31. In a step st35, shown in FIG. 33, it is monitored
whether or not the parallelism between the reference surface 2b of
the second lens 2 and the device reference surface 31a of the
holding unit 31 is kept, using the laser light. If this parallelism
is within a prescribed range, processing transfers to a step st36
and, if otherwise, processing reverts to the step st34 for renewed
setting or sweeping the reference surfaces 2b and 31a. The above
sequence of operations is repeated until the predetermined
parallelism is reached.
[0186] In a step st36, the holding unit 31 is lowered towards the
lens holder 3. In a step st37, the holding unit 31 is halted when
the device reference surface 31a of the holding unit 31 is at a
predetermined distance from the device reference surface 30a of the
base unit 30, as shown in FIG. 36. The position relationships
between the second lens 2 and the lens holder 3 are selected such
that the surface of the second lens 2 towards the optical recording
medium and the surface of the protector 6 of the lens holder 3 will
be spaced apart by a predetermined distance, as shown in FIG.
37.
[0187] If the distance from the step 3a of the lens holder 3 to the
surface of the protector 6, indicated by arrow a in FIG. 37, is
prescribed accurately, the distance between the surface of the
second lens 2 towards the optical recording medium and the surface
of the protector 6 can be prescribed accurately by prescribing the
distance from the device reference surface 30a of the base unit 30
abutted against the step 3a, shown by arrow b in FIG. 37, to the
surface of the second lens 2 towards the optical recording
medium.
[0188] The lens bolder 3 is formed e.g., of a thermosetting resin,
as described above, and is molded so that the distance from the
step 3a to the end face thereof carrying the protector 6 is
approximately within .+-.3 .mu.m. The protector is formed of a
protector material, such as fluorine coating material, having
buffering and low friction coefficients, and is formed to a
thickness precision within approximately less than tens of .mu.m.
Consequently, the precision on the order of approximately tens of
.mu.m is maintained for the distance from the step 3a to the
surface of the protector 6, as shown by arrow a in FIG. 37. The
error in the distance from the device reference surface 30a to the
surface of the second lens 2, as indicated by arrow b in FIG. 37,
is kept to approximately less than several .mu.m by the objective
lens unit manufacturing device. Consequently, the accuracy of
approximately less than tens of .mu.m may be maintained for the
distance between the surface of the second lens 2 and the surface
of the protector 6.
[0189] In the next step st38 of FIG. 34, the lens holder 3,
carrying the second lens 2, is mounted on the device section for
introducing the first lens 1 into the lens holder 3, as shown in
FIG. 38.
[0190] This device section includes a movable base unit 59, capable
of holding the first lens 1 on a device reference surface 58, by
suction, and a holding unit 56, capable of holding the lens holder
3, carrying the second lens 2, by suction on a device reference
surface 55. The device reference surface 58, as an upper surface of
the movable base unit 59, is adjusted so as to be accurately
perpendicular to the optical axis of the first lens 1 placed
thereon.
[0191] The movable base unit 59 is substantially
cylindrically-shaped, with an inner spacing, and holds the first
lens 1 under an air pressure differential produced across the inner
and outer sides on evacuating the inner spacing, by drawing air
therein to outside through a suction through-hole, with the outer
rim 1a of the first lens 1 abutting on the device reference surface
58. The upper end of the movable base unit 59 is closed at this
time by the first lens 1. The lower end of this movable base unit
59 is fitted with a glass cover 34 for hermetically sealing the
inner spacing. The glass cover 34 is tilted relative to the device
reference surface 58 towards the upper end thereof to reflect the
laser light illuminated on the outer rim 1a of the first lens 1 so
as not to produce the stray light.
[0192] The movable base unit 59 is supported by a so-called cross
roll bearing, that is a uni-axial movement stage 36, and is movable
along the optical axes of the first and second lenses 1 and 2. The
amount of movement of this movable base unit 59 may be detected by
for example a magnetic length measurement device 37. The holding
unit 31 is moved by a driving power source 38, such as a pneumatic
cylinder, a linear motor or a stepping motor.
[0193] The holding unit 56 is substantially cylindrically-shaped,
with an inner spacing, and holds the second lens 2 under an air
pressure differential produced across the inner and outer sides on
evacuating the inner spacing by drawing air therein to outside
through a suction through-hole, with the lens holder 3, carrying
the second lens 2, abutting on the device reference surface 55. The
lens holder 3 mounting the second lens 2 is held by the surface
thereof carrying the protector 6 compressing against the device
reference surface 55. The lower end of the holding unit 56 is
closed at this time by the lens holder 3 and the second lens 2. The
upper end of this holding unit 56 is fitted with a glass cover 35
towards its lower end for hermetically sealing the inner spacing.
The glass cover 35 is tilted relative to the device reference
surface 55 towards its upper end to reflect the laser light
illuminated on the outer rim 2a of the second lens 2 so as not to
produce the stray light.
[0194] The holding unit 56 is supported by an inclined stage 57 for
tilt adjustment.
[0195] This device includes a detection system, including a first
laser light source 39, for detecting the parallelism of the outer
rim of the first lens, set on the device reference surface 58 of
the movable base unit 59, relative to the device reference surface
58. As the first laser light source 39, any suitable monochromatic
light source, such as a gas laser or a solid laser, may be used in
addition to a semiconductor laser.
[0196] The light beam radiated from the first laser light source 39
is turned into collimated light, with an enlarged beam diameter, by
a collimator lens and a beam expander 40. The light beam, now
turned into the collimated light, is transmitted through a beam
splitter 41, a mirror 42 and the glass cover 34 at the lower end of
the movable base unit 59 so as to fall on the upper end of the
movable base unit 59. If the first lens is held on the device
reference surface 59 at the upper end of the movable base unit 58,
this collimated light is reflected by the reference surface 1b of
the outer rim 1a of the first lens 1 and thence returned through
the glass cover 34 and the mirror 42 to the beam splitter 41.
[0197] The return light, reflected back from the is reflected back
from the reference surface 1b of the outer rim 1a of the first lens
1, is reflected back from the reflecting surface of the beam
splitter 41 and branched from the return optical path to the first
laser light source 39 to fall through mirror 43 on the first CCD 32
operating as detection means. The image picked up by the first CCD
32 is displayed on the second monitor 33. The collimating of the
light beam from the first laser light source 39 is adjusted on the
imaging surface of the first CCD 32 so that the beam diameter will
be minimized.
[0198] This device includes a detection system, including a second
laser light source 44, for detecting the parallelism of the outer
rim of the second lens, held under suction on the device reference
surface 55 of the holding unit 56, relative to the device reference
surface 55. As the first laser light source 39, any suitable
monochromatic light source, such as a gas laser or a solid laser,
may be used in addition to a semiconductor laser.
[0199] The light beam radiated from the second laser light source
44 is turned into collimated light, with an enlarged beam diameter,
by a collimator lens and a beam expander 45. The light beam, now
turned into the collimated light, is transmitted through a beam
splitter 46, a mirror 41 and the glass cover 35 at the upper end of
the holding unit 56 so as to fall on the lower end of the holding
unit 56. If the second lens 2 is held on the device reference
surface 55 at the lower end of the holding unit 56, this collimated
light is reflected by the reference surface of the outer rim of the
second lens 2 and thence returned through the glass cover 35 and
the mirror 41 to the beam splitter 46. The return light is
reflected back from the reflecting surface of the beam splitter 46
and branched from the return optical path to the first laser light
source 44 to fall through mirror 48 on the second CCD 49 operating
as detection means. The image picked up by the second CCD 49 is
displayed on the second monitor 50. The collimating of the light
beam from the second laser light source 44 is adjusted on the
imaging surface of the second CCD 49 so that the beam diameter will
be minimized.
[0200] This device includes a focusing error signals detecting
optical system 60. Similarly to the optical system used in an
optical pickup device for an optical disc, this focusing error
signal detecting optical system 60 is an optical system for
detecting the focusing error signals by a so-called astigmatic
aberration method or a differential concentric method. This
focusing error signal detecting optical system 60 includes a
semiconductor laser 61, as a light source, and collimates the light
beam, radiated from the semiconductor laser 61, by a collimator
lens 62, and illuminates the resulting collimated light beam
through the beam splitter 63, mirror 64 and the glass cover 34 from
the lower side of the movable base unit 59 into the inside of the
movable base unit 59. If the first lens 1 is set on the device
reference surface 58 of the movable base unit 59, the light beam is
incident on the first lens 1. If the second lens 2 is mounted on
the lens holder 3 by the holding unit 56, the light beam falls on
the second lens 2. The collimated light beam, sequentially incident
on the first and second lenses 1 and 2, is focussed on a point
above the second lens 2. The mirror 64 is reciprocable with respect
to the lower portion of the movable base unit 59, and includes an
aperture corresponding to the effective diameter of the objective
lens unit.
[0201] A glass cover 69, having a reflective surface, is mounted in
the vicinity of the focal point of the objective lens unit, as
shown in FIG. 39. This glass cover 69 is designed to be equal in
thickness and refractive index to the cover layer of the optical
recording medium used in conjunction with the objective lens unit.
The light beam, converged by the first and second lenses 1 and 2,
is reflected by the reflective surface of the glass cover 69 and
returned to the beam splitter 63 through the second lens 2, first
lens 1, glass cover 34 and the mirror 64, as shown in FIG. 38. This
return light is reflected by the reflecting surface of the beam
splitter 63, on the beam splitter 63, and branched from the return
optical path to the semiconductor laser 61. If the astigmatic
method is used, this return light is transmitted through the mirror
65, converging lens 66 and a cylindrical lens 67 to produce
astigmatic aberration so as to be then converged on the light
receiving surface of a photodetector 68.
[0202] The light receiving surface of the photodetector 68 is
divided from the center into four radial areas, outputting
photodetector signals independently of one another. Focusing error
signals and pull-in signals may be generated by calculations based
on the four photodetector signals, output from the photodetector
68, as will be explained subsequently.
[0203] The glass cover 69 is supported by being secured to the
movable part of the voice coil motor as driving means actuatable in
the optical axis direction as shown in FIG. 39. This voice coil
motor has a leaf spring 70. The glass cover 69 and the coil 72 are
carried by this leaf spring 70 so that the glass cover 69 and the
coil 72 are movable as movable parts. The voice coil moto is a
fixed part and includes a magnet 71 in the vicinity of the coil
72.
[0204] In the voice coil motor, the glass cover 69 is moved along
the direction of the optical axis, under the interaction of the
current supplied to the coil 72 and the magnetic field generated by
the magnet 71, by the driving current supplied to the coil 72.
[0205] The leaf spring 70 of the voice coil motor includes a
through-hole 73 for transmission smaller in diameter than the
second lens 2, in order for the laser light radiated from the
second laser light source 44 to be illuminated on the surface of
the second lens 2 proximate to the optical recording medium, as
shown for example in FIGS. 39 and 40. With the detection system,
having the second laser light source 44, the parallelism of the
surface of the second lens 2 close to the optical recording medium
to the device reference surface 58 of the movable base unit 59 can
be monitored through this through-hole 73.
[0206] In a step st39, shown in FIG. 34, the parallelism between
the device reference surface 58 of the movable base unit 59, used
for positioning the first lens 1, and the surface of the second
lens 2, mounted to the lens holder 3, held by the holding unit 56,
which surface is closer to the optical recording medium, is
adjusted. If the parallelism between the surface of the second lens
2 and the device reference surface 58 of the movable base unit 59
is not comprised within the preset range, the inclined stage 57 is
adjusted so that the parallelism between the surface of the second
lens 2 and the device reference surface 58 of the movable base unit
59 will be comprised within the preset range. In a step st40, the
parallelism between the device reference surface 58 and the surface
of the second lens 2 is checked and, if the parallelism is within
the prescribed range, processing transfers to a step st41. If
otherwise, processing reverts to a step st39.
[0207] In a step st41, the first lens 1 is mounted on the device
reference surface 58 of the movable base unit 59, as shown in FIG.
38. In a step st42, shown in FIG. 34, the laser light from the
first laser light source 39, reflected by the reference surface 1b
of the first lens 1, is checked to verify the parallelism between
the reference surface 1b of the first lens 1 and the device
reference surface 58 of the movable base unit 59. At this time, the
mirror 64 of the focusing error signal detecting optical system 60
is receded back from the position below the movable base unit
59.
[0208] If, in a step st43, the parallelism is within the prescribed
range, processing transfers to a step st44. If the parallelism is
not within the prescribed range, processing reverts to a step st41
for re-setting, or the reference surfaces 1b and 58 are swept. This
sequence of operations is repeated until the parallelism is within
the prescribed value.
[0209] When the parallelism of the reference surface 1b of the
first lens 1 with respect to the device reference surface 58 is
confirmed, the mirror 64 of the focusing error signal detecting
optical system 60 is inserted to a position below the movable base
unit 59, as shown in FIG. 38.
[0210] If the refractive curved surfaces and thicknesses of the
respective lenses of the double lens set are all equal to the
design values, the spherical aberration should be zero when the
distance between the two lenses of the set is equal to the design
value. In actuality, however, refractive curved surfaces or the
thicknesses of the respective lenses, possibly affecting the
spherical aberration, are deviated from the design values, due to,
for example, an offset from a design value of the metal die for
molding, or variations in the molding conditions. It is known to
adjust the distance between the two lenses to minimize the
spherical aberration in the assembled state of the double lens set.
In this objective lens set manufacturing device, adjustment may be
made by exploiting this principle, in such a manner that, as the
focusing error signals are detected, the device reference surface
58 is raised to a position where the center value of the focusing
error signals is zero, in order to minimize the spherical
aberration on the objective lens unit under a condition that the
lenses molded using variable metal dies or under variable molding
conditions co-exist.
[0211] That is, in a step st44 shown in FIG. 34, the movable base
unit 59 is uplifted to introduce the first lens 1 set on the device
reference surface 58 into the lens holder 3. In a step st45, the
glass cover 69 is vibrated along the optical axis by the voice coil
motor. That is, when the movable base unit 59 is raised, a
sinusoidal driving voltage, for example, is supplied to the coil 72
of the voice coil motor to vibrate the glass cover 69 along the
optical axis.
[0212] In the optical pickup device of the recording and/or
reproducing apparatus, employing an optical recording medium, the
objective lens unit is vibrated along the optical axis by an
objective lens unit driving actuator to detect the so-called
focusing error signals (S-shaped signal) and a sum signal to be
incident on a detector (pull-in signal) to set the application
timing of focusing servo. In the present manufacturing device, the
objective lens unit is fixed, instead of vibrating the objective
lens unit by the objective lens unit driving actuator, with the
glass cover 69 being forced into vibrations. At this time, the
focusing error signals and the pull-in signals are detected, as
shown in FIGS. 41A to 41C.
[0213] Referring to FIG. 38, with photodetector signals A to D from
the respective areas of the light receiving surface of the
photodetector 68, the signal levels of the focusing error signals
in the case of, for example, an astigmatic method, are defined by
[A+C-(B+D)], while the pull-in level is defined by [A+C+B+D)].
[0214] Referring to FIGS. 41A to 41C, the relationship between the
S-shaped waveform of the focusing error signals and the spherical
aberration of the objective lens unit is hereinafter explained.
With the zero spherical aberration, the S-shaped waveform of the
focusing error signals becomes symmetrical in the up-and-down, with
the focusing error signals becoming zero V for the maximum pull-in
level. If the spherical aberration is of a negative polarity, the
S-shaped waveform of the focusing error signals becomes
asymmetrical in the up-and-down direction, with the focusing error
signals being of negative polarity at the maximum pull-in level, as
shown in FIG. 41A. If conversely the spherical aberration of the
objective lens unit is of a positive polarity, the S-shaped
waveform of the focusing error signals becomes asymmetrical in the
up-and-down, with the focusing error signals becoming of a positive
polarity for the maximum pull-in level, as shown in FIG. 41C.
[0215] The state of the focusing error signal level can be
estimated from the signal level of the focusing errors at the
maximum point of the pull-in level. Alternatively, the state of the
focusing error signal level can be estimated by detecting the level
of the center value of the S-shaped signal of the focusing error
signals.
[0216] The level of the center value of the S-shaped waveform of
the focusing error signals (V center) may be defined by:
V center=(Vt+Vb)/2
[0217] where Vt and Vb are a top voltage and a bottom voltage of
the S-shaped signal of the focusing error signals,
respectively.
[0218] If the spherical aberration of the objective lens unit is
plotted against the center value (voltage) of the S-shaped signal
waveform of the focusing error signals, a constant relationship is
obtained, as shown in FIG. 42. Meanwhile, the graph shows data in
which plural lenses molded using plural metal dies co-exist, with
the numerical aperture (NA) of 0.85, the working wavelength
(.lambda.) of 405 nm and with the effective diameter of 3 mm. Based
on this relationship, it is possible to find a center value of the
S-shaped signal for which the absolute value of the spherical
aberration is minimum.
[0219] In a step st46, shown in FIG. 34, the center value of the
S-shaped signal of the focusing error signals is detected to check
whether or not the so detected value is within a preset range with
respect to a preset value. It the center value is within a
prescribed value, processing transfers to a step st47 and, if
otherwise, processing reverts to a step st44 to repeat the
processing of adjusting the position of the first lens 1.
[0220] In a step st47, the first lens 1 is bonded to the lens
holder 3. The objective lens unit is completed on curing the
adhesive in a step st48.
[0221] Similarly to the conventional single-lens glass molded
objective lens or an objective lens formed of synthetic resin, the
objective lens unit, assembled using the device and the method
according to the present invention, is used for an optical pickup
device. Similarly to the conventional objective lens, the objective
lens unit may be mounted to the objective lens driving mechanism
used for a conventional optical pickup device as shown in FIGS. 43
and 44.
[0222] As the objective lens unit driving mechanism, on which is
loaded the objective lens unit of the present invention, such a
mechanism similar to the conventional mechanism may be used. For
example, in a four-wire type objective lens driving mechanism, in
which the objective lens unit is carried in a cantilevered fashion
with four wires, as shown in FIGS. 43 and 44, a coil bobbin 8,
carrying the objective lens, is movably supported by a base block
10 using four resilient wires 9. A dumper material 11 is provided
on the proximal sides of the wires 9 carried by the base block 10.
A focusing coil 12 and a tracking coil 13 are mounted on the coil
bobbin 8. On the base block 10 are mounted a magnet 14 and a yoke
15. The magnet 14 and the yoke 15 are arranged so that the focusing
col 12 and the tracking coil 13 are placed in the magnetic field
thereby produced.
[0223] When the driving current is supplied to the focusing coil
12, the objective lens unit driving mechanism causes movement of a
coil bobbin 8 along the focusing direction parallel to the optical
axis of the objective lens unit, under the interaction of the
driving current and the magnetic field formed by the magnet 14 and
the yoke 15. Moreover, when the driving current is supplied to the
tracking coil 13, the objective lens unit driving mechanism causes
movement of the coil bobbin 8 along the planar tracking direction
perpendicular to the optical axis of the objective lens unit.
[0224] The optical pickup device controls the driving currents
supplied to the focusing col 12 and the tracking coil 13 to cause
movement of the objective lens unit to control the position of the
objective lens unit in such a manner that the light spot of the
light beam converged by this objective lens unit will be formed at
all times on the signal recording surface of the optical recording
medium to follow the recording track formed on the optical
recording medium.
[0225] If the optical recording medium is an optical disc, the
up-and-down direction in FIG. 43 and the depth-wise direction in
FIG. 44 correspond to the radial direction of the optical disc,
while the left and right direction in FIGS. 43 and 44 correspond to
the tangential direction thereof.
[0226] The optical pickup device, having the objective lens unit
driving mechanism and the objective lens unit according to the
present invention, includes a semiconductor laser (LD) 16,
operating as a light source, as shown in FIG. 45. A
linear-polarized light beam L1, radiated as a divergent light beam
from the semiconductor laser 16, is collimated by a collimator lens
17 and has its optical path bent by 90.degree. by the mirror 18 to
fall on a polarizing beam splitter (PBS) 19. The light beam
transmitted through the polarizing beam splitter 19 is turned into
circular polarized light by a .lambda./4 plate (quarter wave plate)
20 to fall on a beam expander 21 composed of a concave mirror and a
convex lens so as to be thereby expanded in beam diameter to fall
on an objective lens unit 51. This objective lens unit 51 is
supported by an objective lens unit driving mechanism, not shown,
for movement in the focusing direction F.sub.1 parallel to the
optical axis and in a planar tracking direction T.sub.1
perpendicular to the optical axis.
[0227] The light beam incident on the objective lens unit is
converged by this objective lens unit 51 and illuminated on the
signal recording surface of an optical recording medium 110, such
as an optical disc. The light beam L.sub.1, illuminated on the
signal recording surface of the optical recording medium 110, is
modulated in a preset manner by this signal recording surface with
respect to the direction of polarization and reflected to fall on
the objective lens unit 51. A return light beam L.sub.2 is
transmitted through a beam expander 21 and turned by the .lambda./4
plate (quarter wave plate) 20 into linear polarized light of the
direction of polarization perpendicular to the direction of
polarization of the light beam L.sub.1 incident on the optical
recording medium 110. The resulting linear polarized light is
returned to the polarizing beam splitter 19.
[0228] The return light beam L.sub.2 is reflected by the reflecting
surface within the beam splitter 19 to fall on a second polarizing
beam splitter 22. This second polarizing beam splitter 22 is set so
that, in the state in which the return light beam L.sub.2 is not
modulated by the optical recording medium 110, the amount of the
transmitted light will be equal to the amount of the reflected
light. The return light beam L.sub.2, transmitted through the
second polarizing beam splitter 22, is converged by enlarging lens
systems 23 and 24 on a first photodetector (PD1) 25. The return
light beam L.sub.2, reflected by the second polarizing beam
splitter 22, is converged through a converging lens system 26 and a
knife edge 27 to a second photodetector (PD2) 28. Based on the
detection signals of the optical output from the photodetectors 25,
28, variable signals, such as RF signals, focusing error signals or
tracking error signals may be generated to read out information
signals recorded on the optical recording medium 110.
[0229] For detecting the focusing error signals, the so-called
astigmatic method or the so-called differential concentric method
may be used in addition to the above-described knife edge method.
For detecting the tracking error signals, the so-called push-pull
method, or the so-called differential push-pull method (DPP method)
may be used. Moreover, the present optical pickup device is able
not only to read out information signals from the optical recording
medium but also to write information signals on the optical
recording medium 110.
[0230] A recording and/or reproducing apparatus may be constructed
by providing the optical pickup device, described above, and a
recording medium holding mechanism, adapted for holding and
rotationally driving the optical recording medium 110, such as an
optical disc, as shown in FIG. 45. In the recording and/or
reproducing apparatus, shown in FIG. 45, the signals read out by
the optical pickup device from the optical recording medium 110 are
processed by the signal processing circuit to generate RF signals
and various error signals. The signals input to this recording
and/or reproducing apparatus from outside are processed by the
signal processing circuit so as to be written by the optical pickup
device on the optical recording medium 110.
INDUSTRIAL APPLICABILITY
[0231] With the objective lens unit manufacturing method and
device, according to the present invention, in which a lens is
positioned in a cylindrically-shaped lens holder of a synthetic
resin material, using another lens, already mounted and secured to
the lens holder, as a reference, and the one lens is secured to the
lens holder, for setting relative positions of the one and the
other lenses, so that a double lens set comprised of two lenses,
with a numerical aperture (NA) not less than 0.7, may be assembled
efficiently within a short time. Since only the necessary minimum
precision may be required of the lens holder, it is possible to
enlarge the casting tolerance of the lens holder to improve the
yield. Moreover, since the lens interval along the optical axis of
the objective lens unit and the parallelism between the lenses are
maintained by the precision of the objective lens unit
manufacturing device, it is possible to improve the assembling
reproducibility and the yield.
[0232] With the present invention, it is possible to produce an
objective lens unit, with a numerical aperture not less than 0.7,
made up by plural lenses, in which the relative positions between
the lenses may be adjusted to high accuracy and in which the
spherical aberration may be reduced
* * * * *