Method of producing switchable holograms

[0002] The present application claims the benefit of U.S. Provisional Application Ser. No. 60/156,774 filed Sep. 29, 1999.

[0003] The present invention relates generally to holographic devices, and more particularly, to a switchable hologram.

[0004] Switchable holograms are often formed from polymer-dispersed liquid crystal (PDLC) material which have holographic fringes recorded therein. During the recording process, the PDLC material undergoes a phase separation thereby creating fringes, which include regions densely populated by liquid crystal micro-droplets interspersed with regions of clear polymer. The liquid crystal material is interposed between electrodes so that an electrical field can be applied to the hologram. When an electrical field is applied to the hologram, the natural orientation of the liquid crystal droplets is changed, causing the refractive index modulation of the fringes to reduce and the hologram diffraction efficiency to drop to a very low level, thereby effectively erasing the hologram.

[0005] Volume holograms (also known as thick or Bragg holograms) provide high diffraction efficiencies for incident light beams having wavelengths close to the theoretical wavelength satisfying the Bragg diffraction condition, and which are within a few degrees of the theoretical angle that also satisfies the Bragg diffraction condition. Experimental evidence shows that the magnitude of the electric field required to switch these holograms between their active (diffracting) state and passive (non-diffracting) state is dependent upon the slant angle of the holographic fringes, which is defined as the angle between the Bragg surface of the fringes and a normal to the hologram surface.

[0006] In applications in which a hologram is required to operate over a wide range of incidence angles, such as with an eyepiece in an off-axis wearable display device, there is likely to be significant variation in the slant angle over the area of the hologram. When an electric field of a predetermined magnitude is applied to the hologram, there may be areas in which the slant angle prevents the hologram from switching (i.e., the switching threshold due to the slant angle is higher than the applied field). Under these circumstances, the hologram will only be partially erased.

[0007] The magnitude of the electric field may be increased to overcome this problem. However, there may be other areas within the hologram which are then exposed to excessively high voltages (i.e., areas where the switching threshold due to the slant angle is significantly below the applied voltage). This may cause electrical breakdown, delamination (i.e., separation of the hologram from the substrate), or other effects that result in irreversible damage to the hologram.

[0008] A method of producing a switchable hologram generally comprises providing a substrate composed of a liquid crystal material and having two generally opposed surfaces and recording a plurality of holographic fringes composed of liquid crystal droplets in the substrate with one or more of the plurality of holographic fringes positioned such that a surface of the fringes is angled relative to one of the substrate surfaces. The substrate is subjected to an electrical field during recording of the holographic fringes. The electrical field varies across the surface of the substrate so that the liquid crystal droplets are all oriented generally in a common direction.

[0009] Recording of the holographic fringes may further include interposing the hologram between a pair of electrodes each positioned generally adjacent to one of the substrates surfaces. One of the pair of electrodes has a plurality of independently controlled segments. Each of the segments is positioned over an area of the substrate having holographic fringes positioned within a common range of angles relative to the substrate surface. A pair of glass carriers may be interposed between the electrodes and the substrate surfaces.

[0010] The above is a brief description of some deficiencies in the prior art and advantages of the present invention. Other features, advantages, and embodiments of the invention will be apparent to those skilled in the art from the following description, drawings, and claims.

[0011] FIG. 1 is a schematic of an image viewing apparatus having a switchable hologram of the present invention.

[0012] FIG. 2 is a perspective of the switchable hologram FIG. 1 and light source.

[0013] FIG. 3 is a partial front view of the hologram FIG. 2 illustrating an electrode and electric circuit.

[0014] FIG. 4 is a schematic illustrating slant angles of holographic fringes of the switchable hologram of FIG. 2.

[0015] FIG. 5 is a schematic illustrating an exemplary disposition of liquid crystal droplets within the switchable hologram of FIG. 2.

[0016] FIG. 6 is a front view of the switchable hologram of FIG. 1 and electrodes used to apply an electrical field to the hologram during production.

[0017] FIG. 7 is a side view of the switchable hologram of FIG. 6.

[0018] FIG. 8 is a side view of the switchable hologram FIG. 6 illustrating an alternative method of production.

[0019] Corresponding reference characters indicate corresponding parts throughout the several views of the drawings.

[0020] The following description is presented to enable one of ordinary skill in the art to make and use the invention. Descriptions of specific embodiments and applications are provided only as examples and various modifications will be readily apparent to those skilled in the art. The general principles described herein may be applied to other embodiments and applications without departing from the scope of the invention. Thus, the present invention is not to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features described herein. For purpose of clarity, details relating to technical material that is known in the technical fields related to the invention have not been described in detail.

[0021] Referring now to the drawings, and first to FIG. 1, a display system, generally indicated at 10, is shown. The system 10 includes a display device 12, a holographic device generally indicated at 14, and a controller 16. The display device 12 is operable to display an image to be viewed by an observer (eye of the observer shown at 20 in FIG. 1). The holographic device 14 includes a Bragg reflection hologram (recorded on a substrate) 22 interposed between electrodes 24, 26 coupled to the controller 16. The Bragg hologram 22 provides high diffraction efficiencies for incident beams with wavelengths close to the theoretical wavelength satisfying the Bragg diffraction condition and within a few degrees of the theoretical angle which also satisfies the Bragg diffraction condition. By varying the voltage applied across the electrodes 24, 26, the hologram 22 can be switched between an active state and a passive state, as described below. It is to be understood that although the invention is described herein in the context of a display system, the present invention may apply to any switchable holographic element in which there is a variation of slant angle across the optical aperture of the hologram. The optical elements may be used in nonimaging applications such as sensors and beam direction switching devices for optical telecommunications applications, for example.

[0022] The system 10 shown in FIG. 1 is a simplified schematic of a display system. The system 10 may include components such as lenses and mirrors or other optical components as is well known by those skilled in the art. Additional optical elements may also be provided to correct for aberrations, if required. For example, the lenses may include cylinders, prisms, and off-axis aspheric elements to correct for aberrations due to the off-axis, non-symmetric nature of the display system.

[0023] The display system 10 may be configured for use as a head mounted display. The head mounted display includes a headpiece (not shown) designed to be worn by a viewer and the display system 10 for producing wide-angle, electronically generated virtual images to one or both eyes of the viewer. The display system 10 may also be used in a narrow field of view displays. The headpiece may include a frame configured to fit over a viewer's head and a mask which fits over the viewer's eye region, as is well known by those skilled in the art. The display system may comprise left and right optical systems or only one optical system. For example, two optical systems may be used as described in U.S. patent application Ser. No. 09/405,444, filed Sep. 23, 1999, by A. Preston, now U.S. Pat. No. 6,094,283 to provide a variable aspect ratio system. The display system may also include a single image generator and projection system operable to project a left image to a left eye of a viewer and a right image to a right eye of the viewer as described in U.S. patent application Ser. No. 09/439,063, filed Nov. 12, 1999, by M. Popovich.

[0024] The display device 12 produces an image under control of an image generator (not shown) which may provide, for example, video or graphic information. The display device 12 may comprise a flat panel display (e.g., a reflective liquid crystal display (LCD) panel, or other spatial light modulator (SLM) which reflects light produced externally). The image display panel may be a miniature reflective LCD having either a nematic or ferroelectric material on a silicon backplane, for example. The reflective display panel utilizes an external light source to reflect and modulate light off the front of the microdisplay. The display panel may also be based on transmissive display technologies. The display device 14 may be, for example, a miniature reflective silicon backplane device, such as a SVGA (800×600 pixels) device available from Colorado MicroDisplay, of Boulder, Colo. Other devices including a micro-electromechanical system, such as a Digital Light Processor (DLP) using a Digital Micromirror Device™ (DMD) available from Texas Instruments, or a diffractive display device such as a Grating Light Valve™ (GLV) available from Silicon Light Machines (formerly Echelle, Inc.), may also be used as the display device 12. It is to be understood that the display devices listed above are provided only as examples and that other types of display devices may be used without departing from the scope of the invention.

[0025] As previously described and shown in FIGS. 1 and 2, the holographic device 14 includes hologram 22 interposed between a pair of electrodes 24, 26. The hologram 22 is used to control transmitted light beams based on the principles of diffraction. The hologram selectively directs an incoming light beam from light source 30 either towards or away from a viewer and selectively diffracts light at certain wavelengths into different modes in response to a voltage applied to the electrodes 24, 26 (FIGS. 2 and 3). Light passing through the hologram in the same direction that the light is received from the light source 30 is referred to as the zeroth (0th) order mode 38. When no voltage is applied to the electrodes 24, 26, liquid crystal droplets within the holographic device 14 are oriented such that the hologram is present in the element and light is diffracted from the zeroth order mode to a first (1st) order mode 40 of the hologram. When a voltage is applied to the holographic optical element 14 the liquid crystal droplets become realigned effectively erasing the hologram, and the incoming light passes through the holographic device in the zeroth order mode 38.

[0026] It is to be understood that the holographic device 14 may also be reflective rather than transmissive. In the case of a reflective holographic optical element, the arrangement of the holographic device and optical components would be modified to utilize reflective properties of the hologram rather than the transmissive properties described herein.

[0027] The light that passes through the hologram 22 is diffracted to form an image by interference fringes 42 recorded in the hologram (FIGS. 1 and 2). Depending on the recording, the hologram 22 is able to perform various optical functions which are associated with traditional optical elements, such as lenses and prisms, as well as more sophisticated optical operations which would normally require very complex systems of conventional components. The hologram 22 may be configured to perform operations such as deflection, focusing, filtering or aberration correction.

[0028] The holograms 22 are preferably recorded on a photopolymer/liquid crystal composite material (emulsion) such as a holographic photopolymeric film which has been combined with liquid crystal, for example. The presence of the liquid crystal allows the hologram 22 to exhibit optical characteristics which are dependent on an applied electrical field. The photopolymeric film may be composed of a polymerizable monomer having dipentaerythritol hydroxypentacrylate, as described in PCT Publication, Application Serial No. PCT/US97/12577, by Sutherland et al., which is incorporated herein by reference. The liquid crystal may be suffused into the pores of the photopolymeric film and may include a surfactant.

[0029] The diffractive properties of the holographic device 14 depend primarily on the recorded holographic fringes 42 in the photopolymeric film. The holographic fringes 42 may perform a variety of diffractive optical functions such as focusing the image towards the observer's eye. The interference fringes 42 may be created by applying beams of light to the photopolymeric film. Alternatively, the interference fringes 42 may be artificially created by using highly accurate laser writing devices or other replication techniques, as is well known by those skilled in the art. The holographic fringes 42 may be recorded in the photopolymeric film either prior to or after the photopolymeric film is combined with the liquid crystal. In the preferred embodiment, the photopolymeric material is combined with the liquid crystal prior to the recording. In this preferred embodiment, the liquid crystal and the polymer material are pre-mixed and the phase separation takes place during the recording of the hologram, such that the holographic fringes 42 become populated with a high concentration of liquid crystal droplets. This process can be regarded as a “dry” process, which is advantageous in terms of mass production of the switchable holographic device 14.

[0030] The electrodes (electrode layers) 24, 26 are positioned on opposite sides of the substrate 22 and are preferably transparent so that they do not interfere with light passing through the hologram. The electrodes 24, 26 may be formed from a vapor deposition of Indium Tin Oxide (ITO) which typically has a transmission efficiency of greater than 80%, or any other suitable substantially transparent conducting material. The transmission efficiency of the electrodes 24, 26 can be improved to greater than 97% by applying multilayer anti-reflection coatings. The electrodes 24, 26 are connected to an electric circuit 25 operable to apply a voltage to the electrodes, to generate an electric field (FIG. 3). Initially, with no voltage applied to the electrodes 24, 26, the hologram 22 is in its diffractive (active) state and the holographic device 14 diffracts propagating light in a predefined manner. When an electrical field is generated in the hologram 22 by applying a voltage to the electrodes 24, 26 of the holographic device 14, the operating state of the hologram switches from its diffractive state to its passive (inactive) state and the holographic optical element does not optically alter the propagating light. The electrodes 25 are preferably configured to apply a generally constant electrical field across a surface of the substrate.

[0031] The holographic device 14 may also be formed on curved substrates, as described in U.S. patent application Ser. No. 09/416,076, by M. Popovich, filed Oct. 12, 1999.

[0032] FIG. 4 is an enlarged side view of the holographic device 14 of FIG. 1 showing additional detail of the holographic fringes 42 recorded in the hologram 22. Point O₁represents an image displayed on the display device 12 and point O₂represents a conjugate point at which an image of the object is formed by the hologram 22 (FIGS. 1 and 4). Because the points O₁and O ₂are located off-axis, light rays passing from the object to the conjugate point are incident upon the hologram 22 at a wide variety of angles. The inclination of the fringes 42 at any given point in the hologram 22 is uniquely determined by the beam geometry at that point, as is well known by those skilled in the art of optics. The inclination will therefore vary based upon the light incidence angle. There is generally a progressive change in the inclination of the fringes 42 from the bottom to the top of the hologram 22, as viewed in FIG. 4.

[0033] A slant angle μ is defined as the angle between a vector N which is normal to a Bragg surface 54 of the fringe and a vector E which is normal to the surface 56 of the hologram (or substrate) 22. The vector E generally corresponds to a vector of the electric field applied between the electrodes. As shown in FIG. 4, a light ray 60 emitted from the object point O₁is incident upon a lower portion of the hologram at an angle α₁and is diffracted at an angle β₁by fringes 42ahaving a relatively small slant angle μ₁. In contrast, a light ray 62 emitted from the point O₁is incident upon an upper portion of the hologram 22 at an angle α₂and is diffracted at an angle β₂by fringes 42bhaving a larger slant angle μ₂. As shown in FIG. 4, angles, α, β, and μ are all measured relative to the vector E normal to the surface 56 of the hologram 22. It should be noted that the slant angles μ₁, μ₂shown in FIG. 4 are exaggerated for ease of illustration and that the effects of refraction between the holographic medium 22 and the surrounding air have not been taken into account.

[0034] The invention has been described above with respect to a reflection hologram. The Bragg surfaces of reflection holograms tend to be generally parallel with the hologram surface (i.e., an unslanted fringe will have a slant angle of 0°). The above described invention may also be applied to transmission holograms, in which case the Bragg surfaces 54 tend to be generally perpendicular to the hologram (i.e., slant angle of unslanted fringe will be 90°).

[0035] The magnitude of the threshold voltage required to switch a hologram 22 between its active and passive states becomes larger as the thickness of the hologram (substrate) increases, and the optimum switching voltage is very sensitive to small fluctuations in the hologram thickness. Experimental evidence shows that the threshold voltage varies with the slant angle μ. As a consequence, a voltage that is sufficient to switch the fringes 42amay be below the threshold level for the fringes 42b, resulting in only partial erasure of the hologram. On the other hand, a voltage that is sufficient to switch the fringes 42bmay be sufficiently above the threshold for the fringes 42asuch that it causes damage to the hologram 22.

[0036] One possible explanation for this effect is illustrated in the schematic of FIG. 5. The liquid crystal droplets 70 which form the hologram fringes 42 are asymmetric in shape and tend to have their long axes (major axes) aligned parallel to the grating vector (indicated at 74 in FIG. 5) of the fringes. In order to erase the fringes 42, it is necessary to deform the liquid crystal molecules contained in the droplets 70 such that the average index of the droplets matches that of the surrounding holographic medium 22. The molecules are typically aligned parallel to the long axis of the droplet in the case of a transmission hologram. The response of a given droplet 70 to the electrical field applied to the electrodes appears to be a function of the angle of the liquid molecules contained in the droplet relative to the field vector E. Consequently, the switching voltage required to deform the droplet 70 can also be expected to vary with this angle, and therefore, also with the slant angle μ of the fringes 42.

[0037] The precise characteristics of the above-described effect depends upon whether the hologram 22 is reflective or transmissive and in each case on whether it is based on positive or negative anisotropy materials. In the case of a reflection hologram (such as shown in FIG. 4), the Bragg surfaces 54 are generally perpendicular to the intended direction of electric field application. The PDLC material may have negative dielectric anisotropy, such that the liquid crystal molecules contained in the droplets 70 tend to rotate to align in a direction perpendicular to the field direction when an electric field is applied. The molecules in the droplets 70 thus switch from a state in which the molecules are aligned substantially normal to the Bragg surfaces to a state in which they are perpendicular to the applied field in order to effect switching of the hologram 22. In the case of a transmission hologram, the PDLC material has positive anisotropy such that the liquid crystal molecules, which are typically aligned parallel to the long axis of the liquid crystal droplets 70 tend to the become aligned parallel to the field direction when an electric field is applied. In the case of a reflection hologram it is also possible to have positive anisotropy material which provides a random alignment of liquid crystal molecules when no field is applied and alignment in the direction of the field when an electric field is applied. In the case of a transmission hologram, it is preferable to have positive anisotropy materials, however in some cases it may be possible to create distinct on and off states using negative anisotropy materials.

[0038] Another possible explanation (although less likely) is that the slant angle μ of the fringes 42 may give rise to small variations in thickness of the hologram 22. In particular, the concentration of liquid crystal droplets 70 (i.e., the number of droplets per unit length in the direction of the applied electric field) will change with the slant angle μ. This is likely to alter the compressibility of the material in a direction perpendicular to the hologram surface 56, and may also affect shrinkage, which tends to take place in the same direction.

[0039] It is to be understood that there may be other explanations for the above-described effect, however, regardless of the explanation, the present invention may be used to overcome this effect, as described below.

[0040] In order to order to obviate problems associated with this effect, the hologram is produced such that the liquid crystal droplets are oriented in substantially uniform alignment over the entire area of the substrate, independent of the Bragg fringe geometry. The following describes exemplary methods for producing the hologram.

[0041] FIG. 6 is a front view of the hologram and FIG. 7 is a side view of the hologram during production by a first method. In order to record the hologram on the substrate 80, the substrate is subjected to an electric field applied between a pair of electrodes 84, 86 disposed on opposed surfaces of the substrate. The electric field is configured to vary in magnitude across the area of the substrate, based on the slant angle of the hologram fringes 42. The electric field exerts a small perturbation on the liquid crystal droplets 70, thereby reorienting the droplets so that they lie with their major axis in a common direction normal to the direction of the electric field E (FIGS. 4 and 5). The liquid crystal droplets 70 are thus given a substantially uniform alignment with respect to the field direction over the entire area of the substrate 80, independent of the Bragg fringe geometry. This provides a more uniform and efficient switching of the hologram 22 than with conventional holograms and avoids the problem of partial erasure described above.

[0042] The variation in the applied electric field is achieved by arranging from one of the electrodes to be composed of a plurality of segments 48 and by applying a different voltage to each segment by a voltage supply 96 and voltage controllers 50 (FIG. 6). The voltage control may be provided by electrical resistance, for example. Each of the segments 48 extends over a region of the substrate in which the slant angle of the fringes 42 lies within a common predetermined range.

[0043] The range can be determined experimentally by making measurements of switching voltage against slant angle for a set of test holograms. Alternatively, it may be possible to develop a mathematical model which relates switching voltage to the molecular deformation characteristics of liquid crystal droplets. The model would take into account factors such as the shape of the droplet, the refractive index, the concentration of the droplets, the polymer material, additional materials such as surfactants and photoinitiators, etc. [See, for example, the theory published by W U [see B-G Wu, J H Erdmann and J W Doan “Response times and voltages for PDLC light shutters” Liq. Cryst. 5, pp 1453-1465 (1989)] which may provide a basis.]

[0044] Each of the segments 48 is connected to the individual segment controllers 50 so that the voltage applied to the electrode segments varies from one segment to the next. The actual voltage applied to each segment 48 can be determined experimentally or by means of a mathematical model which relates switching voltage to the slant angle μ while taking into account all relevant material characteristics. The equations of the Bragg surface are determined by calculating the interference pattern formed by the interfering waveforms (corresponding to the image and object conjugates). The latter can be calculated from a knowledge of the optical specification of the holographic optical elements. The shapes of the wavefronts could be determined by means of optical design software such as CodeV (available from Optical Research Associates, California) or ZEMAX (available from Focus Software Inc. Tucson, Ariz.). Having determined the Bragg surface equations, it is possible to calculate iso-slant angle contours over the aperture of the hologram. The shapes of the electrodes would match the contours with the number of electrodes actually required being a compromise between complexity of the electrode structure and the number of contour sub-ranges required to give suitable “average” switching voltages within each sub range. The variation in threshold voltage may vary by 10-20%, for example. The voltage may range, for example, from 50-100V depending on thickness of the substrate 80 and orientation of fringes 42. The variation in voltage difference may be achieved by the use of electrical resistances, as is well known by those skilled in the art.

[0045] The required shapes of the electrode segments 48 may be determined based on the desired phase function of the hologram. It is to be understood that the number of segments 48 and shape of the segments may be different than shown herein without departing from the scope of the invention. For example, the segments 48 may be formed as concentric deformed circles or may be curved to match the contour of a lens. The number of segments 48 may vary, for example, from five to ten segments, or may be fewer or greater in number depending on the size of the hologram 80 or variation in slant angle μ of the fringes 42.

[0046] The substrate 80 shown in FIG. 7 has a positive dielectric anisotropy and is intended to form a transmission hologram. In the absence of an electric field, the recordal process tends to align the major axes 74 of the liquid crystal droplets 70 along the direction indicated by arrows 90 (i.e., corresponding to the Bragg surface normal vectors). The electric field E re-orients the major axes 74 of the droplets 70 into the direction indicated by arrows 92 (i.e., normal to the direction of the electric field), such that the droplets across the entire area of the substrate 80 all have substantially the same orientation.

[0047] During operation of the hologram 22, the fringes 42 are activated and deactivated by application of the same electrical voltage across the electrodes 24, 26.

[0048] A substrate used to form a reflection hologram may typically have a negative dielectric anisotropy and the major axes of the liquid crystal droplets 70 are oriented parallel to the direction of the electric field rather than normal thereto as with a transmission hologram. In other respects, the production of the hologram will be the same as described above for the transmission hologram.

[0049] FIG. 8 illustrates an alternative method for producing the hologram 22. The segmented electrode 84 is used only during the recording process, and not for actual switching of the finished hologram. During the recordal process, the substrate 80 is interposed between a pair of thin glass carriers 100, each of which has an electrode 84, 86 deposited on a surface thereof. The electrode 84 is segmented, as described for the previous process. After the recordal process, the segmented electrode 84 is replaced by an unsegmented electrode 26 which is used to apply a generally constant electric field over the surface of the hologram 80 to switch the hologram between its active and passive states.

[0050] Although the present invention has been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations made to the embodiments without departing from the scope of the present invention. Accordingly, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.