CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the priority of the US Provisional Patent Application No. 61/344,059 entitled BEAM STEERING DEVICE filed on 17 May 2010 which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates to an electro optical beam deflection device, and more particularly to a beam deflection device based on electrically switchable holographic gratings.

Applications such as despecklers, 3D displays require compact and efficient means for scanning laser beams. There is much prior art in the field of laser beam scanning. Mechanical approaches based on microelectromechanical systems and moving lenslet arrays are often difficult to integrate into compact optical system. Electrooptical solution based on birefringent liquid crystals have been explored but suffer from low efficiency and limited sweep angles.

There is a requirement for an improved compact electro optical beam deflection device. There is a further requirement for a compact electro optical beam deflection device for providing alternate left and right eye perspective image light from a spatial light modulator (SLM) to provide an autostereoscopic display. There is a further requirement for a compact electro optical beam deflection device that can switch between a first state in which it provides alternate left and right eye perspective image light from a SLM and a second state in which it provides diffused image light for viewing a two dimensional image on said SLM. There is a yet further requirement for a compact electro optical beam deflection device that can provide an angular diversity laser beam despeckler.

SUMMARY OF THE INVENTION

It is a first object of the invention to provide a compact electro optical beam deflection device. It is a further object of the invention to provide a compact elector optical beam deflection device for providing alternate left and right eye perspective image light from a spatial light modulator to provide an autostereoscopic display. It is a further object of the invention to provide a compact electro optical beam deflection device that can switch between a first state in which it provides alternate left and right eye perspective image light from a SLM and a second state in which it provides diffused image light for viewing a two dimensional image on said SLM. It is a yet further object of the invention to a compact elector optical beam deflection device that can provide an angular diversity laser beam despeckler. In one embodiment of the invention a beam deflection device comprises: a first transparent optical substrate with an input surface and an output surface; a second transparent optical substrate with an input surface and an output surface; transparent electrodes applied to the output surface of the first substrate and the input surface of the second substrate; an

subwavelength grating layer recorded in a Holographic Polymer Dispersed Liquid Crystal (HPDLC) material having a planar surface and a second surface shaped to provide an array of prisms; and a polymer layer having a planar surface and a second surface shaped to provide an array of prismatic cavities. The prisms and prismatic cavities have identical geometries, each prism abutting a prismatic cavity. The planar surface of the subwavelength grating layer abuts the output surface of first substrate while the planar surface of the polymer layer abuts the input surface of the second substrate, The transparent electrodes are electrically coupled to a variable voltage generating means.

In one embodiment of the invention at least one of said transparent electrodes is patterned into independently switchable electrode elements having substantially the same cross sectional area as the prisms such that the subwavelength grating prisms may be selectively switched in discrete steps from a fully diffracting to a non diffracting state by an electric field applied across the transparent electrodes. In one embodiment of the invention the refractive index differential between the subwavelength grating layer and the polymer layer is positive when a first voltage is applied and negative when a second voltage is applied and zero when a third voltage is applied. The application of the first, second or third voltages causes light entering the beam deflection device normal to the input surface of the first transparent substrate to emerge from the output surface of the second substrate in first, second or third directions respectively. The first and second directions are at opposing angles to the normal to the output surface and the third direction is parallel to the normal to the output surface.

In one embodiment of the invention one face of each prism is at ninety degrees to said planar surface of the subwavelength grating layer.

In one embodiment of the invention the prisms are provided by a linear array of elements of triangular cross section. In one embodiment of the invention the input surface of the first substrate is optically coupled to a spatial light modulator (SLM), each prism in the subwavelength grating layer substantially overlapping at least one column of pixels of the SLM. In one embodiment of the invention the beam deflection device provides an

autostereoscopic display means for directing alternate right and left eye perspective image light from a spatial light modulator to the right and left eyes of a viewer.

In one embodiment of the invention the beam deflection device provides a means for switching between a first viewing mode in which the beam deflection device provides an autostereoscopic display means for directing alternate right and left eye perspective image light from a spatial light modulator to the right and left eyes of a viewer and a second mode in which the beam deflection device transmits a two dimensional image displayed on the spatial modulator.

In one embodiment of the invention the beam deflection device further comprises a means for expanding the angular deflection provided by the subwavelength grating and polymer layers. In one embodiment of the invention randomly varying voltages are applied across each subwavelength grating prism.

A more complete understanding of the invention can be obtained by considering the following detailed description in conjunction with the accompanying drawings wherein like index numerals indicate like parts. For purposes of clarity details relating to technical material that is known in the technical fields related to the invention have not been described in detail.

BRIEF DESCRIPTION OF THE DRAWINGS FIG.1 is a schematic side elevation view of one embodiment of the invention.

FIG.2 is a schematic side elevation view of a first aspect of one embodiment of the invention. FIG.3 is a schematic side elevation view of a second aspect of one embodiment of the invention. FIG.4 is a schematic side elevation view of a third aspect of one embodiment of the invention. FIG.5A is a schematic plan view of the viewing geometry of an autostereoscopic display provided in one embodiment of the invention.

FIG.5B is a schematic plan view of the viewing geometry of a two dimensional display provided in one embodiment of the invention.

FIG.6 is a schematic side elevation view of a beam angle expansion means used in one embodiment of the invention.

FIG.7 is a schematic side elevation view of a beam angle expansion means used in one embodiment of the invention.

FIG.8A is a chart illustrating the sequential display of left and right eye images on a SLM FIG.8B is a chart illustrating a voltage waveform use in an autostereoscopic display according to the principles of the invention.

FIG.9 is a chart illustrating a random voltage waveform used in one embodiment of the invention. DETAILED DESCRIPTION OF THE INVENTION

It is a first object of the invention to provide a compact electro optical beam deflection device. It is a further object of the invention to provide a compact elector optical beam deflection device for providing alternate left and right eye perspective image light from a spatial light modulator to provide an autostereoscopic displays. It is a further object of the invention to a compact elector optical beam deflection device that can switch between a first state in which it provides alternate left and right eye perspective image light from a SLM and a second state in which it provides diffused image light for viewing a two dimensional image on said SLM. It is a yet further object of the invention to a compact elector optical beam deflection device that can provide an angular diversity laser beam despeckler.

It will be apparent to those skilled in the art that the present invention may be practiced with only some or all aspects of the present invention as disclosed in the following description. For the purposes of explaining the invention well-known features of laser technology and laser displays have been omitted or simplified in order not to obscure the basic principles of the invention.

Parts of the following description will be presented using terminology commonly employed by those skilled in the art of optics and laser displays in particular.

In the following description the terms light, ray, beam and direction will used

interchangeably and in association with each other to indicate the propagation of light energy along rectilinear trajectories. Unless otherwise stated the term optical axis in relation to a ray or beam direction refers to propagation parallel to an axis normal to the surfaces of the optical components described in relation to the embodiments of the invention. It should also be noted that in the following description of the invention repeated usage of the phrase "in one embodiment" does not necessarily refer to the same embodiment.

In one embodiment of the invention illustrated in the schematic side elevation view of FIG. l a beam deflection device 100 comprises: a first transparent optical substrate 101 with an input surface and an output surface; a second transparent optical substrate 102 with an input surface and an output surface; transparent electrodes 122,123 applied to the output surface of the first substrate and the input surface of the second substrate; an subwavelength grating recorded in a HPDLC material forming a layer 103 having a planar surface and a second surface shaped to provide an array of prisms such as the one indicated by 103 A; and a polymer layer 104 having a planar surface and a second surface shaped to provide an array of prisms such as the one indicated by 104 A. The prismatic cavities of the polymer layer are filled by the subwavelength grating prisms. The prisms and prismatic cavities have identical geometries, each prism abutting a prismatic cavity. The planar surface of the subwavelength grating layer abuts the output surface of first substrate while the planar surface of the polymer layer abuts the input surface of the second substrate. The transparent electrodes are electrically coupled to a variable voltage generating means 120 via an electrical switching network generally indicated by 121. The refractive indices of the subwavelength grating layer, polymer layer and the glass comprising the first and second substrates are indicated by symbols ¾ , n _p and n _g . In one embodiment of the invention at least one of said transparent electrodes is patterned into independently switchable electrode elements such as the one indicated by 122. The second electrode 123 may be continuous over the substrate area or may also be patterned in an identical fashion to the first electrode. The electrode elements have substantially the same cross sectional area as the prisms such that the subwavelength grating prisms may be selectively switched in discrete steps from a from a minimum average refractive index to a maximum index. In other embodiments of the invention the voltage across the subwavelength gratings may be varied continuously. In one embodiment of the invention both of the transparent electrodes are continuous such that electric fields may be applied across each subwavelength grating prism simultaneously.

A subwavelength grating as used in the present invention is similar in concept to a Switchable Bragg Grating (SBG). An SBG is formed by recording a volume phase grating, or hologram, in a polymer dispersed liquid crystal (PDLC) mixture. Typically, SBG devices are fabricated by first placing a thin film of a mixture of photopolymerizable monomers and liquid crystal material between parallel glass plates which may be sealed to form a cell. Techniques for making and filling glass cells are well known in the liquid crystal display industry. One or both glass plates support electrodes, typically transparent indium tin oxide films, for applying an electric field across the PDLC layer. A volume phase grating is then recorded by illuminating the liquid material with two mutually coherent laser beams, which interfere to form the desired grating structure. During the recording process, the monomers polymerize and the HPDLC mixture undergoes a phase separation, creating regions densely populated by liquid crystal micro-droplets, interspersed with regions of clear polymer. The alternating liquid crystal-rich and liquid crystal-depleted regions form the fringe planes of the grating. The resulting volume phase grating can exhibit very high diffraction efficiency, which may be controlled by the magnitude of the electric field applied across the PDLC layer. When an electric field is applied to the hologram via transparent electrodes, the natural orientation of the LC droplets is changed causing the refractive index modulation of the fringes to reduce and the hologram diffraction efficiency to drop to very low levels. Note that the diffraction efficiency of the device can be adjusted, by means of the applied voltage, over a continuous range from near 100% efficiency with no voltage applied to essentially zero efficiency with a sufficiently high voltage applied. U.S. Patent 5,942,157 and U.S. Patent 5,751 ,452 describe monomer and liquid crystal material combinations suitable for fabricating SBG devices. Typically, the SBG element is configured with its cell walls perpendicular to an optical axis. An SBG element diffracts incident off-axis light in a direction substantially parallel to the optical axis when in said active state. However, each SBG element is substantially transparent to said light when in said inactive state. An SBG element can be designed to diffract at least one wavelength of red, green or blue light. In the case where the fringes have very low periodicity the SBG gratings becomes a subwavelength grating. In subwavelength gratings the only present diffraction order is the m=0 order. For the purposes of understanding the present invention the grating may bee considered to function as a variable refractive index medium.

In one embodiment of the invention one face of each subwavelength grating prism is at ninety degrees to said planar surface of the subwavelength grating layer.

In one embodiment of the invention the subwavelength grating prisms are configured as a linear array of identical elements of triangular cross section. The ray geometry is illustrated in more detail in the schematic side elevation view of FIG.2 which illustrates ray propagation through one prism of the embodiment of FIG.1. The refractive indices of the subwavelength grating layer, polymer layer and the glass comprising the first and second substrates are again indicated by symbols ¾ , n _p and n _g . The incident ray 520 which is normal to the first substrate passes through the first substrate emerging as ray 521 and strikes the interface between the subwavelength grating prism and the polymer prism at an angle a] to the normal to the interface. From Snell's law the ray is refracted into the polymer prism as the ray 522 at the angle a ₂ which is given by a ₂ = arcsin ((¾/¾) sin ( j)), which is approximately equal to (n _h /n _g )ai. The angle ( i is given by ai = arctan (h/ D), where D is the length of the prism (or period) and h is its height. The ray 521 is next refracted into the glass substrate as the ray 523 . The incidence angle a ₃ at the polymer glass interface is given by a ₂ - ai. Applying Snell's law at the interface the refracted angle a ₄ is given by the equation

Finally, we apply Snell's law once again at the glass to air interface to obtain the output angle a ₅ for the emerging ray 524 which is given by a ₅ =arcsin(n _g sin(a ₄ )) . In FIG.2 the average refractive index of the subwavelength grating layer is greater than that of the polymer layer.

Turning now to FIG.3 we consider the case where the average subwavelength grating index is lower than the polymer index. In this case the refracted ray bends closer to the normal to the subwavelength grating -polymer interface. Hence the angle a ₂ is now smaller than the angle ai with the result that the angle a ₃ is now negative. It will be clear from consideration of FIGS.2- 3 that if in each case the modulus of the difference between the subwavelength grating and polymer index is maintained the refracted angles a ₄ and a ₅ have the same magnitudes as in FIG.2 however they are now in opposing directions. The angular symmetry is achieved by choosing a polymer index which is exactly at the center of the H-PDLC average refractive index swing, which typically extends froml .55 to 1.65. In this case the polymer index should be 1.60. There is no need for a redirection prism to equalise the angles. It should be clear from consideration of FIGS.2-3 that that the resultant angular sweep is twice the prism deflection angle ie it is equal to 2a ₅ . For completeness FIG.4 illustrates the case where the subwavelength grating index is identical to the polymer index, incident light 520 propagating through the prisms without deviation emerging from the second substrate as the light 527.

It should be apparent from consideration of FIGS.2-4 that the discussion of the ray propagation would apply to a prism of any geometry including elongated prisms or pyramidal prisms.

It should also be apparent that although the description refers to the use of a polymer in fixed refractive index prism layer the latter could be fabricated from any other refractive optical material including glasses and plastics commonly used in optical components.

In one embodiment of the invention there is provided an autostereoscopic display in which prismatic subwavelength grating devices according to the principles of the invention switch light from a spatial light modulator (SLM) into left/right viewing perspectives. Typically the SLM is a Liquid Crystal Display (LCD). The input surface of the first substrate is optically coupled to the SLM, each prism in the subwavelength grating layer substantially covering a portion of at least one column of pixels of the SLM. Typically the prism deflection angles provided by the apparatus of FIG.1 will be small. In most cases the total angle swing is below 1-2 degrees. To achieve a viable

autostereoscopic display it is therefore necessary to magnify the small angular deflection of the prisms into an a much larger angular sweep. For example, if we consider an implementation in which a 100 mm wide SLM is viewed normally at a range of 500mm. and assume a viewer inter- pupillary distance (IPD) of 65 mm. it can be shown that the left/right beam separation (ie the full angle sweep to be provided by the beam deflector) is up to 7.4 degrees depending on field position, the largest angle occurring at the centre of the field. Advantageously, the SLM is updated at a frequency of 120Hz so that each eye views the displayed image at the maximum resolution.

In one embodiment of the invention there is provided an autostereoscopic display device that can be switched between two operational modes. The two modes are illustrated in the schematic plan views of FIG.5. The autostereoscopic mode is shown in FIG.5A where the SLM width 107 has a width indicated by the symbol d and the range is indicated by the symbol R. The eyes are positioned at the points 105,106 separated by the Interpulilary Distance indicated by IPD. In this case the beam divergences only need to be wide enough to provide illumination at the left/right eye points at the specified viewing range. The sweep angle required to provide image rays at each eye point is indicated by the symbol β. The second mode in which the display provides 2D imagery is illustrated in FIG.5B. In this case a much greater diffusion angle indicated by Θ is needed. In one embodiment of the invention this additional diffusion may be provided by adding a subwavelength grating layer configured as a switchable diffuser. Typically the maximum distance between the left and right eye during the maximum allowed head movement for the above display example corresponds to an effective IPD of 95mm which is equivalent to total full angle sweep of 10.75 degrees. The maximum angle deflection from normal is approximately 1 1 degrees. The maximum angle that must be provided by the beam deflector is ± 9.5 degrees (19 degrees full angle) about the normal 107 to the SLM surface. To provide full 2D and 3D scanning capability it is estimated from first order optics that a x20 to x25 beam angle expansion is required. In one embodiment of the invention illustrated in FIG.6 there is provided a beam deflection device based on the embodiment of FIG.1 which further comprises an angle expansion means 120. The angular swing from the beam deflection device of FIG.1 indicated again by 100 is indicated by the rays 530 with beam divergence angle 531. The angle expansion means converts the beam 530 into the divergent output beam 533 which is symmetrical around the optical axis 532 defined by the normal to the substrates. The beam divergence angle is indicated by 534. In one embodiment of the invention illustrated in FIG.7 the angle expansion is provided by means of two microlens arrays 121,122 configured as an afocal magnifier. In a typical 3D display application we may assume a microprism angle sweep range of 0.7 degrees based on the subwavelength grating and polymer index values quoted above and a typical output angle dictated by viewing range and IPD considerations of 18 degrees. Therefore, if the focal lengths of the microlenses are fl and f2 the angle enlargement factor is f2/fl= 25x. The relative apertures or F-numbers of the lenses are given by fl# = fl/D and f2# =f2/D. Typically, the fastest relative aperture that can be achieved in a practical array is f2# =1.25. If we assume a lens diameter D in each array of 50 microns, we find that the minimum value of the focal length f2 is 1.25*50 = 62.5 microns. Hence f2=l 562.5 microns. The separation d of the arrays is given by d=fl-|f2|. The spacing between both lenslet arrays is thus: d=T 562.5 - 62.5 microns = 1500 microns. The invention does not assume any particular means for providing the angle expansion means 120. The afocal magnifier of FIG.7 may be based on any optical configurations known to those skilled in the art of optical design. Desirably, the prisms and lenslets should be as small as possible. Note that the sizes of the prisms and lenslet do not need to be identical. The lenslet size should not exceed 50micron. In order that the prisms operate within the refractive regime their aperture should ideally be greater than 40 microns. The invention does not assume any particular type of microlens array. In many applications of the invention refractive solutions are preferred to diffractive optical elements.

FIG.8A is a chart representing the alternating left/right image states 551 of the spatial light modulator as s function of time 550. The left and right image states are indicated by the letters L and R and also by the numerals 552,553 FIG.8B.is a chart illustrating the voltage waveform 554 applied to the subwavelength grating prism element of FIGS.2-3 as a function of time 550. The voltage levels corresponding to the image states 552,553 are indicate by 555,556. I should be noted that since the subwavelength grating index dependence on applied voltage is approximately linear the charts may also represent the index change. The state in which no beam deflection takes place (ie when the subwavelength grating refractive index matches that of the polymer prism layer) is indicated by the dashed line 557.

In one embodiment of the invention the voltages applied to the beam deflector illustrated in FIGS 2-3 may have random waveforms such as the one illustrated in the chart provided in FIG.9. The chart is a plot of voltage applied across the subwavelength grating prism layer plotted against time. One voltage step is indicated by 557. From consideration of FIGS.2-3 it should be apparent that applying a random voltage across a given subwavelength grating prism results in beam deflections randomly distributed around some mean direction in space. Such an

embodiment has many applications. In the case of displays it provides an alternative to using a separate diffuser to increase the cone angle of light the light from a display pixel. The advantage of this approach in relation to the autostereoscopic display described above is that a single subwavelength grating prism layer can be used to provide left and right beam steering for stereo viewing together with direct 2D viewing with an acceptable range of viewing positions. In one embodiment the display may be combined with an eye or head tracker to allow the viewing angles for the 2D mode to be matched to the instantaneous viewing position thereby providing a privacy function.

In one embodiment of the invention the prisms are configured as a linear array of elements of triangular cross section.

In one embodiment of the invention the prisms are configured as a two dimensional array of elements of triangular cross section. In one embodiment of the invention the surface angles of the subwavelength grating prisms have a random distribution.

In one embodiment of the invention the subwavelength grating prisms are each characterised by one of at least two different surface geometries.

In one embodiment of the invention the subwavelength grating prisms are each characterised by one of at least two different surface geometries with the prismatic elements of each surface geometry being distributed uniformly across the prism array. In one embodiment of the invention the apparatus of FIGS.1-3 may be used to provide a means for reducing the speckle of a laser beam incident at the input surface of the first substrate FIGS.1-3 also serve to illustrate such an embodiment of the invention. Since random ray deflections would be required in a despeckler the voltage waveform applied to the prism elements would be similar to the one illustrated in FIG.9. The resulting random distribution of ray directions would reduce speckle according to the well known principle of angular diversity.

In alternative embodiments of the invention the subwavelength grating layer may be replaced by a PDLC material. However, such a material would suffer from the problem of slow response to applied electric fields.

In certain embodiments of the invention the subwavelength grating prisms may incorporate optical power by superimposing the hologram encoding the desired optical function with subwavelength grating. Techniques for superimposing (or multiplexing) more than one optical function into a holographic or diffractive optical element are well known to those skilled in art of holography. The effect of incorporating optical power into the subwavelength grating prisms is equivalent to disposing a microlens array in series with the subwavelength grating prism array. Advantageously, an subwavelength grating element incorporating optical power is fabricated by first designing and fabricating a CGH with the required optical properties and then recording said CGH into the subwavelength grating element. Recording the CGH into the subwavelength grating element essentially means forming a hologram of the CGH using conventional holographic recording techniques well known to those skilled in the art of holography. The present invention does not assume any particular process for fabricating despeckler devices based on the above described principles. The fabrication steps may be carried out used standard etching and masking processes. The number of steps may be further increased depending on the requirements of the fabrication plant used. For example, further steps may be required for surface preparation, cleaning, monitoring, mask alignment and other process operations that are well known to those skilled in the art but which do not form part of the present invention

The invention does not rely on any particular method for electrode patterning. The methods described in the PCT Application No. PCT/IB2008/001909 by the present inventors may be used.

The invention is not limited to any particular type of HPDLC or recipe for fabricating a HPDLC. The HPDLC material currently used by the inventors typically switches at 170us and restores at 320us. The inventors believe that with further optimisation the switching times may be reduced to 140 microseconds.

It should be emphasized that the Figures are exemplary and that the dimensions have been exaggerated. For example thicknesses of the subwavelength grating layers have been greatly exaggerated.

The subwavelength grating s may be based on any crystal material including nematic and chiral types. In particular embodiments of the invention any of the subwavelength grating discussed above may be implemented using super twisted nematic (STN) liquid crystal materials. STN offers the benefits of pattern diversity and adoption of simpler process technology by eliminating the need for the dual ITO patterning process described earlier.

The invention may also be used in other applications such as optical telecommunications.

Although the invention has been described in relation to what are presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not limited to the disclosed arrangements, but rather is intended to cover various modifications and equivalent constructions included within the spirit and scope of the invention.