Title: Objective lens with the diffractive surface for DVD/CD compatible optical pickup
Abstract: Disclosed is an objective lens for a DVD/CD compatible optical pickup, which improves optical efficiency while minimizing aberration occurring due to different disc thicknesses of different optical storage media. The objective lens has first and second aspherical lens surfaces.
Patent Number: 6,992,838 Issued on 01/31/2006 to Park
| Inventors:
|
Park; Cheon-Ho (Kyunggi-Do, KR)
|
| Assignee:
|
Samsung Electro-Mechanics Co., Ltd. (Kyunggi-do, KR)
|
| Appl. No.:
|
738265 |
| Filed:
|
December 17, 2003 |
Foreign Application Priority Data
| Oct 13, 2003[KR] | 10-2003-0071162 |
| Current U.S. Class: |
359/719; 359/741; 369/44.23; 369/112.07 |
| Current Intern'l Class: |
G02B 3/02 (20060101); G02B 13/18 (20060101); G02B 3/08 (20060101); G11B 7/00 (20060101) |
| Field of Search: |
359/718-719,722,741-743
369/442.3,112.01,112.06,112.07,112.08,112.12
|
References Cited [Referenced By]
| Foreign Patent Documents |
| 09145995 | Jun., 1997 | JP.
| |
| 2000081566 | Mar., 2000 | JP.
| |
| 2001051192 | Feb., 2001 | JP.
| |
Primary Examiner: Epps; Georgia
Assistant Examiner: Harrington; Alicia M
Attorney, Agent or Firm: Christensen O'Connor Johnson Kindness PLLC
Claims
What is claimed is:
1. An objective lens for a DVD/CD compatible optical pickup, said objective lens
being a refractive lens having a positive refractive power, said refractive lens including:
a first lens surface on which an optical beam having a different wavelength for
each of a plurality of optical recording media respectively including transparent
substrates of different thicknesses is incident, said first lens surface having
a positive refractive power for refracting the optical beam and thus focusing it
onto an information recording surface formed on a transparent substrate of an optical
recording medium, said first lens surface including a diffractive grating formed
thereon over the entire surface thereof, said first lens surface being an aspherical
surface satisfying the following condition:
##EQU5##
where "f" denotes an effective focal distance of the objective lens, and "Sr"
denotes a distance along an optical axis of the lens from one point at which an
optical beam passing through the optical axis on the last surface of the objective
lens meets the lens surface to another point at which an effective optical beam
passing through the upper side meets the lens surface; and
a second lens surface being an aspherical surface having a positive refractive
power for refracting the optical beam and thus focusing it onto the information
recording surface formed on the transparent substrate of the optical recording medium.
2. The objective lens according to claim 1, wherein the aspherical surfaces of
the first and second lens surfaces satisfy the following two equations:
##EQU6##
where "i" denotes the ordinal number of a surface from a light source, "r
i"
denotes a curvature radius of an i-th surface, "k" denotes a conic constant, "h"
denotes a distance from the optical axis to the lens surface, and "A, B, C, . .
. J" denotes fourth, sixth, eighth . . . twentieth aspherical surface coefficients,
respectively, and
Φ′(
h)=(MOD(
C1h2+C2h4+C3h6
. . . +C,1)×λ
B,
where "Φ′(h)" denotes the actual microscopic shape of the diffractive
lens structure; "h" denotes a distance from the optical axis to the lens surface;
"C
1, C
2, C
3 . . . " denotes second, fourth, sixth
. . . diffractive coefficients, respectively; symbol λ
B is a blaze
wavelength for which the steps of the grating gives an optical path difference
by one wavelength and the diffractive efficiency becomes the maximum at the blaze
wavelength λ
B; symbol C is a constant defining a phase at a boundary
between adjacent rings (0<C≦1); the function MOD(x,y) represents the
remainder when x is divided by y; and the diffractive lens structure is formed
on the base curve that is the lens surface of the refractive lens.
3. The objective lens according to claim 2, wherein the lens has a center thickness
of 2.20 mm, and has refractive indexes of 1.54065 and 1.53713 at wavelengths of
655 nm and 785 nm, respectively, and further has a blaze wavelength of 715 nm,
wherein when the distance h of the first lens surface is in the range of 0 to
1.6 mm, the radius of curvature is 2.117248 mm, the diffractive coefficients C
1,
C
2 and C
3 are 0, -1.731035 and -0.139774, respectively; the
conic constant k is -5.66134E-01; and the aspherical surface coefficient A is 0,
B is -6.33910E-04, C is 3.79512E-05, D is 1.58394E-05, E is -1.08464E-05, and F
is 1.00884E-06,
wherein when the distance h is in the range of 1.6 to 2.0 mm, the radius of curvature
is 2.212176 mm, the diffractive coefficients C
1, C
2 and C
3
are -6.993007, -0.097902, -0.001399, respectively; the conic constant k is
-9.31672E-01; and the aspherical surface coefficient A is 6.80832E-03, B is -1.55240E-04,
C is 2.05226E-04, D is -5.13685E-05, E is 1.28641E-06, and F is 3.13362E-07, and
wherein regarding the second lens surface, r
2 is -8.125557 mm, the
conic constant k is -104.216960, the aspherical surface coefficient A is -0.1317125E-02,
B is 0.167540E-02, C is -0.6788669E-03, D is -0.4133680E-04, E is 0.5953331E-04,
and F is -0.778703E-05.
4. The objective lens according to claim 1, satisfying the following condition:
##EQU7##
where "f" denotes the effective focal distance of the objective lens, and "t"
denotes an interval along the optical axis between front and rear surfaces of the
objective lens.
5. The objective lens according to claim 4, wherein the aspherical surfaces of
the first and second lens surfaces satisfy the following two equations:
##EQU8##
where "i" denotes the ordinal number of a surface from a light source, "r
i"
denotes a curvature radius of an i-th surface, "k" denotes a conic constant, "h"
denotes a distance from the optical axis to the lens surface, and "A, B, C, . .
. J" denotes fourth, sixth, eighth . . . twentieth aspherical surface coefficients,
respectively, and
Φ′(
h)=(MOD(
C1h2+C2h4+C3h6
. . . +C,1)×λ
B,
where "Φ′(h)" denotes the actual microscopic shape of the diffractive
lens structure; "h" denotes a distance from the optical axis to the lens surface;
"C
1, C
2, C
3 . . . " denotes second, fourth, sixth
. . . diffractive coefficients, respectively; symbol λ
B is a blaze
wavelength for which the steps of the grating gives an optical path difference
by one wavelength and the diffractive efficiency becomes the maximum at the blaze
wavelength λ
B; symbol C is a constant defining a phase at a boundary
between adjacent rings (0<C≦1); the function MOD(x,y) represents the
remainder when x is divided by y; and the diffractive lens structure is formed
on the base curve that is the lens surface of the refractive lens.
6. The objective lens according to claim 5, wherein the lens has a center thickness
of 2.20 mm, and has refractive indexes of 1.54065 and 1.53713 at wavelengths of
655 nm and 785 nm, respectively, and further has a blaze wavelength of 715 nm,
wherein when the distance h of the first lens surface is in the range of 0 to
1.6 mm, the radius of curvature is 2.117248 mm, the diffractive coefficients C
1,
C
2 and C
3 are 0, -1.731035 and -0.139774, respectively; the
conic constant k is -5.66134E-01; and the aspherical surface coefficient A is 0,
B is -6.33910E-04, C is 3.79512E-05, D is 1.58394E-05, E is -1.08464E-05, and F
is 1.00884E-06,
wherein when the distance h is in the range of 1.6 to 2.0 mm, the radius of curvature
is 2.212176 mm, the diffractive coefficients C
1, C
2 and C
3
are -6.993007, -0.097902, -0.001399, respectively; the conic constant k is
-9.31672E-01; and the aspherical surface coefficient A is 6.80832E-03, B is -1.55240E-04,
C is 2.05226E-04, D is -5.13685E-05, E is 1.28641E-06, and F is 3.13362E-07, and
wherein regarding the second lens surface, r
2 is -8.125557 mm, the
conic constant k is -104.216960, the aspherical surface coefficient A is -0.1317125E-02,
B is 0.167540E-02, C is -0.6788669E-03, D is -0.4133680E-04, E is 0.5953331E-04,
and F is -0.778703E-05.
7. The objective lens according to claim 1, wherein the diffractive grating formed
on the first lens surface over the entire surface thereof is a sawtooth-shaped
diffractive grating.
8. The objective lens according to claim 7, wherein the aspherical surfaces of
the first and second lens surfaces satisfy the following two equations:
##EQU9##
where "i" denotes the ordinal number of a surface from a light source, "r
i"
denotes a curvature radius of an i-th surface, "k" denotes a conic constant, "h"
denotes a distance from the optical axis to the lens surface, and "A, B, C, . .
. J" denotes fourth, sixth, eighth . . . twentieth aspherical surface coefficients,
respectively, and
Φ′(
h)=(MOD(
C1h2+C2h4+C3h6
. . . +C,1)×λ
B,
where "Φ′(h)" denotes the actual microscopic shape of the diffractive
lens structure; "h" denotes a distance from the optical axis to the lens surface;
"C
1, C
2, C
3 . . . " denotes second, fourth, sixth
. . . diffractive coefficients, respectively; symbol λ
B is a blaze
wavelength for which the steps of the grating gives an optical path difference
by one wavelength and the diffractive efficiency becomes the maximum at the blaze
wavelength λ
B; symbol C is a constant defining a phase at a boundary
between adjacent rings (0<C≦1); the function MOD(x,y) represents the
remainder when x is divided by y; and the diffractive lens structure is formed
on the base curve that is the lens surface of the refractive lens.
9. The objective lens according to claim 8, wherein the lens has a center thickness
of 2.20 mm, and has refractive indexes of 1.54065 and 1.53713 at wavelengths of
655 nm and 785 nm, respectively, and further has a blaze wavelength of 715 nm,
wherein when the distance h of the first lens surface is in the range of 0 to
1.6 mm, the radius of curvature is 2.117248 mm, the diffractive coefficients C
1,
C
2 and C
3 are 0, -1.731035 and -0.139774, respectively; the
conic constant k is -5.66134E-01; and the aspherical surface coefficient A is 0,
B is -6.33910E-04, C is 3.79512E-05, D is 1.58394E-05, E is -1.08464E-05, and F
is 1.00884E-06,
wherein when the distance h is in the range of 1.6 to 2.0 mm, the radius of curvature
is 2.212176 mm, the diffractive coefficients C
1, C
2 and C
3
are -6.993007, -0.097902, -0.001399, respectively; the conic constant k is
-9.31672E-01; and the aspherical surface coefficient A is 6.80832E-03, B is -1.55240E-04,
C is 2.05226E-04, D is -5.13685E-05, E is 1.28641E-06, and F is 3.13362E-07, and
wherein regarding the second lens surface, r
2 is -8.125557 mm, the
conic constant k is -104.216960, the aspherical surface coefficient A is -0.1317125E-02,
B is 0.167540E-02, C is -0.6788669E-03, D is -0.4133680E-04, E is 0.5953331E-04,
and F is -0.778703E-05.
10. The objective lens according to claim 7, wherein a step depth "d" of each
of rings constituting the sawtooth-shaped diffractive grating of the first lens
surface is expressed by the following equation:
##EQU10##
where a refractive index is n, and a design wavelength is λnm.
11. The objective lens according to claim 10, wherein the aspherical surfaces
of the first and second lens surfaces satisfy the following two equations:
##EQU11##
where "i" denotes the ordinal number of a surface from a light source, "r
i"
denotes a curvature radius of an i-th surface, "k" denotes a conic constant, "h"
denotes a distance from the optical axis to the lens surface, and "A, B, C, . .
. J" denotes fourth, sixth, eighth . . . twentieth aspherical surface coefficients,
respectively, and
Φ′(
h)=(MOD(
C1h2+C2h4+C3h6
. . . +C,1)×λ
B,
where "Φ′(h)" denotes the actual microscopic shape of the diffractive
lens structure; "h" denotes a distance from the optical axis to the lens surface;
"C
1, C
2, C
3 . . . " denotes second, fourth, sixth
. . . diffractive coefficients, respectively; symbol λ
B is a blaze
wavelength for which the steps of the grating gives an optical path difference
by one wavelength and the diffractive efficiency becomes the maximum at the blaze
wavelength λ
B; symbol C is a constant defining a phase at a boundary
between adjacent rings (0<C≦1); the function MOD(x,y) represents the
remainder when x is divided by y; and the diffractive lens structure is formed
on the base curve that is the lens surface of the refractive lens.
12. The objective lens according to claim 11, wherein the lens has a center thickness
of 2.20 mm, and has refractive indexes of 1.54065 and 1.53713 at wavelengths of
655 nm and 785 nm, respectively, and further has a blaze wavelength of 715 nm,
wherein when the distance h of the first lens surface is in the range of 0 to
1.6 mm, the radius of curvature is 2.117248 mm, the diffractive coefficients C
1,
C
2 and C
3 are 0, -1.731035 and -0.139774, respectively; the
conic constant k is -5.66134E-01; and the aspherical surface coefficient A is 0,
B is -6.33910E-04, C is 3.79512E-05, D is 1.58394E-05, E is -1.08464E-05, and F
is 1.00884E-06,
wherein when the distance h is in the range of 1.6 to 2.0 mm, the radius of curvature
is 2.212176 mm, the diffractive coefficients C
1, C
2 and C
3
are -6.993007, -0.097902, -0.001399, respectively; the conic constant k is
-9.31672E-01; and the aspherical surface coefficient A is 6.80832E-03, B is -1.55240E-04,
C is 2.05226E-04, D is -5.13685E-05, E is 1.28641E-06, and F is 3.13362E-07, and
wherein regarding the second lens surface, r
2 is -8.125557 mm, the
conic constant k is -104.216960, the aspherical surface coefficient A is -0.1317125E-02,
B is 0.167540E-02, C is -0.6788669E-03, D is -0.4133680E-04, E is 0.5953331E-04,
and F is -0.778703E-05.
13. The objective lens according to claim 10, wherein a mean wavelength 715 nm
of the two wavelengths 655 nm and 785 nm is used as the wavelength of λnm
in calculating the step depth of the sawtooth-shaped diffractive grating of the
first lens surface.
14. The objective lens according to claim 13, wherein the aspherical surfaces
of the first and second lens surfaces satisfy the following two equations:
##EQU12##
where "i" denotes the ordinal number of a surface from a light source, "r
i"
denotes a curvature radius of an i-th surface, "k" denotes a conic constant, "h"
denotes a distance from the optical axis to the lens surface, and "A, B, C, . .
. J" denotes fourth, sixth, eighth . . . twentieth aspherical surface coefficients,
respectively, and
Φ′(
h)=(MOD(
C1h2+C2h4+C3h6
. . . +C,1)×λ
B,
where "Φ′(h)" denotes the actual microscopic shape of the diffractive
lens structure; "h" denotes a distance from the optical axis to the lens surface;
"C
1, C
2, C
3 . . . " denotes second, fourth, sixth
. . . diffractive coefficients, respectively; symbol λ
B is a blaze
wavelength for which the steps of the grating gives an optical path difference
by one wavelength and the diffractive efficiency becomes the maximum at the blaze
wavelength λ
B; symbol C is a constant defining a phase at a boundary
between adjacent rings (0<C≦1); the function MOD(x,y) represents the
remainder when x is divided by y; and the diffractive lens structure is formed
on the base curve that is the lens surface of the refractive lens.
15. The objective lens according to claim 14, wherein the lens has a center thickness
of 2.20 mm, and has refractive indexes of 1.54065 and 1.53713 at wavelengths of
655 nm and 785 nm, respectively, and further has a blaze wavelength of 715 nm,
wherein when the distance h of the first lens surface is in the range of 0 to
1.6 mm, the radius of curvature is 2.117248 mm, the diffractive coefficients C
1,
C
2 and C
3 are 0, -1.731035 and -0.139774, respectively; the
conic constant k is -5.66134E-01; and the aspherical surface coefficient A is 0,
B is -6.33910E-04, C is 3.79512E-05, D is 1.58394E-05, E is -1.08464E-05, and F
is 1.00884E-06,
wherein when the distance h is in the range of 1.6 to 2.0 mm, the radius of curvature
is 2.212176 mm, the diffractive coefficients C
1, C
2 and C
3
are -6.993007, -0.097902, -0.001399, respectively; the conic constant k is
-9.31672E-01; and the aspherical surface coefficient A is 6.80832E-03, B is -1.55240E-04,
C is 2.05226E-04, D is -5.13685E-05, E is 1.28641E-06, and F is 3.13362E-07, and
wherein regarding the second lens surface, r
2 is -8.125557 mm, the
conic constant k is -104.216960, the aspherical surface coefficient A is -0.1317125E-02,
B is 0.167540E-02, C is -0.6788669E-03, D is -0.4133680E-04, E is 0.5953331E-04,
and F is -0.778703E-05.
16. The objective lens according to claim 1, wherein the objective lens's thickness,
defined by a distance between the first and second lens surfaces along the optical
axis, is 2.20 mm or more.
17. The objective lens according to claim 16, wherein the aspherical surfaces
of the first and second lens surfaces satisfy the following two equations:
##EQU13##
where "i" denotes the ordinal number of a surface from a light source, "r
i"
denotes a curvature radius of an i-th surface, "k" denotes a conic constant, "h"
denotes a distance from the optical axis to the lens surface, and "A, B, C, . .
. J" denotes fourth, sixth, eighth . . . twentieth aspherical surface coefficients,
respectively, and
Φ′(
h)=(MOD(
C1h2+C2h4+C3h6
. . . +C,1)×λ
B,
where "Φ′(h)" denotes the actual microscopic shape of the diffractive
lens structure; "h" denotes a distance from the optical axis to the lens surface;
"C
1, C
2, C
3 . . . " denotes second, fourth, sixth
. . . diffractive coefficients, respectively; symbol λ
B is a blaze
wavelength for which the steps of the grating gives an optical path difference
by one wavelength and the diffractive efficiency becomes the maximum at the blaze
wavelength λ
B; symbol C is a constant defining a phase at a boundary
between adjacent rings (0<C≦1); the function MOD(x,y) represents the
remainder when x is divided by y; and the diffractive lens structure is formed
on the base curve that is the lens surface of the refractive lens.
18. The objective lens according to claim 17, wherein the lens has a center thickness
of 2.20 mm, and has refractive indexes of 1.54065 and 1.53713 at wavelengths of
655 nm and 785 nm, respectively, and further has a blaze wavelength of 715 nm,
wherein when the distance h of the first lens surface is in the range of 0 to
1.6 mm, the radius of curvature is 2.117248 mm, the diffractive coefficients C
1,
C
2 and C
3 are 0, -1.731035 and -0.139774, respectively; the
conic constant k is -5.66134E-01; and the aspherical surface coefficient A is 0,
B is -6.33910E-04, C is 3.79512E-05, D is 1.58394E-05, E is -1.08464E-05, and F
is 1.00884E-06,
wherein when the distance h is in the range of 1.6 to 2.0 mm, the radius of curvature
is 2.212176 mm, the diffractive coefficients C
1, C
2 and C
3
are -6.993007, -0.097902, -0.001399, respectively; the conic constant k is
-9.31672E-01; and the aspherical surface coefficient A is 6.80832E-03, B is -1.55240E-04,
C is 2.05226E-04, D is -5.13685E-05, E is 1.28641E-06, and F is 3.13362E-07, and
wherein regarding the second lens surface, r
2 is -8.125557 mm, the
conic constant k is -104.216960, the aspherical surface coefficient A is -0.1317125E-02,
B is 0.167540E-02, C is -0.6788669E-03, D is -0.4133680E-04, E is 0.5953331E-04,
and F is -0.778703E-05.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an objective lens for a DVD/CD compatible optical
pickup, and more particularly to an objective lens for a DVD/CD compatible optical
pickup which can improve optical efficiency while minimizing aberration occurring
due to different disc thicknesses of different optical storage media.
2. Description of the Related Art
An optical disc is used as a medium for recording information, which enables
the
recording/reproducing of information without bringing a recording/reproducing head
into contact with the recording medium, and also improves recording density.
In order to reproduce information recorded on such an optical disc, a laser beam
outputted from an optical pickup as a recording/reproducing head is irradiated
to a signal recording layer of the optical disc, and an optical detector mounted
on the optical pickup detects a laser beam reflected from the signal recording
layer, thereby reading the information.
As an optical disc allowing recording/reproducing of information, there is, for
example, an optical disc employing a phase change recording layer that is irradiated
with a laser beam to alternate between a crystalline state and an amorphous state.
In order to record information on this kind of optical disc, a laser beam modulated
according to a signal of the information to be recorded is irradiated to the signal
recording layer.
There is a DVD (Digital Video Disc or Digital Versatile Disc), as one of the
optical discs that use a phase change recording layer to permit recording/reproducing
of information. The DVD has a recording density greatly improved over a CD (Compact
Disc), an optical disc in wide use. While the CD has a recording track pitch of
1.6 μm, the DVD has a recording track pitch of 0.74 μm, thereby achieving
high recording density.
In order to record information, by laser beam irradiation, on such an optical
disc allowing high-density information recording, it is required to form a much
smaller beam spot on the signal recording layer of the optical disc.
The spot diameter of a laser beam is proportional to the design wavelength of
the laser beam, and inversely proportional to the numerical aperture (NA) of an
objective lens that focuses the laser beam. Therefore, for achieving high recording
density of an optical disc, it is required to increase the numerical aperture of
the objective lens and decrease the wavelength of the laser beam.
In the meantime, a DVDP and a DVD-ROM is attracting attention in imaging/acoustic
fields, as a device capable of performing high density recording/reproducing. For
compatibility, an optical pickup device employed in the DVDP must be able to record
and/or reproduce information when it adopts not only the DVD but also CD family
such as CD, CD-R (CD-Recordable), CD-I and CD-G, as a recording medium.
However, the DVD has been standardized in a different thickness from the
CD family, due to mechanical tolerance for disc tilt and the numerical aperture
of an objective lens. The thickness of the conventional CD family is 1.2 mm, while
the thickness of the DVD is 0.6 mm. Since the thickness of the CD family is different
from that of the DVD, if an optical pickup device for the DVD is applied to the
CD family, the thickness difference causes spherical aberration. This spherical
aberration leads to a failure to obtain enough optical intensity to record an information
signal, or lowers the quality of reproduced signals.
In addition, regarding the wavelength of a light source for reproducing, the
DVD
has also been standardized in a different wavelength range from the CD family.
That is, the wavelength of the reproducing light source for the conventional CD
family is about 780 nm, whereas the wavelength for the DVD is about 650 nm.
Due to the different standardizations, it is impossible for a conventional CDP
to reproduce information recorded on the DVD. Thus, there is a need to develop
the optical pickup device for DVD, which must also be compatible with the conventional
CD family.
In the prior art, a ring-shaped optical disc device has been proposed, which
can
reproduce different kinds of optical discs such as CD and DVD.
Both the CD and the DVD (hereinafter, collectively referred to as an "optical
disc") employ a transparent substrate that has an information recording surface
at one side thereof. The optical disc is formed by attaching two such transparent
substrates to each other so that the respective information recording layers face
each other, or by attaching such a transparent substrate to a protective substrate
so that an information recording layer thereof faces the protective substrate.
In order to reproduce an information signal stored in the optical disc having
such a configuration, it is required for an optical disc device to focus a laser
beam from a light source onto the information recording surface of the optical
disc via the transparent substrate.
The wavelength of a laser beam used for the CD is different from that of the
DVD, as described below. The optical disc device uses an objective lens in order
to focus a laser beam.
As described above, the thickness of a transparent substrate used for the CD
is
1.2 mm, whereas the thickness of a transparent substrate used for the DVD is 0.6
mm. The thickness of the transparent substrate on which the information recording
surface is formed varies depending on the kind of optical disc or the wavelength
of the laser beam.
For an optical disc device for reproducing different kinds of optical discs,
it is required to focus a laser beam onto the information recording surface even
if the thickness of the transparent substrate varies depending on the kind of optical disc.
In new optical disc devices recently suggested, it has been proposed to use a
blue laser having a wavelength of about 400 nm in order to perform reproduction
of information. It is thus expected that the optical disc device can use the CD
for downward compatibility and the DVD now in use, and can also use such a new
optical disc.
It is considered that such a ring-shaped optical disc device is classified into
the following two types. In one type, for each kind of optical disc, an individual
objective lens is installed in a pickup, and the objective lenses are exchanged
according to the kind of optical disc to be used. In the other type, an individual
pickup is installed for each kind of optical disc, and the pickups are exchanged
according to the kind of optical disc to be used.
However, in order to realize a low cost, small size device, it is desirable
to be able to use the same objective lens for any kind of optical disc.
One typical example of such a kind of objective lens can be seen in Japanese
Patent Publication No. Hei9-145995. The objective lens described in this document
has a lens surface divided into three or more concentric ring-shaped lens surfaces
radially arranged, in which the refractive power of one ring-shaped lens surface
of one color is different from that of another ring-shaped lens surface of another color.
Said one ring-shaped lens surface of one color allows a laser beam of a wavelength
to be focused onto an information recording surface of a thin optical disc of 0.6
mm thickness (for example, DVD). On the other hand, said another ring-shaped lens
surface of another color allows a laser beam of the same wavelength to be focused
onto an information recording surface of a thick optical disc of 1.2 mm (for example, CD).
Another typical example can be seen in Japanese Patent Publication No. 2000-81566
(or U.S. Pat. No. 6,118,594). This document has disclosed an optical disc device
that uses a laser beam of a short wavelength (635 nm or 650 nm) for the thin transparent
substrate (DVD), and uses a laser beam of a long wavelength (780 nm) for the thick
transparent substrate (CD). This optical disc device has an objective lens commonly
used for the two laser beams.
This objective lens has a refractive lens surface having a positive refractive
power, on which fine concentric ring-shaped steps are closely formed, thereby obtaining
a diffractive lens structure. This diffractive lens structure is designed to focus
a diffractive laser beam of a short wavelength onto the information recording surface
of a thin transparent substrate (DVD), and to focus a diffractive laser beam of
a long wavelength onto the information recording surface of a thick transparent
substrate (CD).
The diffractive lens structure is also designed to focus a diffractive beam of
the same diffractive order onto the information recording surface, no matter what
kind of diffractive beam it is. The reason why a laser beam of a short wavelength
is used for the DVD is because the recording density of the DVD is higher than
that of the CD and it is thus required to throttle the beam spot to be smaller.
As is well known, the size of an optical spot is proportional to the wavelength,
and inversely proportional to the numerical aperture (NA).
A different ring-shaped objective lens has been proposed in Japanese Patent Publication
No. 2001-51192, which has a phase-compensated lens structure in which ring-shaped
phase shifters are formed on a surface of the lens. Firstly, in this objective
lens, a lens surface, which allows removal of wavefront aberration due to a laser
beam of a wavelength λ
1 (640 nm) used for the DVD, is defined
as a reference lens surface. The surface of this objective lens is divided into
a plurality of ring-shaped refractive surfaces radially arranged. Each ring-shaped
refractive surface is formed to have a predetermined step depth from the reference
lens surface (the i-th step depth from the center of the lens is denoted by "d
i").
Each refractive surface having a step depth (d
i) allows a laser beam
for the DVD to be phase-shifted by an integral multiple (m
i) of the
wavelength (λ
1) with respect to the reference lens surface, thereby
reducing the wavefront aberration of the CD family.
Since it allows an objective lens to be commonly used for both the DVD and
the CD, all the prior art described above has no need to provide means for exchanging
an individual member, including an objective lens, used for each of the DVD and
the CD, so it is advantageous in terms of cost and has a simpler configuration.
However, the prior art described above has the following disadvantages.
First, the objective lens of Japanese Patent Publication No. Hei9-145995 uses different
ring-shaped lens surfaces for the DVD and the CD, so it has many portions invalid
for an incident laser beam, and thus has very low optical utilization efficiency.
In addition, the objective lens of Japanese Patent Publication No. 2000-81566
(or U.S. Pat. No. 6,118,594) uses diffractive beams obtained by the diffractive
lens structure, so it is impossible to simultaneously obtain 100% diffractive efficiency
for each of different wavelengths.
In order for the lens surface to have the diffractive lens structure, it is required
to form minute steps on the lens surface. But, this is liable to be affected by
manufacturing errors. An error at the time of designing the diffractive lens structure
also causes deterioration of the diffractive efficiency.
Due to such diffractive efficiency deterioration or inherent impossibility of
obtaining 100% diffractive efficiency, it is impossible to focus every incident
beam onto the information recording surface formed on the transparent substrate
of the optical disc, which causes an optical loss.
In addition, as described above, the ring-shaped objective lens disclosed in
Japanese
Patent Publication No. 2001-51192 has a phase-compensated lens structure for providing
high optical utilization efficiency. A lens surface, which allows removal of wavefront
aberration due to laser beams used for the DVD, is defined as a reference lens
surface, and the surface of the lens is divided into a plurality of ring-shaped
refractive surfaces radially arranged. In order to reduce wavefront aberration
of laser beams for the CD, a ring-shaped refractive surface is formed to be recessed
from the reference lens surface by a step depth (d
i), which corresponds
to an integral multiple (m
i) of the wavelength (λ1) of laser beams
for the DVD. However, since it is inherently based on the DVD, it is impossible
for only the step formation to sufficiently reduce the wavefront aberration for
laser beams for the CD.
SUMMARY OF THE INVENTION
Therefore, the present invention has been made in view of the above problems,
and it is an object of the present invention to provide an objective lens for a
DVD/CD compatible optical pickup, which can improve optical efficiency while minimizing
aberration occurring due to different disc thicknesses of different optical storage media.
In accordance with the present invention, the above and other objects can be
accomplished
by the provision of an objective lens for a DVD/CD compatible optical pickup, including:
a first lens surface on which an optical beam having a different wavelength for
each of a plurality of optical recording media respectively including transparent
substrates of different thicknesses is incident, said first lens surface being
an aspherical surface having a positive refractive power for refracting the optical
beam and thus focusing it onto an information recording surface formed on a transparent
substrate of an optical recording medium, said first lens surface including a diffractive
grating formed thereon over the entire surface thereof; and a second lens surface
being an aspherical surface having a negative refractive power for refracting the
optical beam and thus focusing it onto the information recording surface formed
on the transparent substrate of the optical recording medium.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects, features and other advantages of the present invention
will be more clearly understood from the following detailed description taken in
conjunction with the accompanying drawings, in which:
FIG. 1 shows the configuration of a compatible optical pickup device to which
the present invention is applied; and
FIGS. 2
a and 2
b are front and side views showing an objective
lens for the compatible optical pickup shown in FIG. 1, according to the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Now, preferred embodiments of the present invention will be described in detail
with reference to the drawings.
FIG. 1 shows the configuration of a compatible optical pickup device to which
the present invention is applied.
As shown in FIG. 1, a two-wavelength light source
10 generates two optical
beams, one having a wavelength of 650 nm for the DVD family and the other having
a wavelength of 780 nm for the CD family. Each of the optical beams for the DVD/CD
families generated by the two-wavelength light source
10 is incident on
a wavelength-selective diffractive grating
20, which diffracts the incident
beam to separate it into three beams, in order to perform tracking/focusing control.
In the case where the three beams separated through the wavelength-selective
diffractive
grating
20 belong to the beam having a wavelength of 780 nm for the CD family,
they are incident on an optical splitter
50 after being subjected to compensation
for coma aberration, which occurs in an objective lens
80 described below,
through an optical axis correction hologram
30. Here, the optical axis correction
hologram
30 allows the respective optical axes of the separated beams to
be coincident with each other to perform the compensation for coma aberration based
on the optical axis deviation.
In the case where the three beams separated through the wavelength-selective
diffractive
grating
20 belong to the beam having a wavelength of 650 nm for the DVD
family, they are incident on the optical splitter
50 without being subjected
to the compensation for the coma aberration, because they do not cause coma aberration
in the objective lens
80.
After being split by the optical splitter
50, an optical beam is totally
reflected at a right angle by a mirror
60. The reflected beam is then incident
on a collimating lens
70, through which the incident beam is changed to
a planar waveform optical beam. After passing through the collimating lens
70,
the optical beam is incident on the objective lens
80, through which the
optical beam is focused onto a pit of an optical disc
100 in which data
is stored.
Here, an actuator
90 functions to perform focusing/tracking control
on the objective lens
80 based on external control signals.
An optical beam reflected from the optical disc
100 is split through the
beam splitter
50 to be directed toward an optical detector
130 described
below. After being split by the beam splitter
50, the beam is incident on
the optical detector
130 having a predetermined PDIC pattern via a sensor
lens
120. The sensor lens
120 causes astigmatism for detecting a
focus error of an optical beam to perform focusing control on the optical detector
130.
Here, in the case where the optical beam to be incident on the optical detector
130 is an optical beam having a wavelength of 780 nm for the CD family,
it is diffracted at a specific angle through a CD detection diffractive grating
110, so as to allow the beam spot thereof to correctly coincide with the
PDIC pattern of the optical detector
130.
When the optical beam generated from the light source
10 is incident
on the optical detector
130 after being reflected by the optical disc
100,
the optical detector
130 converts the incident optical beam into an electrical
signal, so as to record data on the optical disc
100 or reproduce the recorded
data from the disc
100.
FIGS. 2
a and
2b are front and side views showing an objective
lens for an optical pickup, according to an embodiment of the present invention.
The configuration of the objective lens will now be described in more detail
with reference to FIGS. 2
a and
2b. The objective lens includes
a first aspherical lens surface
210 having a positive refractive power and
a second aspherical lens surface
211 having a positive refractive power.
The objective lens is positioned so that the first aspherical lens surface
211
faces a light source, and the second aspherical lens surface
211 is opposite
to the light source. The first aspherical lens surface
210 has a sawtooth-shaped
diffractive grating structure so as to improve optical efficiency while minimizing
aberration occurring due to different thicknesses of optical discs.
When the amount of sag Z of the first lens
210 defines an aspherical
surface thereof, it is expressed by the following equation with a parameter being
the height "h" from the optical axis.
##EQU1##
Here, "i" denotes the ordinal number of a surface from a light source, "r
i"
denotes the curvature radius of an i-th surface, "k" denotes a conic constant,
"h" denotes the distance from the optical axis to the lens surface, and A, B, C,
. . . J denotes fourth, sixth, eighth . . . twentieth aspherical surface coefficients, respectively.
Optical phase difference Φ(h) in a diffractive lens is defined by the
following phase function.
Φ(
h)=(
C1h2+C2h4+C3h6
+C
4h8+C5h10 .
. . )×λ [Expression 2]
Here, "h" denotes the distance from the optical axis to the lens surface, "C
1,
C
2, C
3 . . . " denotes second, fourth, sixth . . . diffractive
coefficients, respectively, and "λ" denotes wavelength of incident light.
An actual microscopic shape of the diffractive lens structure is defined like
a Fresnel lens having a large number of concentric rings. The actual shape Φ′(h)
is defined by subtracting λ×m (m:integer) from Φ(h) as follows.
Φ′(
h)=(MOD(
C1h2+C2h4+
. . . +C,1)×λ
B
Symbol λ
B is a blaze wavelength for which the steps of the
grating gives an optical path difference by one wavelength, and the diffractive
efficiency becomes the maximum at the blaze wavelength λ
B. Symbol
C is a constant defining a phase at a boundary between adjacent rings (0<C≦1).
The function MOD(x,y) represents the remainder when x is divided by y. The diffractive
lens structure is formed on the base curve that is the lens surface of the refractive lens.
The lens has a center thickness of 2.20 mm, and has refractive indexes of 1.54065
and 1.53713 at wavelengths of 655 nm and 785 nm, respectively, and has a blaze
wavelength of 715 nm. When the distance "h" is in the range of 0 to 1.6 mm, C
1,
C
2 and C
3 are 0, -1.731035 and -0.139774, respectively.
In this case, k is -5.66134E-01, A is 0, B is -6.33910E-04, C is 3.79512E-05,
D is 1.58394E-05, E is -1.08464E-05, and F is 1.00884E-06.
In addition, when the distance "h" is in the range of 1.6 to 2.0 mm, C
1,
C
2 and C
3 are -6.993007, -0.097902 and -0.001399, respectively.
In this case, k is -9.31672E-01, A is 6.80832E-03, B is -1.55240E-04, C is 2.05226E-04,
D is -5.13685E-05, E is 1.28641E-06, and F is 3.13362E-07.
On the other hand, regarding the second aspherical lens surface
211, r
2
is -8.125557, k is -104.216960, A is -0.1317125E-02, B is 0.167540E-02, C is -0.6788669E-03,
D is -0.4133680E-04, E is 0.5953331E-04, and F is -0.778703E-05.
When "f" denotes the effective focal distance of the objective lens, and "Sr"
denotes the distance along the optical axis from one point at which an optical
beam passing through the optical axis on the last surface of the objective lens
meets the lens surface to another point at which an effective optical beam passing
through the upper side meets the lens surface, the objective lens must satisfy
the following condition.
##EQU2##
When "f" denotes the effective focal distance of the objective lens, and "t"
denotes an interval along the optical axis between the front and rear surfaces
of the objective lens, the objective lens must satisfy the following condition.
##EQU3##
A sawtooth-shaped diffractive grating is formed on the first lens surface
210.
It can be seen from the front view of FIG. 2
a that the grating is formed
on the first lens surface
210 over the entire surface thereof.
The phase distribution λ(h) of the diffractive grating of the first lens
210 is defined only with an even-order function as shown in the above Expression 2.
Consequently, since the optical path difference between an optical
path in a medium of refractive index (n) and an optical path in the air is given
as (n-1), the step depth d of each of the rings (diffractive elements) constituting
the diffractive lens is expressed by the following equation when the design wavelength
is λ nm.
##EQU4##
Alternatively, the step depth d is an integral multiple of the above
value. According to the present invention, the mean wavelength 715 nm of the two
wavelengths 655 nm and 785 nm is used as the wavelength λ in calculating
the step depth d.
The objective lens according to the present invention minimizes aberration caused
by different optical disc thicknesses between different optical storage media,
so as to greatly improve the performance of the objective lens, and increase the
optical efficiency thereof, and also minimize performance degradation due to the
variance of temperature. These improved results are shown in the following Table 1.
| TABLE 1 |
|
| |
Partial Diffractive |
Full Diffractive |
| Annular Type |
Type |
Type |
|
| DVD optical |
DVD optical |
DVD optical efficiency: 100 |
| efficiency: 74 |
efficiency: 97 |
| CD optical |
CD optical |
CD optical efficiency: 100 |
| efficiency: 56 |
efficiency: 70 |
|
For easy comparison between the optical efficiencies of different diffractive
types, this table shows the comparison results, assuming that the full diffractive
type according to the present invention has an optical efficiency of 100. It can
be seen from this table that the present invention is very advantageous.
As apparent from the above description, an objective lens for a DVD/CD compatible
optical pickup according to the present invention minimizes aberration caused by
different optical disc thicknesses of different optical storage media, thereby
greatly improving the performance thereof.
In addition, the objective lens according to the present invention improves the
optical efficiency thereof and minimizes performance degradation due to temperature variation.
Although the preferred embodiments of the present invention have been disclosed
for illustrative purposes, those skilled in the art will appreciate that various
modifications, additions and substitutions are possible, without departing from
the scope and spirit of the invention as disclosed in the accompanying claims.
*
