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`US005552916A
`
`United States Patent
`
`[19]
`
`[11] Patent Number:
`
`5,552,916
`
`O’Callaghan et al.
`
`[45] Date of Patent:
`
`Sep. 3, 1996
`
`[54] DIFFRACTIVE LIGHT MODULATOR
`[75]
`Inventors: Michael J. 0’Callaghan, Louisville;
`Igfigk A" Handschy’ Boulder’ both of
`'
`
`_
`_
`[73] Asslgneei Dmplaytechv Inca Boulderv C°1°-
`
`[21] Appl. No.2 8,764
`[22]
`Filed:
`Jan. 25, 1993
`
`Related U.S. Application Data
`'
`Continuation-in—part of Ser. No. 578,647, Sep. 7, 1990, Pat.
`No. 5,182,665.
`
`[63]
`
`Int. Cl.“ ........................................................ G02F 1/13
`[51]
`[52] U.S. Cl.
`................................. 359/95; 359/73; 359/53;
`.
`[58] Field of Search ...................
`
`S39/9543>,9é0,17(§id
`’
`’
`’
`
`’
`
`[56]
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`7/1974 Beard -
`$824,002
`'
`3343331 10/1974 Bmel at 31'
`gags it a1‘a'1
`1/1983 Cc1’m:°et‘:1.
`_'
`3/1984 Fergason .
`12/1985 Fergason .
`1/1986 Clark at 31”
`4/1986 Fergason _
`5/1936 1:
`_
`g/1935 1:322:32 _
`1/1987 Huignard et al. .
`3/1989 Clark et al. .
`3/1989 Handschy et a1.
`6/1989 Clark et al.
`.
`7/ 1939 Yamanobe Ct 211-
`9/1989 G00db)’ 61 31-
`-
`12/1990 Gibbons et a1.
`5/1991 Sakata.
`1/1993 O’Ca11aghan et al.
`
`'
`
`.
`
`.
`
`-
`
`OTHER PUBLICATIONS
`3:33;‘f;'§1S:°‘i1‘;‘“g “gm ‘"1 a1‘g“me“" Name "°1' 351’
`Patel et al, “Electrically controlled polarization—independent
`li uid—c
`stal Fresnel Lens arra s, ” O tics Letters vol. 16,
`Nq 7
`ry
`1991
`532 594
`p
`.
`, pp.
`o.
`, Apr. 7,
`—
`Wayne M. Gibbons et al, “Surface—mediated alignment of
`nematic liquid crystals with polarized laser light, ” Nature,
`1. 351 May 2, 1991, pp. 49-50.
`=
`
`V°
`
`L‘ t
`t‘
`d
`t
`.
`is C0“ "me °n “ex page)
`(
`Primary Examiner—Dona1d T. Hajec
`Assistant Exam’-"e"—".ran H0
`Attorney, Agent, or Fzrm—Cushman Darby & Cushman
`
`[57]
`
`ABSTRACT
`
`An arrangement (apparatus ‘and method) for selectively
`modulating Incldgnt unpolarlzed 11ght_PaSSmg through a
`biieiilgisamf:“::1::m°;2i.ii,:::i:P::,
`rau
`cu e
`avingo e
`ore ir
`y
`corresponding to first and second alignment regions The
`birefringent layer corresponding to the first alignment region
`has a first optic axis selectably set in a first orientation and
`a second orientation. The birefringent layer corresponding to
`the second alignment region has a second optic axis select-
`ably set in a third orientation and a fourth orientation. A
`switching means controls the optical axis states of the
`birefringent material by applying switching voltages to the
`areas of the birefringent layer. Light having passed through
`the birefringent layer at locations having the first orientation
`has ‘a dilferent phase from, and same polarization as, light
`having passed through locations with the third orientation,
`independent of a polarization state of the incident light. As
`a result, the birefringent material has a uniform state at
`locations where the corresponding optic axes between the
`first and second alignment regions are parallel, and a dif-
`fracting state produced by an interaction between the light
`having passed through areas having the first orientation and
`the light having passed through areas having the second
`orientation. The arrangement
`is effective for reflective
`modulat_ors, multilayer modulators or polarization-preserv-
`ing modulators, and has applications for intensity modula-
`tion, blurring modulation and beam steering.
`
`4’367’924
`4,435,047
`4 556 289
`4’563’059
`4:579:43
`4,596,445
`4,605,511
`4,639,091
`4,813,767
`4,813,771
`4,840,463
`4,850,681
`4,867,539
`4,974,941
`5,013,141
`5,182,665
`
`................... 359/95
`
`30 Claims, 14 Drawing Sheets
`
`ELECTRICAL
`DRIVING
`MEANS
`
`
`
`
`COMMON
`ELECTRODE
`
`
`
`FNC 1022
`
`FNC 1022
`
`

`
`5,552,916
`Page 2
`
`OTHER PUBLICATIONS
`
`Williams G, et al, “Electrically controllable Liquid Crystal
`Fresnel Lens, ” Proceedings of SPIE, vol. 1168, No.
`352-357, et al., Current Developments in Optical Engineer-
`ing and Commercial Optics, Aug. 1989.
`Mochizuki et al, “Elimination of Crosstalk in Highly Mul-
`tiplexed STN—LCDs by Using Conducting Orientation
`Films” , SID 90 Digest, pp. 84-87.
`Rieker, T. P. et al., “Layer and Director Structure in Surface
`Stabilized Ferroelectric Liquid Crystal Cells with Non—pla-
`nar Boundary Conditions, ” vol. 6, No. 5, 1989, pp.
`565-576.
`Nakaya et al, “Electrooptic Bistability of a Ferroelectric
`Liquid Crystal Device Prepared Using Charge-Transfer
`Complex—doped Polyimide-Orientation Films” Japanese
`Journal of Appl. Phys., vol. 28, No. 1, Jan. 1989, pp.
`Ll16—l8.
`Channin et al, “Rapid Turn Olf in Triode Optical Gate
`Liquid Crystal Devices,” Applied Physics Letters, vol. 28,
`No. 6, Mar. 15, 1976, pp.300-302.
`Wu et al, “Physical Properties of Diphenyldiacetylenic Liq-
`uid Crystals, ” J. Appl. Phys., 65(11), Jun. 11, 1989, pp.
`4372-4376.
`
`Patel et al, “Alignment of Liquid Crystals Which Exhibit
`Cholesteric to Smectic C* Phase Transitions,” J. Appl. Phys.
`59(7), Apr. 1,1986, pp. 2355-2360.
`
`Armitage et al., k “Liquid—Crystal Diiferentiating Spatial
`Light Modulator, ” Proceedings of SPIE, vol. 613, Nonlinear
`Optics and Applications, Jan. 1986, pp. 165-171.
`
`Cotts et al, “Appendix F: Compilation of Polymer Conduc-
`tivity Data,” Electrically Conductive Organic Polymers for
`Advanced Applications, 1986, pp. 176-202.
`
`Doane, J. W. et al, “Polymer Dispersed Liquid Crystals for
`Display Application ” , Molecular Crystals and Liquid
`Crystals, vol. 165, 1988 pp. 511-532.
`
`Cognard, Jacques, “Alignment of Nemati Liquid Crystals
`and Their Mixtures ”, Molecular Crystals and Liquid Crys-
`tals, Supplement Series, 1982, pp. 64-68.
`
`Mao, C. C. et al., “Low Power, High Speed Optical Phase
`Conjugation Using Chiral Smectic Opticaly Addressed Spa-
`tial Light Modulators,” Optic News, vol. 15, Sep. 1989, pp.
`A-38, Abst. TuA4.
`
`

`
`U.S. Patent
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`'
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`Sep. 3, 1996
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`Sheet 1 of 14
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`5,552,916
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`FIG‘. IA
`(PRIOR ART)
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`

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`U.S. Patent
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`Sep. 3, 1996
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`Sheet 2 of. 14
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`5,552,916
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`F/6.2
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`U.S. Patent
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`Sep. 3, 1996
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`Sheet 3 of 14
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`5,552,916
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`U.S. Patent
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`Sep. 3, 1996
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`Sheet 4 of 14
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`5,552,916
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`ELECTRICAL
`DRHHNG
`
`MEANS
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`

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`U.S. Patent
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`Sep. 3, 1996
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`Sheet 5 of 14
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`5,552,916
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`U.S. Patent
`
`Sep. 3, 1996
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`Sheet 6 of 14
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`5,552,916
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`ELECTRICAL A
`
`DRIVING
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`46
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`

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`U.S. Patent
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`Sep. 3, 1996
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`Sheet 7 of 14
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`5,552,916
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`F /6'.‘ /0
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`

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`U.S. Patent
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`Sep. 3, 1996
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`Sheet 8 of 14
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`5,552,916
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`

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`U.S. Patent
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`Sep. 3, 1996
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`Sheet 9 of 14
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`5,552,916
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`

`
`U.S. Patent
`
`5,552,916
`
`FIG. I4(a)
`
`F/G. /4(b)
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`U.S. Patent
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`Sep. 3, 1996
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`Sheet 11 of 14
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`5,552,916
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`

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`U.S. Patent
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`Sep. 3, 1996
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`Sheet 12 of 14
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`5,552,916
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`Sheet 13 of 14
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`5,552,916
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`U.S. Patent
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`Sep. 3, 1996
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`Sheet 14 of 14
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`5,552,916
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`

`
`1
`DIFFRACTIVE LIGHT MODULATOR
`
`5,552,916
`
`2
`
`This is a continuation-in-part application of application
`Ser. No. 07/578,647, filed Sep. 7, 1990, now U.S. Pat. No.
`5,182,665.
`
`BACKGROUND OF THE INVENTION
`
`Government Contract Clause: This invention was made
`
`with Government support under contract F2960l—89-C-0075
`awarded by the U.S. Air Force. The Government may have
`certain rights in this invention.
`
`FIELD OF THE INVENTION
`
`This invention relates to light modulators which control a
`light beam in response to external electrical signals. In
`particular, the present invention relates to a light modulator
`having an electrically switchable diffraction grating. Such
`modulators have a variety of uses, for example, as intensity
`modulators, beam steerers, image-blurring modulators, etc.
`
`DESCRIPTION OF THE ART
`
`There are many examples of light modulators in the prior
`art. Common types include electromechanical shutters,
`acousto-optic modulators, and electro-optic modulators. The
`electromechanical shutters typically use an electric motor or
`actuator to move an opaque member in or out of the light’s
`path. The acousto-optic modulators diffract
`light of the
`refractive index grating produced by a sound wave travelling
`through a transparent solid. The electro-optic modulators
`use the efl°ect of electrically-induced refractive index
`changes to modulate polarized light. Examples of electro-
`optic modulators are those that use ferroelectric liquid
`crystals (FLCs) as the modulating medium, as disclosed in
`U.S. Pat. Nos. 4,367,924, 4,563,059, 4,813,767, and 4,840,
`463 to Clark and Lagerwall, and in U.S. Pat. No. 4,813,771
`to Handschy and Clark. These FLC modulators have the
`advantage of low-power, low-voltage operation over most
`electro-optic modulators that use solid materials such as
`LiNbO3.
`The varying characteristics of these modulator technolo-
`gies gives each certain advantages and disadvantages. The
`electromechanical modulators have perfect optical charac-
`teristics, passing all incident light without attenuation in
`their open state, and completely stopping all incident light in
`their closed state. They have the disadvantages of relatively
`slow switching time (typically not faster than a few milli-
`seconds) and high switching energy. Further, electro-me-
`chanical modulators capable of independently controlling
`selected parts of their aperture are possible in principle, but
`difiicult in practice because of their complexity and poor
`reliability.
`The acousto-optic modulators are much faster (typical
`bandwidths of many MHz) and more reliable than mechani-
`cal modulators, but they also require high drive power
`(typically 1 watt) to operate, and again can be made to
`modulate in selected portions of their aperture only with
`difiiculty.
`The electro-optic modulators can also be quite fast, while
`consuming less drive power than acousto-optic modulators.
`Electro-optic modulators capable of independently modu-
`lating defined portions of their apertures are simply con-
`structed by placing electrode patterns adjacent to the modu-
`lating material. This principle is used to make the well-
`known liquid crystal displays. However, a significant
`
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`disadvantage of conventional electro-optic modulators is
`that they only modulate polarized light. Unpolarized light
`will not be modulated at all by conventional prior-art elec-
`tro-optic devices. As a result, an unpolarized source must
`first be polarized, causing at least a 50% loss in intensity,
`before the light can be modulated by an electro-optic modu-
`lator.
`
`Electro-optic modulators that use the principles of light
`diffraction or light scattering are known in the art, and have
`the advantage that they can modulate unpolarized light to a
`lesser or greater extent. For example, U.S. Pat. No. 4,639,
`091 to Huignard et. al. teaches the use of an electrically
`switchable grating in nematic liquid crystals. Such a device
`will dilfract one polarization component of an incident
`unpolarized light beam. A polarization-independent electri-
`cally controllable Fresnel lens has been disclosed by G.
`Williams, N. J. Powell, and A. Purvis,
`in “Electrically
`Controllable Liquid Crystal Fresnel Lens,” published in
`Current Developments in Optical Engineering and Commer-
`cial Optics; Proceedings of the SPIE, vol. 1168, pages
`352-357, 1989. This device used two variable—birefringence
`nematic layers having their alignment directions crossed.
`While the Williams et. al. device could completely focus
`unpolarized light,
`its two-layer structure would make the
`fabrication of devices needing fine patterns difiicult.
`Devices employing liquid crystal droplets embedded in a
`polymer matrix, such as taught by Ferguson in U.S. Pat. Nos.
`4,435,047, 4,579,423, 4,606,611, 4,596,445, and 4,556,289,
`and by J. W. Doane, A. Golemme, J. L. West, J. B. White-
`head, Jr., and B. G. Wu, in “Polymer Dispersed Liquid
`Crystals for Display Application,” published in Molecular
`Crystals and Liquid Crystals, vol. 165, pages 5112, 1988,
`can completely scatter unpolarized light by a single layer by
`virtue of the layer being thick enough that the incident light
`encounters many randomly oriented liquid crystal droplets.
`These devices have the drawback that for efiicient scattering
`the liquid crystal droplet must be comparable in size to the
`light’s wavelength. Thus, since nematic liquid crystal
`switching time increases as the square of the droplet size,
`these devices become impractically slow when the droplet
`size is optimized for infrared wavelengths. This disadvan-
`tage is shared by the non-droplet nematic devices as well.
`Finally, difiractive modulators using FLCs have been
`disclosed, for example by C. C. Mao, K. M. Johnson, G.
`Moddel, K. Amett, and M. A. Handschy at the 1989 Annual
`Meeting of the Optical Society of America, held Oct. 15-20
`in Orlando, Fla. (abstract published in Optics News. vol. 15,
`September, 1989, page A-38, abstract TuA4). The device
`disclosed by Mao et. al. used the fringe pattern produced by
`two interfering coherent light beams to activate an amor-
`phous silicon photosensor, which in turn switched an adja-
`cent FLC film. Thus, the incident interference pattern was
`replicated into a grating in the FLC. Mao et. al. diffracted
`polarized laser light ofi" this grating. The diffraction effi-
`ciency varied strongly with time after the applied electronic
`drive signal made the amorphous silicon photosensitive,
`increasing from zero to a peak value, and then declining
`back to zero again. However, the peak value was always
`small.
`
`SUMMARY OF THE INVENTION
`
`The principal object of the present invention is to provide
`an electro-optic light modulator capable of completely
`modulating unpolarized incident light.
`Another object of the present invention includes provid-
`ing a light modulator capable of eifecting modulation over
`a selected portions of its apertures.
`
`

`
`3
`
`4
`
`5,552,916
`
`Still another object of the present invention is to provide
`a light modulator capable of rapidly modulating light, even
`when the light has a relatively long wavelength.
`Ferroelectric liquid crystals are desirable electro-optic
`materials because of their strong optical interaction arising
`from their large, permanent birefringence (An>O.l) and their
`microsecond switching time in response to low applied
`voltages. Since FLC switching depends on the sign of the
`applied electric field the FLCs can be driven ON as well as
`OFF, and hence need not sufi'er from the slowness that
`affects most
`thick nematic liquid crystal devices. As
`described earlier, thick nematic liquid crystal devices have a
`refractive index which varies with the polarization direction
`of the light, so that their most straightforward applications as
`light modulators require polarized light.
`The present invention shows that FLCs can be used to
`make switchable diifraction gratings that, when properly
`constructed, completely diflract unpolarized incident light.
`The switchable diffraction grating according to the present
`invention includes two plates and a birefringent
`layer
`between the two plates which includes an optic axis having
`two selectable optic axis orientation states. The two orien-
`tations both make substantially the same angle to the per-
`pendicular to the plates. When light passes through the
`birefringent layer at locations having the first orientation
`state, the light has a different phase from light which has
`passed through locations of the birefringent layer having the
`second orientation state. As a result of this phase difference,
`difiraction is produced by an interaction between the light
`having passed through the first orientation and the light
`having passed through the second orientation.
`This diffraction takes place without disturbing the polar-
`ization states of the light. Specifically, the light passing
`through locations having the first and second orientations of
`the birefringent layer have substantially the same polariza-
`tions, independent of the polarization state of the incident
`light and independent of whether the incident light falls on
`the area of the birefringent layer having the first or second
`orientation.
`
`These orientations of the optical axis of the birefringent
`material can be controlled to either be parallel or at an angle
`(e.g., perpendicular). When the optical axes are in parallel
`throughout
`the birefringent material,
`the incident
`light
`passes through with no modulation in phase. However, when
`the optical axes are at an angle, diffraction occurs.
`The orientations of the optical axes are controlled by
`groups of electrodes which are controlled by an electrical
`driving means. These groups of electrodes can be indepen-
`dently controlled, enabling a variety of dififraction patterns.
`As a result, the diffractive light modulator of the present
`invention is effective in a variety of applications, including
`intensity modulation, blurring modulation and beam steer-
`mg.
`
`In addition, the orientation of the optical axes are aligned
`in accordance with the FLC alignment direction. The light
`modulator can have an alignment pattern that comprises
`dilferent alignment regions each elfecting the respective
`FLC alignment. Thus, the optic axes can be oriented accord-
`ing to both the corresponding FLC alignment region and the
`groups of electrodes.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`These and other objects and advantages of the invention
`will become more apparent and more readily appreciated
`from the following detailed description of the presently
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`preferred exemplary embodiments of the invention taken in
`conjunction with the accompanying drawings, of which:
`FIGS. 1A and 1B are schematic representations of a
`prior-art FLC device which has a film of smectic C* liquid
`crystal between transparent electrode plates;
`FIG. 2 is a schematic representation of a diifraction
`grating formed in an FLC film according to an embodiment
`of the present invention;
`FIG. 3 shows the diffraction of light oil“ the FLC grating
`of FIG. 2;
`FIG. 4 shows in cross section the structure of an FLC
`difiractive modulator;
`FIG. 5(a) shows a face view of an electrode pattern of the
`FLC grating of FIG. 2;
`FIG. 5(b) shows a face view of an electrode pattern of a
`low-tilt FLC grating according to another embodiment of the
`present invention;
`FIG. 5(a) is an enlarged view of a portion of FIG. 5(b).
`FIG. 6 shows a modulator with tilted smectic layers from
`plates with obliquely evaporated alignment layers in the
`antiparallel configuration according to a second embodiment
`of the present invention;
`FIG. 7 shows the construction of a switchable-grating
`modulator with an electrode pattern that covers only one
`region according to a third embodiment of the present
`invention;
`
`FIG. 8 shows an electrode arrangement according to a
`fourth embodiment of the present invention comprising a
`uniform electrode layer adjacent the substrate topped by a
`second patterned electrode layer, with an insulating layer
`between the first and second electrode layers;
`FIG. 9 shows an electrode pattern where the voltage of
`each electrode, and hence of each overlying FLC strip, can
`be controlled independently of the others according to a fifth
`embodiment of the present invention;
`FIG. 10 is a waveform pattern for alternately scanning a
`modulator of the present invention into its uniform (nondif-
`fracting) and grating (diffracting) states;
`FIG. 11 shows an electrode pattern for producing hex-
`agonal dilfraction patterns according to a sixth embodiment
`of the present invention;
`FIG. 12 shows a diffractive modulator configured to
`operate in reflection according to a seventh embodiment of
`the present invention;
`FIG. 13A—13E show the construction of an eighth
`embodiment of the present invention employing two FLC
`films;
`'
`FIGS. 14(a) and 14(b) show a ninth embodiment of the
`present invention including a reflective modulator incorpo-
`rating a single FLC film with 45° switching between optic
`axis states, and a compound nrirror comprising a reflector
`and a quarter-wave plate;
`FIGS. 15(a), 15(b), and 15(c) show intensity modulators
`incorporating the diffractive light modulator of the present
`invention;
`
`FIGS. 16(a), 16(b) and 16(c) show an image detecting
`system in accordance with the present invention with a lens
`producing an image of the incident scene on the image
`detector;
`
`FIG. 17 is a schematic representation of a pseudo-random
`region pattern for producing a blurring modulator according
`to the present invention;
`FIG. 18 is a detailed view of an electrode pattern for
`implementing the pattern of FIG. 17 in a single layer of
`conductive material; and
`
`

`
`5
`
`6
`
`5,552,916
`
`FIG. 19 is a detailed view of an electrode pattern dividing
`the region pattern of FIG. 17 up into lines.
`
`DETAILED DESCRIPTION OF THE PREFERRED
`EMBODIMENTS
`
`Modulator Concept
`The geometry of the standard prior-art ferroelectric liquid
`crystal (FLC) electro-optic device is shown in FIG. 1A and
`1B. A film of smectic *C liquid crystal is located between
`two parallel electrode plates 10 and 12 and is made up of
`smectic layers 14. Voltages applied between the plates 10
`and 12 produce electric fields perpendicular to the plates
`(hereafter longitudinal), whereby the direction of the applied
`field reverses when the sign of the applied voltage changes.
`As shown in FIGS. 1A and 1B, the selected FLC optic axis
`11 (fr) lies in the plane of the electrode plates 10 and 12
`(hereafter transverse) at a different orientation depending on
`the applied field direction. Also, the ferroelectric polariza-
`tion (F) changes orientation depending on the applied field
`direction.
`Whereas the birefringence in conventional electro-optic
`devices is induced by the applied field, the birefringence of
`the FLC device is permanent and is produced by the field-
`induced rotation, in the transverse plane, of the optic axis. In
`spite of its differences from conventional electro-optic
`devices, the FLC device of FIG. 1A and 1B shares the
`property of requiring polarized light to produce modulation.
`FIG. 2 shows, in a schematic style, an FLC device that
`modulates unpolarized light according to the present inven-
`tion. A film 20 of FLC having a thickness dis divided into
`strips 22 along a grating axis x, with each strip being
`switchable into either of two optic axis states 16. FIG. 2
`assumes that the smectic layer normal lies parallel to the face
`of film 20, i.e., in the x—y plane; however, since the smectic
`layer may be tilted as shown in FIG. 6, reference numeral 16
`is more precisely defined as the projection of the optic axis
`states onto the plane x—y. The angle 6 in FIG. 2 is defined
`as the angle between the projection of the FLC smectic layer
`normal and the grating axis x. The width of the strips
`alternate between a and l—a, producing a spatially periodic
`pattern with period 1. Although for clarity only a few strips
`are shown, in fact the device works best when the strip
`widths are small, as explained in more detail below, so that
`in practice the device may comprise many strips. The strips
`22 are made up of a first region 24 having the width a, and
`a second region 26 having the width l—a. When the first
`region 24 and the second region 26 both have the same optic
`axis orientation, then the FLC film 20 forms a uniform,
`birefringent window. When the first region 24 has the
`opposite optic axis state from the second region 26, then the
`FLC film 20 forms a grating, which can diffract incident
`light.
`FIG. 3 shows the FLC film 20 and an incident ray 30.
`Since the optical change from one strip 22 to the next is
`sharp, the incident ray 30 is diffracted into many orders of
`diffracted rays 32, with the diffraction angle Bm of the m"‘
`order being defined by the usual relation
`
`10
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`sin[im=rn}Jl,
`
`60
`
`(1)
`
`where ?t is the vacuum wavelength of the light.
`Although the effect produced on an incident light beam 30
`by a strip in either state depends on the polarization of the
`incident beam, the intensity pattern of the diffracted light 32
`depends only on the differences between the effects pro-
`duced by the strips in the two different optic axis states.
`
`65
`
`Since the effect of this difference is independent of the
`incident polarization state, the diifraction of the grating state
`of the film is also independent of the polarization state of the
`incident light. As a result, the diifracting device can diffract
`even unpolarized light. Of course, the uniform film produces
`no sensible effect (at least without a polarization analyzer)
`on incident light of any polarization state.
`Since the diffraction of light is produced by interference
`of the rays passing through the first region with rays passing
`through the second region, the diffracted intensity is maxi-
`mized when the two sets of rays interfere to the maximum
`extent. This in turn occurs when their polarization state is
`most nearly the same. Thus, maximal diffraction occurs
`when the light passing through the first region differs only in
`phase, ideally by an amount of 113 radians, or an odd integral
`multiple thereof, from light passing through the second
`region. Although a polarization-independent phase grating is
`usually implemented as a relief grating in an optically
`isotropic material, we teach below how it can in fact be
`implemented in switchable, optically anisotropic electro-
`optic ferroelectric liquid crystals (FLCs).
`To produce maximum diffraction efficiency, the FLC film
`thickness d should be such that the film forms a half-wave
`
`plate, i.e., such that And/7»=‘/2, where An is the birefringence
`of the plate. Further, the strips should be of equal width
`(a=l/2), and the angle between the selected FLC optic axis
`states (Zn!) should be 90°. The optical properties of the
`modulator can be appreciated in more detail using the Jones
`calculus for anisotropic optics well known in the art. Here,
`the incident and transmitted light electric field amplitudes,
`-—>
`E i and E0, respectively, are related by the matrix equation
`(2)
`
`= W
`
`15,,
`
`5.,
`
`where the actual electric fields e depend on the ampli-
`tudes E as
`.
`—>
`~)
`.
`-) —)
`6 Fe iex(ld—wr); E 0:6 0ez(kI—w1)’
`
`(3)
`
`with the wavevector k defined as k=211:/7». In the above
`
`representation, the waveplate lies parallel to the x—y plane,
`with the light propagating along z at normal incidence. The
`simple FLC device of FIGS. 1A and 1B has the Jones matrix
`
`W(¢) =
`
`e“T’2cos2¢ + e‘T’2sinZ¢
`
`(e"T’2—e'T’2)sin¢cos¢
`
`(e"T’2 — e‘T’2)sinc]>eos¢
`
`e"T’3sin2¢+e‘T’2cos2¢
`
`)
`
`(4)
`
`where q) is the angle between the optic and coordinate axes.
`The angular retardance F is determined by the light wave-
`length 7», waveplate thickness d, and birefringence An as
`I‘=21:And/7». The device of the present invention shown in
`FIG. 2 has a transmittance alternating between W1=W(6+\|;)
`and W2=W(6—\p) with period 1. This transmittance can be
`described as a Fourier series,
`
`W00 5
`
`2 mmelnimall;
`m:—oo
`
`(5)
`
`each term in the series produces a diffracted wave travelling
`in a different direction given by equation (1). The coefficient
`of the m"‘ term is given by:
`
`(1/2)l(W1+ W2) + (W1 - W2)(2l1/1 '-1)]
`
`m = 0
`
`(6)
`
`(i/21tm)(W1 — W2)(1 — e~2m'~w/I)
`mm =
`Making a=l/2 permits the amplitude of the undiffracted wave
`(m=0) to be reduced to zero by making W1=—W2. In fact,
`using the form of W in equation (4) gives, for a=l/2, the
`
`m at 0.
`
`

`
`7
`
`8
`
`5,552,916
`
`transmittance term producing the undiifracted wave as:
`too =
`
`cos(l‘/2) - i sin(l"/2)cos6cos2\y
`—sin(I‘/2)sin9cos2\tI
`
`(
`
`—sin(l“/2)sin6cos2\4I
`cos(l"/2) + i sin(I‘/2)cos6cos2\4I
`
`(7)
`
`).
`
`5
`
`For an incident monochromatic plane wave, the fraction
`1,,/Ii, of the incident optical power 1,. undilfracted is then
`
`I,/I,=1—sin2(I‘/2)sin2 2w
`
`(8)
`
`and the fraction diffracted in the m"' order is 1,,/I;[4/(m1t)2]
`(l—I,,/Ii).
`This shows that for equal strip widths, 2\lI=90° between
`the two FLC optic axis states, and half-wave thickness
`(l"=1t), the device of FIG. 2 produces a 1: phase shift between
`alternate strips, and thereby completely diffracts incident
`light, modulation of even unpolarized incident light. For
`incident light of appreciable spectral width, i.e., non-mono-
`chromatic,
`the device will not
`in general be an exact
`half-wave plate for all the incident wavelengths. However, if
`the thickness d is chosen to make the film half-wave for a
`
`wavelength K0 near the middle of the incident light’s spec-
`trum, the modulator can still dilfract substantially all of the
`incident light if the spectral width is not too broad, as can be
`seen from how slowly equation (8) varies near its first
`minimum. For light of narrow spectral band, the modulator
`will work equally well when its thickness is such as to
`produce any odd-integral multiple of half-wave retardance.
`FLC Cell Construction
`A cross section of a structure implementing the modulator
`of FIG. 2 is shown in FIG. 4. The modulator comprises two
`transparent plates 40, 42, each coated with a transparent
`electrode layer 44 and 46 and a liquid crystal alignment layer
`48 and 50. For use of the modulator in visible light, the
`plates 40 and 42 could favorably be made of glass, or
`polymer. For use in the infrared, the plates 40 and 42 could
`be made of one of the many infrared-transparent materials
`known in the art, such as the semiconductors silicon, ger-
`manium, gallium arsenide, or the insulators zinc selenide,
`zinc sulphide, sapphire, etc. On the insulating substrates the
`electrodes 44 and 46 could be deposited layers of indium-tin
`oxide, as are commonly used in the art to form electrodes
`transparent to visible and infrared light, with the further
`favorable property that they can conveniently be patterned
`by photolithographic etching techniques well known in the
`art. Alternately, for use in the infrared, deposited layers of
`doped semiconductors such as silicon or germanium could
`be used for the electrodes 44 and 46. On the semiconducting
`substrates the electrodes could be made by the techniques
`known in the integrated circuit art of dopant diifusion, ion
`implanting, or epitaxial growth of doped layers. Again,
`photolithographic techniques are known for making such
`electrodes according to a desired pattern.
`The liquid crystal alignment layers 48 and 50 might
`typically be a thin layer of polymer film which has been
`rubbed in a single direction with a cloth, or be a thin layer
`of obliquely vacuum-deposited silicon oxide. The plates 40
`and 42 are faced together to form a gap which contains the
`FLC film 20. The gap spacing can be defined by spacer
`particles distributed throughout the gap or confined to the
`seal ring.
`As shown in FIG. 4, the electrode layer 46 on one plate
`is continuous, and forms the common electrode, while the
`electrode layer 44 on the other plate is patterned into stripes
`52 to allow electric fields alternating in direction to be
`applied to alternate strips 22 of the FLC film.
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`FIG. 5(a) shows a face view of the electrode pattern of
`FIG. 4 to produce the FLC strips of FIG. 2. The electrode
`layer 44 is divided up into stripes 52 by a thin insulating gap
`54, shown here as a black line. The stripe electrodes 52 are
`connected together into a first group 56 and a second group
`58 which are then both connected to electrical driving means
`60 along with the common electrode 46, such that, according
`to the state of the driving means 60, both groups of electrode
`strips 56 and 58 can have the same voltage relative to the
`common electrode, or the voltage on the two groups 56 and
`58 can be of opposite sign, so that the voltage on the stripes
`spatially alternates in sign relative to the common electrode
`46. The light to be modulated is directed so that it passes
`through the transparent plates 40 and 42 and FLC film 20.
`When the state of the driving means 60 is such that both
`groups of electrodes 56 and 58 have the same sign of voltage
`relative to the common plate 46 then the FLC film 20 is in
`its uniform state, and the light passes through undiffracted.
`When the driving means 60 produces opposite signs of
`voltage on the two groups of electrodes relative to the
`common plate 46 then the alternate stripes 52 are switched
`to opposite optic axis states, and the device is in its grating
`state, diffracting the incident light. The techniques for form-
`ing the conductive and alignment layers, for spacing and
`sealin