WO2000028354A2 - Light source locator using switchable holograms - Google Patents
Light source locator using switchable holograms Download PDFInfo
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- WO2000028354A2 WO2000028354A2 PCT/US1999/024246 US9924246W WO0028354A2 WO 2000028354 A2 WO2000028354 A2 WO 2000028354A2 US 9924246 W US9924246 W US 9924246W WO 0028354 A2 WO0028354 A2 WO 0028354A2
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B5/00—Optical elements other than lenses
- G02B5/32—Holograms used as optical elements
Definitions
- the present invention generally relates to light source location, and more particularly to an apparatus and a method for light source location using switchable holograms
- military and surveillance systems may desire to track planes or missiles in the sky
- the heat generated by the engines of the plane or missile may cause the target to be an infrared light source
- the plane or missile may reflect sunlight causing the plane or missile to be a visible light source
- the wavelength charactenstics of the light source may also allow an identification of the light source
- the engines of different types of planes may emit different infrared wavelengths
- Such a military or surveillance system may also be required to track multiple targets simultaneously over a large angular field
- a military or surveillance system may also be required to locate the source of laser emissions from, for example, laser range finders or other laser-assisted military equipment
- Robotic systems may also need to locate light sources For instance, in a factory a robotic system may locate an object it needs usmg light emitted from the object The object may naturally be a light source or a light source may be attached to the object to aid the robotic system in location of the object Additionally, different objects to be tracked by a robotic system may
- SHOE switchable holographic optical element
- the SHOE may be used to direct light from a light source onto a detector Utilizing the position at which light is incident on the detector and the predetermined ray deviation characteristics of the SHOE, a bearing to the light source may be determined
- Use of a plurality of SHOEs allows light sources to be located within large angular fields a substantially hemispherical angular field for example
- different SHOEs may be used to direct light of different wavelength bandwidths onto the detector thereby allowing light sources within different wavelength bandwidths to be located
- the SHOEs are preferably a Bragg hologram comprising a polymer dispersed liquid crystal material
- a Bragg hologram is preferred due their high diffraction efficiency
- a sw itchable Bragg hologram fabricated using a polymer dispersed liquid crystal material is further preferred due to their fast switching rate, low switching voltage and high diffraction efficiency
- a fast switching rate allows the SHOEs to rapidly cycled through, a low switching voltage reduces demands placed on the electronics required to operate the SHOE, and a high diffraction efficiency allows the SHOE to effectively diffract light onto a detector
- An embodiment of the light source locator apparatus includes a plurality of SHOEs, a detector, and a processing circuit coupled to the SHOEs and the detector
- Each of the SHOEs may have the same wavelength bandwidth or they may have different wavelength bandwidths
- Each SHOE has a field of view corresponding to the angular bandwidth of the hologram When a SHOE is deactivated, it allows light to be transmitted substantially unal
- FIG 1 is a cross-sectional view of an electrically switchable hologram made of an exposed polymer dispersed liquid crystal (PDLC) material made in accordance with the teachings of the description herem,
- PDLC polymer dispersed liquid crystal
- FIG 2 is a graph of the normalized net transmittance and normalized net diffraction efficiency of a hologram made in accordance with the teachmgs of the description herein (without the addition of a surfactant) versus the rms voltage applied across the hologram,
- FIG 3 is a graph of both the threshold and complete switching rms voltages needed for switching a hologram made in accordance with the teachmgs of the description herein to minimum diffraction efficiency versus the frequency of the rms voltage,
- FIG 4 is a graph of the normalized diffraction efficiency as a function of the applied electric field for a PDLC material formed with 34% by weight liquid crystal surfactant present and a PDLC material formed with 29% by weight liquid crystal and 4% by weight surfactant,
- FIG 5 is a graph showing the switching response time data for the diffracted beam in the surfactant- contammg PDLC matenal in FIG 4
- FIG 6 is a graph of the normalized net transmittance and the normalized net diffraction efficiency of a hologram
- FIG 7 is an elevational view of typical experimental arrangement for recordmg reflection gratings
- FIGS 8a and 8b are elevational views of a reflection grating, made in accordance with the teachings of the descnption herein, having periodic planes of polymer channels and PDLC channels disposed parallel to the front surface in the absence of a field (FIG 8a) and with an electric field applied (FIG 8b) wherein the liquid- crystal utilized in the formation of the gratmg has a positive dielectric amsotropy,
- FIGS 9a and 9b are elevational views of a leflection grating, made in accordance with the teachings of the description herein, having periodic planes of polymer channels and PDLC channels disposed parallel to the front surface of the gratmg in the absence of an electric field (FIG 9a) and with an elecrnc field applied (FIG 9b) wherein the liquid crystal utilized in the formation of the grating has a negative dielectric amsotropy,
- FIG 10a is an elevational view of a reflection grating, made in accordance with the teachings of the descnption herein, disposed within a magnetic field generated by Helmholtz coils,
- FIGS 10b and 10c are elevational views of the reflection gratmg of FIG 10a in the absence of an electric field (FIG 10b) and with an electric field applied (FIG 10c),
- FIGS 11a and l ib are representative side views of a slanted transmission gratmg (FIG 11a) and a slanted reflection gratmg (FIG 1 lb) showing the orientation of the grating vector G of the periodic planes of polymer channels and PDLC channels,
- FIG 12 is an elevational view of a reflection grating, made in accordance with the teachings of the descnption herein, when a shear stress field is applied thereto,
- FIG 13 is an elevational view of a subwavelength gratmg, made m accordance with the teachmgs of the description herein, having penodic planes of polymer channels and PDLC channels disposed perpendicular to the front surface of the gratmg,
- FIG 14a is an elevational view of a switchable subwavelength, made in accordance with the teachings of the descnption herein, wherein the subwavelength grating functions as a half wave plate whereby the polarization of the mcident radiation is rotated by 90°,
- FIG 14b is an elevational view of the switchable half wave plate shown in FIG 14a disposed between crossed polanzers whereby the incident light is transmitted,
- FIGS 14c and 14d are side views of the switchable half wave plate and crossed polarizes shown in FIG 14b and showing the effect of the application of a voltage to the plate whereby the polarization of the light is no longer rotated and thus blocked by the second polarizer,
- FIG 15a is a side view of a switchable subwavelength grating, made in accordance with the teachings of the descnption herein, wherein the subwavelength grating functions as a quarter wave plate whereby plane polarized light is transmitted through the subwavelength gratmg, retroreflected by a mirror and reflected by the beam splitter,
- FIG 15b is a side view of the switchable subwavelength gratmg of FIG 15a and showing the effect of the application of a voltage to the plate whereby the polarization of the light is no longer modified, thereby permitting the reflected light to pass through the beam splitter
- FIGS 16a and 16b are elevational views of a transmission grating, made in accordance with the teachings of the description herein, having periodic planes of polymer channels and PDLC channels disposed perpendicular to the front face of the gratmg in the absence of an electric field (FIG 16a) and with an electric field applied (FIG 16b) wherein the liquid crystal utilized in formation of the gratmg has a positive dielectric amsotropy,
- FIG 17 is a side view of five subwavelength gratings wherein the gratings are stacked and connected electncally m parallel thereby reducing the switching voltage of the subwavelength grating,
- FIG 18 is a schematic diagram of a light source locator system accordmg to a first embodiment
- FIG 19 is a schematic diagram of a portion of the light source locator system shown in FIG 18 in which the overlap of the fields of views is illustrated
- FIG 20 is a schematic diagram of a light source locator system according to a second embodiment
- FIG 21 is a schematic diagram of a light source locator system accordmg to a third embodiment
- FIG 22 is a schematic diagram of a light source locator system accordmg to a fourth embodiment
- FIG 23 is a flow diagram for a method for a processing circuit to determine a bearing to a light source
- FIG 24 is a flow diagram for a method for a processmg circuit to tnangulate a position of a light source
- FIG 25 is a flow diagram for a method for a processing circuit to activate and deactivate a plurality of SHOEs in a light source locator system
- the present mvention employs holographic optical elements formed, in one embodiment, from a polymer dispersed liquid crystal (PDLC) material compnsmg a monomer, a dispersed liquid crystal, a cross- linking monomer, a co-initiator and a photo-initiator dye These PDLC materials exhibit clear and orderly separation of the liquid crystal and cured polymer, whereby the PDLC material advantageously ides high quality optical elements
- the PDLC materials used in the holographic optical elements may be formed in a single step
- the holographic optical elements may also use a unique photopolyme ⁇ zable prepolymer material that permits in situ control over characteristics of resulting gratings, such as domain size, shape, density, ordenng and the like
- methods and materials taught herem can be used to prepare PDLC materials for optical elements comprising switchable transmission or reflection type holographic gratings
- the resulting embodiment of PDLC material may have an amsotropic spatial distribution of phase- separated LC droplets within the photochemically cured polymer matrix
- Prior art PDLC materials made by a single-step process can achieve at best only regions of larger LC bubbles and smaller LC bubbles m a polymer matrix
- the large bubble sizes are highly scattering which produces a hazy appearance and multiple ordermg diffractions, in contrast to the well-defined first order diffraction and zero order diffraction made possible by the small LC bubbles of one embodiment of PDLC material m well-defined channels of LC- ⁇ ch material
- Reasonably well-defined alternately LC- ⁇ ch channels, and nearly pure polymer channels in a PDLC matenal are possible by multistep processes, but such processes do not achieve the precise morphology control over LC droplet size and distribution of sizes and widths of the polymer and LC- ⁇ ch channels made possible by one embodiment of PDLC material
- the same may be prepared by coatmg the mixture between two mdium-tin-oxide (ITO) coated glass slides separated by spacers of nominally 10-20 ⁇ m thickness
- ITO mdium-tin-oxide
- the sample is placed m a conventional holographic recording setup
- Gratings are typically recorded using the 488 nm line of an Argon ion laser with intensities of between about 0 1-100 mW/cm 2 and typical exposure times of 30-120 seconds
- the angle between the two beams is varied to vary the spacing of the intensity peaks, and hence the resulting grating spacing of the hologram Photopolyme ⁇ zation is induced by the optical intensity pattern
- SPIE Society of Photo-Optical Instrumentation Engineers
- the features of the PDLC material are influenced by the components used in the preparation of the homogeneous starting mixture and, to a lesser extent, by the intensity of the mcident light pattern
- the prepolymer material comprises a mixture of a photopolyme ⁇ zable monomer, a second phase material, a photo-initiator dye, a co-initiator, a chain extender (or cross-linker), and, optionally, a surfactant
- two major components of the prepolymer mixture are the polymenzable monomer and the second phase material, which are preferably completely miscible Highly functionalized monomers may be preferred because they form densely cross-linked networks which shnnk to some extent and to end to squeeze out the second phase material As a result, the second phase material is moved amsotropically out of the polymer region and, thereby, separated into well-defined polymer-poor, second phase-rich regions or domains Highly functionalized monomers may also be preferred because the extensive cross-linking associated with such monomers yields fast kmetics, allowing the hologram to form relatively quickly, whereby the second phase material will exist m domains of less than approximately 0 1 ⁇ m
- acrylates such as tnethyleneglycol diacrylate, tnmethylolpropane t ⁇ acrylate, pentaerythntol tnacrylate, pentaerythntol tetracrylate, pentaerythntol pentacrylate, and the like can be used in the present invention.
- tnethyleneglycol diacrylate tnmethylolpropane t ⁇ acrylate
- pentaerythntol tnacrylate pentaerythntol tetracrylate
- pentaerythntol pentacrylate and the like can be used in the present invention.
- an approximately 1.4 mixture of tn-to penta-acrylate facilitates homogeneous mixing while providing a favorable mixture for forming 10-20 ⁇ m films on the optical plates
- the second phase material of choice for use m the practice of the present invention is a liquid crystal (LC) This also allows an electro-optical response for the resulting hologram
- LC liquid crystal
- concentration of LC employed should be large enough to allow a significant phase separation to occur in the cured sample, but not so large as to make the sample opaque or very hazy. Below about 20% by weight very little phase separation occurs and diffraction efficiencies are low.
- the concentration of LC may be increased to 35% by weight without loss m optical performance by adjustmg the quantity of surfactant Suitable liquid crystals contemplated for use m the practice of the present mvention may include the mixture of cyanobiphenyls marketed as E7 by Merck, 4'-n-pentyl-4-cyanob ⁇ phenyl, 4'-n-heptyl-4- cyanobiphenyl, 4'-octaoxy-4-cyanob ⁇ phenyl, 4'-penryl-4-cyanoter ⁇ henyl, oc-methoxybenzyl ⁇ dene-4'- butylaniline, and the like Other second phase components are also possible
- the polymer dispersed liquid crystal matenal employed in the practice of the present mvention may be formed from a prepolymer material that is a homogeneous mixture of a polymenzable monomer compnsmg drpentaeryth ⁇ tol hydroxypentacrylate (available, for example, from Polysciences, Inc , War ⁇ ngton, Pennsylvania), approximately 10-40 wt% of the liquid crystal E7 (which is a mixture of cyanobiphenyls marketed as E7 by Merck and also available from BDH Chemicals, Ltd , London, England), the chain- extending monomer N-vmylp-yrrolidmone (“NVP”) (available from the Aldnch Chemical Company, Milwaukee Wisconsin), co-initiator N-phenylglycine (“NPG”) (also available from the Aldnch Chemical Company.
- a prepolymer material that is a homogeneous mixture of a polymenzable monomer compnsmg drpentaeryth ⁇ to
- Rose bengal is also available as rose bengal sodium salt (which must be estenfied for solubility) from the Aldnch Chemical Company This system has a very fast curing speed which results m the formation of small liquid crystal micro- droplets
- the mixture of liquid crystal and prepolymer material are homogenized to a viscous solution by suitable means (e g , ultrasomfication) and spread between mdium-tin-oxide (ITO) coated glass sides with spacers of nominally 15-100 ⁇ m thickness and, preferably, 10-20 ⁇ m thickness
- ITO mdium-tin-oxide
- the ITO is electrically conductive and serves as an optically transparent electrode
- Preparation, mixing and transfer of the prepolymer material onto the glass slides are preferably done in the dark, as the mixture is extremely sensitive to light
- the co-mitiator employed m the practice of the present invention controls the rate of curing m the free radical polymerization reaction of the prepolymer matenal Optimum phase separation and, thus, optimum diffraction efficiency m the resultmg PDLC material, are a function of curing rate It has been found that fa ⁇ orable results can be achieved utilizing co-mitiator in the range of 2-3% by weight Suitable co-initiators include N-phenylglycme, tnethyl amme, t ⁇ ethanolamine, N,N-d ⁇ methyl-2,6-dnsopropyl aniline, and the like
- Suitable dyes and dye co-mitiator combinations that may be suitable for use m the present invention, particularly for visible light, include eosin and triethanolamme, camphorquinone and N- phenylglycme, fluorescein and triethanolamme, methylene blue and triethanolamme or N-phenylglycme, ervthrosm B and triethanolamme, mdolmocarbocyanine and t ⁇ phenyl borate, lodobenzospiropyran and t ⁇ ethylamme, and the like
- the chain extender (or cross linker) employed m the practice of the present mvention may help to increase the solubility of the components m the prepolymer material as well as mcrease the speed of polymerization
- the chain extender is preferably a smaller vinyl monomer as compared with the pentacrylate, whereby it can react with the acrylate positions in the pentacrylate monomer, which are not easily accessible to neighboring pentaacrylate monomers due to steric hindrance
- reaction of the chain extender monomer with the polymer increases the propagation length of the growing polymer and results m high molecular weights
- suitable chain extenders can be selected from the following N-vinylpyrrohdinone, N-vmyl pyndme acrylonit ⁇ le, N-vmyl carbazole, and the like
- Suitable surfactants include octanoic acid, heptanoic acid, hexanoic acid, dodecanoic acid, decanoic acid, and the like
- PDLC matenals used in the present invention may also be formed usmg a liquid crystalline bifunctional acrylate as the monomer ("LC monomer”)
- LC monomers have an advantage over conventional acrylate monomers due to their high compatibility with the low molecular weight nematic LC matenals, thereby facilitating formation of high concentrations of low molecular weight LC and yielding a sample with high optical quality
- the presence of higher concentrations of low molecular weight LCs in the PDLC material greatly lowers the switching voltages (e g , to ⁇ 2V/ m)
- Another advantage of using LC monomers is that it is possible to apply low AC or DC fields while recording holograms to pre-a gn the host LC monomers and low molecular weight LC so that a desired orientation and configuration of the nematic directors can be obtained in the LC droplets.
- the chemical formulate of several suitable LC monomers are as follows:
- Semifluorinated polymers are known to show weaker anchoring properties and also significantly reduced switching fields. Thus, it is believed that semifluorinated acrylate monomers which are bifunctional and liquid crystalline may find suitable application in the present invention.
- FIG. 1 there is shown a cross-sectional view of an electrically switchable hologram 10 made of an exposed polymer dispersed liquid crystal material made according to the teachings of this description.
- a layer 12 of the polymer dispersed liquid crystal material is sandwiched between a pair of indium-tin-oxide coated glass slides 14 and spacers 16.
- the interior of hologram 10 shows Bragg transmission gratings 18 formed when layer 12 was exposed to an interference pattern from two intersecting beams of coherent laser light.
- the exposure times and intensities can be varied depending on the diffraction efficiency and liquid crystal domain size desired. Varying the concentrations of photo-initiator, co-initiator and chain- extending (or cross-linking) agent can control liquid crystal domain siz;e.
- the orientation of the nematic directors can be controlled while the gratings are being recorded by application of an external electric field across the ITO electrodes.
- the scanning electron micrograph shown in FIG. 2 of the referenced Applied Physics Letters article, and incorporated herein by reference, is of the surface of a grating which was recorded in a sample with a 36 wt% loading of liquid crystal using the 488 nm line of an argon ion laser at an intensity of 95 mW/cm 2 .
- the size of the liquid crystal domains is about 0.2 ⁇ m and the grating spacing is about 0.54 ⁇ m.
- This sample which is approximately 20 ⁇ m thick, diffracts light in the Bragg regime.
- FIG. 2 is a graph of the normalized net transmittance and normalized net diffraction efficiency of a hologram made according to the teachings of his disclosure versus the root mean square voltage ("Vrms") applied across the hologram.
- ⁇ is the change in first order Bragg diffraction efficiency.
- ⁇ T is the change in zero order transmittance.
- FIG. 2 shows that energy is transferred from the first order beam to the zero-order beam as the voltage is increased.
- the peak diffraction efficiency can approach 100%, depending on the wavelength and polarization of the probe beam, by appropriate adjustment of the sample thickness.
- the minimum diffraction efficiency can be made to approach 0% by slight adjustment of the parameters of the PDLC material to force the refractive index of the cured polymer to be equal to the ordinary refractive index of the liquid crystal.
- FIG. 3 is a graph of both the threshold rms voltage 20 and the complete switching rms voltage 22 needed for switching a hologram made according to the teachmgs of this disclosure to minimum diffraction efficiency versus the frequency of the rms voltage.
- the threshold and complete switching rms voltages are reduced to 20 Vrms and 60 Vrms, respectively, at 10 kHz Lower values are expected at even higher frequencies
- FIG 6 is a graph of the normalized net transmittance and normalized net diffraction efficiency of a hologram made accordmg to the teachings of this disclosure versus temperature
- polymer dispersed liquid crystal materials described herem successfully demonstrate the utility for recording volume holograms of a particular composition for such polymer dispersed liquid crystal systems
- a PDLC reflection grating is prepared by placing several drops of the mixture of prepolymer material 1 12 on an mdium-tm oxide coated glass slide 114a A second indium-tin oxide coated slide 114b is then pressed against the first, thereby causing the prepolymer material 112 to fill the region between the slides 114a and 114b
- the separation of the slides is maintained at approximately 20 ⁇ m by utilizing uniform spacers 118
- Preparation, mixing and transfer of the prepolymer material is preferably done in the dark
- a mirror 116 may be placed directly behind the glass plate 114b The distance of the mirror from the sample is preferably substantially shorter than the coherence length of the laser
- the PDLC material is preferably exposed to the 488 nm line of an argon-ion laser, expanded to fill the entire plane of the glass plate, with an intensity of approximately 0 1-100 mWatts/c ⁇ r with typical exposure times of 30-120 seconds Constructive and destructive interference within the
- the prepolymer matenal utilized to make a reflection gratmg compnses a monomer, a liquid crystal, a cross-linking monomer, a co-mitiator, and a photo-mitiator dye
- the reflection gratmg may be formed from prepolymer material compnsmg by total weight of the monomei dipentaerythntol hydroxypentacrylate (DPHA), 35% by total weight of a liquid crystal comprising a mixture of cyano biphenyls (known commercially as "E7"), 10% by total weight of a cross-linking monomer compnsmg N- vinylpyrro dmone ("NVP"), 2 5% by weight of the co-initiator N-phenylglycme (“NPG”) and 10 5 to 10 s gram moles of a photo-initiator dye comprising rose bengal ester
- DPHA monomei dipentaerythntol hydroxypentacrylate
- NDP
- the morphology of the reflection grating differs significantly In particular, it has been determined that, unlike transmission gratings with similar liquid crystal concentrations, very little coalescence of individual droplets was evident Further more, the droplets that were present in the material were significantly smaller having diameters between 50 and 100 nm Furthermore, unlike transmission gratings where the liquid crystal-rich regions typically compnse less than 40% of the grating, the liquid crystal-rich component of a reflection grating is significantly larger Due to the much smaller periodicity associated with reflection gratings, I e , a narrower grating spacing ( ⁇ 02 microns), it is believed that the time difference between completion of curmg m high intensity versus low intensity regions is much smaller It is also believed that the fast polymerization, as evidenced by small droplet diameters, traps a significant percentage of the liquid crystal m the matrix during gelation and precludes any substantial growth of large droplets or diffusion of small droplets into larger domams
- the reflection notch In PDLC materials that are formed with the 488 nm lme of an argon ion laser, the reflection notch typically has a reflection wavelength at approximately 472 nm for normal mcidence and a relatively narrow bandwidth The small difference between the wnting wavelength and the reflection wavelength (approximately 5%) indicates that shrinkage of the film is not a significant problem Moreover, it has been found that the performance of such gratmgs is stable over periods of many months In addition to the materials utilized m the one embodiment described above, it is believed that suitable
- PDLC materials could be prepared utilizing monomers such as t ⁇ ethyleneglycol diacrylate, tnmethylolpropanet ⁇ acrylate, pentaerythntol tnacrylate, pentaerythntol tetracrylate, pentaerythntol pentacrylate, and the like Similarly, other co-initiators such as tnethylamme, triethanolamme, N,N-d ⁇ methyl- 2,6-dnsopropylan ⁇ l ⁇ ne, and the like could be used instead of N-phenylglycme Where it is desirable to use the 458 nm, 476 nm, 488 nm or 514 nm lines of an Argon ion laser, that the photo-mitiator dyes rose bengal sodium salt eosin, eosm sodium salt, fluorescein sodium salt and the like will give favorable results Where the 633 nm lme is utilized, methylene blue will find ready application Finally,
- FIG 8a there is shown an elevational view of a reflection gratmg 130 made m accordance with this disclosure havmg periodic planes of polymer channels 130a and PDLC channels 130b disposed parallel to the front surface 134 of the gratmg 130
- the symmetry axis 136 of the liquid crystal domains is formed m a direction perpendicular to the periodic channels 130a and 130b of the grating 130 and perpendicular to the front surface 134 of the gratmg 130
- the symmetry axis 136 is already m a low energy state in alignment with the field E and will reorient
- reflection gratings formed in accordance with the procedure described above will not normally be switchable
- a reflection grating tends to reflect a narrow wavelength band, such that the grating can be used as a reflection filter
- the reflection grating is formed so that it will be switchable
- switchable reflection gratings can be made utilizing negative dielectric amsotropy LCs (or LCs with a low cross-over frequency), an applied magnetic field, an applied shear stress field, or slanted gratmgs.
- liquid crystals havmg a negative dielectric amsotropy ( ⁇ ) will rotate m a direction perpendicular to an applied field
- the symmetry axis 136 of the liquid crystal domains formed with a liquid crystal havmg a negative ⁇ will also be disposed in a direction perpendicular to the penodic channels 130a and 130b of the grating 130 and to the front surface 135 of the gratmg
- the symmetry axis of the negative ⁇ liquid crystal will distort and reonent m a direction perpendicular to the field E, which is perpendicular to the film and the periodic planes of the grating
- the reflection gratmg can be switched between a state where it is reflective and a state where it is transmissive
- the following negative ⁇ liquid crystals and others are expected to find ready applications in the methods and devises of the present
- Liquid crystals can be found in nature (or synthesized) with either positive or negative ⁇ Thus, it is possible to use a LC which has a positive ⁇ at low frequencies, but becomes negative at high frequencies
- the frequency (of the applied voltage) at which ⁇ changes sign is called the crossover frequency
- the cross-over frequency will vary with LC composition, and typical values range from 1-10 kHz
- the reflection gratmg may be switched
- low crossover frequency materials can be prepared from a combination of positive and negative dielectric amsotropy liquid crystals
- a suitable positive dielectric liquid crystal for use m such a combination contams four nng esters as shown below
- a strongly negative dielectric liquid crystal suitable for use in such a combmation is made up of pyndazmes as shown below
- switchable reflection gratmgs can be formed using positive ⁇ liquid crystals
- FIG 10a such gratings are formed by exposing the PDLC startmg material to a magnetic field during the curing process
- the magnetic field can be generated by the use of Helmholtz coils (as shown m FIG 10a), the use of a permanent magnet, or other suitable means
- the magnetic field M is oriented parallel to the front surface of the glass plates (not shown) that are used to form the grating 140
- the symmetry axis 146 of the liquid crystals ill orient along the field while the mixture is fluid
- the field may be removed and the alignment of the symmetry axis of the liquid crystals will remain unchanged (See FIG 10b )
- FIG 10c When an electric field is applied, as shown in FIG 10c the positive ⁇ liquid crystal will reorient in the direction of the field, which is perpendicular to the front surface of gratmg and to the periodic channels of the grating
- FIG 11a depicts a slanted transmission grating 148 and FIG l ib depicts a slanted reflection grating 150
- a holographic transmission grating is considered slanted if the direction of the gratmg vector G is not parallel to the grating surface
- the gratmg is said to be slanted if the gratmg vector G is not perpendicular to the gratmg surface
- Slanted gratmgs have many of the same uses as non- slanted gratmg such as visual displays, mirrors, lme filters, optical switches, and the like
- slanted holographic gratings are used to control the direction of a diffracted beam
- a slanted gratmg is used to separate the specular reflection of the film from the diffracted beam
- a slanted gratmg has an even more useful advantage
- the slant allows the modulation depth of the grating to be controlled by an electric field when using either tangential or homeotropic aligned liquid crystals This is because the slant provides components of the electric field in the directions both tangent and perpendicular to the gratmg vector
- the LC domam symmetry axis will be oriented along the gratmg vector G and can be switched to a direction perpendicular to the film plane by a longitudinally applied field E This is the typical geometry for switchmg of the diffraction efficiency of the slanted reflection gratmg
- Neutral density filters can then be placed in optical contact with the back faces of the pnsms using index matching fluids so as to frustrate back reflections which would cause spurious gratmgs to also be recorded
- a conventional beam splitter splits the mcident laser beam into two beams which are directed to the front faces of the pnsms, and then overlapped in the sample at the desired angle The beams thus enter the sample from opposite sides
- This pnsm coupling technique permits the light to enter the sample at greater angles
- the slant of the resulting gratmg is determined by the angle which the prism assembly is rotated (l e , the angle between the direction of one incident beam and the normal to the prism front face at which that beam enters the prism)
- switchable reflection gratmgs may be formed in the presence of an applied shear stress field
- a shear stress would be applied along the direction of a magnetic field M This could be accomplished, for example, by applying equal and opposite tensions to the two ITO coated glass plates which sandwich the prepolymer mixture while the polymer is still soft This shear stress would distort the LC domams in the direction of the stress, and the resultant LC domam symmetry axis will be preferentially along the direction of the stress, parallel to the PDLC planes and perpendicular to the direction of the applied electric field for switchmg
- Reflection grating prepared in accordance with this descnption may find application m color reflective displays, switchable wavelength filters for laser protection, reflective optical elements and the like
- PDLC materials can be made that exhibit a property known as form birefringence whereby polanzed light that is transmitted through the grating will have its polarization modified
- Such gratmgs are known as subwavelength gratings, and they behave like a negative uniaxial crystal, such as calcite, potassium dihydrogen phosphate, or lithium niobate, with an optic axis perpendicular to the PDLC planes
- FIG 13 there is shown an elevational view of a transmission gratmg 200 made m accordance with this description having periodic planes of polymer planes 200a and PDLC planes 200b disposed perpendicular to the front surface 204 of the gratmg 200
- the optic axis 206 is disposed perpendicular to polymer planes 200a and the PDLC planes 200b
- Each polymer plane 200a has a thickness t p and refractive mdex ri p
- each PDLC plane 200b has a thickness
- the gratmg will exhibit form birefringence
- the magnitude of the shift m polarization is proportional to the length of the gratmg
- the length of the subwavelength gratmg should be selected so that
- the length of the subwavelength grating should be selected so that
- the reflected light will be reflected by the beam splitter 218 If an appropriate switching voltage is applied, as shown in FIG 15b, the reflected light will pass through the beam splitter and be retroreflected on the incident beam
- FIG 16a there is shown an elevational view of a subwavelength gratmg 230 recorded in accordance with the above-described methods and having periodic planes of polymer channels 230a and PDLC channels 230b disposed perpendicular to the front surface 234 of grating 230
- the symmetry axis 232 of the liquid crystal domains is disposed in a direction parallel to the front surface 234 of the gratmg and perpendicular to the penodic channels 230a and 230b of the gratmg 230
- the symmetry axis 232 distorts and reonents in a direction along the field E, which is perpendicular to the front surface 234 of the gratmg, and parallel to the periodic channels 230a and 230b of the gratmg 230
- subwavelength gratmg 230 can be switched between a state where
- n e 2 - n 0 2 -[(f PDLC ) (f p ) (n PDLC 2 - u p 2 )] / [f PDLC n PDLC 2 + f p n p 2 ]
- n 0 the ordinary mdex of refraction of the subwavelength gratmg, n e the extraordinary mdex of refraction, n PDLC the refractive mdex of the PDLC plane, the refractive index of the polymer plane n L, C the effective refractive index of the liquid crystal seen by an incident optical wave,
- the effective refractive index of the liquid crystal, n LC is a function of the applied electric field, having a maximum when the field is zero and value equal to that of the polymer, n P , at some value of the electric field, ⁇ , MA
- ⁇ n -[(fp DLC ) (f p ) (n PDLC 2 - ii p 2 )] / [2n AVG (f PDLC n PDLC 2 + f p n p 2 )]
- n AVG (n. + n 0 ) 12
- the refractive index of the PDLC plane n PDLC is related to the effective refractive index of the liquid crystal seen by an incident optical wave, n LC , and the refractive mdex of the surrounding polymer plane, n P , by the following relation
- Np DL c n P + f LC [n LC - n P ]
- f LC is the volume fraction of liquid crystal dispersed in the polymer within the PDLC plane
- f u [V LC / (V LC + V P )].
- n LC 1 7
- n P 1.5
- the net birefringence, ⁇ n, of the subwavelength gratmg is approximately 0 008
- the length of the subwavelength gratmg should be 50 ⁇ m for a half-wave plate and a 25 ⁇ m for a quarter-wave plate
- the refractive mdex of the liquid crystal can be matched to the refractive mdex of the polymer and the birefringence of the sub
- the plates can be switched between the on and off (zero retardance) states on the order of microseconds
- cunent Pockels cell technology can be switched in nanoseconds with voltages of approximately 1000-2000 volts, and bulk nematic liquid crystals can be switched on the order of milliseconds with voltages of approximately 5 volts
- the switching voltage of the subwavelength gratmg can be reduced by stackmg several subwavelength gratings 220a-220e together, and connecting them electrically m parallel
- stackmg several subwavelength gratings 220a-220e together and connecting them electrically m parallel
- Subwavelength gratings in accordance with the this description are expected to find suitable application in the areas of polarization optics and optical switches for displays and laser optics, as well as tunable filters for telecommunications, colo ⁇ metry, spectroscopy, laser protection, and the like
- electncally switchable transmission gratings have many applications for which beams of light must be deflected or holographic images switched Among these applications are.
- a switchable hologram is one for which the diffraction efficiency of the hologram may be modulated by the application of an electric field, and can be switched from a fully on state (high diffraction efficiency) to a fully off state (low or zero diffraction efficiency)
- a static hologram is one whose properties remam fixed mdependent of an applied field In accordance with this description, a high contrast status hologram can also be created In this variation of this description, the holograms are recorded as described previously The cured polymer film is then soaked m a suitable solvent at room temperature for a short duration and finally dried For the liquid crystal E7, methanol has shown satisfactory application.
- a high birefringence static sub-wavelength wave-plate can also be formed Due to the fact that the refractive mdex for air is significantly lower than for most liquid crystals, the corresponding thickness of the half-wave plate would be reduced accordingly Synthesized wave- plates m accordance with this description can be used m many applications employing polarization optics, particularly where a matenal of the appropriate birefringence that the appropriate wavelength is unavailable, too costly, or too bulky
- polymer dispersed liquid crystals and polymer dispersed liquid crystal material includes, as may be appropriate, solutions in which none of the monomers have yet polymerized or cured, solutions m which some polymenzation has occurred and solutions which have undergone complete polymerization
- polymer dispersed liquid crystals which grammatically refers to liquid crystals dispersed in a fully polymerized matrix
- polymer dispersed liquid crystals is meant to mclude all or part of a more grammatically correct prepolymer dispersed liquid crystal material, or a more grammatically correct starting material for a polymer dispersed liquid crystal material
- FIG 18 shows a schematic diagram of a light source locator according to a first embodiment
- System 300 includes switchable holographic optical elements (SHOEs) 302, detector 304, and processing circuit 306 coupled to SHOEs 302 and detector 304
- SHOEs switchable holographic optical elements
- System 300 may be used to determine the bearing to one or more light sources
- System 300 may be incorporated into a variety of systems such as military systems for tracking targets and locating laser emissions, surveillance systems for monitoring activity, and robotic systems for tracking an object
- SHOEs 302 may comprise any type of switchable holographic elements and preferably comprise switchable Bragg holograms fabricated using a polymer dispersed liquid crystal material due to their fast switching rate, low switching voltage, and high diffraction efficiency If SHOEs 302 comprise switchable Bragg holograms, they may be either the transmission type (l e , light is incident on a first surface and diffracted light exits a second surface) or the reflective type (I e , light is mcident on a first surface and diffracted light exits from the first surface) SHOEs 302 are preferably transmission-type Bragg holograms due to their large spectral bandwidth SHOEs 302 shown FIG 18 are of the transmission type The angular bandwidth of each SHOE 302 may be approximately 15° to 20° m air (l e , the angular bandwidth corresponds to a cone with a half angle of approximately 7 5° to 10°) The SHOEs may be sensitive (1 e , diffract) the same wavelength bandwidth or may
- Each SHOE 302 has a corresponding field of view 308 Field of view 308 is determined by the angular bandwidth of SHOE 302 which is defined by the recordmg conditions of SHOE 302 Although only three SHOEs 302 are shown m FIG 18, system 300 may contam more than or less than this amount There may a sufficient number of SHOEs 302 such that their field of views 308 cover a substantially full hemispherical viewing area Adjacent field of views 308 may partially overlap one another or each field of view 308 may be substantially unique
- SHOE 302 When SHOE 302 is deactivated, all light incident on SHOE 302 is transmitted substantially unaltered through SHOE 302 When SHOE 302 is activated, a portion of the light incident on SHOE 302, within the angular and wavelength bandwidth of SHOE 302, is diffracted such that the diffracted light will be incident on detector 304 Additionally there may be an optical system 310 where optical system 310 is configured to correct any aberrations of SHOEs 302 Optical system 310 may mclude lenses mirrors, and or non-switchable holographic optical elements Processing circuit 306 activates and deactivates SHOEs 302 If SHOEs 302 are polymer dispersed liquid crystals as described in the preceding section, circuit 306 activates SHOEs 302 by applying a first voltage and deactivates SHOEs 302 by applying a second voltage Circuit 306 is preferably configured such that only one SHOE 302 is activated at any given time Circuit 306 also monitors the output of detector 304 which mdicates the location at which light is
- light source 312 is located within field of view 308a of SHOE 302a
- Light source 312 is located such that it has a bearing from system 300 indicated by angle 314 If circuit 306 activates SHOE 302a, then light from light source 312 will be diffracted by SHOE 302 and the light will follow path 316 such that the light will be incident on detector 304 at location 318 Circuit 306 may then receive a signal from detector 304 such that circuit 306 can determine that light was incident on detector 304 at location 318 Circuit 306 may then calculate bearing 314 to light source 312 using location 318 and the ray deviation charactenstics of SHOE 302a
- circuit 306 There are two basic modes in which circuit 306 may operate In the first mode, circuit 306 will activate each SHOE 302 m order For example, the order of activation may be SHOEs 302a, 302b, 302c, 302a, 302b, etc While each SHOE 302 is activated, circuit 306 may calculate a bearing or bearmgs to any light source or sources withm the field of view of that SHOE 302 Alternatively, circuit 306 may only activate the SHOE or SHOEs in which their field of views have a high probability of containing a light source For example, if there is a light source within field of view 308a, circuit 306 may only activate SHOE 302a or may alternatively activate SHOEs 302a and 302b m order to continually determine a bearing to the light source
- circuit 306 may triangulate a position of a light source usmg the bearmgs to the light source determined usmg two different SHOEs 302
- FIG 19 shows a portion of the light source locator depicted m FIG 18
- the fields of view 308a and 308b are arranged such that they partially overlap and a light source 320 is shown contained within the region of overlap Due to the slightly different positions of SHOEs 302a and 302b, the bearing calculated to light source 320 will be slightly different for each SHOE 302 Circuit 306 may first activate SHOE 302a and find bearmg 322 and then circuit 306 may activate SHOE 302b and find bearing 324 Usmg bearmgs 322 and 324, circuit 306 may then triangulate the position of light source 320
- system 300 may mclude a second set of SHOEs and detector coupled to circuit 306 Circuit 306 may use the second set of SHOEs and detector to determine a second bearing to a light source Circuit 306 may then use the first and second set of bearmg to triangulate a position to the light source
- FIG 20 shows a schematic diagram of a light source locator according to a second embodiment
- System 330 includes SHOEs 302, detector 304, and processing circuit 306
- the principal difference between systems 300 and 330 is that system 300 tiles SHOEs 302 next to one another while system 330 stacks SHOEs 302 together
- FIG 21 illustrates a schematic diagram of a light source locator accordmg to a third embodiment
- System 340 includes SHOEs 302, detector 304, and processing circuit 306
- system 300 uses transmission-type SHOEs while system 340 uses reflection-type SHOEs
- Optical system 342 which may mclude a mirror, directs diffracted light from SHOEs 302 onto detector 304
- optical system 342 may be omitted and detector 304 may be placed at the position of optical system 342
- FIG 22 depicts a schematic diagram of a light source locator according to a fourth embodiment
- System 350 includes SHOEs 302, SHOEs 352 detector 304, and processing circuit 306
- SHOEs 302 have a first wavelength bandwidth while SHOEs 352 have a second wavelength bandwidth
- System 350 may be particularly useful for locating a light source that emits at two distinctive wavelengths or for locatmg multiple light sources that emit at different wavelengths
- System 350 could be scaled to include additional SHOEs that operate at additional wavelength bandwidths
- system 350 is shown for transmission-type SHOEs, reflective-type SHOEs may also be used Additionally, SHOEs 302 and 352 could be arranged differently (l e , different arrangements of stacking and tiling)
- FIG 22 shows a preferred embodiment in which SHOEs 302 and 352 are paired up
- SHOEs 302a and 352a share a common field of view 308a Circuit 306 operates system 350 such that only
- FIG 23 is a flow diagram for a method for a processing circuit to determine a beanng to a light source This method will be described in reference to system 300 of FIG 18 but could be used for any of the embodiments of light source locators discussed above Initially, circuit 306 activates a SHOE (e g , SHOE 302a) by applymg a first voltage to SHOE 302a (box 360) If a light source is located withm field of view 308a, SHOE 302a diffracts light from that light source onto detector 304 Circuit 306 then receives a signal from detector 304 indicating a location on detector 304 at which the light was mcident (box 362) Circuit 306 then calculates a beanng to the light source depending on the location at which the light was mcident on detector 304 and a predetermined ray deviation charactenstic of SHOE 302a (box 364)
- a SHOE e g , SHOE 302a
- SHOE 302a diffracts light from that
- FIG 24 is a flow diagram for a method for a processing circuit to triangulate a position of a light source This method will be descnbed in reference to system 300 of FIG 18 but could be used for any of the embodiments of light source locators discussed above Initially, circuit 306 determines a first bearing to a light source withm a field of view of a first SHOE (e g , SHOE 302a) (box 370) Circuit 306 then determines a second beanng to a light source within a field of view of a second SHOE (e g SHOE 302b) (box 372) Field of views 308a and 308b must partially overlap for this method to work and the light source must be located withm the area of overlap Circuit 306 determines the first and second bearing using the method described in reference to FIG 23 Finally, circuit 306 may use the first bearmg and the second bearmg to triangulate the position of the light source (box 374)
- first SHOE e g , SHOE 302a
- circuit 306 determine
- FIG 25 is a flow diagram for a method for activating and deactivating a plurality of SHOEs m a light source locator system This method will be described in reference to system 300 of FIG 18 but could be used for any of the embodiments of light source locators discussed above Initially, a first SHOE (e g , SHOE 302a) is activated by circuit 306 applymg a first voltage to SHOE 302a (box 380) Any light diffracted by SHOE 302a is detected by detector 304 and used by circuit 306 to determine a bearmg to a light source (box 382) SHOE 302a is then deactivated by applymg a second voltage to SHOE 302a (box 384) Next, a second SHOE (e g SHOE 302b) is activated by circuit 306 applying a first voltage to SHOE 302b (box 386) Any light diffracted by SHOE 302b is detected by detector 304 and used by circuit 306 to determine a bearmg to a light source (box 38
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- Physics & Mathematics (AREA)
- General Physics & Mathematics (AREA)
- Optics & Photonics (AREA)
- Holo Graphy (AREA)
- Length Measuring Devices By Optical Means (AREA)
Abstract
Description
Claims
Priority Applications (1)
Application Number | Priority Date | Filing Date | Title |
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AU28425/00A AU2842500A (en) | 1998-10-16 | 1999-10-15 | Light source locator using switchable holograms |
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
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US10447498P | 1998-10-16 | 1998-10-16 | |
US60/104,474 | 1998-10-16 |
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WO2000028354A2 true WO2000028354A2 (en) | 2000-05-18 |
WO2000028354A3 WO2000028354A3 (en) | 2000-10-05 |
WO2000028354A9 WO2000028354A9 (en) | 2001-01-04 |
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PCT/US1999/024246 WO2000028354A2 (en) | 1998-10-16 | 1999-10-15 | Light source locator using switchable holograms |
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AU (1) | AU2842500A (en) |
WO (1) | WO2000028354A2 (en) |
Cited By (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
ES2622336A1 (en) * | 2017-03-30 | 2017-07-06 | Universidad De Alicante | Holographic sensor for detection of adulterants in essential oils and method of obtaining such sensor (Machine-translation by Google Translate, not legally binding) |
CN115857178A (en) * | 2023-03-01 | 2023-03-28 | 南昌虚拟现实研究院股份有限公司 | Holographic optical waveguide lens and preparation method thereof |
Citations (3)
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US5289264A (en) * | 1991-09-26 | 1994-02-22 | Hans Steinbichler | Method and apparatus for ascertaining the absolute coordinates of an object |
US5684612A (en) * | 1993-06-11 | 1997-11-04 | Board Of Trustees Of The Leland Stanford Junior University | Method and system for maintaining and controlling the signal-to-noise ratio of hologams recorded in ferroelectric photorefractive materials |
WO1998004650A1 (en) * | 1996-07-12 | 1998-02-05 | Science Applications International Corporation | Switchable volume hologram materials and devices |
-
1999
- 1999-10-15 AU AU28425/00A patent/AU2842500A/en not_active Abandoned
- 1999-10-15 WO PCT/US1999/024246 patent/WO2000028354A2/en active Application Filing
Patent Citations (3)
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US5289264A (en) * | 1991-09-26 | 1994-02-22 | Hans Steinbichler | Method and apparatus for ascertaining the absolute coordinates of an object |
US5684612A (en) * | 1993-06-11 | 1997-11-04 | Board Of Trustees Of The Leland Stanford Junior University | Method and system for maintaining and controlling the signal-to-noise ratio of hologams recorded in ferroelectric photorefractive materials |
WO1998004650A1 (en) * | 1996-07-12 | 1998-02-05 | Science Applications International Corporation | Switchable volume hologram materials and devices |
Non-Patent Citations (1)
Title |
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SUTHERLAND R L ET AL: "ELECTRICALLY SWITCHABLE VOLUME GRATINGS IN POLYMER-DISPERSED LIQUID CRYSTALS" APPLIED PHYSICS LETTERS,US,AMERICAN INSTITUTE OF PHYSICS. NEW YORK, vol. 64, no. 9, 28 February 1994 (1994-02-28), pages 1074-1076, XP000425907 ISSN: 0003-6951 cited in the application * |
Cited By (4)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
ES2622336A1 (en) * | 2017-03-30 | 2017-07-06 | Universidad De Alicante | Holographic sensor for detection of adulterants in essential oils and method of obtaining such sensor (Machine-translation by Google Translate, not legally binding) |
WO2018178435A1 (en) * | 2017-03-30 | 2018-10-04 | Universidad De Alicante | Holographic sensor for detection of adulterants in essential oils and method of obtaining such sensor |
CN115857178A (en) * | 2023-03-01 | 2023-03-28 | 南昌虚拟现实研究院股份有限公司 | Holographic optical waveguide lens and preparation method thereof |
CN115857178B (en) * | 2023-03-01 | 2023-06-16 | 南昌虚拟现实研究院股份有限公司 | Holographic optical waveguide lens and preparation method thereof |
Also Published As
Publication number | Publication date |
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WO2000028354A9 (en) | 2001-01-04 |
AU2842500A (en) | 2000-05-29 |
WO2000028354A3 (en) | 2000-10-05 |
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