U.S. patent application number 09/946757 was filed with the patent office on 2002-04-25 for optical head having two semiconductor lasers of different wavelength, an objective lens focusing laser beams on different thickness sustrates, and an annular phase shifter decreasing focused laser beam spot aberration.
Invention is credited to Arimoto, Akira, Shimano, Takeshi.
Application Number | 20020048250 09/946757 |
Document ID | / |
Family ID | 26399802 |
Filed Date | 2002-04-25 |
United States Patent
Application |
20020048250 |
Kind Code |
A1 |
Shimano, Takeshi ; et
al. |
April 25, 2002 |
Optical head having two semiconductor lasers of different
wavelength, an objective lens focusing laser beams on different
thickness sustrates, and an annular phase shifter decreasing
focused laser beam spot aberration
Abstract
An objective lens and an optical head including the objective
lens for reading information contained on substrates having
different thicknesses using laser beams having different
wavelengths. The objective lens includes an annular phase shifter
for decreasing an aberration of a focused spot of each of the laser
beams. The annular phase shifter can be optimally combined with the
objective lens having inner and outer regions each having a
different substrate thicknesses.
Inventors: |
Shimano, Takeshi;
(Tokorozawa-shi, JP) ; Arimoto, Akira; (Tokyo,
JP) |
Correspondence
Address: |
ANTONELLI TERRY STOUT AND KRAUS
SUITE 1800
1300 NORTH SEVENTEENTH STREET
ARLINGTON
VA
22209
|
Family ID: |
26399802 |
Appl. No.: |
09/946757 |
Filed: |
September 6, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09946757 |
Sep 6, 2001 |
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09768310 |
Jan 25, 2001 |
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09768310 |
Jan 25, 2001 |
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09501682 |
Feb 10, 2000 |
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6256284 |
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09501682 |
Feb 10, 2000 |
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09041768 |
Mar 13, 1998 |
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6215756 |
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Current U.S.
Class: |
369/112.26 ;
G9B/7.12 |
Current CPC
Class: |
G11B 7/0909 20130101;
G11B 7/12 20130101; G11B 7/127 20130101; G11B 2007/0006 20130101;
G11B 7/13922 20130101; G11B 7/1374 20130101 |
Class at
Publication: |
369/112.26 |
International
Class: |
G11B 007/135 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 13, 1997 |
JP |
9-058800 |
Jun 20, 1997 |
JP |
9-163743 |
Claims
We claim:
1. An objective lens for focusing two laser beams having different
wavelengths through substrates having different thicknesses for the
respective wavelengths, comprising: an annular phase shifter for
decreasing an aberration of a focused spot of each of the two laser
beams.
2. An objective lens according to claim 1, wherein said substrates
include a substrate having a 1.2 mm thickness and a substrate
having 0.6 mm thickness.
3. An objective lens according to claim 1, wherein said annular
phase shifter is integrally formed in the objective lens.
4. An objective lens according to claim 1, wherein said laser beams
include a laser beam having a wavelength of 650 nm and a laser beam
having a wavelength of 780 nm.
5. An objective lens for focusing two laser beams having different
wavelengths through substrates having different thicknesses f or
the respective wavelengths, comprising: an annular phase shifter
for decreasing an aberration of a focused spot of each of the two
laser beams, Wherein the two laser beams are focused without
aberrations by an inner region and an outer region of said
objective lens.
6. An objective lens according to claim 5, wherein said substrates
include a substrate having a 1.2 mm thickness and a substrate
having 0.6 mm thickness.
7. An objective lens according to claim 5, wherein said annular
phase shifter is integrally formed in the objective lens.
8. An objective lens according to claim 5, wherein said laser beams
include a laser beam having a wavelength of 650 nm and a laser beam
having a wavelength of 780 nm.
9. An optical head comprising: two semiconductor lasers for
generating two laser beams having different wavelengths; an
objective lens for focusing two laser beams having different
wavelengths through substrates having different thicknesses for the
respective wavelengths, wherein said objective lens comprises: an
annular phase shifter for decreasing an aberration of a focused
spot of each of the two laser beams; a diverger which diverges a
beam reflected from an optical disk on an optical path which
extends from said semiconductor lasers to the optical disk; and a
detector which detects a focused spot position control signal and a
data signal from the reflected beam diverged by said diverger.
10. An optical head according to claim 9, wherein said substrates
include a substrate having a 1.2 mm thickness and a substrate
having 0.6 mm thickness.
11. An optical head according to claim 9, wherein said annular
phase shifter is integrally formed in the objective lens.
12. An optical head according to claim 9, wherein said laser beams
include a laser beam having a wavelength of 650 nm and a laser beam
having a wavelength of 780 nm.
13. An optical head comprising: two semiconductor lasers for
generating two laser beams having different wavelengths; an
objective lens for focusing two laser beams having different
wavelengths through substrates having different thicknesses for the
respective wavelengths, wherein said objective lens comprises: an
annular phase shifter for decreasing an aberration of a focused
spot of each of the two laser beams; wherein the two laser beams
are focused without aberrations by an inner region and an outer
region of said objective lens; a diverger which diverges a beam
reflected from an optical disk on an optical path which extends
from said semiconductor lasers to the optical disk; and a detector
which detects a focused spot position control signal and a data
signal from the reflected beam diverged by said diverger.
14. An optical head according to claim 13, wherein said substrates
include a substrate having a 1.2 mm thickness and a substrate
having 0.6 mm thickness.
15. An optical head according to claim 13, wherein said annular
phase shifter is integrally formed in the objective lens.
16. An optical head according to claim 13, wherein said laser beams
include a laser beam having a wavelength of 650 nm and a laser beam
having a wavelength of 780 nm.
17. An optical head comprising: at least two semiconductor lasers
having different wavelengths; an objective lens for focusing beams
having the respective wavelengths on optical disks having different
substrate thicknesses; an annular phase shifter for decreasing both
aberrations of focused spots having the respective wavelengths;
diverging means for diverging a beam reflected from an optical disk
from an optical path which extends from said semiconductor lasers
to the optical disk; and detecting means for detecting a focused
spot position control signal and a data signal from the reflected
beam diverged by said diverging means.
18. An optical head according to claim 17, wherein said substrates
include a substrate having a 1.2 mm thickness and a substrate
having 0.6 mm thickness.
19. An optical head according to claim 17, wherein said annular
phase shifter is integrally formed in the objective lens.
20. An optical head according to claim 17, wherein said laser beams
include a laser beam having a wavelength of 650 nm and a laser beam
having a wavelength of 780 nm.
21. An optical head comprising: at least two lasers having
different wavelengths; an objective lens for focusing beams having
the respective wavelengths on optical disks having different
substrate thicknesses, said objective lens having different
substrate thicknesses for focusing the beams without aberrations in
its inner and outer regions; an annular phase shifter for
decreasing both aberrations of focused spots having the respective
wavelengths; diverging means for diverging a beam reflected from an
optical disk from an optical path which extends from said
semiconductor lasers to the optical disk; and detecting means for
detecting a focused spot position control signal and a data signal
from the reflected beam diverged by said diverging means.
22. An optical head according to claim 21, wherein said substrates
include a substrate having a 1.2 mm thickness and a substrate
having 0.6 mm thickness.
23. An optical head according to claim 21, wherein said annular
phase shifter is integrally formed in the objective lens.
24. An optical head according to claim 21, wherein said laser beams
include a laser beam having a wavelength of 650 nm and a laser beam
having a wavelength of 780 nm.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to an optical disk apparatus
for optically reading information from an optical storage medium.
More particularly the present invention relates to an optical head
for reading signals from optical disks having different substrate
thicknesses by using light sources having different optical
wavelengths, and an objective lens for use in such optical
head.
[0002] Optical disks have recently been making remarkable advances
as large capacity-removable-information storage media. Accordingly,
writing-reading methods, storage densities and disk sizes have
taken on great diversity. Thus, it is becoming very difficult to
ensure compatibility among the different systems. Among other
things, CDs (Compact Disc(s)) are presently most popular, and CD-Rs
(Compact Disk-Recordable(s)) which are recordable CDs having
reading compatibility with CDs are becoming equally as popular. It
is desired that development of new optical disks meet the important
demand for compatibility with such CDs and CD-Rs.
[0003] Recently DVDs (Digital Video Disk(s)), the next generation
of high density ROM following CDs and CD-Rs, have been introduced
to the market. To increase the storage density of a DVD, the
numerical aperture (NA) of an objective lens is increased from 0.45
for the conventional CDs up to 0.6.
[0004] Letting .lambda. be the wavelength of a laser source to be
used, the size of a focused spot on an optical disk is proportional
to .lambda./NA, so that as the wavelength is made shorter and the
NA larger, the size of the beam spot can be made smaller. If the
size of the beam spot is small, it is possible to read high-density
information pits with good quality, so that the storage density of
the optical disk can be increased.
[0005] In light of the above, the wavelength of a semiconductor
laser used for DVDs is 650 nm instead of 780 nm for CDs. However,
since an increase in the NA sharply increases coma which occurs
when a disk tilts, and rather degrades the beam spot, it is
impossible to excessively increase the NA. For this reason, DVDs
have a substrate thickness of 0.6 mm thinner than 1.2 mm of CDs so
that the NA can be increased and the accompanying coma due to a
disk tilt can be reduced. However, since the substrate thickness of
DVDs differs from that of CDs, if a CD is read with a DVD-dedicated
objective lens, a spherical aberration will occur and the beam spot
will defocus. This occurs because objective lenses for optical
disks are respectively intended for particular substrate
thicknesses and are beforehand designed to have spherical
aberrations which compensate for the particular substrate
thicknesses.
[0006] Conventional apparatus for solving the above problem is
described in, for example, Optical Review, Vol. 1, No. 1 (1994) pp.
27-29). In this conventional apparatus, a hologram is formed on the
surface of an objective lens for 0.6 mm disks, and a CD is read
with diffracted light, while a DVD is read with transmitted light.
The pattern of the hologram is beforehand designed so as to
compensate for spherical aberration which occurs during CD-read.
However, in this conventional apparatus, since the hologram is
used, a beam spot for DVDs is produced even during a CD-read
operation, whereas a beam spot for CDs is produced even during a
DVD-read operation. In addition, a beam reflected from a disk is
again diffracted. This leads to the disadvantage of unavoidable
loss of light power.
[0007] A second conventional apparatus is described in Mitsubishi
Electric Co. Ltd. News Release, No. 9507 (Jun. 21, 1995). In the
second conventional apparatus, both an objective lens for 0.6 mm
disks and an objective lens for 1.2 mm disks are provided on an
optical head, and the two lenses are switched when needed by a
movable actuator. However, in this example, since the two lenses
are switched when needed, there are problems such as an increase in
cost due to the use of two lenses, the reproducibility of the
positions of the lenses, and the degradation of response
characteristics due to a large and heavy actuator.
[0008] A third conventional apparatus is described in Nikkei
Electronics, Jan. 29, 1996 (No. 654), pp. 15-16. In the third
conventional apparatus an aperture limitation using a liquid
crystal is provided, and during a CD-read operation, the NA is
reduced to 0.35 so as to reduce aberration. Since a semiconductor
laser of wavelength 635 nm is used for both CDs and DVDs, the NA
for CDs can be reduced to some extent. There is, however, a
disadvantage in that this method cannot be used for reading CD-Rs
whose reflectance becomes quite low for a beam of wavelength
shorter than 780 nm.
[0009] A fourth conventional apparatus is described in Japanese
Patent Application No. 342203/1995. The fourth conventional
apparatus provides an objective lens in which the inner and outer
regions are given different optimized substrate thicknesses, so as
to realize compatibility between both DVDs and CDs at a wavelength
of 650 nm. However, if a CD is to be read at a wavelength of 780
nm, this boundary NA needs to be made at least 0.45 or more, but
this case leads to the disadvantage that the aberration for
DVD-read becomes extremely large.
SUMMARY OF THE INVENTION
[0010] An object of the present invention is to provide an optical
disk apparatus having an optical head for reading signals from
optical disks having different substrate thickness by using light
sources having different optical wavelengths.
[0011] Another object of the present invention is to provide an
objective lens for use in an optical head for reading signals from
optical disks having different substrate thicknesses by using light
sources having different optical wavelengths.
[0012] Yet another object of the present invention is to provides
an optical head for reading a CD having a substrate thickness of
1.2 mm by using a beam of wavelength 780 nm as well as a DVD having
a substrate thickness of 0.6 mm by using a beam wavelength of 780
nm, without loss of light power, at low cost and with high
precision.
[0013] The present invention provides an objective lens for
focusing two laser beams having different wavelengths on an optical
disks having different thicknesses. Integrally added to objective
lens is an annular phase shifter for decreasing aberrations of
focused spots of the respective wavelengths.
[0014] A second embodiment of the present invention provides an
objective lens having different substrate thicknesses in the inner
and outer regions of the objective lens for focusing a laser beam
without aberration. Integrally added to the objective lens is an
annular phase shifter for decreasing aberrations of focused spots
of laser beams of different wavelengths.
[0015] A third embodiment of the present invention provides an
optical head which includes at least two semiconductor lasers
having different wavelengths, a diverging apparatus for diverging a
beam reflected from an optical disk from an optical path which
extends from the semiconductor lasers to the optical disk, and a
detector for detecting a focused spot position control signal and a
data signal from the reflected beam diverged by the diverging
apparatus. The optical head further includes an objective lens for
focusing beams having the respective wavelengths on optical disks
having different substrate thicknesses.
[0016] A fourth embodiment of the present invention provides an
optical head which includes at least two semiconductor lasers
having different wavelengths, an objective lens for focusing beams
having the respective wavelengths on optical disks having different
substrate thicknesses, a diverging apparatus for diverging a beam
reflected from an optical disk from an optical path which extends
from the semiconductor lasers to the optical disk, and a detector
for detecting a focused spot position control signal and a data
signal from the reflected beam diverged by the diverging apparatus.
The optical head further includes an annular phase shifter for
decreasing aberrations of focused spots having the respective
wavelengths.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] The present invention will be more apparent from the
following detailed description, when taken in conjunction with the
accompanying drawings, in which:
[0018] FIG. 1 is a diagram illustrating an objective lens according
to the present invention;.
[0019] FIG. 2 is a diagram illustrating a wavefront shape of a
spherical aberration;
[0020] FIG. 3 is a diagram illustrating a wave aberration shape
obtained from an annular phase shifter;
[0021] FIG. 4 is a diagram illustrating a wave aberration shape
obtained from an inverted annular phase shifter;
[0022] FIG. 5 is a table indicating phase shift for a CD under
conditions which do not affect a DVD;
[0023] FIG. 6 is a graph illustrating spot performance for a
CD-read operation when a dual and a phase shifter are combined;
[0024] FIG. 7 is a graph illustrating a RMS wavefront aberration
occurring during a DVD-read operation;
[0025] FIG. 8 is a diagram illustrating a dual optimum substrate
lens with which an optimized inverted annular phase shifter is
formed integrally;
[0026] FIG. 9 is a graph illustrating a variation in spot
performance for a CD-read operation due to a shift of a CD-read
wavelength;
[0027] FIG. 10 is a graph illustrating a RMS wavefront aberration
for a wavelength shift during a DVD-read operation;
[0028] FIG. 11 is a graph illustrating wave-aberration shapes for a
CD-read operation;
[0029] FIG. 12 is a graph illustrating wave-aberration shapes for a
DVD-read operation;
[0030] FIGS. 13A and 13B are a graph and a table illustrating the
result of calculations on spot shapes;
[0031] FIG. 14 is a diagram illustrating an embodiment of an
optical head of the present invention;
[0032] FIG. 15 is a diagram illustrating an embodiment of the
present invention in which an objective lens and an annular phase
shifter are integrated in a hybrid form; and
[0033] FIG. 16 are tables of the specification and shape of a DVD
lens.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0034] Embodiments of the present invention will be described below
with reference to the accompanying drawings.
[0035] FIG. 1 is a diagram illustrating an objective lens according
to the present invention. The construction of an objective lens
according to the present invention makes use of a DVD objective
lens 1. According to the present invention a doughnut-shaped
annular phase shift region 101 is added to the DVD objective lens
1. The annular phase shift region 101 may be formed as a thin film,
or the DVD objective lens can be worked directly into such a shape
in advance. Since a normal DVD lens is designed to have no
aberration for a substrate thickness of 0.6 mm, when a DVD is to be
read with a laser beam of wavelength 650 nm, an aberration is made
as small as possible by the phase shifter. On the other hand, when
a CD having a substrate thickness of 1.2 mm is to be read with a
laser beam of wavelength 780 nm, a spherical aberration due to a
substrate thickness of 0.6 mm is decreased.
[0036] The manner in which an aberration decreases will be
qualitatively described below. FIG. 2 is a diagram illustrating the
wavefront shape of a spherical aberration for an optimized focus
position. In FIG. 2, the horizontal axis represents the coordinates
of the pupil radius of the objective lens, while the vertical axis
represents a wave aberration. A beam spot to be used for reading a
CD with a DVD objective lens has a wavefront shape such as that
approximately expressed by a quartic function, because of the
difference in substrate thickness between the CD and the DVD. FIG.
3 is a diagram illustrating a wavefront shape obtained from an
annular phase shifter. It can be seen that the maximum value of the
aberration is made smaller by the annular phase shifter.
[0037] The aberration of a DVD must not become large when it is
read by using the above-described DVD objective lens. One method to
compensate for this is to use the difference between a wavelength
for a CD-read operation and a wavelength for a DVD-read operation
so that a phase shift occurs only when a CD is being read and no
phase shift occurs when a DVD is being read. For this purpose,
letting .lambda.1 be the wavelength for a CD-read operation,
.lambda.2 the wavelength for a DVD-read operation, and .phi. a
phase shift occurring during a CD-read operation, the following is
provided:
[0038] Eq. 1
(n+.phi.).lambda.1=m.lambda.2 (n, m: integer)
[0039] The integers m and n may be selected to satisfy the above
equation. If there is no appropriate m or n, the manner of the
phase shift may also be altered as shown in FIG. 4. In this case, a
wavefront shape identical to that shown in FIG. 3 can be realized
by applying a phase shift of -.phi. to the region other than the
annular phase shift region. Therefore, in this case the following
is provided:
[0040] Eq. 2
(n-.phi.).lambda.1=m.lambda.2 (n, m: integer)
[0041] From this equation, for example, if .lambda.1 is set to 780
nm and .lambda.2 650 nm, the phase shift .phi. in each region is as
shown in FIG. 5. If the phase shift is selected in this manner, it
is possible to decrease the spherical aberration during a CD-read
operation without at all affecting the wavefront for a DVD-read
operation. The term "inverted annular phase shifter" used herein is
a name which takes into account the case in which a phase shift is
realized by a phase lag, such as when a film having a larger
refractive index than air is added. In a case where a phase shift
can be realized by a phase lead, as by grinding a lens, the annular
phase shift region may be directly formed by grinding. Since either
case is equivalent, both cases will be hereinafter referred to as
the inverted annular phase shifter.
[0042] The shape of the annular phase shifter and the optimization
of the phase shift will be described below. The Strehl intensity
which is the main peak intensity of a beam spot having an
aberration normalized with the main peak intensity of an
aberration-free spot is available as an evaluation index of a beam
spot. However, with such Strehl intensity, a difference in NA in
the presence of an aperture limitation does not appear. For this
reason, the ratio of the main peak intensity of a beam spot to a
total light power incident on the pupil of an objective lens is
adopted as a new evaluation index when an aperture limitation is
present. For example, even for the same aperture diameter, such
evaluation index becomes larger as the NA becomes larger, the spot
diameter becomes smaller or the main peak intensity becomes larger.
1 spot leak intensity total incident light power in the objective
lens pupil = 0 2 0 R e i ( r , ) r r 2 0 2 0 R r r = 0 2 0 R e i (
r , ) r r 2 0 2 0 R r r 2 0 2 0 R r r 2 0 2 0 R r r = I st ( R 2 )
2 R 2 = I st R 2 Eq . 3
[0043] It can be seen from Eq. 3 that the new evaluation index is
proportional to the product of the Strehl intensity and the second
power of the aperture limitation radius R normalized with the
radius of the full-aperture of the objective lens. In the
following, .eta. denotes the value obtained by multiplying the
Strehl intensity by the second power of the normalized aperture
limitation radius. In a normal CD pickup, since a wavelength of 780
nm is used and an objective lens NA is 0.45, if there is no
aberration for a DVD objective lens NA of 0.6,
.eta.=1.times.(0.45/0.6).sup.2=0.56, and .eta.=0.45 at 0.8 which is
the lower limit of the Strehl intensity based on Marechal's
criterion. Regarding spherical aberration due to substrate
thickness error, a fourth-order spherical aberration is given by: 2
W 40 = d 8 n 2 - 1 n 3 ( NA ) 4 Eq . 4
[0044] and a sixth-order spherical aberration is given by: 3 W 60 =
d 32 n 4 + 2 n 2 - 3 n 5 ( NA ) 6 Eq . 5
[0045] In these equations, n denotes a refractive index. From these
equations, an aberration, obtainable by adding an annular phase
shifter which causes a phase lag of .phi. between a radius R1 and a
radius R2, is expressed as follows: 4 W = { W 60 6 + W 40 4 + W 20
2 + W 00 W 60 6 + W 40 4 + W 20 2 + W 00 + ( 0 R 1 , R 2 ) ( R 1 R
2 ) Eq . 6
[0046] The Strehl intensity can be approximated as follows: 5 I st
= 1 - ( 2 W rms ) 2 = 1 - { 2 ( W 2 _ - ( W _ ) 2 ) } Eq . 7
[0047] Therefore, from this equation, R1, R2 and .phi. for a
maximum .eta. as well as the NA, W20 and W00 of the aperture
limitation are obtained. Actually, numerical-formula processing
software was used to analytically obtain W20 and W00 and to
numerically obtain R1, R2 and .phi. as well as the NA of the
aperture limitation. As the result, it was found that when the
inner and outer diameters of the annular phase shifter were NA0.20
and NA0.42 and the NA of the aperture limitation was 0.456 and the
phase shift was 0.265 .lambda. (.lambda.=780 nm), .eta.=0.48 became
a maximum and larger than .eta.=0.45 based on Marchal's criterion.
On the other hand, when optimization was performed with only the
aperture limitation without using the annular phase shifter,
.eta.=0.45 was a maximum at NA0.39. In other words, this is
equivalent to an improvement of from 0.61 to 0.86 in the Strehl
intensity in terms of NA0.45. For this phase shift, the aberration
occurring during a DVD-read operation was 0.33 .lambda.
(.lambda.=650 nm) in terms of RMS wavefront aberration. This is
almost equivalent to the working accuracy of lenses, and it can be
considered that no problem occurs in practice.
[0048] If this optimized phase shift of 0.265 .lambda. is compared
with the previously described phase shift which does not affect a
DVD-read operation, it can be seen that the closest phase shift is
0.333 .lambda. of the inverted annular phase shifter for m=2 and
n=2 or of the annular phase shifter for m=4 and n=3. However, as m
becomes larger, the step of a film or lens which causes a phase
shift becomes thicker and the deviation of a phase shift when a
wavelength shift occurs in a semiconductor laser becomes larger.
Therefore, the inverted annular phase shifter is more preferably
herein. When the shape of a phase shifter which was fixed at this
phase shift which does not affect a DVD-read operation was
obtained, .eta.=0.47 was a maximum at an inner diameter of NA0.20,
an outer diameter of NA0.44, and an aperture limitation of NA0.48.
This is almost equal to the above-described optimized phase
shift.
[0049] The effects of the annular phase shifter applying to the DVD
objective is confirmed by ray tracing. FIG. 16 shows the
specifications and the shape of the example DVD lens, where R, k,
A4, A6, A8 and A10 are the paraxial curvature radius, conical
constant, 4-th, 6-th, 8-th and 10-th order of aspherical
coefficients, respectively. The surface shape is defined using
these parameters and radial coordinate r by 6 z ( r ) = r 2 R + R 2
( K + 1 ) r 2 + A 1 r 4 + A 2 r 6 + A 3 r 8 + A 4 r 10 Eq . 8
[0050] where the shape is assumed to be symmetrical to the axis.
When the collimated light of wavelength of 780 nm is focused
through CD substrate of thickness 1.2 mm without the annular phase
shifter, the root mean square (RMS) wave front aberration of the
spot was 0.1279.lambda. (.lambda.=780 nm) in the aperture of NA
0.45. By applying the annular phase shifter of 0.3333 .lambda.
(.lambda.=780 nm) to this lens, however, the aberration was reduced
to 0.07366 .lambda.(.lambda.=780 nm). On the other hand, when the
collimated light of wavelength of 650 nm is focused through DVD
substrate of thickness 0.6 nm with the annular phase shifter, the
RMS wave front aberration of the spot was accordingly less than
0.001 .lambda.(.lambda.=650 nm) in the aperture of NA 0.6.
[0051] The above description has been made on the assumption that
the aperture limitation is employed, but this does not necessarily
mean that an actual aperture is needed. Actually, it can be
considered that the above-described process is almost equivalent to
specifying an evaluation area of a pupil when an optimized focus
position is to be obtained by using an RMS wavefront aberration as
an evaluation function. If a focus error is adjusted so that the
RMS wavefront aberration becomes as small as possible within the
area of the aperture limitation, the aberration of light outside
the area of the aperture limitation naturally becomes larger and
the slope of the wavefront also becomes larger. For this reason,
the rays in such area cross a focal plane at a position greatly
offset from a focus. Therefore, the presence of such rays is almost
equivalent to the absence of the rays in terms of a focused
spot.
[0052] If only the annular phase shifter is used in the
above-described manner, spot performance will be improved but a
Strehl intensity of 0.86 for NA0.45 may not completely suffice when
account is taken of a degradation of a spot due to misalignment of
optical parts, disk tilt, focus error or the like. For this reason,
in combination with the above construction, different substrate
thicknesses to be optimized may be provided inside and outside a
lens. Such lens is hereinafter referred to as the dual optimum
substrate lens (DOSL). This lens was invented by the present
inventors as a method of realizing compatibility between both DVDs
and CDs at a wavelength of 650 nm. This method is disclosed in
Japanese Patent Application No. 342203/1995. However, such lens has
the disadvantage that it is necessary to make this dual NA at least
NA0.45 or more for the purpose of reading CDs at a wavelength of
780 nm, and in this case, the aberration for DVD-read becomes
extremely large.
[0053] To solve such disadvantages, a phase shifter and the dual
optimum substrate lens were combined to optimize the shape of the
phase shifter, a phase shift, an inside-outside boundary radius and
an inside substrate thickness at the same time, and it has been
found out that such combination has the effect of decreasing both
aberrations which occur due to the dual optimum substrate lens
during a CD-read operation at a wavelength of 780 and during a
DVD-read operation at a wavelength of 650 nm, thereby further
improving the spot performance for a CD-read operation. This
combination will be described below.
[0054] The wave aberration due to the combination of the dual
optimum substrate lens and the phase shift is expressed as follows:
7 W = { W 601 6 + W 401 4 + W 201 2 + W 001 ( 0 R 1 ) W 601 6 + W
401 4 + W 201 2 + W 001 + ( R 1 R 2 ) W 602 6 + W 402 4 + W 202 2 +
W 002 + ( R 2 R 3 ) W 602 6 + W 402 4 + W 202 2 + W 002 ( R 3 R 4 )
Eq . 8
[0055] In this equation, R1 denotes the inner diameter of the
annular phase shifter, R2 the boundary radius, R3 the outer
diameter of the annular phase shifter, and R4 the radius of the
aperture limitation. The disk substrate thickness required to
remove an aberration differs between inner and outer regions
separated by the boundary radius, and the inner region is 0.6 mm
thick for a DVD-read operation, while the outer region is optimized
to be between 0.6 mm thick and 1.2 mm thick. Accordingly, the
discriminator "1" or "2" is affixed to each of the aberration
coefficients W60 and W40 for spherical aberration for the purpose
of discrimination between the inner and outer regions. The focus
errors W201 and W202 are determined from a spherical aberration so
as to minimize the RMS wavefront aberrations of the inner region
and the outer region, and the constant terms W001 and W002 are
determined so that the average values of the wave aberration of the
inner and outer regions become the same, thereby optimizing the
total RMS wavefront aberration. The differences between W201 and
W202 and between W001 and W002 are determined by the difference
between the inner and outer corresponding substrate thicknesses of
the lens, and W201 and W001 were also analytically obtained by
numerical-formula processing software under conditions for
minimizing the RMS wavefront aberration under the conditions of the
phase shifter which were given W202 and W002.
[0056] Further, regarding the given inner corresponding substrate
thickness and the boundary radius R2, conditions for maximizing n
were obtained by numerically changing R1, R3, R4 and the phase
shift. The result is shown in FIG. 6. In FIG. 6, the horizontal
axis represents the boundary radius of the dual optimum substrate
lens and the vertical axis represents .eta., and the result of
calculations under optimized conditions for different central
substrate thicknesses is plotted. In the graph, dashed lines
respectively indicate a CD having no aberration, a lower limit
level equivalent to a Strehl intensity of 0.8, the above optimized
phase shifter, and a fixed phase shifter. These lines cannot be
plotted with points on the graph because no dual optimum substrate
lens is used. RMS wavefront aberration occurring during a DVD-read
operation at that time is shown in FIG. 7.
[0057] As can be seen from a comparison of FIGS. 6 and 7, as the
central substrate thickness is made closer to 1.2 mm, the
performance for CDs becomes higher and the aberration for DVDs
increases. Therefore, a decision as to which of these points should
be selected as an optimum point depends on the distribution of
various margins of the system. It is considered, however, that it
is almost possible to accept a CD performance of .eta.=0.526 (0.94
in terms of the Strehl intensity for CDs) and a DVD RMS wavefront
aberration of 0.030 for a central substrate thickness of 0.76 mm
and a boundary radius of NA0.45. When the phase shifter is not
provided, the evaluation factor of a CD and DVD aberration are
0.414 and 0.031, respectively. Accordingly, the aberration of the
light DDot for both a CD and a DVD are decreased. Furthermore, at
this point, the maximum value of the CD performance and the minimum
value of the DVD aberration coincide with each other. The phase
shift of the annular phase shifter at this time is 0.2985
.lambda.(.lambda.=780 nm), the inner diameter is NA0.2145, and the
outer diameter is NA0.45 which coincides with the boundary radius
NA.
[0058] FIG. 8 is a diagram illustrating a dual optimum substrate
lens with which an inverted annular phase shifter is formed
integrally. Since the inverted annular phase shifter is formed
integrally with the lens, the region of the annular phase shifter
is recessed. Although a step for the dual optimum substrate lens is
also formed on a disk-side surface having a comparatively moderate
curvature, this step may be provided on only an image side.
[0059] FIG. 9 illustrates the value of the CD-read spot performance
.eta. affected by a shift of a CD-read wavelength. Although the
range of the horizontal axis is .+-.20 nm, the wavelength range in
which the value of .eta. shifts due to temperature variations or
the like seems to be about .+-.10 nm. The degradation within the
wavelength range is from .eta.=0.53 to approximately .eta.=0.52 at
a wavelength shift of -10 nm, and is almost negligible because of a
variation of from 0.93 to about 0.92 in terms of the Strehl
intensity for NA0.45. In addition, FIG. 9 illustrates the values of
the previously-described optimized annular phase shifter and the
fixed annular phase shifter.
[0060] FIG. 10 illustrates a RMS wavefront aberration for a
wavelength shift during a DVD-read operation at a wavelength of 650
nm, and the aberration increases from 0.030 .lambda. up to 0.036
.lambda. at a wavelength shift of -10 nm when the dual optimum
substrate lens and the optimized annular phase shifter are
combined. It can be considered that this increase is also within a
fully allowable range. In addition, FIG. 10 illustrates the
aberrations of the previously-described optimized annular phase
shifter and the fixed annular phase shifter. Regarding the fixed
annular phase shifter, since its phase shift is selected so that no
aberration occurs for DVDs, the aberration is 0 at a wavelength
shift of 0. Regarding the optimized annular phase shifter, since
its phase shift is offset from a phase shift which causes no
aberration for DVDs, the wave aberration linearly varies toward a
wavelength shift which corresponds to the phase shift.
[0061] FIG. 11 is a graph illustrating wave-aberration shapes for a
CD-read operation at a wavelength of 780 nm. For each of the
wave-aberration shapes, since a focus error is optimized within the
NA range of an aperture limitation and the horizontal axis
represents the coordinates of the pupil radius over a full-aperture
of NA0.6, the aberrations become extremely large in their
peripheral portions. On the other hand, since the vertical axis
represents the aberrations in a folded form within the range of
.+-.0.5 .lambda., the peripheral portions seem to be sharply
vibrating. These aberrations are suppressed over an NA wider than
when a focus error is optimized with only the aperture limitation.
Furthermore, the rise of the wavefront outside the range of the
aperture limitation NA is also sharp, and it is expected that the
effect of the aperture limitation becomes more remarkable because
of such a large aberration.
[0062] FIG. 12 illustrates wave aberrations for a DVD-read
operation at a wavelength of 650 nm. Since the wave aberration for
only the aperture limitation and that for only the fixed phase
shifter, both of which are shown in FIG. 11, become completely zero
in FIG. 12, FIG. 12 only illustrates the case in which the dual
optimum substrate lens and the optimized phase shifter are combined
and the case of only the optimized phase shifter. From the fact
that the aberration does not become zero even in the outermost
portion in which no aberration at all occurs, it is seen that a
small focus error occurs over the entire pupil. This is because the
total RMS wavefront aberration becomes small owing to the small
focus error if the phase shift caused by the phase shifter is
regarded the aberration. In any case, the value of the vertical
axis of the graph is considerably small and the distinctiveness of
the wavefront shape is reduced to an actually negligible degree of
RMS wavefront aberration.
[0063] FIG. 13A illustrates the result of calculations on spot
shapes. In the graph, the horizontal axis represents the spot size
of an intensity which is exp (-2) times the peak intensity of a
spot, while the vertical axis represents the value of the intensity
of a side-lobe normalized with the main peak intensity of the spot.
Accordingly, since it is desirable that both the spot and the
side-lobe be small, it follows that a plotted point nearer to the
bottom left of the graph corresponds to a spot of higher
resolution. Assuming that the intensity distribution of the pupil
of the objective lens is a symmetrical Gaussian distribution, the
shown calculation result is that obtained when the ratio of a lens
diameter to the range of the intensity of exp (-2) times the
intensity of the center of the Gaussian distribution in the pupil
is 0.1 and the ratio of the intensity of the peripheral portion of
the lens to the intensity of the central portion thereof is
0.98.
[0064] In FIG. 13B, a white circle denotes a normal CD lens having
no aberration, and as the position of a plotted point is nearer to
the white circle, read-out performance becomes closer to that of
the normal CD lens. Each black square denotes a normal DVD lens
with only an aperture limitation. As such black squares, there are
plotted three points which respectively correspond to the case in
which the aperture limitation is actually inserted, the case in
which the aperture limitation is omitted at that focus position,
and the case in which the aperture limitation is omitted and the
focus position is shifted so that the spot peak intensity becomes a
maximum. Any of the cases is inferior in spot resolution to the
normal CD lens having no aberration. Each block triangle denotes
the case in which only the optimized annular phase shifter is
inserted, and there are similarly three plotted points.
[0065] Although the spot size is considerably improved as compared
with the case of the aperture limitation only, the side-lobe
becomes considerably large if there is no aperture limitation. Each
white square denotes the case in which the dual optimum substrate
lens and the optimized annular phase shifter are combined. Although
there are similarly three plotted points, it is seen that these
three plotted points are considerably close to one another. In
other words, it is seen that in this case it makes no matter
whether there is an aperture limitation or not, and since the
aberration of a beam outside the range of a virtual aperture
limitation sharply increases, the forming of a spot is not
substantially affected. In this case, the beam spot is slightly
smaller in spot size and slightly larger in side-lobe than the
normal CD lens having no aberration.
[0066] The reason why the value of .eta. which is the evaluation
index of the spot performance, is almost equal or slightly inferior
to the normal CD lens having no aberration is presumed to be that
the effect of the side-lobe which is not completely decreased is
canceled by reducing the spot size. In addition, the result of
calculations on a spot for a DVD-read operation is plotted with a
white triangle and diamond at the bottom left. The diamond denotes
a spot for reading a DVD without aberration, and the triangle
denotes the case in which the optimized dual optimum substrate lens
and the optimized annular phase shifter are combined. Spot shapes
for DVDs are almost the same.
[0067] FIG. 14 illustrates an embodiment of an optical head. Light
from semiconductor lasers 41 and 42 of different wavelengths are
combined by dichromatic mirror 20 and formed into parallel light by
a collimator lens 5. The elliptical beam is formed into a circular
beam by beam forming prisms 61 and 62. If the efficiency of the
optical system is sufficiently high or the track pitch of a disk is
wider than the gap between a main lobe of a beam spot and a first
dark line on the disk, the beam forming prisms can also be
advantageously omitted in terms of the number of component parts or
for the purpose of decreasing crosstalk between adjacent tracks.
The beam is transmitted through a beam splitter 71 and reflected by
an erect mirror 8, and is then focused on an optical disk 10 by an
objective lens 3 according to the present invention. The objective
lens 3 is provided on a two dimensional actuator 9. The optical
disk 10 may be a CD or a DVD. The two-dimensional actuator 9 moves
in the direction of a disk radius in response to a tracking error
signal and positions the beam spot on a track, and also moves in
the direction of the optical axis in response to a focus error
signal and positions a focus position on the disk.
[0068] The reflected beam again passes through the objective lens 3
and the erect mirror 8, and is reflected by the beam splitter 71
and conducted toward a detecting optical system.
[0069] The beam transmitted through a beam splitter 72 is formed
into a focused beam by a focusing lens 111, and enters a beam
splitter 73. The beam transmitted through the beam splitter 73 is
transmitted through a cylindrical lens 12 and is made incident on a
four-split photodetector 13. A differential signal obtained from
the sum signals of the diagonal components of this split
photodetector is outputted from a differential amplifier 141 as a
focus error signal.
[0070] The beam reflected by the beam splitter 73 is also made
incident on a two-split photodetector 15, and a differential signal
obtained from the outputs of the two-split photodetector 15 is
outputted from a differential amplifier 142 as a tracking error
signal. The beam reflected by the beam splitter 72 is focused on a
photodetector 16 by a focusing lens 112, and the signal
photoelectrically converted by the photodetector 16 is amplified by
an amplifier 17 so that a data signal is obtained. The data signal
may be detected from a sum signal of the outputs from the detector
for detecting a servo signal. In this case, the servo signal may be
detected by band-limiting a signal detected up to a signal band,
through a low-pass filter or the like. The servo detecting optical
system is one example, and another system may also be used.
[0071] Although the above description has referred to the
embodiment in which the annular phase shifter is formed integrally
with the objective lens, FIG. 15 shows another embodiment in which
a DVD objective lens 18 and an independent annular phase shifter 19
are integrated in a hybrid form as an optical head which is
incorporated in a two-dimensional actuator. Since it is assumed
that this embodiment is substituted for only a portion
corresponding to the optical system of FIG. 14 which extends from
the erect mirror to the disk, FIG. 15 shows only the corresponding
portion.
[0072] By using an annular phase shifter or by optimally combining
the annular phase shifter and an objective lens having inner and
outer regions each having a different substrate thickness which
causes no aberration, it is possible to read a DVD having a
substrate thickness of 0.6 mm with a laser beam of wavelength 650
nm and a CD having a substrate thickness of 1.2 mm with a laser
beam of wavelength 780 nm by one lens without the need for an
aperture limitation. Thus, using the present invention it is
possible to provide a small-size inexpensive optical head.
[0073] While the present invention has been described in detail and
pictorially in the accompanying drawings it is not limited to such
details since many changes and modifications recognizable to those
of ordinary skill in the art may be made to the invention without
departing from the spirit and the scope thereof.
* * * * *