Abstract:
A device that provides light beam switching, agile steering of the light beam over a range of angles, and generation of arbitrary wavefront shapes with high spatial and temporal resolution. The agile device can include a volume diffractive structure comprising Bragg planes having one refractive index and the Bragg planes separated by regions containing an active optical medium. Electrodes (which may be the Bragg planes themselves, or may be arrayed adjacent to the active optical medium) are used to control the electric field intensity and direction across the structure, and thereby control the diffraction efficiency of the structure and the local phase delay imposed on a diffracted wavefront. Means are provided for addressing the many thousands of electrodes required for precise and rapid wavefront control. Applications include free-space atmospheric optical communications, near-eye displays, direct-view 3D displays, optical switching, and a host of other applications.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 12/061,390, filed Apr. 2, 2008, now U.S. Pat. No. 8,014,050, which is a nonprovisional of U.S. Provisional Application No. 60/909,515 filed Apr. 2, 2007. U.S. patent application Ser. No. 12/061,390 is hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The invention relates to optical switches, beam steerers, and traveling lenses for such purposes as switching, shaping, and angularly steering beams for near-eye displays, direct-view 3D displays, and other display, control, or communication applications. 
     BACKGROUND OF THE INVENTION 
     One object of this invention is to provide a fast optical beam steerer with no mechanical moving parts such as oscillating mirrors, and to use the beam steerer alone or in combination with other beam steerers in near-eye displays or other display, control or communications applications. 
     In this disclosure, “beam steerer” refers to any device that redirects the direction of propagation of electromagnetic radiation in a continuously time-varying fashion. That is, for purposes of this disclosure, a device that is inherently non-continuous such as a sequence of switchable prisms or diffraction gratings would not be considered a beam steerer, but a variable-index prism or a continuously variable-focus lens would be considered a beam steerer. Although this disclosure will usually use the term “light” for convenience to refer to electromagnetic radiation, it should be understood that the term is intended to include visible, ultraviolet, infra-red, deep infra-red, terahertz, microwave, and other such electromagnetic radiation. Similarly, such terms as “wavelength of light” are intended to refer generally to a referenced property of electromagnetic radiation (i.e., wavelength). Accordingly, the term “beam steerer” refers generally to any continuously variable optical element, including for example traveling lenses, variable prisms, variable focal length lenses, deformable mirrors, oscillating mirrors, spatial phase modulators, and so on. 
     In the prior art, there are many different kinds of beam steerers implemented, including:
         Acousto-optic Bragg cells   Acousto-optic pulsed traveling lenses   Acousto-optic phase modulators   Variable electro-optic prisms   Variable focus liquid crystal lenses   Liquid crystal spatial phase modulators   Liquid crystal variable focus mirrors   Kerr cells   Deformable mirrors       

     Also in the prior art, there are various switched optical devices such as H-PDLC switchable holograms and POLICRYPS switchable gratings; and the prior art also includes electro-optically variable diffraction efficiency gratings. 
     SUMMARY OF THE INVENTION 
     The present invention among its preferred embodiments steers a light beam through a continuous range of angles by diffracting the light beam through a diffractive structure and superimposing a locally spatially periodic varying phase or amplitude modulation onto the diffracted light beam. Without the modulation, the light beam would be diffracted into one or more fundamental Nth order diffracted beam directions by the diffractive structure. However, locally spatially periodic varying modulation causes the light beam to diffract into “sidebands”: angles spaced to either side of the original Nth order diffracted beam directions by an amount that depends on the spatial period of the modulation, the wavelength of the diffracted light, and the angle of the fundamental Nth order beam direction. Hereafter within this disclosure I refer to the “sidebands” as Mth sub-orders of Nth fundamental diffracted orders. 
     By varying the spatial period of the modulation, it is possible to vary the direction into which a particular Mth suborder of a particular Nth fundamental order Nth order beam is diffracted. By varying the spatial period in a linear fashion, it is possible to direct a collimated beam into other directions while keeping it collimated. By spatially varying the spatial frequency in a quadratic fashion, it is possible to diffract a collimated beam to a focus or diffract it as if from a virtual focus. By spatially varying the spatial frequency in a quadratic fashion while translating the pattern of spatial frequency variation across the device at a constant velocity, it is possible to effect a traveling lens. In general, with this invention it is possible to control the diffracted wavefront in the Mth suborder of the Nth fundamental order with high spatial and temporal resolution by controlling the spatial frequency of modulation both temporally and spatially. 
     One version of the invention as an agile holographic phased array for electromagnetic radiation includes a first diffractive structure that diffracts electromagnetic radiation into one or more fundamental diffractive orders. A spatial and temporal modulator adjusts the phase shift undergone by electromagnetic radiation diffracted from the diffractive structure in a predetermined spatial and temporal pattern. 
     The spatial and temporal pattern can have (a) a spatial resolution finer than 1000 wavelengths of the electromagnetic radiation and (b) a temporal resolution finer than 1 second, such that the spatial and temporal modulator can generate diffraction suborders from the one or more fundamental diffracted orders. The direction of propagation of the suborders can be determined by the spatial frequency of the spatial and temporal pattern of phase shift. 
     Predetermined zones of the first diffractive structure are preferably switchable. The spatial and temporal modulator can be a second diffractive structure whose period and amplitude are independently controllable in multiple regions on the second structure. Together, the first diffractive structure and the spatial and temporal modulator preferably comprise a single electro-optically controllable layer. 
     The first diffractive structure is preferably configured to diffract electromagnetic radiation out of a totally internally reflected mode within a high refractive index medium, primarily into a single diffracted order that exits the high refractive index medium into a lower refractive index medium. The first diffractive structure is further preferably configured to diffract electromagnetic radiation of different wavelengths independently. 
     The electro-optically controllable layer preferably includes a series of transparent dielectric vanes arrayed on a surface and separated by distances smaller than 1000 wavelengths of the electromagnetic radiation and larger than one-half of a wavelength of the electromagnetic radiation. The vanes are preferably tilted at an angle to the surface normal and separated by spaces filled with electro-optically active material. In addition, the controllable layer includes an array of transparent electrodes proximate to the controllable layer and the electrodes are addressable so that a predetermined spatial and temporal pattern of voltages can be applied to the electrodes, thereby imposing a corresponding spatial and temporal pattern of phase and/or amplitude modulation onto electromagnetic radiation diffracted by the agile holographic phased array. 
     Alternatively, the first diffractive structure and the spatial and temporal modulator can be separate but proximately positioned devices. Preferably, the first diffractive structure comprises a POLICRYPS or H-PDLC structure. In particular, the first diffractive structure preferably includes a series of transparent electrically conductive vanes arrayed on a surface and separated by distances smaller than 1000 wavelengths of the electromagnetic radiation and larger than one-half of a wavelength of the electromagnetic radiation. The vanes are tilted at an angle to the surface normal and separated by spaces filled with electro-optically active material. The vanes also serve as electrodes for imposing electric fields across the electro-optically active material in a direction perpendicular to the vane surfaces. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
         FIG. 1  is a side view of a holographic phased array with progressively enlarged breakaway sections. 
         FIG. 2  is a greatly enlarged side view of a diffractive structure within a layer of the phased array. 
         FIGS. 3   a - 3   c  are side views of the same diffractive structure with superimposed bar charts showing relative amounts of voltage applied to electrodes of the array and resulting paths of light rays steered by the array. 
         FIG. 4  is a side view of the phased array showing a range of angles through which the light can be steered. 
         FIGS. 5   a  and  5   b  are side views of an alternative holographic phased array with  FIG. 5   b  showing light diffracted through different orders and suborders. 
         FIG. 6  is a side view of another alternative holographic phased array having a separate diffractive element and modulator for providing two stages of beam steering. 
         FIG. 7  is a diagram of a prior art device comprising a recording layer containing three superimposed holograms. 
         FIG. 8  shows the application of a phased array with three holograms for steering three different wavelengths of light. 
         FIG. 9  depicts two phased arrays, such as shown in  FIGS. 5 and 6 , oriented at right angles for scanning a beam in two orthogonal planes. 
         FIGS. 10   a - 10   c  of an alternative diffractive structure within the phased array in which electrically conductive transparent vanes are used as electrodes with  FIGS. 10   b  and  10   c  including voltage bar charts representing the vanes being individually addressed. 
         FIG. 11  shows ray bundles propagating through the phased array of  FIG. 8  from light sources at opposite ends of a light source array. 
         FIG. 12  depicts an alternative orientation of the phase array for displaying columns of information. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A preferred embodiment of the invention is a holographic phased array  10  as shown in  FIG. 1 . A glass substrate  12  has a continuous transparent electrode  14  on one surface. The glass may be for example fused silica or fused quartz having a refractive index of around 1.5. On the transparent electrode  14 , a layer  16  contains a diffractive structure composed of tilted vanes  18  of dielectric material (e.g., a cured photopolymer) alternating with spaces  20  filled with nematic liquid crystal. The diffractive layer  16  can be 2 microns thick with (a) the vanes  18  having a matching axial length (i.e., a length in the thickness direction of the diffractive layer  16 ), a transverse width of 250 nanometers, and an inclination of 10 degrees from the substrate&#39;s surface normal and (b) the interstitial spaces  20  also having a matching 2 micron length and a transverse width of 250 nanometers. Thus, the period of the diffractive structure within the diffractive layer  16  is 500 nanometers. On or adjacent to diffractive layer  16  is an array  22  of individually addressable transparent strip electrodes  24 , each approximately 400 nanometers wide and separated by 100 nanometers. Overlying the array  22  of strip electrodes  24  is a glass cover plate  26  having approximately the same refractive index as the substrate  12 , the dielectric material of the vanes  18 , and the ordinary refractive index of the liquid crystal material within the interstitial spaces  20 . 
     A nonconductive transparent material, which has a refractive index close to that of the transparent electrodes  14  and  24 , may be used as a planarization layer  28  to minimize unwanted diffraction from the strip electrodes  24 . In addition, an antireflection layer (not shown) can be applied between the planarization layer  28  and the diffractive layer  16  to minimize reflections from the interface between them. 
     Light  30  from rows of red, green, and blue LEDs  32   r ,  32   g , and  32   b , respectively, enters the substrate  12  through a lens  34 , travels longitudinally through the substrate  12  to impinge upon a holographic optical element  36 , where the light  30  diffracts toward the diffractive structure in the layer  16  at an angle of 75 degrees to the substrate normal. If the liquid crystal in the spaces  20  is in its ordinary state so that its refractive index matches that of the dielectric vanes  18 , the light passes through layer  16  without diffraction, strikes a glass/air interface  38  of cover plate  26 , and is totally internally reflected from interface  38  back into the substrate  12 . 
     However, if the liquid crystal in the spaces  20  is in its extraordinary state, for example because of a voltage difference between electrodes  24  in the array  22  and electrode  14 , the light  30  will be diffracted from the diffractive structure in layer  16  and will exit the cover plate  26  as a diffracted beam  40  in one or more directions α, such as a direction perpendicular to the cover plate  26 . 
       FIG. 2  is a close-up view of the diffractive structure in layer  116 , showing vanes  18  separated by spaces  20  filled with liquid crystal material. 
       FIGS. 3   a ,  3   b  and  3   c  illustrate the beam steering function of the device in  FIG. 1 . In each of  FIGS. 3   a ,  3   b , and  3   c , the holographic phased array  10  of  FIG. 1  is shown with the cover glass  26  removed, and a bar chart  42  is shown on top of the electrode array  22 , with the bar height indicating the voltage to which each electrode  24  is charged relative to the uninterrupted electrode  14 . In  FIG. 3   a , all of the electrodes  24  in the array  22  are charged to the same voltage, while the uninterrupted electrode  14  is charged to a higher or lower voltage than the electrode array  22 . In this case, the diffractive layer  16  will contain a nearly uniform electric field directed normal to the substrate  12 , and essentially all diffracted light will exit the cover plate in a single direction as a +1 diffracted order beam  44  as shown in  FIG. 3   a  because the phase and amplitude of the light wavefront is essentially constant in the diffracted beam  44 . 
     However, if the electrodes  24  in the array  22  are charged in such a way that the voltage follows a sawtooth spatial pattern as indicated in  FIG. 3   b , the light  30  will be diffracted additionally into at least one sub-order  46  or  48 , in accordance with well-known diffraction principles, and probably diffracted as well into other sub-orders. The +1 sub-order  46 , the zero (fundamental) order  44 , and the −1 sub-order  48  are shown as vectors in  FIGS. 3   a ,  3   b , and  3   c , with length indicating relative intensity. This additional diffraction into sub-orders will occur whether or not the diffraction efficiency of the diffractive structure in layer  16  varies with electrode voltage, as long as the optical phase delay for light diffracted from the structure is dependent on electrode voltage. Similarly, it will occur if only the diffraction efficiency of the structure is modulated by electrode voltage, because an amplitude variation at one point in the propagation of a wave is converted to a phase variation downstream and thus to a variation in the direction of propagation. 
     In  FIG. 3   c , a voltage pattern corresponding to a Fresnel lens is imposed on the electrodes  24 , resulting in a distribution of light primarily into a single sub-order  56  that converges to a point  50 . A possible distribution is indicated by the length of the vectors representing the other sub-orders  58 . In the example of  FIG. 3   c , most of the light originally diffracted into the +1 fundamental order  54  is diverted into the −1 sub-order  56  because of the voltage pattern. Careful selection of the profile of the voltage pattern and care to ensure that the phase delay at each peak in the pattern is very close to 1 wavelength of the light, with that the amount of phase delay corresponding to the desired phase function, can result in as much as 90% or more of the light being concentrated into this −1 sub-order. 
       FIG. 4  illustrates the potential scanning range a of diffraction angle of the +1 sub-order of the +1 fundamental diffracted order in the device of  FIG. 1 . Note that a diffraction grating  62  has been added to pre-disperse the spectral components of the incident light so that diffractive elements  36  and  16  will reverse the dispersion, resulting in minimal spectral dispersion. For light having a nominal wavelength of 700 nanometers, incident on the active layer  16  at the angle of 70 degrees, the first order fundamental diffracted order  64  exits the cover plate  26  at an angle normal to the substrate  12 . The largest possible angle of deviation from the +1 fundamental diffracted order into the +1 suborder, given an electrode spacing of 500 nanometers, is plus or minus 90 degrees. The smallest angle of deviation is zero, so the range of deviation, or the scanning range, of the +1 sub-order is from minus 90 degrees to +90 degrees from normal. It will be evident to any skilled practitioner in the art of diffractive element design that shorter spatial periods in the voltage pattern will correspond to wider deviation angles for the −1 sub-order, and that the amount of light distributed into each of the various sub-orders will depend on the precision of wavefront shaping, which in turn depends on the number of electrodes available to define each region of the voltage pattern. Fewer electrodes (shorter periods) will result usually in (a) less precision and (b) a larger proportion of light being distributed among other suborders. In order to obtain a full 180 degree scanning range, electrode spacing must be less than about 0.7× the wavelength of light being manipulated. 
     Another embodiment does not require the total internal reflection of  FIG. 1 . In the device of  FIG. 5 , an array  22  of electrodes, a glass cover  26 , and a diffractive layer  16  essentially identical to that of  FIG. 1  are mounted on a glass plate substrate  72 , and light  70  enters the substrate  72  through a diffraction grating  74 . The incident light cannot be totally internally reflected by the front surface of the glass cover  26 , so a large portion of the light  70  transmits through the glass cover  26  as the beam  76  when the diffractive layer  16  is in its “off” state. However, if the thickness, vane spacing and tilt within the diffractive layer  16  are selected according to well-known principles of Bragg diffraction to correspond to the wavelength of the light  70 , refractive index distribution in the layer  16 , and the desired direction of fundamental first-order diffracted light, then a very large fraction of incident light  70  (e.g., up to 95% or more) will be diffracted into the +1 fundamental order  78  when the active layer is in its “on” state. If the thickness of layer  16  is very small, light may be diffracted into several fundamental diffracted orders as shown in  FIG. 5   b.    
     In  FIG. 5   b , the fundamental zero order diffracted beam  76  is labeled b, the fundamental plus one order diffracted beam  78  is labeled j, and the fundamental plus two order diffracted beam  80  is labeled s. There is no minus one fundamental diffracted order because the zero order is at too extreme an angle. Around the zero order fundamental diffracted order b, there are several sub-orders shown: a, c, d and e, which are the minus one, plus one, plus two and plus three sub-orders. There is no minus two sub-order of the zero-order fundamental order because of the extreme angle of the zero order beam, although there could be higher sub-orders than the plus three sub-order. Around the plus one fundamental order j there are six sub-orders shown, the minus sub-orders labeled g, h, and i, and the plus sub-orders labeled k, m, and n. Similarly, around the plus two fundamental order s are shown four suborders: three minus sub-orders labeled p, q, and r, and one plus sub-order labeled t. There are no further plus sub-orders because the angle of the second fundamental diffracted order is too extreme. 
     Nonetheless, a properly shaped sawtooth modulation such as that indicated in  FIG. 3   b  can guide a large fraction of light into a single sideband a, c, d, e, f, g, h, i, k, l, m, n, p, q, r, or t of one of the single fundamental diffracted orders b, j, and s. By proper selection of the modulation depth (maximum phase delay) and slope in the device of  FIG. 3   b , light can be diverted primarily into any desired sideband. 
     Another embodiment, illustrated in  FIG. 6 , uses separate diffractive element  86  and modulator  88 . In this embodiment, Bragg planes in the diffractive element  86  (which may be, for example, a switchable H-PDLC or POLICRYPS grating or hologram) are tilted and spaced appropriately to diffract incident light  90  into a particular fundamental first diffracted order  94 . That diffracted order is then re-diffracted upon modulation by the modulator  88 , into a first sub-order  96 . The line representing the predominant direction of light propagation at each stage is the heaviest; directions of light propagation that carry less light energy are represented by thinner lines. Separating the fundamental diffractive function and the modulation function in this way has both advantages and disadvantages. An advantage is that the Bragg plane tilt, spacing, and thickness in each element can be optimized for the desired predominant direction and wavelength of propagating light at that element. A disadvantage is that the cost of a two-element beam steerer will usually be greater than that of a one-element beam steerer. 
     In the prior art, different Bragg holograms (physically superimposed or physically separated) can diffract light of different angular incidence and/or different wavelength independently into the same or different directions as illustrated in  FIG. 7  where the holograms  100 ,  102 , and  104  are superimposed within a single recording layer  106 . Note that each of the holograms  100 ,  102 , and  104  diffracts a particular wavelength beam  108 ,  110 , or  112  efficiently only when the beam impinges from a specific angle. 
     In  FIG. 8 , three Bragg holograms are recorded in a POLICRYPS or H-PDLC structure of a diffractive layer  116  designed to receive red, green, and blue light respectively at the same TIR angle. In this configuration, each Bragg hologram diffracts its respective color of light into the same direction normal to the surface of the cover glass. If a thin hologram such as a surface relief hologram were used instead, it would diffract all three colors into different directions because of their difference in wavelength. Because Bragg holograms can be made essentially invisible to any light except light of a particular wavelength propagating in a particular direction, the three superimposed Bragg holograms used in  FIG. 8  will each act only on their respective colors of light. 
       FIG. 8  is the application of an agile holographic optical phased array  120  such as that illustrated in  FIG. 1  to a near-eye display. In this embodiment, the LED line arrays  122   r ,  122   g , and  122   b  display one row of RGB video information at a time. Lens  124  and holographic optical element  126  collimate the beams from the LEDs vertically, and collimate each LED beam in an azimuthal direction corresponding to the position of the LED in its line array  122   r ,  122   g , or  122   b . The beams illustrated in  FIG. 8  represent the beams from the central LEDs in the red, green, and blue arrays. A default diffractive element  128  of the diffractive layer  116  is configured to diffract and focus exiting light to a line  130 , a few degrees below the viewer&#39;s eye at an angle of about 5 degrees below horizontal. A beam steerer  132  or other traveling lens is superimposed within the diffractive layer  116  to recollimate the exiting light vertically toward the pupil of a user&#39;s eye from a moving virtual window section  134  of the diffractive layer  116 . The virtual window  134  formed by TIR switches in the diffractive layer  116  and the beam steerer  132  is controlled to traverse a vertical distance of around one centimeter about 30 times per second at a uniform speed while the LED line arrays  122   r ,  122   g , and  122   b  display one row of RGB video frame information every 1/30,000th of a second, displaying 1,000 rows per frame, 30 frames per second. 
     Yet another embodiment of the invention, illustrated in  FIG. 9 , uses two beam steerers  142  and  144  such as those in  FIGS. 5 and 6  in series but rotated 90 degrees relative to each other, to scan a beam in two axes. 
     Another embodiment of the invention uses electrically conductive transparent vanes  152  separated by spaces  154  filled with liquid crystal as illustrated in  FIG. 10   a . In this case, the electrically conductive transparent vanes  152  are used as electrodes (connected to leads  156  and  158 ) to apply an electric field perpendicular to the vane surfaces. This has the advantage of providing increased switching or modulation speed. This configuration is particularly suitable for fast switching or modulation of ferroelectric liquid crystals. 
     Two approaches to fabricating the structures of  FIG. 10   a  are particularly useful. One approach is to use the POLICRYPS process, employing an oligomer that polymerizes to form an electrically conductive polymer. H-PDLC and POLICRYPS holograms are fabricated by exposing a layer of mixed photopolymerizable oligomer and liquid crystal to light in a stable interference pattern. During photopolymerization, the monomer diffuses toward regions in the layer being irradiated, because monomer is being removed from solution and bound into the polymer matrix there. The important difference between POLICRYPS and H-PDLC is that the POLICRYPS process is modified to ensure nearly complete removal of unreacted monomer from the regions between polymerized “vanes”  152 . This leaves only liquid crystal in the spaces  154  separating the vanes  152 . 
     If the polymer in the vanes  152  is electrically conductive and if there are no parasitical low-resistivity electrical connections between vanes  152 , the vanes  152  can serve as individually addressable electrodes. A layout like that illustrated in  FIG. 10   b , in which surface stripe electrodes  156  and  158  are directly at the edges of the vanes  152 , can be used to electrically address the individual electrodes vanes  152  via capacitive coupling or direct electrical contact. 
     An alternative approach is lithographic. For example, well-known electron beam or XUV lithography techniques may be used to create high aspect ratio submicron grating structures in dielectric, metallic, or electrically conductive transparent materials. These gratings may then be filled with liquid crystals or other electro-optically active materials to form the electrically controllable diffractive structures of  FIG. 10   b  and the other embodiments of this invention. 
     Alternative embodiments of the invention include applications of agile holographic optical phased arrays as lenses, beam steerers, display engines, and LIDAR antennae. In each case, one or more selected sidebands can be focused, steered, distorted or otherwise operated on by spatially and temporally modulating the phase or amplitude of one or more fundamental diffracted orders from a diffractive device. 
       FIG. 11  illustrates the azimuthal ray bundles that would be produced in the embodiment of  FIG. 8  from the LEDs at the two ends of the red LED array  122   r.    
     The number of lines per frame and the number of frames per second of course determines the vertical speed of the virtual window  134 . Moreover, the display can be rotated 90 degrees so that the LED line array  122   r  displays columns of image information while a beam scanner  140  determines the column to be displayed at each instant, as illustrated in  FIG. 12 . 
     It is important to note that a Bragg hologram with extreme diffraction angles acts as a polarizer, so applications that employ down-stream optics should be designed with polarization effects in mind. 
     The array of electrodes can be a 2-D array, addressed by any suitable means such as photo-controlled. 
     “Transparent” refers to transparency with respect to the particular range of electromagnetic radiation wavelengths being manipulated by the agile holographic phased array. For example, if the agile holographic phased array is being used to manipulate visible light, “transparent” means transparent to visible light. If the agile holographic phased array is being used to manipulate infrared or terahertz radiation, “transparent” means transparent to infrared or terahertz radiation, respectively.