WO2000028354A2 - Light source locator using switchable holograms - Google Patents

Abstract

An apparatus and method is described for locating light sources using holographic techniques. The apparatus includes a plurality of switchable holographic elements (SHOEs) (302), a detector (304), and a processing circuit (306) coupled to the holographic elements and detectors. The processing circuit activates one of the SHOEs thereby causing the SHOE to diffract any light incident on the SHOE within its field of view onto the detector. The detector is two-dimensional array that outputs a signal corresponding to the location at which light is incident on the detector. Utilizing the location at which the light is incident on the detector and the predetermined ray deviation characteristics of the SHOE, the processing circuit can calculate a bearing to the light source. The processing circuit may activate each of the plurality of SHOEs in a predetermined sequence to check for light sources in a large angular field of view.

Description

TITLE: LIGHT SOURCE LOCATOR USING SWITCHABLE HOLOGRAMS

BACKGROUND OF THE INVENTION

1 Field of the Invention

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

2 Description of the Relevant Art

Many applications require the location of light sources For example, 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 Additionally, during the day 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 For example, 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 be equipped with lights having different colors to allow the robotic system to distinguish different objects depending on the color of the light attached to the object It is therefore desired to detect the location of a light source It is also desired to detect multiple light sources over an extremely large angular field It is further desired to locate light sources withm different wavelength bandwidths

SUMMARY OF THE INVENTION The problems outlined above are in large part addressed by an apparatus in which a switchable holographic optical element (SHOE) is used to locate a light source 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 Additionally, 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

Although any type of SHOE may be used, 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 unaltered When the SHOE is activated, it diffracts light onto the detector The detector is a two- dimensional array detector that outputs a signal corresponding to the location at which light is mcident on the detector For example, the detector may be a charge-coupled detector (CCD) array From the location that light is incident on the detector and the predetermined ray deviation characteristics of the SHOE, the circuit can calculate a bearing to the light source The SHOEs may be either the transmission or reflective type Additionally, the SHOEs may be stacked together, tiled next to one another, or some combination thereof In addition to the apparatus descπbed above, a method is contemplated herein The processing circuit activates only one SHOE at a time The circuit then calculates a beanng for any light source withm the field of view of that SHOE The circuit may activate and deactivate each SHOE within the plurality of SHOEs in a predetermined sequence Alternatively, the circuit may only activate those SHOEs for which there is a high probability of a light source being located within their field of view Additionally, if the field of views of the SHOEs overlap, the circuit may triangulate a position of a light source based upon two bearings to the light source determined usmg two different SHOEs

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the invention will become apparent upon readmg the following detailed description and upon reference to the accompanying drawings in which

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,

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, and

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

While the invention is susceptible to vanous modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herem be described in detail It should be understood, however, that the drawing and detailed descnption thereto are not mtended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

1 Switchable Hologram Materials And Devices

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 photopolymeπzable prepolymer material that permits in situ control over characteristics of resulting gratings, such as domain size, shape, density, ordenng and the like Furthermore methods and materials taught herem can be used to prepare PDLC materials for optical elements comprising switchable transmission or reflection type holographic gratings

Polymer dispersed liquid crystal materials, methods, and devices contemplated for use in the present invention are also described m R L Sutherland et al "Bragg Gratings in an Acrylate Polymer Consisting of Periodic Polymer dispersed Liquid-Crystal Planes " Chemistry of Materials, No 5, pp 1533-1538 (1993), in R L Sutherland et al , "Electrically switchable volume gratings in polymer dispersed liquid crystals," Applied Physics Letters, Vol 64, No 9, pp 1074-1076 (1994), and T J Bunmng et al , "The Morphology and Performance of Holographic Transmission Gratings Recorded in Polymer dispersed Liquid Crystals," Polymer, Vol 36, No 14, pp 2699-2708 (1995), all of which are fully incorporated by reference into this Detailed Descnption U S Patent application Serial Nos 08/273, 436 and U S Patent 5,698,343 to Sutherland et al , titled "Switchable Volume Hologram Matenals and Devices," and "Laser Wavelength Detection and Energy Dosrmetry Badge," respectively, are also incorporated by reference and include background material on the formation of transmission gratings inside volume holograms The process by which a hologram for use in one embodiment of the present invention, may be formed is controlled pπmaπly by the choice of components used to prepare the homogeneous starting mixture, and to a lesser extent by the intensity of the incident light pattern In one embodiment of polymer dispersed liquid crystal (PDLC) matenal employed in the present invention creates a switchable hologram in a single step A feature of one embodiment of PDLC material is that illumination by an inhomogeneous, coherent light pattern initiates a patterned, amsotropic diffusion (or counter diffusion) of polymenzable monomer and second phase material, particularly liquid crystal (LC) Thus, alternating well-defined channels of second phase-rich material, separated by well-defined channels of a nearly pure polymer, can be produced m a single-stop process

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-πch material Reasonably well-defined alternately LC-πch 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-πch 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 μm thickness 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/cm² 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 Photopolymeπzation is induced by the optical intensity pattern A more detailed discussion of exemplary recording apparatus can be found in R L Sutherland, et al , "Switchable holograms in new photopolymer-hquid crystal composite matenals," Society of Photo-Optical Instrumentation Engineers (SPIE), Proceedings Reprint, Volume 2402, reprinted from Diffractive and Holographic Optics Technology II (1995), incorporated herein by reference

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 In one embodiment, the prepolymer material comprises a mixture of a photopolymeπzable monomer, a second phase material, a photo-initiator dye, a co-initiator, a chain extender (or cross-linker), and, optionally, a surfactant

In one embodiment, 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 μm

Highly functionalized monomers, however, are relatively viscous As a result, these monomers do not tend to mix well with other materials, and they are difficult to spread into thin films Accordmgly, it is preferable to utilize a mixture of penta-acrylates m combmation with di-, tπ-, and/or tetra-acrylates in order to optimize both the functionality and viscosity of the prepolymer material Suitable acrylates, such as tnethyleneglycol diacrylate, tnmethylolpropane tπacrylate, pentaerythntol tnacrylate, pentaerythntol tetracrylate, pentaerythntol pentacrylate, and the like can be used in the present invention. In one embodiment, it has been found that an approximately 1.4 mixture of tn-to penta-acrylate facilitates homogeneous mixing while providing a favorable mixture for forming 10-20 μm 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 The 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. Above about 35% by weight, the sample becomes highly scattenng, reducing both diffraction efficiency and transmission Samples fabricated with approximately 25% by weight typically yield good diffraction efficiency and optical clarity In prepolymer mixtures utilizing a surfactant, 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-cyanobιphenyl, 4'-n-heptyl-4- cyanobiphenyl, 4'-octaoxy-4-cyanobιphenyl, 4'-penryl-4-cyanoterρhenyl, oc-methoxybenzylιdene-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 drpentaerythπtol hydroxypentacrylate (available, for example, from Polysciences, Inc , Warπngton, 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. Milwaukee Wisconsm), and the photo-initiator dye rose bengal ester, (2,4,5,7-tetraιodo-3',4',5',6'- tetrachlorofluoresceιn-6-acetate ester) marketed as RBAX by Spectragraph, Ltd , Maumee, Ohio) 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 μm thickness and, preferably, 10-20 μm thickness 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 sensitivity of the prepolymer materials to light intensity is dependent on the photo-initiator dye and its concentration A higher dye concentration leads to a higher sensitivity In most cases, however, the solubility of the photo-initiator dye limits the concentration of the dye and, thus, the sensitivity of the prepolymer material Nevertheless, it has been found that for more general applications, photomitiator dye concentrations in the range of 0 2-0 4% by weight are sufficient to achieve desirable sensitivities and allow for a complete bleaching of the dye in the recording process, resulting in colorless final samples Photo-initiator dyes that may be useful in generating PDLC materials are rose bengal ester (2,4,5,7-tetraιodo-3',4',5',6'- tetrachlorofiuoresceιn-6-acetate ester), rose bengal sodium salt, eosin, eosm sodium salt, 4,5-duodosuccmyl fluorescein, camphorquinone, methylene blue, and the like These dyes allow a sensitivity to recordmg wa\ elengths across the visible spectrum from nominally 400 nm to 700 nm Suitable near-mfrared dyes, such as cationic cyamne dyes with tnalkylborate anions having absorption from 600-900 nm as well as merocyanine dyes derived from spiropyran may also find utility m the present invention

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, tπethanolamine, N,N-dιmethyl-2,6-dnsopropyl aniline, and the like

Other 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 tπphenyl borate, lodobenzospiropyran and tπethylamme, 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 Thus, reaction of the chain extender monomer with the polymer increases the propagation length of the growing polymer and results m high molecular weights It has been found that chain extender m general applications m the range of 10-18% by weight maximizes the performance in terms of diffraction efficiency In the one embodiment, it is expected that suitable chain extenders can be selected from the following N-vinylpyrrohdinone, N-vmyl pyndme acrylonitπle, N-vmyl carbazole, and the like

It has been found that the addition of a surfactant material, namely, octanoic acid, in the prepolymer material lowers the switching voltage and also improves the diffraction efficiency In particular, the switching voltage for PDLC materials containing a surfactant are significantly lower than those of a PDLC material made without the surfactant While not wishing to be bound by any particular theory, it is believed that these results may be attributed to the weakening of the anchoring forces between the polymer and the phase-separated LC droplets SEM studies have shown that droplet sizes in PDLC materials including surfactants are reduced to the range of 30-50nm and the distnbution is more homogeneous Random scattering in such materials is reduced due to the dominance of smaller droplets, thereby increasmg the diffraction efficiency Thus, it is believed that the shape of the droplets becomes more spherical m the presence of surfactant, thereby contnbutmg to the decrease in switching voltage

For more general applications, it has been found that samples with as low as 5% by weight of surfactant exhibit a significant reduction in switching voltage It has also been found that, when optimizing for low switching voltages, the concentration of surfactant may vary up to about 10% by weight (mostly dependent on LC concentration) after which there is a large decrease in diffraction efficiency, as well as an mcrease m switchmg voltage (possibly due to a reduction in total phase separation of LC) Suitable surfactants include octanoic acid, heptanoic acid, hexanoic acid, dodecanoic acid, decanoic acid, and the like

In samples utilizing octanoic acid as the surfactant, it has been observed that the conductivity of the sample is high, presumably owmg to the presence of the free carboxyl (COOH) group in the octanoic acid As a result, the sample mcreases in temperature when a high frequency (~2 kHz) electncal field is applied for prolonged penods of time Thus, it is desirable to reduce the high conductivity introduced by the surfactant, without sacrificing the high diffraction efficiency and the low switchmg voltages It has been found that suitable electrically switchable gratings can be formed from a polymenzable monomer, vmyl neononanoate ("VN") C₈H₁₇C0₂CH=CH₂, commercially available from the Aldnch Chemical Co in Milwaukee, Wisconsin Favorable results have also been obtamed where the chain extender N-vinylpyrrohdmone ("NVP") and the surfactant octanoic acid are replaced by 6 5% by weight VN VN also acts as a cham extender due to the presence of the reactive acrylate monomer group In these vanations, high optical quality samples were obtamed with about 70% diffraction efficiency, and an applied field of βVlμ could electrically switch the resulting gratmgs

PDLC matenals used in the present invention may also be formed usmg a liquid crystalline bifunctional acrylate as the monomer ("LC monomer") The 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:

• CH₂=CH-COO-(CH₂)₆0-C₆H C₆H₅-COO-CH=CH₂ • CH₂=CH-(CH₂)₈-COO-C₆H₅-COO-(CH₂)₈-CH=CH₂

• H(CF₂)_l0CH₂O-CH₂-C(=CH₂)-COO-(CH₂CH₂O)₃CH₂CH₂O-COO-CH₂C(=CH₂)- CH₂O(CF₂)₁₀H

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.

Referring now to 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/cm². The size of the liquid crystal domains is about 0.2 μm and the grating spacing is about 0.54 μm. This sample, which is approximately 20 μm 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. Δη is the change in first order Bragg diffraction efficiency. ΔT 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. There is a true minimum of the diffraction efficiency at approximately 225 Vrms. 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.

By increasing the frequency of the applied voltage, the switching voltage for minimum diffraction efficiency can be decreased significantly. This is illustrated in FIG. 3, which 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

Smaller liquid crystal droplet sizes have the problem that it takes high switching voltages to switch their orientation As described in the previous paragraph, using alternating current switching voltages at high frequencies helps reduce the needed switching voltage As demonstrated in FIG 4, it has been found that addmg a surfactant (e g , octanoic acid) the prepolymer material in amounts of about 4%-6% by weight of the total mixture results in sample holograms with switching voltages near 50Vrms at lower frequencies of 1-2 kHz As shown in FIG 5, it has also been found that the use of the surfactant with the associated reduction in droplet size reduces the switching time of the PDLC materials Thus, samples made with surfactant can be switched on the order of 25-44 microseconds Without wishing to be bound by any theory, the surfactant is believed to reduce switching voltages by reducing the anchoring of the liquid crystals at the interface between liquid crystal and cured polymer

Thermal control of diffraction efficiency is illustrated in FIG 6 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

The 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

As shown m FIG 7, 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 Preferably, the separation of the slides is maintained at approximately 20 μm by utilizing uniform spacers 118 Preparation, mixing and transfer of the prepolymer material is preferably done in the dark Once assembled, 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/cπr with typical exposure times of 30-120 seconds Constructive and destructive interference within the expanded beam establishes a penodic intensity profile through the thickness of the film

In one embodiment, 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 10⁵ to 10 ^s gram moles of a photo-initiator dye comprising rose bengal ester Further, as with transmission gratings, the addition of surfactants is expected to facilitate the same advantageous properties discussed above m connection with transmission gratings It is also expected that similar ranges and variation of prepolymer starting material will find ready application in the formation of suitable reflection gratings It has been determined by low voltage, high resolution scanning electron microscopy ("LVHRSEM") that the resulting material comprises a fine grating with a periodicity of 165 nm with the grating vector perpendicular to the plane of the surface Thus, as shown schematically m FIG 8a, grating 130 includes periodic planes of polymer channels 130a and PDLC channels 130b which run parallel to the front surface 134 The gratmg spacing associated with these penodic planes remains relatively constant throughout the full thickness of the sample from the air/film to the film substrate interface

Although interference is used to prepare both transmission and reflection gratings, 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

Analysis of the reflection notch in the absorbance spectrum supports the conclusion that a penodic refractive index modulation is disposed through the thickness of the film 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 tπethyleneglycol diacrylate, tnmethylolpropanetπacrylate, pentaerythntol tnacrylate, pentaerythntol tetracrylate, pentaerythntol pentacrylate, and the like Similarly, other co-initiators such as tnethylamme, triethanolamme, N,N-dιmethyl- 2,6-dnsopropylanιlιne, 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, it is believed that other liquid crystals such as 4 -pentyl-4-cyanobιphenyl or 4'-heptyl-4-cyanobιphenyl, can be utilized

Referring again to 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 Thus, when an electric field E is applied, as shown m FIG 8b, the symmetry axis 136 is already m a low energy state in alignment with the field E and will reorient Thus, reflection gratings formed in accordance with the procedure described above will not normally be switchable

In general, a reflection grating tends to reflect a narrow wavelength band, such that the grating can be used as a reflection filter In one embodiment, however, the reflection grating is formed so that it will be switchable More particularly, 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.

It is known that liquid crystals havmg a negative dielectric amsotropy (Δε) will rotate m a direction perpendicular to an applied field As shown in FIG 9a, the symmetry axis 136 of the liquid crystal domains formed with a liquid crystal havmg a negative Δε 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 However, when an electric field E is applied across such gratmgs, as shown m FIG 9b, the symmetry axis of the negative Δε 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 As a result the reflection gratmg can be switched between a state where it is reflective and a state where it is transmissive The following negative Δε liquid crystals and others are expected to find ready applications in the methods and devises of the present invention

Liquid crystals can be found in nature (or synthesized) with either positive or negative Δε Thus, it is possible to use a LC which has a positive Δε at low frequencies, but becomes negative at high frequencies The frequency (of the applied voltage) at which Δε changes sign is called the crossover frequency The cross-over frequency will vary with LC composition, and typical values range from 1-10 kHz Thus, by operating at the proper frequency, the reflection gratmg may be switched It is expected that 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

"-^"O^{^}O^""™

Both liquid crystal materials are available from LaRoche & Co , Switzerland By varying the proportion of the positive and negative liquid crystals m the combmation, crossover frequencies form 1 4-2 3 kHz are obtamed at room temperature Another combination suitable for use m the present embodiment is a combmation of the following p-pentylρhenyl-2-chloro-4-(ρ-ρentylbenzoyloxy) benzoate and benzoate These materials are available from Kodak Company

In still more detailed aspects, switchable reflection gratmgs can be formed using positive Δε liquid crystals As shown in 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 Preferably, 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 As a result, the symmetry axis 146 of the liquid crystals ill orient along the field while the mixture is fluid When polymerization is complete, the field may be removed and the alignment of the symmetry axis of the liquid crystals will remain unchanged (See FIG 10b ) When an electric field is applied, as shown in FIG 10c the positive Δε 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 In a holographic reflection grating, 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

Primarily, slanted holographic gratings are used to control the direction of a diffracted beam For example, m reflection holograms a slanted gratmg is used to separate the specular reflection of the film from the diffracted beam In a PDLC holographic gratmg, 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 In particular, for the reflection grating, 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

When recording slanted reflection gratmgs, it is desirable to place the sample between the hypotenuses of two right-angle glass prisms 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)

As shown m FIG 12, switchable reflection gratmgs may be formed in the presence of an applied shear stress field In this method, 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

In one embodiment, 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 Referring now to 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, and each PDLC plane 200b has a thickness t_PDLC and refractive index n_PDLC

Where the combined thickness of the PDLC plane and the polymer plane is substantially less than an optical wavelength (I e (t_{PDLC +} t_p) « λ), the gratmg will exhibit form birefringence As discussed below, the magnitude of the shift m polarization is proportional to the length of the gratmg Thus, by carefully selectmg the length, L, of the subwavelength gratmg for a given wavelength of light, one can rotate the plane of polarization or create circularly polarized light Consequently, such subwavelength gratings can be designed to act as a half-wave or quarter-wave plate, respectively Thus, an advantage of this process is that the birefringence of the material may be controlled by simple design parameters and optimized to a particular wavelength, rather than relying on the given birefringence of any matenal at that wavelength

To form a half- wave plate, the retardance of the subwavelength grating must be equal to one-half of a wavelength, I e , retardance = λ/2, and to form a quarter- wave plate, the retardance must be equal to one-quarter of a wavelength, l e , retardance = λ/4 It is known that the retardance is related to the net birefringence, I Δn | , which is the difference between the ordinary mdex of refraction, n₀, and the extraordinary mdex of refraction n_c, of the sub-wavelength grating by the following relation

Retardance = | Δn | L = | n_e - n₀ | L

Thus, for a half- wave plate, I e. a retardation equal to one-half of a wavelength, the length of the subwavelength gratmg should be selected so that

L = λ / (2 | Δn | )

Similarly, for a quarter- wave plate, i.e., a retardance equal to one-quarter of a wavelength, the length of the subwavelength grating should be selected so that

L = λ / (4 I Δn I )

If, for example, the polarization of the incident light is at an angle of 45° with respect to the optic axis 210 of a half-wave plate 212, as shown in FIG 14a, the plane polarization will be preserved, but the polarization of the wave exiting the plate will be shifted by 90° Thus, referring now to FIG 14b and 14c, where the half- wave plate 212 is placed between cross-polarizers 214 and 216, the incident light will be transmitted If an appropriate switching voltage is applied, as shown m FIG 14d, the polarization of the light is not rotated and the second polarizer will block the light For a quarter-wave plate plane polarized light is converted to circularly polarized light Thus, refernng now to FIG 15a, where quarter-wave plate 217 is placed between a polarizing beam splitter 218 and a mirror

219, 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

Referring now to 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 As shown in FIG 16a, 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 Thus, when an electric field E is applied across the grat g, as shown in FIG 15b, 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 As a result, subwavelength gratmg 230 can be switched between a state where it changes the polarization of the incident radiation and a state in which it does not Without wishing to be bound by any theory, it is currently believed that the direction of the liquid crystal domain symmetry 232 is due to a surface tension gradient which occurs as a result of the amsotropic diffusion of monomer and liquid crystal dunng recording of the grating, and that this gradient causes the liquid crystal domam symmetry to onent m a direction perpendicular to the periodic planes

As discussed m Born and Wolf, Principles of Optics, 5^th Ed , New York (1975) and incorporated herein by reference, the birefringence of a subwavelength grating is given by the following relation

n_e ² - n₀ ² = -[(f_PDLC) (f_p) (n_PDLC ² - u_p ²)] / [f_PDLC n_PDLC ² + f_pn_p ²]

Where

n₀ the ordinary mdex of refraction of the subwavelength gratmg, n_e the extraordinary mdex of refraction, ⁿPDLC 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,

^±PDLC tpDLC ' (tpDLC ⁺ ^p) V (t^pDLC ⁺ *^p)

Thus, the net birefrmgence of the subwavelength grating will be zero if n_PDLC= n_P

It is known that 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, Ε,_MA Thus, by application of an electric field, the refractive mdex of the liquid crystal, n_LC, and hence, the refractive index of the PDLC plane can be altered Using the relationship set forth above, the net birefringence of a subwavelength gratmg will be a minimum when n_PDLC is equal to n_P, 1 e when n_LC = n_P Therefore, if the refractive index of the PDLC plane can be matched to the refractive index of the polymer plane, 1 e. n_PDLC = n_P, by the application of an electric field, the birefringence of the subwavelength grating can be switched off

The following equation for net birefringence, 1 e | Δn | = | n_e - n₀1 , follows from the equation given in Born and Wolf (reproduced above)

Δn = -[(fp_DLC) (f_p) (n_PDLC ² - ii_p ²)] / [2n_AVG (f_PDLC n_PDLC ² + f_pn_p ²)]

where n_AVG = (n. + n₀) 12

Furthermore, it is known that 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_DLc = n_P + f_LC [n_LC - n_P]

Where 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)].

By way of example, a typical value for the effective refractive index for the liquid crystal in the absence of an electnc field is n_LC = 1 7, and for the polymer layer n_P, = 1.5. For the gratmg where the thickness of the PDLC planes and the polymer planes are equal (l e t_PDLC = t_P, f_PDLC = 0.5 = f_P) and f_LC = 0 35, the net birefringence, Δn, of the subwavelength gratmg is approximately 0 008 Thus, where the mcident light has a wavelength of 0.8 μm, the length of the subwavelength gratmg should be 50 μm for a half-wave plate and a 25 μm for a quarter-wave plate Furthermore, by application of an electric field of approximately 5 V/μm, the refractive mdex of the liquid crystal can be matched to the refractive mdex of the polymer and the birefringence of the subwavelength gratmg turned off Thus, the switching voltage, V„, for a half-wave plate is on the order of 250 volts, and for a quarter-wave plate approximately 125 volts

By applymg such voltages, the plates can be switched between the on and off (zero retardance) states on the order of microseconds As a means of comparison, 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 In an alternauve embodiment, as shown m FIG 17, the switching voltage of the subwavelength gratmg can be reduced by stackmg several subwavelength gratings 220a-220e together, and connecting them electrically m parallel By way of example, it has been found that a stack of five gratmgs each with a length of 10 μm yields the thickness required for a half- wave plate It should be noted that the length of the sample is somewhat greater than 50 μm, because each grating includes an mdium-tin-oxide coating which acts as a transparent electrode The switching voltage for such a stack of plates, however, is only 50 volts

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, coloπmetry, spectroscopy, laser protection, and the like Similarly, electncally switchable transmission gratings have many applications for which beams of light must be deflected or holographic images switched Among these applications are. Fiber optic switches, reprogrammable NxN optical interconnects for optical computing, beam steenng for laser surgery, beam steenng for laser radar, holographic image storage and retrieval, digital zoom optics (switchable holographic lenses), graphic arts and entertainment, and the like

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. Other potential solvents include alcohols such as ethanol, hydrocarbons such as hexane and heptane, and the like. When the material is dried, a high contrast status hologram with high diffraction efficiency results. The high diffraction efficiency is a consequence of the large index modulation m the film (Δn~0.5) because the second phase domains are replaced with empty (air) voids (n~l)

Similarly, m accordance with this description 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

The term 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 Those skilled m the art will clearly understand that the use herem of the standard term used m the art, "polymer dispersed liquid crystals" (which grammatically refers to liquid crystals dispersed in a fully polymerized matrix) 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

2 Light Source Locator Using Switchable Holograms

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 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 be sensitive to different wavelength bands Wavelengths that may be used for locatmg a light source range from the infrared to the ultraviolet Detector 304 may comprise a two-dimensional detector anay capable of outputtmg a signal indicative of the location at which a light is incident on the detector An example of a suitable two-dimensional detector array is a charge-coupled detector (CCD) array Other examples mclude photo capacitors, photo resistors, photo diodes, etc Each of the example arrays includes a plurality of detector elements (e g , CCD, photo capacitors, etc ) arranged in rows and columns Further, it is noted that FIG 18 shows one detector array 304 shared by each SHOE 302 It is to be understood that a separate detector havmg a one or more detector elements may be provided for each SHOE 302, where each detector element is in data communication with control circuit 306

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

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 incident on detector 304 Depending on the location on which light is incident on detector 304 and a predetermined ray deviation characteristics of SHOE 302 that diffracts the light together with additional optics 310, circuit 306 can calculate a bearing to a light source Circuit 306 may also spatially filter the output signal of detector 302 to eliminate background signal

By way of an example, 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

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

If field of views 308 overlap, 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

Additionally, 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 Portions of system 330 shown in FIG 20 similar to that of system 300 shown m FIG 18 are labeled identically 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 Portions of system 340 shown m FIG 21 similar to that of system 300 shown m FIG 18 are labeled identically System 340 includes SHOEs 302, detector 304, and processing circuit 306 The principal difference between systems 300 and 330 is that 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 Alternatively, 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 Portions of system 350 shown m FIG 22 similar to that of system 300 shown in FIG 18 are labeled identically 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 Although 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 For example, SHOEs 302a and 352a share a common field of view 308a Circuit 306 operates system 350 such that only one of SHOEs 302 and 352 are active at any given tune

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)

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)

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 388) SHOE 302b is then deactivated by applying a second voltage to SHOE 302b (box 390) All of SHOEs 302 in system 300 may be activated and deactivated in a predetermined sequence (such as 302a, 302b, 302c, 302a, 302b, etc ) Such a sequencing may be particularly advantageous for tracking all light source in the field of view of system 300 Alternatively, the first SHOE to be activated may be the SHOE whose field of view has the highest probability of containing a light source The second SHOE activated may then be the SHOE whose field of view has the next highest probability of contaming a light source This method allows system 300 to concentrate on areas most likely to contain light sources so that bearings to the light sources may be determined more frequently

While the present invention has been described with reference to particular embodiments, it will be understood that the embodiments are illustrated and that the invention scope is not so limited Any variations, modifications, additions and improvements to the embodiments descnbed are possible These variations, modifications, additions and improvements may fall withm the scope of the mvention as detailed withm the following claims

Claims

WHAT IS CLAIMED IS:

1 An apparatus comprising a switchable holographic optical element (SHOE), wherein the SHOE is configured to receive and diffract a light originating from a pomt in space when activated, and wherein the SHOE is configured to receive and substantially transmit the light, without diffraction, originating from the point in space when deactivated, a detector configured to receive and detect the diffracted light, and a circuit coupled to the detector, wherein the circuit is configured to calculate a bearing corresponding to the pomt in space relative to the apparatus m response to the detector receiving and detecting the diffracted light

2 The apparatus of claim 1, wherein the point m space is one of a plurality of points in space

3 The apparatus of claim 2, wherein the plurality of points in space is defined by all points located within a cone having a half angle of approximately 7 5° to 10° wherem an apex position of the cone substantially coincides with a position of the apparatus

4 The apparatus of claim 1 , wherem the detector compnses a two-dimensional detector array, wherem the two-dimensional detector array comprises a plurality of detector elements arranged in columns and rows, wherem one of the detector elements is configured to receive and detect the diffracted light

5. The apparatus of claim 1 wherein the SHOE comprises a hologram, wherem the hologram is formed by exposing an mterference pattern inside a polymer-dispersed liquid crystal material, wherein polymer-dispersed liquid crystal material compnses before exposure a polymenzable monomer, a liquid crystal, a cross-linkmg monomer, a comitiator, and a photoimtiator dye

6 The apparatus of claim 1 , further comprising a second SHOE, wherem the second SHOE is configured to receive and diffract a second light originating from a second point m space when activated, and wherein the second SHOE is configured to receive and substantially transmit the second light, without diffraction, ongmatmg from the second point in space when deactivated, wherein the detector is configured to receive and detect the diffracted second light from the second SHOE, and wherein the circuit is configured to calculate a second bearing corresponding to the second point in space relative to the apparatus in response to the detector receiving and detecting the diffracted second light; wherein the point in space is one of a plurality of points in space, and; wherein the second point in space is one of a plurality of second points in space;.

7. A method, comprising: diffracting light incident on a SHOE from a point in space; detecting the diffracted light; and calculating a bearing corresponding to the point in space in response to detecting the diffracted light.

8. A method, comprising: a first SHOE receiving a first light form a point in space; calculating a first bearing corresponding to the point in space in response to the first SHOE receiving the first light; a second SHOE receiving a second light form the point in space; calculating a second bearing corresponding to the point in space in response to the second SHOE receiving the second light; and triangulating the point in space as a function of the first and second bearings.

9. The method of claim 35, further comprising: activating the first SHOE, wherein the first SHOE diffracts the first light in response to activating the first SHOE; deactivating the first SHOE, wherein the first SHOE transmits the first light without diffraction in response to deactivating the first SHOE.

10. An apparatus comprising: a hologram, wherein the hologram is configured to receive and diffract a light originating from a point in space when activated, and wherein the hologram is configured to receive and substantially transmit the light, without diffraction, originating from the point in space when deactivated; a detector configured to receive and detect the diffracted light; and a circuit coupled to the detector, wherein the circuit is configured to calculate a bearing corresponding to the point in space relative to the apparatus in response to the detector receiving and detecting the diffracted light.