United States Patent [19]
`Hasman et al.
`
`IIIII IMI III IIIII IIIII IIIII IIIII UlII IIIII IIIII IIIII IIIIII III IIIII IIII
`US005526338A
`[11] Patent Number:
`[45] Date of Patent:
`
`5,526,338
`Jun. 11, 1996
`
`[54] METHOD AND APPARATUS FOR STORAGE
`AND RETRIEVAL WITH MULTILAYER
`OPTICAL DISKS
`
`[75]
`
`[73]
`
`Inventors: Erez Hasman, Kiron; Asher A.
`Friesem, Rehovot, both of Israel
`
`Assignee: Yeda Research & Development Co.
`Ltd., Rehovot, Israel
`
`[21] Appl. No.: 402,227
`
`[22] Filed:
`
`Mar. 10, 1995
`
`Int. CI.° ........................................................ GlIB 7/00
`[51]
`[52] U.S. CI ............................. 3691109; 369194; 3691102;
`3691112; 369/275.1; 369144.23
`[58] Field of Search ................................. 369144.23, 109,
`369/102, 94, 103, 99, 112, 116, 275.1,
`275.3, 275.4; 3651124, 125
`
`[56]
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`4/1993 Rosen et al ............................... 369/94
`5,202,875
`5,251,198 1011993 Striekler .................................... 369/94
`5,255,262 10/1993 Best et al ............................. 369/275.1
`5,373,499 1211994 Imaino et al ........................ 369/275.4
`111995 Best et al ............................. 3691275.1
`5,381,401
`411995 Holtslag et al ...................... 369144.23
`5,408,453
`
`OTHER PUBLICATIONS
`
`Computer--Originated Aspheric Holographic Optical Ele-
`ments--R. C. Fairchild et al, Opt. Eng. vol. 21 No. 1 (1982)
`pp. 133-140.
`
`Depth Response of Conlocal Optical Microscopes--T. R.
`Code, et al., Opts. Letters, vol. 11, No. 12 (1986) pp.
`770--772.
`Design of Wave Length-Division .... Y. Amitai Opts.
`Comm. (1993) pp. 24--28.
`Analytic Design of Hybrid Diffractive--Refractive Achro-
`mats--N. Davidson et al, Appl. Opt. vol. 32, No. 25 pp.
`4770-4774.
`Efficient Multilevel Phase Holograms For CO2 Lasers--E.
`Hasman, et al. Opt. Soc. Am.-dgptics Letters (1991) pp.
`423--425.
`MultiFuncfional Holographic Elements For Surface
`Meas.--E. Hasman, et al. Opt. Eng. vol. 31, No. 2, (1992)
`pp. 363-368.
`
`Primary Examiner--Loha Ben
`Attorney, Agent, or Firm--Mark M. Friedman
`
`[57]
`
`ABSTRACT
`
`A multilayer optical disk system, which includes an optical
`disk unit having a number of connected optical disks. A
`number of light sources, such as diode lasers, are used to
`provide a number of light beams of different wavelengths. A
`wavelength multiplexer combines the light beams into a
`single beam which is then axially dispersed so that light of
`different wavelengths are simultaneously focuses onto the
`different optical disks. A wavelength demultiplexer splits
`light reflected from the optical disks according to wave-
`length to produce separate beams which are then separately
`detected.
`
`35 Claims, 8 Drawing Sheets
`
`SPLITTER
`\ i
`
`""1
`t
`
`I
`I
`
`,
`
`\If
`
`CONFOCAL IMAGING
`/Cl0
`
`94B-I-----’-~ / I \ I
`
`\i I
`
`L~O -~ 19~’~ ~ ~.107101 109"~ \/ ~///95B 96B
`
`LG Electronics, Inc. et al.
`EXHIBIT 1007
`IPR Petition for
`U.S. Patent No. RE43,106
`
`

`
`U.S. Patent
`
`Jun. 11, 1996
`
`Sheet 1 of 8
`
`5,526,338
`
`SOURCE
`
`WAVELENGTH
`MULTIPLEXING
`
`5 BEAM
`
`LDO
`(DOE)
`3
`
`WDD
`6
`
`_1 WAVELENGTH l
`
`-IDEMULTIPLEXINGI
`
`ClO
`
`CONFOCAL
`IMAGING
`OPTICAL
`
`DETECTORS
`
`MOD
`4
`
`,, ~;~
`
`MULTILAYER DISK
`
`FIG.1
`
`

`
`U.S. Patent
`
`Jun. 11, 1996
`
`Sheet 2 of S
`
`5,526,338
`
`LIGHT SOURCE
`&: WMD
`
`21
`
`29
`
`DIODE
`LASER
`
`26.j~
`
`32
`
`31
`
`/
`
`X2 X3 X4
`
`o.c..o,,
`
`SPLITTER 30
`
`FIG.2
`
`WMD
`
`33
`\
`
`I
`
`38
`
`34
`
`31
`
`DIODE
`LASER
`
`AIR
`
`4C
`
`FIG.3
`
`

`
`U.S. Patent
`
`Jun. 11, 1996
`
`Sheet 3 of 8
`
`5,526,338
`
`DIODE
`LASER
`
`/WMD
`
`45 f
`X1 X2 X3 (_
`COMBINED BEAM
`
`43
`
`HOE
`
`FIG.4
`
`130
`
`~^
`
`97
`
`~.
`’
`
`~,o
`
`20
`
`150
`
`\^
`
`FIG.9B
`
`

`
`U.S. Patent
`
`Jun. 11, 1996
`
`Sheet 4 of 8
`
`5,526,338
`
`50
`
`J
`
`LDO
`
`/
`
`51
`
`p
`
`i |1
`
`DOE’~
`X4
`54
`
`\ ~",,L///
`\Xl/"A.. .....
`
`53
`X2
`
`6O
`
`_i/
`X2
`Xl,,
`}’3 X4
`
`61
`
`II II II
`
`,p-
`
`II
`
`N N N
`
`N
`
`FIG.5
`
`LDO
`
`/
`
`HYBRID
`DIFFRACTIVE- REFRACTIVE
`
`S ELEMENT
`
`F4 F3
`
`F2 FI
`
`62j
`X1
`
`\
`63
`X2
`
`FIG.6
`
`

`
`U.S. Patent
`
`Jun. 11, 1996
`
`Sheet 5 of 8
`
`5,526,338
`
`},1 },2 },3 },4
`
`MOD
`
`MULTILAYER OPTICAL
`DISK SELECTIVE
`WAVELENGTH MEDIA
`
`FIG.7
`
`--71
`
`f
`
`"72
`
`--73
`
`f
`
`-’74
`
`DICHROIC
`FILTERS
`
`/
`/
`
`\
`
`/’\
`
`FIG.8
`
`PARTIALLY
`TRANSPARENT
`LAYERS
`
`

`
`U.S. Patent
`
`Jun. 11, 1996
`
`Sheet 6 of 8
`
`5,526,338
`
`0
`
`

`
`U.S. Patent
`
`Jan. 11, 1996
`
`Sheet 7 of 8
`
`5,526,338
`
`WDD & CIO & DETECTORS
`
`PIANO-CONVEX LENS
`201
`
`201
`
`I
`
`3~2
`
`X3
`
`PINHOLES
`207 208
`209 210
`
`3O0
`DETECTORS
`
`202
`LINEAR GRATING
`
`FIG. 10
`
`SOURCE
`40~ LIGHT
`III
`
`.THE
`
`MOBILE PART
`
`407
`//,..,.,-~
`CUSING
`/ BLOCK
`
`~
`
`PENTA- PRISM
`--/~~408
`/
`
`404
`BEAM SPLITTER MOD420)
`
`~410
`FOCUSING LENS
`
`FIG.11
`
`

`
`U.S. Patent
`
`Jan. 11, 1996
`
`Sheet S of S
`
`5,526,338
`
`SINGLE
`DIODE-
`LASER
`
`BEAM ,~~
`SPLITTER
`301
`
`~/90
`
`’~~295
`
`307 308
`
`312
`
`315
`
`)
`
`BIFOCAL LENS
`3OO
`
`306B
`
`L318
`
`313
`
`/l~’~’’~ PINHOLE
`
`DETECTOR
`
`316
`
`FIG.12
`
`302
`TWO LAYERS
`
`BINARY RELIEF GRATING
`5OO
`
`\
`
`nl
`
`V--l
`
`n2
`
`FIG.13
`
`

`
`5,526,338
`
`1
`METHOD AND APPARATUS FOR STORAGE
`AND RETRIEVAL WITH MULTILAYER
`OPTICAL DISKS
`
`FIELD AND BACKGROUND OF THE
`INVENTION
`
`The present invention relates to optical storage disks, and,
`in particular, to methods and apparatus for the retrieval of
`high density information in optical storage disks with high
`readout bit-rate.
`The density of optical memory is normally described in
`two dimensional formats and is typically quantified in bits
`per square millimeter. The upper limits on information
`density are set by the diffraction of light. Specifically, the
`minimum diameter of a light spot formed at a focal point is
`about X/(2NA), where ~ is the wavelength and NA is the
`numerical aperture of the focused beam. Consequently, the
`information density is approximately (NA/L)2 correspond-
`ing to 109 to 101° bits on one surface of a typical 4.7 inch
`(120 mm) diameter optical disk. Optical disks are available
`on which information may be stored on both sides of the
`disk. The description herein is equally applicable for both
`single-sided and double-sided optical disks.
`Advances in computer technology call for increased
`memory capac!ty and shortened access time. Current work
`in the optical storage field is geared towards quadrupling the
`capacity of the disks by utilizing blue-green diode lasers.
`Significant improvement in memory capacity requires
`increasing the volume density of the storage medium. Pres-
`ently, such storage density increases are achieved by exploit-
`ing optical tapes. However, optical tapes are serial access
`devices with limited access speed which greatly reduces the
`attractiveness of this approach relative to optical disks
`whose geometry makes it possible to use random access
`techniques.
`The optical disk, as a two-dimensional optical storage
`device, is currently the most widespread physical format for
`optical storage. The volume data density of optical disks can
`be increased by adding a third physical dimension. This can
`be done by using a multilayer optical disk, i.e., by axially
`stacking a number optical disks. In order to utilize the third
`physical dimension, an unconventional optical head must be
`used.
`Several stacked optical disk systems have been proposed.
`IBM Almaden Research Center of San-Jose, Calif., propose
`a unique volumetric method for increasing optical disk
`capacities which is disclosed in U.S. Pat. Nos. 5,202,875 and
`5,255,262, incorporated herein be reference for all purposes
`as if fully set forth herein. The IBM approach involves
`bonding together individual disks in a stack with spacers
`provided between adjacent disks to define a gap between
`adjacent disks. At any one time, a movable lens in the optical
`disk drive focuses a laser on one surface of one of the disks
`in order to read data. The focus of the laser is changed
`repeatedly to sequentially read data from the various disk
`surfaces. Each disk, or at least all but the farthest disk from
`the laser source, must be partially transparent so that the
`laser can be used to read a disk which lies beyond one or
`more other disks. Each disk surface, however, must also be
`sufficiently reflective to allow the data to be read.
`Unfortunately, there are several limitations in using the
`IBM approach. By increasing the number of disks, the signal
`to noise ratio (SNR) is generally reduced and the interlayer
`crosstalk becomes significant. Moreover, the time required
`
`2
`to focus the optical head on the desired disk surface, which
`involves accelerating the head from a fixed position, dis-
`placing it, and decelerating the head to a new rest position,
`requires significant time and greatly limits the data access
`time of the device.
`There is thus a widely recognized need for, and it would
`be highly advantageous to have, a method and apparatus
`which will alleviate the above limitations by providing
`multilayer optical disk storage with high volume storage
`capacity and high bit-rate readout of the stored data.
`
`10
`
`SUMMARY OF THE INVENTION
`
`15
`
`The present invention offers a three-dimensional optical
`disk storage with significantly decreased erosstalk (high
`signal to noise ratio), and greatly reduced access time
`(increased bit-rate readout).
`The present invention is characterized in that two or more,
`20 and preferably all, of the disks are read simultaneously using
`a parallel readout. This approach offers a significant access
`time advantage over the movable lens approach described
`above, regardless of whether partially transparent disks or
`selective wavelength disks are used.
`25 The present invention is of a novel three dimensional
`optical storage device which includes a multilayer optical
`disk. In one embodiment, the media of each layer are
`wavelength dependent, such that each layer may have high
`transmission for one wavelength or band of wavelengths and
`30 high reflectivity for a different wavelength or band.
`The optical head of a system according to the present
`invention includes an incident light system which combines
`two or more light sources, which may be diode lasers, each
`light source emitting a beam of light of a different wave-
`35 length which is related to the selective wavelength media of
`the multllayer optical disk, for readout of the desired disk
`information.
`The combining of several beams into a single beam, is
`effected using a suitable wavelength multiplexing device
`40 which may include diehroie beam combiners or a holo-
`graphic/diffractive optical element.
`The combined beam is focused on the multilayer optical
`disk using suitable axially dispersing optics, so that a
`45 multiplicity of axially displaced foei along the spinning axis
`of the disks are obtained, depending on the wavelengths of
`the various lasers source. The dispersing optics includes an
`on-axis diffractive lens or hybrid diffractive-refractive opti-
`.cal element. The beam from each source of a different
`50 wavelength, such as diode laser beams, is focused on a
`corresponding optical disk surface and is reflected back from
`the surface to produce a reflected beam which includes
`information related to the zone of the optical disk surface
`being read.
`55 The reflected light is detected by a confocal configuration
`which gives accurate depth discrimination. In addition, the
`refieeted light is discriminated by wavelength, using, for
`example, diehroic beam-splitters, prisms or off-axis diffrac-
`tive elements (holographic elements), and is focused on a
`60 corresponding photodiode or photodiode array, which each
`photodiode detecting information stored on one of the
`optical disk surfaces. The detection is performed simulta-
`neously on two or more layers, resulting in high bit-rate
`readout.
`65 In order to radially scan a multilayer compact disk system
`according to the present invention it is proposed, in a
`preferred embodiment of the preset invention, to use a scan
`
`

`
`5,526,338
`
`3
`concept which obviates the need to displace the light source
`and the detection unit.
`According to further features in an alternative embodi-
`ment of the present invention, the multilayer optical disk are
`partially transparent rather than being wavelength selective. 5
`The use of partially transparent disks simplifies the produc-
`tion process and lowers the cost of the system. However,
`everything else being equal, the use of such a system
`requires the use of higher power diode lasers and leads to
`some reduction in the signal to noise ratio. Use of confoeal 10
`readout configuration results in adequate signal to noise ratio
`for a suitable number of disk.
`In an alternative embodiment of the present invention, a
`bifocal focusing element is used for obtaining two focused
`spots on an optical disk device which includes two disks. 15
`The bifocal focusing element is based on a hybrid diffrac-
`tive-refractive element. In this ease, partially transparent
`layers are used. The readout also contains a confocal con-
`figuration so as to make it possible to retrieve information
`from the two layers simultaneously, with adequate signal to
`noise ratio.
`
`20
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`25
`
`35
`
`40
`
`The invention is herein described, by way of example
`only, with reference to the accompanying drawings,
`wherein:
`HG. 1 is a block diagram of the three dimensional optical
`storage and readout system which is based on multi-wave- 30
`length light discrimination;
`FIG. 2 illustrates a wavelength multiplexing device which
`is based on dichroic beam-splitters;
`FIG. 3 illustrates a wavelength multiplexing device which
`is based on dichroic prisms assembly;
`FIG. 4 illustrates a wavelength multiplexing device which
`is based on holographic optical element;
`FIG. 5 illustrates axially dispersing optics based on an
`on-axis diffractive optical element;
`FIG. tl illustrates axially dispersing optics based on an
`hybrid diffractive-refractive optical element;
`FIG. 7 illustrates a multilayer optical disk which is based
`on selective wavelength media;
`FIG. 8 illustrates a multilayer optical disk which is based 45
`on partially transparent layers;
`FIG. 9A illustrates a readout principle which is based on
`dichroie beam-splitters and confocal imaging optics;
`FIG. 9B illustrates a Colmnon Confocal Filter operation; 50
`FIG. 10 illustrates a readout principle which is based on
`a combined diffractive-refractive element and the confocal
`technique;
`FIG. 11 illustrates a proposed scanning configuration;
`FIG. 12 illustrates a readout principle which is based on 55
`single diode laser, bifocal focusing element and the confocal
`technique;
`FIG. 13 illustrates a binary relief grating.
`
`60
`
`DESCRIPTION OF THE PREFERRED
`EMBODIMENTS
`
`The principles and operation of optical disk systems and
`methods according to the present invention may be better 65
`understood with reference to the drawings and the accom-
`panying description.
`
`4
`Referring now to the drawings, FIG. 1 shows a block
`diagram of a three dimensional optical storage system
`(3-DOSS) of the present invention. The 3-DOSS includes an
`optical head (driver) and a multilayer optical disk assembly
`(MOD) 4.
`The optical head includes the illumination and the readout
`systems. The illumination system includes a light source 1,
`which is preferably supplied by a plurality of diode lasers,
`each emitting at a different wavelength. The illumination
`system further includes a wavelength multiplexing device
`(WMD) 2, for combining the laser beams into a single beam.
`Finally, the illumination system includes axially (or longi-
`tudinally) dispersing optics (LDO) 3, which may be in the
`form of an on-axis diffractive optical element (DOE) or a
`hybrid diffractive-refractive optical element.
`The readout system consists of beam-splitter 5, which
`reflects the back reflected light in the direction of the
`wavelength demultiplexing device 6, designated by WDD;
`confocal imaging optic (CIO) 7, and detectors 8, for readout
`of the information simultaneously from all the layers.
`A key feature of one embodiment of the present invention
`is the use of a plurality of diode lasers, or other light sources,
`each emitting at a different wavelength. Currently, many
`semiconductor diode lasers are available which span the
`entire visible spectrum and the near IR spectrum. Illustrative
`of such diode lasers are those sold by Spectra Diode Lab
`(U.S.A)---InGaAs lasers for wavelengths of 910-1020 rim,
`GaAIAs lasers for 780-860 nm and A1GalnP laser for
`630-670 rim. Electromagnetic radiation of various wave-
`lenghts may be used. Preferably, such radiation will be in the
`visible, ultraviolet and/or infrared ranges. For convenience,
`the term ’light’ is used in the specification and claims, it
`being understood that the term ’light’ as used herein is
`intended to cover all electromagnetic radiation and is not
`limited to the visible spectrum..
`Also known are zinc selenide (ZnSe) diode lasers which
`generate blue-green beams and work is being conducted on
`3-5 nitride compounds, such as gallium nitride, which have
`already been used in blue light-emitting diodes.
`Frequency conversion, with nonlinear materials and novel
`techniques, of laser diode output at the near IR to the
`blue-green wavelength is also possible. Israel-Soreq Nuclear
`Research Center and its commercial arm, Isorad Ltd., devel-
`oped a compact blue laser capable of a continuous output
`power of 3.6 mw at 427 nm by doubling a standard GaAs
`diode laser. Their nonlinear conversion device is based on an
`integrated KTP waveguide.
`Any suitable assembly of light sources may be used with
`the present invention. The sources may be diode lasers with
`a large wavelength separation, for example, 50-100 nm, or
`a small range, for example, 10-50 nm. For example, in the
`wavelength multiplexer illustrated in FIG. 2, the diode lasers
`21, 22, 23, 24 (in this example only four diodes are used) can
`be, respectively, a Sanyo SDL-3038-011 laser with wave-
`length 7~1=635 rim, a Toshiba TOLD 9140 laser with wave-
`length Xz=680 ran, a Sharp LT025MD laser with wavelength
`L3=780 nm and a Sharp LT015MD laser with wavelength
`~4=830 nm. Preferably, the preferred diode lasers are polar-
`ized.
`Various means may be used in the wavelength multiplexer
`to combine the light beams from the various sources. For
`example, one could utilize dichroic beam-splitters (as in
`FIGS. 2-3) or a holographic optical element (as in FIG. 4).
`The dichroic beam-splitters selectively reflect and trans-
`mit light depending on the wavelength of the incident light.
`Dichroic beam-splitters (or interference filters) are typically
`
`

`
`5,526,338
`
`5
`
`15
`
`25
`
`5
`produced using multilayer thin film coating technology.
`Illustrative of dichroic beam-splitters which are commer-
`cially available are those produced by Elop Electroopties
`Industries Ltd., Israel, or those produced by Melles Griot
`(similar to 03-BDS product series).
`Interference filters are of two basic types---edge filters
`and bandpass filters. Edge filters can be a short-wave pass or
`a long-wave pass filter. A long-wave pass filter transmits
`longer wavelengths and reflects shorter wavelengths while a
`short-wave pass filter transmits shorter wavelengths and lo
`reflects longer wavelengths. A bandpass filter transmits
`wavelengths in a desired wavelength interval, while refiect-
`ing wavelengths which are outside the interval. The trans-
`mission and reflection of interference filters also depends on
`the angles of incidence.
`In the preferred embodiment of a wavelength multiplexer
`illustrated in FIG. 2, the beams emitted by diode lasers, 21,
`22, 23, 24 and passed through collimating lenses 25, 26, 27
`and 211 are combined by dichroic beam-splitters of the edge
`filter type 29, 31) and 31. The edge filters are short wave- 20
`length pass, where XI<~<L3<~.,~. For example, the dichroie
`beam-splitter 3t1 transmits the light of X1 and Lz and reflects
`the light of 7~. The combined beam travels along the optical
`axis 32, where it is assumed to be polarized at the "p" plane.
`The polarization makes it possible to achieve higher oper-
`ating effieiencies.
`FIG. 3 is a schematic depiction of another embodiment of
`a wavelength multiplexer which may be used in the present
`invention. The multiplexer of FIG. 3 uses a diehroic prisms 30
`assembly with wavelenght selective coatings, such as that
`produced by Optec s.r.1. (Via Canova 10 20017 RHO Italy).
`The Optec device effects wavelength separation using an
`assembly of prisms for either three wavelengths (Optec
`product number DS-2787.102) or four wavelengths (Optec 35
`product number DS 193.101), where the optional wave-
`length separation can be in a bandpass of 10 nm. The
`three-wavelength version of the Optec device is shown in
`FIG. 3.
`In FIG. 3 the light sources are, for example, three diode 4o
`lasers 31, 32, 33 and their respective collimating lenses 34,
`35 and 36. The emission wavelengths of these lasers pref-
`erably satisfy the relationship ~,1<L2<~. The Optee wave-
`length multiplexer of FIG. 3 includes three prisms 37, 38 and
`39. The surfaces between prisms 37 and 38 and between 45
`prisms 38 and 39 are coated with suitable interference filters
`40 and 41. Here, the dielaroic ~ilm, 411, reflects the ~,~ light
`and transmits the rest, whereas the dichroic film, 41, reflects
`and transmits 2~z. A thin air gap (about 0.03 mm of
`thickness), between the prisms, 37 and 38, is necessary in 50
`order to achieve total internal reflection of the 7% light. The
`assembly of prisms of FIG. 3 is more convenient as a
`wavelength multiplexer than the configuration of FIG. 2
`which includes individual dichroic beam-splitters. More-
`over, the use of small incidence angles which is possible 55
`with the configuration of FIG. 3 substantially reduces polar-
`ization problems.
`FIG. 4 illustrates a wavelength multiplexing device WMD
`which is based on a holographic optical element (HOE). It
`is possible to combine a plurality of diode lasers using a 60
`single HOE. Furthermore, thick holograms can be realized
`with diffraction efficiencies approaching 100%, with
`70-95% being routinely achieved. In addition, the holo-
`graphic multiplexers can operate with parallel light, requir-
`ing the use of external lenses for the diode lasers. Alterna- 65
`tively, the HOE can operate directly with the diverging wave
`from the diode lasers, thereby performing the functions of
`
`6
`collimation, wavelength combination and reshaping the out-
`put wavefront with a single element.
`In the embodiment illustrated by FIG. 4, the wavelength
`multiplexing device is in the form of the HOE 44 which
`combines, for example, three diode lasers 41, 42, 43 having
`wavelength ~,~, ~, ~, respectively. It is possible to use
`phase holographic film such as bleach silver (AgHal) emul-
`sions, dichromated gelatin emulsion or polymeric films
`(commercially available from Eastman Kodak, Agfa and Du
`Pont) for achieving high diffraction efficiency.
`The combined beam (~i, L2, ~) is denoted by 45. It must
`be emphasized here that the performance of the HOE is ideal
`when the readout geometry and the wavelength are identical
`to the recording geometry and wavelength. In the absence of
`such identity it may be necessary to resort to computer
`generated holograms (see, R. C. Fairchild and J. R. Fienup,
`"Computer originated aspheric holographic optical ele-
`ments", Opt. Eng. 21,133 1982).
`Several companies produce holographic/diffractive ele-
`ments. Among them are Kaiser Optical Systems Inc. and
`POC Physical Optics Corporation of the U.S.A. It is also
`possible to realize wavelength multiplexing using planar
`optics configurations which are based on a substrate-mode
`holographic elements (see, Y. Amitai, "Design of wave-
`length multiplexing/demultiplexing using substrate-mode
`holographic elements, Opt. Comm. 98, 24, 1993), or by
`diffractive elements (kinoform).
`FIG. 5 illustrates an example of axially dispersing optics
`for use in the present invention. The axially dispersing optics
`focus the combined beam on the appropriate portion of the
`multilayer optical disk. Any suitable axially dispersing
`optics may be used. Preferably, the axially dispersing optics
`is an on-axis diffraetive optical element (DOE) (as in FIG.
`5), or a hybrid diffractive-refractive optical element (as in
`FIG. 6).
`The diffractive optical element can be generated as a
`computer-generated component using lithography, etching
`or thin film deposition. Optical elements based on diffraction
`were known long before the advent of lasers and before the
`introduction of holograms Dennis Gabor. The first diffrac-
`tion based lens was a Fresnel Zone Plate constructed by Lord
`Rayleigh in 1871. However, the full potential of diffraction-
`based optical elements could only be exploited following the
`development of the lasers.
`Two independent issues are associated with diffractive
`optical elements. One involves the design of the optimal
`two-dimensional phase function, ~, for obtaining the desired
`optical performance. The second problem is the optimiza-
`tion of the diffraction etficiency.
`When a wave impinges on a diffractive element, various
`diffracted orders are generated with typieally only one of
`these being desired. The ratio of diffracted power into the
`desired order (in general, the first diffracted order) over the
`incident power is knows as the diffraction efficiency. In
`order to ensure that the diffraction efficiency is high, it is
`necessary to record the diffractive optical element in special
`materials and/or to exploit special recording techniques. The
`diffractive optical elements can be used for aberration cor-
`rection, wavefront shaping and scanning, as well as for beam
`focusing, collimation, deflection and dispersing optics.
`It is possible to obtain a formula for the wavelength
`dependence of the focal length for a diffractive optical
`element. This can be done as follows: For an idealized
`quadratic DOE, the transmission function t(x,y) is given by,
`
`{1}
`
`

`
`5,526,338
`
`7
`where x and y are the coordinates in the plane of the thin
`lens, 7~o is the wavelength of the light, and fo is the focal
`length of the lens. However, when such a diffractive optical
`element is illuminated using another wavelength, ~, one
`obtains the well known result that the focal length fQ,) is 5
`given by,
`
`8
`FIG. 6 illustrates a hybrid lens 61 which is illuminated
`with the combined beam, 60, containing light of wave-
`lengths XI, Z2, ~3, ~,4. The hybrid lens 61 includes a
`piano-convex refractive lens with a diffractive element
`(kinoform) on the planar surface. The hybrid lens 61 serves
`to focus converging beams k~ 62, L2 63, ~,3 64, ~,4 65.
`High diffraction efficiencies for diffractive optical ele-
`ments can be obtained with kinoforms which are constructed
`{2} as surface relief gratings, with grooves having a graded
`
`This equation indicates that the focal length is inversely
`proportional to the wavelength which is the desired axially
`dispersion of the DOE.
`ff one assumes that the diffractive optical element with
`quadratic phase function is illuminated with the combined
`beam 50, which is made up of wavelengths ~-l, ~, L~, ~-4
`where ~q<~<~<k4, the result is several converging beams,
`each focused on a different plane. The diffractive optical
`element is designed to focus the beam of wavelength ~,l at
`the focal length of fa, 52, so the wavelength L2 is focused on
`fz=f1~JL.~, 53; wavelength ~,3 is focused on fa=flk,/~, 54,
`and wavelength ~.4 is focused on f4=flX1/’A4, 54. In this
`example the layers of the optical disk are placed at zl=fi,
`z~=f2, z3=f3, z4--f4. Utilizing a diffractive optical element
`with quadratic phase function, it is possible to obtain the
`distance between two adjacent focal planes by the relation,
`
`Af=-Az=-fo(A~/~o)
`
`131
`
`A classical objective lens for a compact disc player typically
`has a focal length of 8 mm and a diameter of 7.5 nun
`(NA=0.45). Substituting in Eq. {3} A~=50 nm, hv=500 nm
`(~%/L=0.1) and fo=8 mm, the result is if=800 gin. In order
`to reduce if while keeping the same effective focal length,
`it is necessary to combine diffractive and refractive ele-
`ments. If one assumes that the multiwavelength light illu-
`minates a refractive lens with a focal length fr(~.) and a
`diffractive optical element with a focal length faQ-) in
`cascade. Assuming negligible separation between the refrac-
`tive and diffractive elements, we resort to a simple lens
`combination equation,
`
`i/m~i)= i/fa(1)+i/L(~)
`
`{41
`
`where F(X) is the desired focal distance of the combined
`lens. Using Eq. {2) and neglecting the dispersion of the 45
`refractive lens fr(7~)=f,, the more general relation to Eq. {3}
`is obtained as follows:
`
`10 shape. Indeed, the diffraction efficiencies ofkinoforms hav-
`ing blazed relief gratings can reach 100 percent. An early
`fabrication process for obtaining the desired surface relief
`involves a single photornask with variable optical density for
`controlling the etching rate of some substrate. Unfortunately,
`15 such a process does not provide the required accuracies for
`controlling the graded shape and the depth of the surface
`relief grooves. In a more suitable process, the single pho-
`tomask with the variable density is replaced by a multiplicity
`of simpler binary photomasks so the graded shape is
`20 approximated by multilevel binary steps. To ensure that high
`diffraction efliciencies are obtained, the errors due to the
`depth and width of the step levels must be taken into
`account, (E. Hasman, N. Davidson and A. A. Friesem,
`"Efficient multilevel phase holograms for COs lasers" Opt.
`7_5 Lett. 16, 423, 1991).
`FIG. 7 illustrates a preferred embodiment of the multi-
`layer optical disk assembly (MOD), 70. The MOD 70
`includes a number of discrete disks or layers, which may
`resemble, or be identical to, conventional optical disks with
`30 conventional phase information. In the basic embodiment of
`the present invention each of the layers is made of a selective
`wavelength medium. In FIG. 7 the dichroic filters are short
`wavelength pass. For example, assume the MOD is illumi-
`nated with light of wavelengths ~q, L2, ~, ~,a, where
`35 ~,1<~<~<&4 ¯ The upper layer, denoted by 71, reflects the
`light of wavelength ~.4 and transmits light of wavelengths ~-3,
`~, X~. The layer 72 reflects the light of wavelength k3 and
`transmits that of wavelengths 3,2 and ~,l. The layer 73 reflects
`light of wavelength ~ and transmits that of wavelength ~.1.
`40 Finally, the layer 74 reflects light of wavelength k~.
`It is possible to form a selective wavelength medium
`using dichroie thin film coating technology. The thickness of
`each layer can be hundreds of microns.
`FIG. 8 illustrates a second embodiment of an MOD 80
`according to the present invention. In this embodiment the
`l~,,~w ~ na.rt~all~, t~n~n~nt ~ath~r than h~i~a ~t~r~l~n~th
`selective, as in the basic embodiment. Partially transparent
`layers were disclosed in U.S. Pat No. 5,255,262 which was
`described above. In this embodiment each of layers 81, 82,
`83, 84 partially reflects the incidence beams. It must be
`emphasized that in this case too, light of a number of
`wavelengths is used with each wavelength light being
`focused on a different layer. Thus, light of wavelength 2~ is
`focused on layer 84, light of wavelength L2 is focused on 83,
`55 light of wavelength ~-a is focused on 82, and light of
`wavelength ~.4 is focused on the upper layer 81.
`Regardless of whether a wavelength selective (FIG. 7) or
`partially transparent (FIG. 8) MOD is used, the data surfaces
`are typically ROM (Read Only Memory) which are formed
`60 directly into the substrate at the production phase. However,
`it is possible to coat the substrate with a suitable writable
`optical storage film such as, for example, WORM (Write
`Once Read Many) or one of the various erasable optical
`storage films such as phase change or magneto-optical (see,
`65 U.S. Pat No. 5,255,262). In order to realize a CD-WORM,
`it is necessary to finely tune and balance the absorption of
`the layers for the writing mode. The readout mode can be
`
`AF=(Fc/Io)[Fo(~’~o)]
`
`{5} 50
`
`where Fo-F(L=Xo), and fo=fa(L=2~o). For example, if one is
`interested in AF=Az=200 lain, and with AX/X=0.1 and Fo=8
`mm, then F_,q. {5} yields fo=32 mm and f,-10.6 ram.
`For realizing a hybrid combination of refractive-diffrac-
`tive lens, it is possible to use either separate lenses or a
`single hybrid difffractive-refractive element. The single
`hybrid configuration is particu