U.S. patent number RE44,397 [Application Number 12/100,854] was granted by the patent office on 2013-07-30 for single objective lens for use with cd or dvd optical disk.
This patent grant is currently assigned to Konica Corporation. The grantee listed for this patent is Barry G. Broome, Jenkin A. Richard. Invention is credited to Barry G. Broome, Jenkin A. Richard.
United States Patent |
RE44,397 |
Broome , et al. |
July 30, 2013 |
Single objective lens for use with CD or DVD optical disk
Abstract
An optical disk reader or read/write system for CD or DVD
formats. First and second laser diodes operating at different
wavelengths have their output beams collimated and directed at a
single element objective lens, and are then reflected off the disk
back through the lens to a photodetector. The single element
objective lens has a central aperture zone and an outer aperture
zone, the central zone being profiled to operate at a first
numerical aperture at approximately 0.45 and the output beam of the
first laser diode is confined to the central aperture zone. The
outer aperture zone together with the central aperture zone are
profiled to operate at a second numerical aperture, for example
0.60 wherein the output beam of the second laser diode has ray fans
extending across the full aperture of the single element objective
lens. A diffractive is formed on one surface of the single element
objective lens and provides sufficient aspheric surface power for
spherical aberration correction as well as correction for
spherochromatism. The diffractive also provides sufficient
correction for spherical aberration and spherochromatism that the
single element objective lens achieves diffraction-limited image
quality for both CD and DVD formats.
Inventors: |
Broome; Barry G. (Oceanside,
CA), Richard; Jenkin A. (Guang Dong, CN) |
Applicant: |
Name |
City |
State |
Country |
Type |
Broome; Barry G.
Richard; Jenkin A. |
Oceanside
Guang Dong |
CA
N/A |
US
CN |
|
|
Assignee: |
Konica Corporation (Tokyo,
JP)
|
Family
ID: |
22119752 |
Appl.
No.: |
12/100,854 |
Filed: |
April 10, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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10189259 |
Jul 5, 2002 |
Re. 40329 |
|
|
Reissue of: |
09074474 |
May 7, 1998 |
6088322 |
Jul 11, 2000 |
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Current U.S.
Class: |
369/112.26;
369/112.2; 369/112.08; 369/44.37; 369/94; 369/112.03; 369/44.24;
369/118 |
Current CPC
Class: |
G11B
7/1275 (20130101); G11B 7/1353 (20130101); G11B
7/13922 (20130101); G11B 7/1374 (20130101); G11B
2007/0006 (20130101) |
Current International
Class: |
G11B
7/135 (20060101) |
Field of
Search: |
;369/112.26,44.24,44.37,94,118,112.08,112.12,112.2,112.03
;359/691,356,357 |
References Cited
[Referenced By]
U.S. Patent Documents
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Other References
M Shinoda et al., "Optical Pick-up for DVD", IEEE, vol. 42, No. 3,
Aug. 1996. cited by examiner .
M.R. Feldman et al., "Diffractive optics for packaging of laser
diodes and fiber-optics", IEEE, 1996, pp. 1278-1283. cited by
examiner .
Gerber et al., "Versatile Objective Lens with Adjustable Correction
for Different Wavelengths and Substrate Thicknesses for Testing
Optical Disks", Applied Optics, vol. 36, No. 11, Apr. 10, 1997, pp.
2414-2420. cited by applicant.
|
Primary Examiner: Pardo; Thuy
Attorney, Agent or Firm: Finnegan, Henderson, Farabow,
Garrett & Dunner, L.L.P.
Parent Case Text
.Iadd.This is a divisional of U.S. application Ser. No. 10/189,259,
filed on Jul. 5, 2002 now U.S. Pat. No. Re. 40,329, the contents of
which are incorporated herein by reference (now U.S. Pat. No. Re.
40,329), which is a reissue of U.S. application Ser. No.
09/074,474, filed on May 7, 1998 (now U.S. Pat. No.
6,088,322)..Iaddend.
Claims
What is claimed is:
.[.1. An optical disk reader or optical read/write system capable
of operating in either a compact disk (CD) or digital versatile
disk (DVD) format, comprising: disk support and drive means capable
of supporting and driving either a compact disk having a disk
substrate of thickness Y or a digital versatile disk having a disk
substrate of thickness X, a first laser diode operating with an
output beam having a first wavelength, a second laser diode
operating with an output beam having a second wavelength different
from said first wavelength, optical means for either directing the
output beam of said first laser diode at a said compact disk when
carried by said disk support and drive means or directing the
output beam of said second laser diode at a said digital versatile
disk when carried by said disk support and drive means, a single
element objective lens optically positioned between said disk
support and drive means on one end and said first and second laser
diodes on another end, said single element objective lens having a
central aperture zone and an outer aperture zone, said central
aperture zone being profiled to operate at a first numerical
aperture (NA) and said output beam of said first laser diode being
optically confined to said central aperture zone, said outer
aperture zone together with said central aperture zone being
profiled to operate at a second numerical aperture (NA) and wherein
said output beam of said second laser diode has ray fans extending
across the full aperture of said lens, and diffractive means
carried by said single element objective lens, said diffractive
means providing sufficient aspheric surface power for spherical
aberration correction and providing correction for
spherochromatism..].
.[.2. The apparatus of claim 1 wherein said lens has first and
second surfaces, and said first surface is located closer to said
disk support and drive means than said second surface and said
diffractive means is carried by said second surface..].
.[.3. The apparatus of claim 1 wherein said lens has first and
second surfaces, and said first surface is located closer to said
disk support and drive means than said second surface and said
diffractive means is carried by said first surface..].
.[.4. The apparatus of claim 1 wherein said diffractive means
provides sufficient correction for spherical aberration and for
spherochromatism that said single element objective lens achieves
diffraction-limited image quality for both CD and DVD
formats..].
.[.5. An optical disk reader or optical read/write system capable
of operating in either a compact disk (CD) or digital versatile
disk (DVD) format, comprising: disk support and drive means capable
of supporting and driving either a compact disk having a disk
substrate of thickness 2X or a digital versatile disk having a disk
substrate of thickness X, a first laser diode operating with an
output beam wavelength of approximately 780 nm, a second laser
diode operating with an output beam wavelength of approximately 650
nm, optical means for either directing the output beam of said
first laser diode at a said compact disk when carried by said disk
support and drive means or directing the output beam of said second
laser diode at a said digital versatile disk when carried by said
disk support and drive means, a single element objective lens
optically positioned between said disk support and drive means on
one end and said first and second laser diodes on another end, said
single element objective lens having first and second surfaces,
said first surface having an aspheric profile, said single element
objective lens having a central aperture zone and an outer aperture
zone, said central aperture zone being profiled to operate at
approximately a 0.45 numerical aperture (NA) and said output beam
of said first laser diode being optically confined to said central
aperture zone, said outer aperture zone together with said central
aperture zone being profiled to operate at approximately a 0.60
numerical aperture (NA) and wherein said output beam of said second
laser diode has ray fans extending across the full aperture of said
lens, and diffractive means carried by said single element
objective lens, said diffractive means providing sufficient
aspheric surface power for spherical aberration correction and
providing correction for spherochromatism..].
.[.6. The apparatus of claim 5 wherein said diffractive means has a
predetermined depth to optimize diffraction efficiency for both
laser diode wavelengths..].
.[.7. The apparatus of claim 6 wherein said first surface is
located closer to said disk support and drive means than said
second surface and said diffractive means is carried by said second
surface..].
.[.8. The apparatus of claim 7 wherein said diffractive means
provides sufficient correction for spherical aberration and for
spherochromatism that said single element objective lens achieves
diffraction-limited image quality for both CD and DVD
formats..].
.Iadd.9. An optical disk reader or optical read/write system
capable of operating in different disk formats, each disk format
having a different substrate thickness, comprising: disk support
and drive means capable of supporting and driving the different
disk formats; a first laser diode operating with an output beam
having a first wavelength; a second laser diode operating with an
output beam having a second wavelength different from the first
wavelength; and optical means including a single element objective
lens for either directing the output beam of the first laser diode
at one disk of the different disk formats when carried by the disk
support and drive means or directing the output beam of the second
laser diode at the other disk of the different disk formats when
carried by the disk support and drive means, the single element
objective lens optically positioned between the disk support and
drive means on one end and the first and second laser diodes on
another end, the single element objective lens comprising a central
aperture zone, an outer aperture zone, and a diffractive surface,
the central aperture zone being profiled to contribute to a first
numerical aperture (NA) operation and a first laser diode
operation, and to contribute to a second numerical aperture (NA)
operation and a second laser diode operation, the outer aperture
zone being profiled to contribute to the second numerical aperture
(NA) operation and the second laser diode operation, and the
diffractive surface providing sufficient aspheric surface power for
spherical aberration correction, and comprising a positive powered
diffractive surface which corrects spherical aberration due to
different substrate thickness of the optical disks..Iaddend.
.Iadd.10. The optical disk reader or optical read/write system of
claim 9, wherein each diffractive ray, for reading or writing for
the different disk formats, has the same diffraction
order..Iaddend.
.Iadd.11. The optical disk reader or optical read/write system of
claim 10, wherein the same diffraction order is a first diffraction
order..Iaddend.
.Iadd.12. The optical disk reader or optical read/write system of
claim 9, wherein the diffractive surface has a predetermined depth
having an optimum wavelength dependent on a predetermined
wavelength between the first wavelength and the second
wavelength..Iaddend.
.Iadd.13. The optical disk reader or optical read/write system of
claim 9, wherein the diffractive surface is based on a polynomial
phase function comprising a non-zero fourth power term which
controls spherical aberration correction..Iaddend.
.Iadd.14. The optical disk reader or optical read/write system of
claim 9, wherein the diffractive surface diffracts the output beam
having the first wavelength and diffracts the output beam having
the second wavelength..Iaddend.
.Iadd.15. The optical disk reader or optical read/write system of
claim 9, wherein the single element objective lens is a molded
cyclic olefin copolymer plastic lens..Iaddend.
.Iadd.16. An optical disk reader or optical read/write system
capable of operating in different disk formats, each disk format
having a different substrate thickness, comprising: disk support
and drive means capable of supporting and driving the different
disk formats; a first laser diode operating with an output beam
having a first wavelength; a second laser diode operating with an
output beam having a second wavelength different from the first
wavelength; and a single element objective lens optically
positioned between the disk support and drive means on one end and
the first and second laser diodes on another end, the single
element objective lens comprising diffractive surface, a central
aperture zone, and an outer aperture zone, the central aperture
zone for a first numerical aperture (NA) either directing the
output beam of the first lased diode at one disk of the different
disk formats when carried by the disk support and drive means or
directing together with the outer aperture zone for a second
numerical aperture (NA) the output beam of the second laser diode
at the other disk of the different disk formats when carried by the
disk support and drive means, and the diffractive surface providing
sufficient aspheric surface power for spherical aberration
correction, and comprising a positive powered diffractive surface
which corrects spherical aberration due to different substrate
thickness of the optical disks..Iaddend.
.Iadd.17. The optical disk reader or optical read/write system of
claim 16, wherein each diffractive ray, for reading or writing for
the different disk formats, has the same diffraction
order..Iaddend.
.Iadd.18. The optical disk reader or optical read/write system of
claim 17, wherein the same diffraction order is a first diffraction
order..Iaddend.
.Iadd.19. The optical disk reader or optical read/write system of
claim 16, wherein the diffractive surface has a predetermined depth
having an optimum wavelength dependent on a predetermined
wavelength between the first wavelength and the second
wavelength..Iaddend.
.Iadd.20. The optical disk reader or optical read/write system of
claim 16, wherein the diffractive surface is based on a polynomial
phase function comprising a non-zero fourth power term which
controls spherical aberration correction..Iaddend.
.Iadd.21. The optical disk reader or optical read/write system of
claim 16, wherein the diffractive surface diffracts the output beam
having the first wavelength and diffracts the output beam having
the second wavelength..Iaddend.
.Iadd.22. The optical disk reader or optical read/write system of
claim 16, wherein the single element objective lens is a molded
cyclic olefin copolymer plastic lens..Iaddend.
Description
BACKGROUND AND BRIEF SUMMARY OF THE INVENTION
The present invention relates to a single objective lens that can
be used with either CD optical disks or DVD optical disks. Several
different formats of optical disk are known in the prior art. The
two most commonly used formats are the CD and the DVD. These two
optical disk formats store different data densities; the DVD uses a
much smaller track and much smaller "pits" to record a higher data
density. The CD (Compact Disk) appears in wide use as both a CD-DA
(Company Disk-Digital Audio) and a CD-ROM (Compact Disk-Read Only
Memory); the format is identical for these two species. The DVD
(Digital Versatile Disk) appears in use as a digital video (movie)
storage or an extended computer memory product.
Data records on both CD and DVD formats are in "pits" formed in a
reflective surface of the disk. These "pits" are actually in the
form of short "trenches" that lie along a track that spirals around
the disk surface. The CD "pit" is typically 0.50 micrometer (uM)
wide and between 0.83 to 3.05 uM long. The track pitch is 1.6 uM
and the depth of the "pit" is 0.20 uM. To achieve higher data
density, the DVD "pit" is typically 0.3 uM wide and between 0.40 to
1.5 uM long. The track pitch is 0.74 uM and the "pit" depth is 0.16
uM. The CD can reliably record about 650 MB of digital data whereas
the DVD can reliably record about 4.7 GB of digital data on one
side of the disk (both sides can be used on a DVD).
The width and depth of the CD "pit" was determined by A early
optical fabrication technology which limited the objective lens to
0.45 NA (Numerical Aperture), and by early laser diode technology
(a 780 nm emission line). Because cost-effective objective lenses
could be made no faster than 0.45 NA (i.e. a relative aperture of
f/1.11) and lower wavelength laser diode emission lines were not
available, the size of a diffraction-limited laser spot image was
limited to 1.0 uM at the Full-Width-Half-Maximum intensity points
(FWHM). The CD "pit" depth is chosen to be one-fourth of the laser
wavelength (0.20 uM) and the "pit" width is chosen to be about half
the laser spot diameter (0.50 uM). This arrangement permits about
half of the wavefront in the laser spot to reflect from the bottom
of the "pit" and about half of the wavefront to reflect from the
surface surrounding the "pit." The two reflected components are
half a wavelength out of phase so they interfere destructively. No
signal is returned to the objective lens when a "pit" is present.
When no "pit" is present, the full wavefront reflects from the
surrounding surface and light is returned to the objective
lens.
This is the digital encoding process for most optical disks.
There are other subtle effects that this encoding process
introduces such as diffraction at the edges of the pit, but the
interference process is thought to be the principal phenomenon.
The newer DVD format has been enabled by two technology
developments; a 650 nm laser diode has become commercially viable
and 0.60 NA objective lenses have become cost-effective. The A
combination of these two factors produces a diffraction-limited
laser spot with 0.64 uM FWHM, so the DVD "pit" width becomes 0.32
uM and the "pit" depth becomes 0.16 uM.
Several optical disk products have been produced in the prior art
that combine CD and DVD formats in the same optical reader. In
order to achieve this goal, the prior art uses two laser diodes
plus two lenses and moves either one set (laser diode plus
objective for CD format) or the other set (laser diode plus
objective for DVD format) over the disk that is to be read. No
prior art single objective design is known that can operate with
either the CD or DVD formats.
The invention of this application is a single lens that can operate
with either the CD format (with 780 nm laser diode) or with the DVD
format (with 650 nm laser diode). No moving parts are required with
this invention because the appropriate laser diode can be turned on
electrically and introduced to the objective lens via a dichroic
beamsplitter or a grating structure.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic representation of a typical prior art CD
objective lens;
FIG. 2 shows the wavefront error of the prior art objective lens
shown in FIG. 1;
FIG. 3 is a graphical representation of the depth of focus defined
as the RMS wavefront error of the prior art lens of FIG. 1;
FIG. 4 shows a single objective lens according to the present
invention and related system components operating with either a CD
format (0.45 NA ray fan and thick disk substrate) or a DVD format
(0.60 NA ray fan and thin disk substrate);
FIG. 5 shows a schematic representation of one embodiment of the
single objective lens according to the present invention using
aspheric surfaces;
FIG. 6 is a graphical representation of the wavefront errors of the
single objective lens shown in FIG. 5;
FIG. 7 is a graphical representation showing the depth of focus
defined as the RMS wavefront error for the single objective lens
shown in FIG. 5;
FIG. 8 is a schematic representation of a second and preferred
embodiment of the present invention using one diffractive and one
aspheric surface;
FIG. 9 is a graphical representation showing the wavefront errors
for the lens design shown in FIG. 8;
FIG. 10 is a graphical representation showing the depth of focus
properties of the system shown in FIG. 8;
FIG. 11 is a graphical representation of the percentage of light
focused by a diffractive surface showing wavelength dependency;
and
FIG. 12 is an exaggerated representation of the diffractive surface
used in the preferred embodiment shown in FIG. 8.
DETAILED DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a typical prior art CD objective operating at 0.45 NA
and with a 780 nm laser diode source. This objective uses injection
molded PMMA plastic plus aspheric surfaces on both sides of the
lens. The objective forms a diffraction-limited image on the rear
surface of a 1.2 mm thick polycarbonate plastic cover on the
CD.
FIG. 2 shows the wavefront error of the prior art system of FIG. 1
(the horizontal axis is the dimension across the lens aperture and
the vertical axis is the wavefront error). The Marechal condition
for a diffraction-limited optical system is 0.070 RMS waves. This
prior art lens has a 0.035 RMS wavefront error and is
diffraction-limited by this criterion. This RMS wavefront error is
equivalent to a 0.140 P-V wavefront error and the Rayleigh
criterion for a diffraction-limited lens is a wavefront error of
less than 0.250 PV waves, so the lens is diffraction-limited by
this criterion as well.
FIG. 3 shows the RMS wavefront error of the prior art system of
FIG. 1 as a function of the depth of focus. Because the objective
must be rapidly auto-focused during reading operations, there must
be a useful depth of focus where the objective performance is
essentially diffraction-limited. This prior art nominal design is
essentially diffraction-limited over a +/-1.5 micrometer range.
When the objective is manufactured, fabrication tolerances reduce
performance and the useful depth of focus is reduced to about
+/-1.0 micrometer. The essentially diffraction-limited depth of
focus requirement forces very stringent fabrication tolerances on
this class of objective lens.
FIG. 4 shows the first embodiment of the objective lens design of
the present invention that could operate with both CD and DVD
formats. Lens 20 has a large aperture that permits ray fans for
either a 0.45 NA (and 780 nm laser diode) operation or a 0.60 NA
(and 650 nm laser diode) operation. This figure shows that the
central zone of the lens must be used to control the 0.45 NA and
780 nm laser diode operation and that the outer zone can be
independently designed for the 0.60 NA and 650 nm laser diode
operation. However, the central zone will also contribute to the
0.60 NA and 650 nm laser diode operation and this is the reason
that prior art objectives designers have not been able to use a
single element objective for both CD and DVD reader systems. As
shown in FIG. 4, disk 30 may either be a DVD format disk or a CD
format disk. Disk support and drive means shown generally as 40
includes a conventional drive plate 41, spindle 42 and drive motor
43 as known in the art. First and second laser diodes 51 and 52,
respectively, operate with output beams of approximately 780 nm and
650 nm, respectively. The laser diode output beams pass through
beam-splitters 71 and 72 and are directed towards collimating lens
60. Light 61 exiting the collimating lens 60 passes through single
element objective lens 20, is reflected from the CD or DVD disk,
and is deflected by beam-splitter 72 onto photodetector 80, where
changes in output power are utilized to read the disk, as is known
in the art. It is significant that the single element objective
lens 20 of the present invention is positioned between the
beam-splitter .[.70.]. .Iadd.71 .Iaddend.and disk 30 in a pathway
of collimated light. Several of the prior art systems position the
objective lens in a pathway of non-collimated light requiring that
the placement of the objective lens be very precise. The placement
of components shown in FIG. 4 can be varied without departing from
the invention and alternate beam-splitters and collimators may be
used. Although the embodiments shown and discussed herein disclose
lasers 51 and 52 operating at 780 nm and 650 .[.mn.].
.Iadd.nm.Iaddend., it is to be understood that the invention can be
applied to the general case wherein lasers can be operated with
different output wavelengths including shorter wavelength lasers as
they become commercially available. Another significant aspect of
the single element objective lens 20 as used in the present
invention is that the lens is a single optical element in contrast
to the typical two element prior art design which utilizes either
an objective lens and hologram or an objective lens and a second
lens element. Full alignment of both elements in the prior art
requires alignment of five degrees of freedom of the two combined
elements (centration of both elements and two degrees of tilt for
each element), whereas the use of the single element, fixed
objective lens 20 of the present invention greatly simplifies
alignment of the lens.
The first embodiment of the present invention is shown in greater
detail in FIG. 5. This is a molded COC (Cyclic Olefin
Copolymer) plastic lens 20 with aspheric first surface 21 and
aspheric second surface 22. This invention uses the fact that the
polycarbonate disk cover plate 30 varies from 0.6 mm in the DVD
format 31 to 1.2 mm in the CD format 32 and that the spherical
aberration introduced by the plate is twice as large for the CD
format. Concurrently, the objective DVD format NA is 0.60 and
introduces nearly 2.4 times the spherical aberration that the CD
format 0.45 NA introduces to the system. The spherical aberration
of the cover plate and the spherical aberration of the objective,
therefore, work in concert for the CD and for the DVD systems to
produce similar amounts of system spherical aberration. Although
the amount of spherical aberration for the two systems is similar,
the distribution of spherical aberration across the aperture of the
lens is different for the two systems and this limits the
aberration correction to a less than diffraction-limited condition.
In addition, the CD and DVD systems operate at different
wavelengths and the refractive index of the plastic changes with
wavelength in such a way that the distribution of spherical
aberration across the lens aperture also changes with wavelength.
Optical designers recognize this condition as spherochromatism.
The first embodiment of this invention utilizes the discovery that
a single element objective lens can be used for both CD and DVD
operation because the amount of spherical aberration for the two
systems is similar and can be controlled to nearly
diffraction-limited levels by the correct choice of aspheric
surface profiles in the central zone 25 and in the outer zone 26 of
the objective.
FIG. 5 shows the first embodiment objective. The 0.45 NA, 780 nm
ray fans are shown passing through the central zone 25 of the lens
aperture. The 0.60 NA, 650 nm ray fans are shown extending across
the full aperture of the lens, which includes the central zone 25
and outer zone 26. Although the diameter of the outer zone appears
only slightly larger than the central zone diameter, nearly 0.5 of
the energy in the DVD system resides in this outer zone. The
ability to independently modify these outer zone surface profiles
gives the designer a strong control of the DVD system aberrations
that is independent of the CD system aberrations. The two different
cover plate thicknesses are shown in this figure. The laser diodes
and disk drive are not shown.
The first surface 21 and second surface 22 shown in FIG. 5 can be
described in the following mathematical terms: a first aspheric
surface defined as:
.rho..times..function..times..rho..times..times..times..times..times..tim-
es..times. ##EQU00001## and a second surface having an aspheric
profile defined as:
.rho..times..function..times..rho..times..times..times..times..times..tim-
es..times. ##EQU00002##
Where sag represents sagittal height, and
.rho. ##EQU00003## .rho. ##EQU00003.2## .times.<<
##EQU00003.3## .times.<< ##EQU00003.4##
.times..times..times..times..times..times..times..times..times..times..ti-
mes..times..times..times..times..times..times..times..times..times..times.-
.times..times..times..times..times..times..times..times..times..times..tim-
es..times..times..times..times..times..times..times..times..times..times..-
times..times. ##EQU00003.5## the vertex curvatures .rho..sub.1 and
.rho.2 satisfy
<.rho..rho.< ##EQU00004##
FIG. 6 shows the wavefront errors of the first embodiment objective
(shown in FIG. 5) for both the CD and DVD operating conditions.
Note that the P-V wavefront error for the DVD case is about the
Rayleigh limit of 0.250 wave.
FIG. 7 shows the RMS wavefront error for the system of FIG. 5
through the depth of focus and verifies that the nominal system is
at the limit of being diffraction-limited and that there is
essentially no margin for fabrication tolerances. The first
embodiment is a theoretically viable solution but it requires very
tight manufacturing processes to produce economic yields.
The preferred embodiment uses a diffractive surface on one side of
the objective. Diffractive surfaces introduce an additional
aberration-correction feature that refractive aspheric surfaces
cannot provide. Diffractive surfaces change the wavefront
differently for different wavelengths. A positive powered
diffractive surface bends longer wavelength light more than shorter
wavelength light. This is the opposite behavior of a refractive
aspheric surface. This new aberration-correction feature permits a
single element objective lens to correct most of the
spherochromatism that limits the performance of a simple refractive
aspheric lens.
FIG. 8 shows the preferred embodiment single element objective lens
120. The first surface 121 nearest the disk is aspheric and the
second surface 122 furthest from the disk has a diffractive surface
imposed on a spherical base curve. The diffractive surface provides
the same aspheric correction of spherical aberration provided by a
refractive aspheric surface but also provides spherochromatism
correction. The objective has a slightly different back focal
distance for the two wavelengths of interest but this is
unimportant because the autofocus mechanism brings the objective to
its best focus.
Diffractive surfaces are known in the prior art where they are
widely used to correct the chromatic aberration of a singlet
operating over a broad spectral band or to correct the spherical
aberration of a singlet over a very narrow spectral band. The use
of a diffractive surface to correct .[.sperochromatism.].
.Iadd.spherochromatism .Iaddend.of a singlet operating at two
different wavelengths is not known in the prior art.
A diffractive surface consists of microscopic grooves in the
surface of an optical element. The grooves are widest at the center
of the optical element and progressively decrease groove width
toward the periphery of the element. The groove width is similar in
magnitude to the wavelength of light being used, so the grooves act
as a diffraction grating to bend the light. The bending of light is
due to diffraction rather than refraction (as produced by Fresnel
lenses). Because the groove widths become smaller near the element
periphery, the incident wavefront bends more near the edge of the
optical element than at the center and the wavefront is therefore
focused by diffraction.
Because diffraction is wavelength dependent, the wavefront focusing
changes with wavelength to correct chromatic aberration. Because
the rate at which the groove widths change can be adjusted to make
the surface behave like an aspheric refractive surface, spherical
aberration can be corrected.
FIG. 12 shows an exaggerated view of the diffractive surface. The
actual groove depth is about 1.0 micrometer. The diffractive
surface is described by a polynomial phase function which expresses
how many waves of optical path are added or subtracted from each
radial zone of the wavefront. The polynomial phase function is
.times..times. ##EQU00005## ##EQU00005.2## .times. .times.
.times.<<.times. .times..times..times..times.<<
##EQU00005.3##
The first surface 121 shown in FIG. 8 can be described
mathematically as follows: a first aspheric surface defined as:
.rho..times..function..times..rho..times..times..times..times..times..tim-
es..times. ##EQU00006## the second surface 122 has a spherical
profile on which is imposed a diffractive surface 122d. The
diffractive surface 122d has a polynomial phase function with at
least the second and fourth power terms non-zero where
Phase=C.sub.2r.sup.2+C.sub.4r.sup.4
FIG. 9 shows the wavefront error for the diffractive objective of
FIG. 8. It is significant that the wavefront error vertical scale
is ten times more sensitive than the prior plots.
The wavefront error is essentially zero and the more sensitive
scale is needed to see any wavefront error in this plot.
FIG. 10 shows the depth of focus properties of the diffractive
objective of FIG. 8. The performance of the 0.45 NA, 780 nm system
is better than the prior art. This permits a slightly greater
fabrication tolerance margin compared to prior art objective
lenses. The 0.60 NA, 650 nm nominal system depth of focus is about
+1.0 micrometer. After fabrication tolerances are considered, the
depth of focus will be on the order of .+-.0.7 micrometer. This is
equivalent to the depth of focus that can be achieved by a 0.60 NA,
650 nm objective that only operates with a DVD format reader.
FIG. 11 shows an important feature of diffractive surfaces. The
percentage of light that is focused by a diffractive surface is
wavelength dependent and several different images can be produced
in different diffraction orders. The proper choice of the
diffractive surface depth will cause essentially all of the energy
in one wavelength to be in the image of the preferred first
diffraction order. Because the optimum depth is wavelength
dependent and the laser diodes operate at 780 nm and 650 nm, not
all of the energy in these two wavelengths can be directed into
their respective first order images. The depth of the diffractive
surface of this invention is, therefore, chosen midway between
these two wavelengths at a wavelength value of 715 nm.
FIG. 11 shows that 0.97 of the energy is directed to the respective
first order images when this condition is met. The remaining 0.03
of the energy is primarily directed into the zero diffraction order
and is distributed over a large area of the optical disk and
produces a negligible background signal.
Modifications of design may be made without departing from the
invention. For example, the diffractive surface may be carried by
the lens surface 21 closest to the disk. Various types of
collimators and beam-splitters may be used as well as laser diodes
of various wavelengths. Various materials may be used for the
objective lens, including glass and PMMA.
* * * * *