WO2016142707A1

WO2016142707A1 - Holographically-projected virtual retinal display

Info

Publication number: WO2016142707A1
Application number: PCT/GB2016/050642
Authority: WO
Inventors: Gideon RAEBURN; Manuel ULIBARRENA; Michael Medora
Original assignee: Colour Holographic Limited
Priority date: 2015-03-10
Filing date: 2016-03-09
Publication date: 2016-09-15
Also published as: GB201504042D0; GB2539166A

Abstract

An optical device has a holographic lens element mounted onto a waveguide and configured to direct light incident thereon to a convergent location at a predetermined distance from the waveguide. Each region of the holographic lens element is configured to couple out light that is incident on the region from the waveguide to the convergent location. This makes it possible to project directly a light field with wide field of view via a small spatial aperture without loss of information. Also described is a method of propagating light using a waveguide and holographic lens element and a method of fabricating a holographic lens element.

Description

HOLOGRAPHICALLY-PROJECTED VIRTUAL RETINAL DISPLAY

FIELD OF THE INVENTION [0001] The invention relates to an optical device and method for projecting an image onto a user's retina, and particular using a waveguide mounted holographic lens element.

BACKGROUND TO THE INVENTION [0002] Virtual displays, overlaying virtual information on a user's environment, have received significant interest and research in recent years, with much advancement in the development of technologies for this purpose.

[0003] One such area of development has involved the use of Head-Up Displays (HUD), wherein a screen located in the user's line of sight has projected onto it computer generated information. For instance, this is commonly incorporated into aircrafts, providing pilots with essential information, such as aircraft specifics on the windshield in front of them, without the requirement to move their gaze.

[0004] Despite their potential for widespread use, HUDs do exhibit inherent drawbacks that make them not ideal in all situations. For instance, HUD possess a lack of portability, requiring large screens to project the images onto, in order to cover a wide field of view for the user. Therefore, where it is desirable to provide virtual information on the move, conventional HUDs would be less than ideal.

[0005] For more transportable applications, devices currently exist for providing portable virtual displays incorporated onto wearable headsets, such as glasses. These devices include small displays projected in the corner of a user's field of view, such as in Google's Google Glass™ product. The user then sees this projected image in their peripheral field of view, but with the limitation that the user has to divert attention from the central field of view and direct gaze towards the image to discern its content.

[0006] An alternative to the projection of the virtual image onto a screen, as utilised in the above two mentioned technologies, has been the development of technology for projecting an image directly onto the retina itself using directional rays of light, known as Virtual Retinal Display (VRD) or Scanning Retinal Display (SRD). This provides the advantage that the user retains a full view of their environment and the virtual image simultaneously, as the image is not limited to a small region in front of the eye. Furthermore, as the image is reconstructed within the retina itself, rather than on a screen in front of the user, this can address potential security issues arising from its interception from a third party viewing the image. [0007] In order to project an image onto the retina, the light comprising the image needs to be focused to a small enough size that it can enter the eye through the pupil. Currently available VRD devices utilise conventional optics, which limits the ability to achieve this successfully, as a significant beam convergence is more difficult to obtain. Also, the transmission optical systems used can suffer from distortions arising from diffraction by lens imperfections and occlusions, for example. Moreover, the conventional optics that are typically used in such systems tend to be bulky, making their use in portable wearable devices unsuitable. Furthermore, such bulky devices can inhibit the user's ability to see beyond the digital image and the equipment displaying the image. In augmented reality systems, for example, it is desirable to display a digital image to the user while still allowing the user to see their real surroundings. While it would therefore be preferable for the imaging device to be transparent or hidden from the user, existing VRD devices cannot achieve this goal.

[0008] It would therefore be desirable to provide a virtual retinal display system that is compact, portable, and with improved focusing capabilities, such as the ability to direct light representing a wide field of view to a sufficiently small size that it can enter the eye. SUMMARY OF INVENTION

[0009] In a first aspect of the invention there is provided an optical device, including:

a waveguide comprising a light-coupling portion for coupling light into the waveguide, and configured to propagate the light in a direction along the waveguide; and,

a holographic lens element formed on at least a portion of a surface of the waveguide, the holographic lens element being configured to direct light incident thereon to a convergent location at a predetermined distance from the waveguide,

wherein the holographic lens element comprises a plurality of regions along the direction of the light propagation, wherein each region is configured to couple out light that is incident on the region from the waveguide to said convergent location.

[0010] By mounting a holographic lens element onto a waveguide and configuring the lens element to direct light incident thereon to a convergent location at a predetermined distance from the waveguide, it is possible to project directly a light field with wide field of view via a small spatial aperture without loss of information. This is achieved by each region of the holographic lens element being configured to couple out light that is incident on the region from the waveguide to the convergent location. [0011] The combination of the proposed holographic lens and waveguide provides an optical device that is lighter and more compact than a device constructed using conventional optics. While projecting light directly into a user's retina, for example, would typically require large cumbersome optics, the proposed optical can achieve this in a format that is suitably compact and lightweight to mount on a user.

[0012] In some embodiments the convergent location comprises an area in space smaller than the size of a pupil of an eye. In some embodiments the convergent location is substantially a single point in space. In some embodiments the holographic lens element is configured such that when a user's eye is positioned with its pupil in proximity to the convergent location, the light directed to the convergent location falls on at least a portion of a retina of the eye.

[0013] In this way the optical device can be used to project an image with wide field of view directly onto the retina of a user's eye without loss of information or focus, thereby achieving a Virtual Retinal Display of high quality. [0014] In some embodiments the optical device comprises a light source configured to produce one or more collimated beams of light, each collimated beam of light being associated with a pixel of an image.

[0015] Each region of the plurality of regions in the holographic lens element may correspond to a pixel of the image, wherein the optical device is configured to direct each of the collimated beams of light onto the region corresponding to the associated pixel.

[0016] In this way an optical image comprising a plurality of pixels can be projected and delivered via the waveguide-mounted holographic lens element through a narrow aperture.

[0017] In preferred embodiments each of the collimated beams of light is produced by a laser. Moreover, each of the collimated beams of light may be produced by a plurality of lasers, each of the plurality of lasers having a different colour. In this way a high quality colour image may be projected.

[0018] In some embodiments the light source comprises one or more beam scanning mirrors configured to sweep the beams of light across the regions of the holographic lens. In this way the image is built up by raster scanning a beam from one or more sources. In some preferred embodiments the light source is a picoprojector. [0019] In some embodiments the light source may comprises an array of one or more light emitting diodes, 'LEDs'. When a linear array is used raster scanning in only one dimension is required to produce an image. [0020] In some embodiments the holographic lens element is characterised by a low f-number, preferably below 1.0, and more preferably below 0.5. In this way, the holographic lens is capable of converging light to a point over a very short distance, allowing for the optical device to be positioned close to the user. [0021] In some embodiments light is propagated along the waveguide through total internal reflections. In some embodiments the holographic lens element is bonded and index-matched to the waveguide. In this way, light can propagate a great length along the length of a waveguide with limited losses to the light. [0022] In some embodiments the holographic lens element is one of a reflection hologram and a transmission hologram. A reflection hologram is particularly preferably when the optical device is being used as a transparent or semi- transparent device, as it can be designed to have a limited effect on background light passing through the holographic lens.

[0023] In some embodiments the waveguide is less than 25mm thick, and preferably less than 4mm thick. In this way, the optical device can be thin and lightweight enough to mount to a user's head with minimal discomfort. Furthermore, reduced dimensions may also result in reduced manufacturing costs.

[0024] In some embodiments, the optical device further comprises a light- coupling element at the light-coupling portion, the light coupling element being one or more of: a spherical lens, a cylindrical lens, and a coupling hologram. In this way, light from a light source can be efficiently coupled into the waveguide with limited loss.

[0025] In some embodiments the light-coupling portion is a surface substantially perpendicular to the direction of light propagation. In some embodiments the light-coupling portion is a surface parallel to the direction of light propagation. In some embodiments the light-coupling element is configured to turn light substantially 90° into the waveguide. In some embodiments the waveguide is configured to propagate light at a range of angles along the waveguide. The proposed optical device is versatile in its available configurations, as light sources can be positioned at various orientations relative to the waveguide, depending on the optical device's proposed use.

[0026] In some embodiments each region is further configured to only couple out light incident on the region at a predetermined angle. In some embodiments the predetermined angle for one of the plurality of regions is different to the predetermined angle for another of the predetermined regions.

[0027] In this way, different regions of the holographic lens will couple light out depending on the angle of light incident upon it. While earlier attempts at waveguide holograms use repeated total internal reflections to cause a collimated light source to interact with points along a hologram with lowering levels of intensity, the proposed approach allows a plurality of light rays of various angles of incidence to propagate along the holographic element by total internal reflection such that each light ray of a particular angle of incidence can interact with a region of the holographic element to produce an image.

[0028] In some embodiments the holographic lens is fabricated by recording the interference of reference beam of light and a divergent light source. In some embodiments the divergent light source comprises a laser light source and an optical focusing device, preferably a microscope objective. In this way, the properties of a highly divergent optical device (like a microscope objective) can be effectively encoded into a thin and efficient holographic lens for use in the proposed optical device.

[0029] According to another aspect of the present invention, the applicant makes available a method of propagating light comprising: coupling light into a waveguide such that the light propagates along the waveguide; directing light to a convergent location at a predetermined distance from the waveguide via a holographic lens element formed on at least a portion of a surface of the waveguide, wherein the holographic lens comprises a plurality of regions and each region couples out light that is incident on the region from the hologram to the convergent location at the predetermined distance from the waveguide.

[0030] According to yet another aspect of the present invention, the applicant makes available a method of fabricating a holographic lens element comprising: forming a divergent object beam by focussing a laser beam using a microscope objective; interfering the divergent object beam with a reference laser beam; and, recording the interference with a holographic recording medium.

[0031] In some embodiments the microscope objective has a magnification of at least x60, and preferably at least x100. [0032] In some embodiments the method of fabricating a holographic lens element further comprises the step of spatially filtering the object beam at the focus of the microscope objective.

[0033] In some embodiments the reference beam is collimated or divergent.

BRIEF DESCRIPTION OF DRAWINGS

[0034] Examples of the invention will now be described in detail with reference to the accompanying drawings, in which: [0035] Figure 1 and 1A are top down views of an optical device in accordance with the proposed solution;

[0036] Figure 2 is a top down view of an optical device in accordance with the proposed solution;

[0037] Figure 3 is a top down view of an optical device in accordance with the proposed solution; [0038] Figure 4 is a schematic diagram indicating an example VRD in accordance with the proposed solution;

[0039] Figure 5A illustrates an arrangement for fabricating a low f-number holographic lens element in accordance with the proposed solution; and, Figure 5B illustrates the subsequent illumination of the holographic lens element with a reconstruction beam.

DETAILED DESCRIPTION [0040] Example optical devices in accordance with the proposed solution will now be described with reference to the drawings. Figure 4 illustrates schematically an overall VRD system of the type contemplated here. The system comprises an optical projector 401 for projecting an image, and which is coupled to optical projection elements 404 for delivering the image into a user's eye 405. Further details of the projector will be described later, but we commence by describing examples of the optical projection elements proposed in the solution provided.

Optical Projection Elements [0041] Reference will now be made to Figure 1 which is a top down view of an optical device in accordance with the proposed solution.

[0042] The optical device comprises at least a volume holographic element, herein referred to as a holographic lens element 104, for projecting an image, and a waveguide 101 for directing light from a projector 102 to the holographic lens 104.

[0043] In the example embodiments described herein, the waveguide is planar substrate with a two-dimensional surface for setting a two-dimensional holographic lens element. However, the waveguide may take any form that enables the control of propagation of light. For example, the waveguide may be a fibre optic element. [0044] In the example embodiment of Figure 1 , the waveguide 101 , is an optically transparent thin slab with a polished edge 103. A diverging light source from a projector 102 may be incident on the polished edge 103, with the extreme outer beams of the angular spread of source light being indicated by beams 107 (depicted as broken line) and 108 (solid line). These enter the waveguide 101 at different angles of incidence and travel along the waveguide. These incoming beams 105 are confined in the waveguide by their respective angles, as these angles are above the critical angle of the waveguide. Those beams entering the waveguide below the critical angle will not totally internally reflect along the waveguide, and so will not propagate to the holographic lens element 104.

[0045] The projector 102 is preferably a MEMS scanning laser picoprojector, as already known in the art. Each light beam from the projector 102 preferably originates from an individual pixel of an image. Multiple pixels, and hence multiple beams of light are incident on the waveguide and holographic lens. The beams of light represent parallel ray bundles, typically due to the laser source used, such that each pixel is represented as a collimated ray of light.

[0046] The holographic lens element 104 is bonded to the waveguide slab surface and is preferably index-matched with the waveguide 101 , so as to allow efficient coupling of the light with minimal reflection losses. As shown, the holographic lens element operates in a reflection mode to couple light out of the waveguide through the opposing face of the waveguide. However, if suitably fabricated, the holographic lens element 104 may alternatively be placed on the opposite surface of the waveguide and couple light out in a transmission mode. In the present example, the holographic lens element 104 is specifically manufactured so that it allows light to exit the waveguide 101 when the incident beam is at the correct position and angle of incidence on the holographic lens 104. In this regard it replicates the beam angle at which the region of the holographic lens element 104 was referenced by the reference beam used in making the holographic lens element.

[0047] The holographic lens element 104 is constructed to cause the beams constituting the image to converge to a size sufficiently small, that they can pass through a small aperture 109, such as the pupil of the eye, and then to expand and project the image directly onto the retina 100 at a desired size. Accordingly, as each incident light beam corresponds to a pixel of the image, the purpose of the convergence of light beams is to reduce the image to a size that can enter a small aperture, such as the eye's pupil, when viewed at a close distance.

[0048] It is therefore desirable that the holographic lens element 104 is characterised by a low f-number, and corresponding short focal length, in order to enable this convergence. As such, the holographic lens element 104 typically employed may have an f number of around 1.0 or even lower, such as 0.5. Further details on the fabrication of the holographic lens element are described later. The minimum size of the converging image can be located either before or after entering the pupil of the eye in order to reduce a concentration of light flux on the corneal area, before expanding to a desirable size on the retina.

[0049] In this example embodiment, with an incident diverging light source, a plurality of beams propagate along the waveguide with ranging angles of incidence. The holographic lens is fabricated so that different regions are responsive to different angles of incidence of incoming light, out of the clusters of light beams propagating along the waveguide some may be coupled out at regions of the holographic lens closer to the light source, while others may be coupled out at regions of the holographic element further from the light source.

[0050] As illustrated in Figure 1 and 1A, the holographic lens element 104 is fabricated such that regions closer to the light source couple out light beams with low angles of incidence, while regions further from the light source couple out light beams that have progressively larger angles of incidence.

[0051] The holographic lens element 104 may be made up of a continuum of infinitesimally small regions, each region being configured to interact with a different infinitesimal graduation of angle of incidence. Certain regions of the holographic lens element will interact strongly with certain angles of incidence, but may still interact to a lesser degree with angles of incidence other than the preferred angle, for example, within a tolerance of the preferred angle. [0052] The selectivity of the holographic lens element 104 to different angles of incidence can be adjusted as appropriate. For example, a thicker holographic lens may be more suitable for embodiments where a greater degree of angle fidelity is required. For instance, in Figure 1 , beam 108 is not reflected out of the waveguide when first incident with the holographic lens, it is only on its second incidence that it is projected out of the waveguide. Conversely, beam 17 is emitted on its first incidence with the waveguide. [0053] The embodiment illustrated in Figure 1A allows a large field of view to be observed with the diverging beams 107 and 108 entering the waveguide without collimation, for instance directly from the projector.

[0054] The diverging light source enters the waveguide at a polished edge. Various means of coupling the light to the waveguide are envisioned, for instance, via a bevel, or a prism. A coupling means may be used to couple a diverging light source to the waveguide, or may couple a non-divergent light source into diverging beams within the waveguide. The coupling means may be configured such that light enters the waveguide above the critical angle (to ensure total internal reflection) and introduce a variation of angle of incidence, for example up to 30 degrees from the critical angle.

[0055] An alternative embodiment is shown in Figure 2, in which the diverging light beams entering into the waveguide are received by holographic lens coupler 203 operating in reflection mode. The divergent nature of the input beams 205 is preserved by the holographic lens input coupler 203, causing diffracted beams to continue down the waveguide by total internal reflection as in the first embodiment. [0056] As shown in Figure 2, a first beam 208 travels by total internal reflection down the length of the waveguide. When incident on the output coupling holographic lens element 204 the first beam 208 continues to travel via total internal reflection until it meets the point on the hologram where the correct image point and the appropriate angle of acceptance is reached. A second beam 207 incident at a different angle, illustrates the opposite case to first beam 208, wherein after travelling along the waveguide by total internal reflection, it is immediately coupled out of the waveguide when incident on the output coupling holographic lens element 204, owing to its propagation angle corresponding to the appropriate angle to match the conditions previously encoded into the holographic lens.

[0057] The output coupling holographic lens element 204 shown in Figure 2 is of the same nature of that in Figure 1 , namely that it is characterised by a low f- number in order to converge the light beams, each representing a single pixel of an image, to a small point, such that it can project an image into the receiving eye.

[0058] The light-coupling holographic lens is positioned on one of the surfaces of the waveguide and operates to direct the source light 90° into the waveguide so it propagates in the correct direction. Other light coupling means may be used to achieve a similar effect, thereby allowing source light to be positioned at the surface of the waveguide rather than the edge, as was the case in Figure 1. [0059] Introducing a variation of angles into the waveguide allows the holographic lens to respond differently along the length of the holographic lens. Such a variation of angles need not be limited to point-like diverging light sources, but may be achieved using a single dimension of variation, such as through the use of cylindrical lenses, which may be particularly useful in a planar waveguide configuration.

[0060] The proposed solution is envisioned to be applicable to a range of proportions, suitable for the application. For example, the substrate may be about 4mm thick, the holographic element about 15pm thick, the waveguide about 15cm long (along the direction of propagation) and 10cm across, while the active region of the holographic element may be 8cm long and 8cm across. As discussed, these are only example proportions, as much larger or smaller dimensions are envisioned. For example, the holographic element may be much thicker to provide volume-holography, for example it may be 50pm thick. [0061] The embodiments disclosed in Figures 1 , 1A and 2 all allow the coupling in of divergent light sources, and their subsequent transmission through total internal reflection along the waveguide. This has the advantage that a particularly wide field of view can be observed. Light rays entering the waveguide will reflect by total internal reflection along the waveguide with differing angles and, as mentioned above, the light may propagate many total internal reflections along the waveguide and holographic lens element before it couples out via the holographic lens element. Therefore, the size (length) of the holographic lens element can be much longer than would typically be the case were a collimated light beam to be incident, which would reflect at one point of incidence on the holographic lens element.

[0062] Nevertheless, there are situations where collimated propagation along the waveguide is desirable and Figure 3 shows such an alternative embodiment. In this example the device comprises an input coupling holographic lens element 303 and an output coupling holographic lens element 304, each bonded to a slab waveguide 301. The two elements are located conveniently apart so that one holographic lens element 303 is positioned to accept the image beams 305 and couple them into the waveguide 301 for propagation along its length, and the other holographic lens element 304 is located to couple the beams out of the waveguide 301 and converge them to a small region of space 309 at a distance from the waveguide. Both holographic elements are shown as operating in reflection mode, but suitably fabricated and positioned holographic elements may alternatively operate in a transmission mode.

[0063] As shown in Figure 3, diverging light beams 305 from a projector 302 impinge on the waveguide 301 and are received by the input coupler holographic lens element 303. Unlike in the previous embodiments, the input coupling holographic lens 303 in Figure 3, is constructed such that diverging input beams 305 are diffracted into collimated beams 307, 308, which then propagate along the waveguide 301 by total internal reflection, at the same angle of incidence, toward the output coupling holographic lens element 304. [0064] As in previous embodiments the output coupling holographic lens element 304 is constructed such that the collimated light 307, 308 incident on it is diffracted and converged to a size sufficiently small that it can pass through a small aperture 309, such as the pupil of an eye, and then expand and project the image on the retina 300 at a desired size. Accordingly, as each incident light beam corresponds to a pixel of the image, the purpose of the convergence of the light is again to allow it to pass through a small aperture, such as the eye, when viewed at a close distance. [0065] Therefore, as in previous embodiments, the holographic lens will preferably have a low f-number, and hence short focal length, to enable this convergence. The minimum size of the converging image can be located either before or after entering the pupil of the eye in order to reduce a concentration of light flux on the corneal area, before expanding to a desirable size on the retina. Conveniently, the input coupling and output coupling holographic lens elements 303 and 304 may provide conjugates of one another and be fabricated at the same time.

[0066] Again, the proposed solution is envisioned to be applicable to a range of proportions, particularly suitable for small head mounted devices. In the example embodiment of Figure 3, the waveguide substrate is about 2.5mm thick, the holographic lens is 6pm thick, the waveguide is 10cm long (along the direction of propagation) and 3cm across, while the active region of the holographic lens is 1 cm to 2cm long and 2cm across. As discussed, these are only example proportions, as much larger or smaller dimensions are envisioned.

[0067] While the holographic lens elements described in the embodiments above may be configured to interact with a single wavelength (colour) of light, they may also be designed to couple out multiple wavelengths, such as required for an red, green, blue (RGB) image projection system. Alternatively, separate bonded holographic lens elements could be configured in a stacked arrangement, either with a common waveguide or with their own waveguide, whereby each holographic lens element or waveguide-holographic lens element combination could be responsive to certain frequencies of light. In this way an RGB image projection system may also be realised.

Image Projector

[0068] As described above, the proposed solution provides a method of routing light from a projector in order to project the image onto the retina of a user's eye using a waveguide-mounted holographic lens element. Suitable projectors could include, but are not limited to, laser-illuminated MEMS picoprojectors, such as the commercially available MicroVision picoprojector.

[0069] Figure 4 shows a VRD system according to embodiments of the proposed solution. The system comprises a projector 401 coupled to optical projection elements 404 delivering an image to a user's eye 405. In the preferred embodiment the projector 401 comprises a beam scanning mirror 403 and a laser source 402, which can be formed from three lasers in an RGB format, to create a full colour image. Light from the laser source 402 is incident on the beam scanning mirror, which may comprise a single mirror driven biaxially in the x-y plane, thereby forming pixels representing a 2-D image. This can be driven in a raster-like scanning fashion, sweeping across in a line by line array to build up the image. The projected image is created upon modulating the output of the laser source synchronously with the position of the scanned beam. Individual laser within the source will typically be modulated differently to achieve the desired RGB colour mix. [0070] The raster scanning produces an image output from the projector with a beam corresponding to each individual pixel of the image. The beams of light represent parallel ray bundles, typically due to the laser source used, such that each pixel is represented as a collimated ray of light. The beams of light can then be passed out of the projector 401 and into the optical projection 404 apparatus, such as a waveguide-mounted holographic lens element according to the present solution, wherein they are subsequently delivered to the user's eye 405, where the image is projected onto the retina. [0071] Typical pixel sizes of 40 or 60 m are envisaged, with sizes as small as 5 pm desirable. The lasers used to generate the beams are typically small diode lasers operating in a CW mode, and are of sufficiently low power, such as nW to pW, to ensure there is no damage to the user's eye. However, any suitable laser that can be envisaged could be implemented.

[0072] The scanning speed employed is typically at least 60Hz, such that the image is updated quick enough that the eye, and thus the brain, perceives a fully formed image. However, faster scanning rates would also be desirable. The scanning is preferably biaxial scanning; however separate scanning for horizontal and vertical directions can be implemented in some embodiments.

[0073] Such devices require no focusing by conventional lenses, since each pixel of the image is represented by a collimated beam of light that increases with size proportional to the rate that the scanned image size grows. Therefore, such a device can project an image that is always in focus, where the increase in image size is solely due to the angle subtended by the scanner.

[0074] Accordingly, in the proposed solution, the rays from the picoprojector can be converged and expanded, whilst the overall image remains in focus, allowing for image projection without blur when the ray bundles are expanded onto the retina.

[0075] An alternative projector for use with the proposed solution may use a one dimensional line array of microsized LEDs, wherein each LED represent a single pixel and is capable of ultra-rapid intensity modulation. The image is created upon scanning using an oscillating uniaxial mirror synchronised to the pixel modulation, resulting in a two dimensional image being formed. In this particular type of display, the image does not expand so significantly in the direction of the line array, and as such, is suitable for very compact near to eye displays.

[0076] A further alternative projector for use with the proposed solution may use a two dimensional array of microsized collimated LEDs, wherein each LED represents a single pixel and is capable of ultra-rapid intensity modulation. In this type of display, the image does not expand so significantly in either direction, but does rely on a very compact 2-D LED array that can be modulated appropriately

[0077] The projector, is preferably located close to the waveguide, at the entrance end, to ensure divergence of the light beams does not limit the field of view observed. For example, the projector could be as close as 15mm distant. In a further embodiment the picoprojector could be further at 50mm distant from the waveguide.

[0078] The applicant has envisioned several uses for this virtual retinal display projection, particularly using the above-mentioned improved waveguided holographic lens element arrangements.

[0079] As the proposed approach uses holographic lens element, much less free space is required than using conventional optics, and therefore very compact, transportable virtual retinal display systems can be produced. Furthermore, the low f-number holographic lens enables convergence of the light beams to a size small enough to enter a user's eye, and at a short enough distance from the waveguide for the envisaged applications.

[0080] In the proposed embodiments discussed above, the holographic lens element operates in a reflection mode. Therefore, beams of light incident on the holographic lens from the waveguide are coupled out to converge to a location on the same side from which the beam was incident onto the holographic element. More specifically, this means that the beams do not pass through to the other side of the holographic lens before converging. [0081] However, as mentioned previously, the holographic lens element is not limited to a reflection holographic lens and may be of another type, such as transmission holographic lens. In such a configuration, the light beams incident on the holographic lens from the waveguide pass through the element and are diffracted as they do so, whereby they converge at a location on the opposite side of the lens element to which the beam was incident.

[0082] The proposed virtual retinal display projection using a holographic lens arrangement can, for example, be used as a means for projecting into a user's eye a display that has been generated by a computer processor and used to drive the optical projector. In this way the system may operate similarly to a Head Mounted Display (HMD) or an Augmented Reality (AR) glasses. [0083] More specifically the computer processor could be a communication device, with the projected image including details such as photos, calendars, contacts, emails, or any other function typically associated with modern communication devices. [0084] Furthermore, the set up may be combined with means of interacting with the communication device, in such a way that a user can perform actions on the move whilst having the view of the mobile device overlaid over their normal surroundings. This is achievable due to the transparency and flat nature of the holographic lens element, and permits a wide field of view due to the image being projected onto the retina directly, and not projected in a small area in the line of sight of the user as is common in other technologies. The reflection mode used in the preferred implementations also does not suffer from crosstalk, as can occur in devices operating in a transmission mode. [0085] In addition, as the image is projected directly onto the retina itself the user does not have to actively focus onto the image in order to view it clearly.

[0086] An alternative application could include incorporation into a headset for watching films projected directly into the user's eyes. Further applications could incorporate the system into uses for scientific visual displays, such as to provide an enhanced interface to the user, overlaying of a particular image on top of an object in the user's line of sight. For instance, the device could be used for surgical procedures, wherein a surgeon requires patient's vitals in their field of view in real time to help them make informed decisions. Alternatively, for a mechanic upon viewing an engine, the projected image may provide information as to the components in the engine and details on them.

[0087] A further use could be in the tourism industry, wherein upon viewing a place of significant interest an image corresponding to the place may be projected into the eye to provide the user with, for example, background information relating to that place.

[0088] The proposed system can also be particularly advantageous in security implementations, as the image is projected directly onto the retina of the user and not on a screen in the user's line of sight. Therefore, a third party would not be able to readily view the image.

[0089] In the preferred embodiments, the proposed system is for projection of the image onto the retina of the eye. However, it is not strictly limited to this application and the holographic lens element and waveguide system could be used for any application, wherein it is desirable to converge beams of light to a small size, such that they can pass through a small aperture. [0090] A plethora of real-life applications are envisioned for the proposed holographic lens virtual retinal display system, each taking advantage of the benefits of reduced-size, portability, improved focusing capabilities, transparency, and improved image formation. Fabrication of Low f-number Holographic Lens Element

[0091] Figure 5A is a schematic diagram illustrating an example configuration for fabricating a holographic lens element in accordance with the proposed solution. [0092] As explained above, the holographic lens element is configured such that it possesses a low f-number, below 1.0, and with values smaller than 0.5 achievable, whereby it can converge parallel ray bundles, such as produced by laser picoprojectors, to a size small enough to enter into the pupil of the eye. [0093] A hologram is typically created by directing two light sources at a holographic recording medium, the "reference beam", and a further light source, usually termed the Object beam'. Interference at the holographic recording medium between these two beams is encoded within the recording medium as a phase and/or amplitude modulation, wherein upon illumination by a further beam of light, known as the reconstructing (or interrogating) beam, a holographic effect is produced, whereby the reconstruction beam is diffracted to reconstruct the object.

[0094] In the present case the object is a simple point-like object producing a highly divergent object beam, which interferes with a reference beam to generate a holographic lens element with very low f-number. One approach proposed for fabricating such a holographic lens is illustrated in Figure 5A, where a highly diverging coherent laser beam 6 is incident on a holographic light sensitive plate or film 505. The highly diverging coherent laser beam 506 is formed from passing a laser beam 501 from a laser source 504, through a microscope objective 502, and is further spatially filtered, for instance using a pinhole 503. Thus, a tight convergence is achieved, and subsequently a large divergence of the laser beam when it is incident on the holographic plate 505.

[0095] The microscope objective 502 is preferably very powerful, for instance in some embodiments a x60 objective would be used, while in alternative embodiments a x100 objective would be used, such that the desired convergence of the beams is achieved. The size of the pinhole to achieve spatial filtering would typically be 3pm in some embodiments, whereas in other embodiments a larger pin hole, such as 5pm may be used. The spatial filter assembly serves to eliminate high spatial frequency noise resulting from diffraction, lens imperfections and occlusions, for example

[0096] A second beam, the reference beam 507, is incident on the other side of the holographic material, wherein the interaction between the beam and the laser beam 506 causes an interference pattern which is then encoded in the holographic material as phase and/or amplitude modulations, thus creating a holographic lens.

[0097] In the example shown in Figure 5A, the reference beam 507 is a collimated beam. However, alternative beam types could be used according to the particular application of the holographic lens and the way it is to be illuminated. For instance, the reference beam could be a diverging or a converging beam, in one dimension or multiple dimensions, in either free space or for use in a waveguide. A beam with a more advanced profile could also be used, for instance using a computer generated spatial light modulator (SLM), to generate for instance a cylindrical beam. Thus, the reference beam may be appropriately configured to produce a holographic lens element suitable for subsequent illumination in a particular format. [0098] Once fabricated, the recorded hologram can be used to reconstruct the object, as shown in Figure 5B, wherein a reconstructing beam 508 is incident on the recorded hologram. As can be seen from Figure 5B, this reconstructing beam 508 is identical in form to the reference beam 507 used in fabricating the hologram, but is incident on the opposing side of the hologram. A highly converging beam 509 is thus formed on the same side as the incident reconstructing beam 508, thereby reconstructing the object, for instance on the retina 510. In this way the recorded hologram acts as a low f-number lens in the manner required in the proposed solution. [0099] The method of fabricating a low f-number holographic lens using a collimated reference beam as shown in Figure 5 is thus particularly well suited for producing the holographic lens element used in the device embodiment shown in Figure 3. The collimated reference beam 507 may be incident on the holographic plate 505 via a glass block, or prism, to ensure the light rays are at the correct angle to produce a holographic lens element for subsequent use and illumination with a waveguiding format. In this way, the reference beam corresponds to a collimated beam propagating in the material (e.g. glass) from which the waveguide is made. [0100] Moreover, for the embodiment shown in Figure 3, the input coupling holographic lens element 303 and the output coupling holographic lens element 304 can be produced from the same fabricated holographic lens. The output coupling holographic lens element 304, is reconstructed by the method outlined above to achieve a converging beam for projecting the image into the user's eye. Conversely, the input coupling holographic lens element performs the conjugate task, such that a diverging input beam from a converged point is transformed into a collimated beam within the waveguide. [0101] Similar methods can be used to construct the holographic lens element required for the device embodiments shown in Figures 1 , 1 A and 2. Specifically, since these embodiments require divergent beam input, an appropriate holographic lens element can be fabricated in the manner shown in Figure 5A by replacing the collimated reference beam 507 with a suitably diverging beam. In this instance the reference beam is required to be incident on the holographic plate 505 at different angles of incidence along the length of the holographic plate 505. This may be achieved by orientating the holographic plate 505 with respect to the beam, for instance using a prism, or glass block, to refract the reference beam 507, such that a range of beam angles are encoded onto the holographic element.

[0102] It is further noted that, while fabrication of holographic lens elements is described above in connection with a reflection mode of operation, it is possible to adapt the method to produce a holographic lens for use in a transmission mode of operation simply by illuminating the holographic recording medium from the same side with both the object and reference beam. The recorded hologram is then reconstructed by a beam from the opposite side in transmission mode. Thus, the holographic lens element may be mounted onto a waveguide for use in VRD systems of the type described previously, but operating in transmission mode.

[0103] Low f-number holographic lenses of the type described above can be used for many applications, for instance in a virtual retinal display system according to the proposed solution. However, their use for other applications could also be envisaged, including as simple off axis free space holographic elements.

[0104] It is to be understood that the present disclosure includes permutations of combinations of the optional features set out in the embodiments described above. In particular, it is to be understood that the features set out in the appended dependent claims are disclosed in combination with any other relevant independent claims that may be provided, and that this disclosure is not limited to only the combination of the features of those dependent claims with the independent claim from which they originally depend.

Claims

1. An optical device, including:

2. The optical device of claim 1 , wherein the convergent location comprises an area in space smaller than the size of a pupil of an eye.

3. The optical device of claim 1 or 2, wherein the convergent location is a single point in space

4. The optical device of any preceding claim, wherein the holographic lens is configured such that when an eye is positioned with its pupil in proximity to the convergent location, the light directed to the convergent location falls on at least a portion of a retina of the eye

5. The optical device of any preceding claim, further comprising a light source configured to produce one or more collimated beams of light, each collimated beam of light being associated with a pixel of an image

6. The optical device of claim 5 , wherein each region of the plurality of regions in the holographic lens element corresponds to a pixel of the image, and wherein the optical device is configured to direct each of the collimated beams of light onto the region corresponding to the associated pixel.

7. The optical device of claim 5 or 6, wherein each of the collimated beams of light is produced by a laser.

8. The optical device of claim 5 or 6, wherein each of the collimated beams of light is produced by a plurality of lasers, each of the plurality of lasers having a different colour.

9. The optical device of any of claims 5 to 8, wherein the light source comprises one or more beam scanning mirrors configured to sweep the beams of light across the regions of the holographic lens

10. The optical device of any of claims 5 to 9, wherein the light source is a picoprojector.

1 1. The optical device of claim 5 or 6. wherein the light source comprises an array of one or more light emitting diodes, 'LEDs'.

12. The optical device of any preceding claim, wherein the holographic lens element is characterised by a low f-number, preferably below 1.0, and more preferably below 0.5.

13. The optical device of any preceding claim, wherein light is propagated along the waveguide through total internal reflections.

14. The optical device of any preceding claim, wherein the holographic lens element is bonded and index-matched to the waveguide.

15. The optical device of any preceding claim, wherein the holographic lens element is one of a reflection hologram and a transmission hologram.

16. The optical device of any preceding claim, wherein the waveguide is less than 25mm thick, and preferably less than 4mm thick.

17. The optical device of any preceding claim, further comprising a light- coupling element at the light-coupling portion, the light coupling element being one or more of: a spherical lens, a cylindrical lens, and a coupling hologram.

18. The optical device of any of claims 1 to 17, wherein the light-coupling portion is a surface substantially perpendicular to the direction of light propagation.

19. The optical device of any of claims 1 to 17, wherein the light-coupling portion is a surface parallel to the direction of light propagation.

20. The optical device of claim 19, when dependent on claim 17, wherein the light-coupling element is configured to turn light substantially 90° into the waveguide.

21. The optical device of any preceding claim, wherein the waveguide is configured to propagate light at a range of angles along the waveguide.

22. The optical device of claim 21 , wherein each region is further configured to only couple out light incident on the region at a predetermined angle.

23. The optical device of claim 22, wherein the predetermined angle for one of the plurality of regions is different to the predetermined angle for another of the predetermined regions.

24. The optical device of any preceding claim, wherein the holographic lens is fabricated by recording the interference of reference beam of light and a divergent light source.

25. The optical device of claim 24, wherein the divergent light source comprises a laser light source and an optical focusing device, preferably a microscope objective.

26. A method of propagating light comprising:

coupling light into a waveguide such that the light propagates along the waveguide;

directing light to a convergent location at a predetermined distance from the waveguide via a holographic lens element formed on at least a portion of a surface of the waveguide, wherein the holographic lens comprises a plurality of regions and each region couples out light that is incident on the region from the hologram to the convergent location at the predetermined distance from the waveguide.

27. A method of fabricating a holographic lens element comprising:

forming a divergent object beam by focussing a laser beam using a microscope objective;

interfering the divergent object beam with a reference laser beam; and, recording the interference with a holographic recording medium.

28. The method of claim 27, wherein the microscope objective has a magnification of at least x60, and preferably at least x100.

29. The method of claim 27 or claim 28, further comprising the step of spatially filtering the object beam at the focus of the microscope objective.

30. The method of any one of claims 27 to 29, wherein the reference beam is collimated or divergent.

31. An optical device as hereinbefore described.