Patent Description:
Referring to <FIG>, there is shown a rough sketch of conventional optical aperture expansion using diffractive components in a waveguide. In the current figure, incoming light (image) is vertical from outside the page into the page. Coupling-in element <NUM> couples the incoming light into lateral expansion element <NUM>, which expands the light laterally (from left to right in the current figure). Then the laterally expanded light is coupled into vertical expansion element <NUM> which expands the light vertically (from top to bottom in the current figure), and couples-out the light to a user (eye of a viewer).

Conventional diffractive elements introduce chromatic dispersion where light-rays with different wavelengths diffract at different angles. To reduce chromatic dispersion narrowband light sources (such as lasers) can be used. A more practical solution is to design the diffractive components to cancel the dispersion of each other. <CIT> discloses a spectacle lens for a display device that can be fitted on the head of a user and generate an image. The spectacle lens comprises a lightguide with a coupling-in and a coupling-out section. The lens provides aperture expansion using various configurations of diffractive and partially-reflective components that also compensate for color-dependent diffraction.

Referring to <FIG>, there is shown a diagram of the <FIG> diffraction directions of light propagating in the angular domain (angular space). Dashed arrows and solid arrows show two different exemplary wavelengths. The starting angle at area <NUM> represents the angle of the light rays as the light rays impinge on the first diffractive element (coupling-in element <NUM>) to be coupled into the lightguide. Area <NUM> represents the direction of the light rays after coupling in element <NUM>, Area <NUM> after lateral expansion element <NUM>, and the area <NUM> also represents the angle of the light rays after coupling out of the lightguide by vertical expansion element <NUM>. The direction of the light rays entering the lightguide is equal to the direction of the light rays coupled out of the lightguide in order to minimize chromatic dispersion. It is apparent that different wavelengths will have different directions as the different wavelengths of light propagate within the lightguide, and will have the same direction when output from the lightguide.

<FIG> illustrates a conventional prior art folding optics arrangement, wherein the substrate <NUM> is illuminated by a display source <NUM>. The display is collimated by a collimating optics <NUM>, e.g., a lens. The light from the display source <NUM> is coupled into substrate <NUM> by a first reflecting surface <NUM>, in such a way that the main ray <NUM> is parallel to the substrate plane. A second reflecting surface <NUM> couples the light out of the substrate and into the eye of a viewer <NUM>. Despite the compactness of this configuration, this configuration suffers significant drawbacks. In particular, only a very limited FOV can be achieved.

Referring now to <FIG> there is shown a side view of an exemplary light-guide optical element (LOE). To alleviate the above limitations, an array of selectively reflecting surfaces can be used, fabricated within a light-guide optical element (LOE). The first reflecting surface <NUM> is illuminated by a collimated display light ray (beams) <NUM> emanating from a light source (not shown) located behind the device. For simplicity in the current figures, only one light ray is generally depicted, the incoming light ray <NUM> (also referred to as the "beam" or the "incoming ray"). Other rays of incoming light, such as beams 18A and 18B may be used to designate edges of the incident pupil, such as a left and right edge of an incoming light pupil. Generally, wherever an image is represented herein by a light beam, it should be noted that the beam is a sample beam of the image, which typically is formed by multiple beams at slightly differing angles each corresponding to a point or pixel of the image. Except where specifically referred to as an extremity of the image, the beams illustrated are typically a centroid of the image.

The reflecting surface <NUM> reflects the incident light from the source such that the light is trapped inside a lightguide <NUM> by total internal reflection. The lightguide <NUM> is also referred to as a "waveguide", "planar substrate" and a "light-transmitting substrate. " The lightguide <NUM> includes at least two (major) surfaces parallel to each other, shown in the current figure as a back (major) surface <NUM> and a front (major) surface 26A. Note that the designation of "front" and "back" with regard to the major surfaces (<NUM>, 26A) is for convenience of reference, as the lightguide <NUM> is normally symmetric (so references to the major surfaces <NUM>, 26A can be switched, with the same result). Lightguide <NUM> is referred to in the context of this document as a one-dimensional (1D) waveguide, guiding the injected image in only one dimension between one pair of parallel faces (in this case, the major surfaces <NUM>, 26A).

Incoming light ray <NUM> enters the substrate at a proximal end of the substrate (right side of the figure). Light propagates through the lightguide and one or more facets, normally at least a plurality of facets, and typically several facets, toward a distal end of the lightguide (left side of the figure). Light propagates through the lightguide in both an initial direction <NUM> of propagation, and another direction <NUM> of propagation.

After several reflections off the surfaces of the substrate <NUM>, the trapped waves reach an array of selectively reflecting surfaces <NUM>, which couple the light out of the substrate into the eye <NUM> of a viewer. In alternative configurations, the selectively reflecting surfaces <NUM> are immediately after light ray <NUM> enters the substrate, without first reflecting off the surfaces of the substrate <NUM>.

Internal, partially reflecting surfaces, such as selectively reflecting surfaces <NUM> are generally referred to in the context of this document as "facets. " In the limit, facets can also be entirely reflecting (<NUM>% reflectivity, or a mirror, for example the last facet at the distal end of a substrate), or minimal-reflecting. For augmented reality applications, the facets are partially reflecting, allowing light from the real world to enter via upper surface 26A, traverse the substrate including facets, and exit the substrate via lower surface <NUM> to the eye <NUM> of the viewer. For virtual reality applications, the facets may have alternative reflectivities, such as the first coupling in mirror having <NUM>% reflectivity, as the image light from the real world does not have to traverse this mirror. The internal partially reflecting surfaces <NUM> generally at least partially traverse the lightguide <NUM> at an oblique angle (i.e., neither parallel nor perpendicular) to the direction of elongation of the lightguide <NUM>.

References to reflectivity are generally with respect to the nominal reflectivity. The nominal reflectivity being the total reflection needed at a specific location in the substrate. For example, if the reflectivity of a facet is referred to as <NUM>%, generally this refers to <NUM>% of the nominal reflectivity. In a case where the nominal reflectivity is <NUM>%, then <NUM>% reflectivity results in the reflectivity of the facet being <NUM>%. One skilled in the art will understand the use of percentages of reflectivity from context of use. Partial reflection can be implemented by a variety of techniques, including, but not limited to transmission of a percentage of light, or use of polarization.

<FIG> illustrate a desired reflectance behavior of selectively reflecting surfaces. In <FIG>, the ray <NUM> is partially reflected from facet <NUM> and coupled out 38B of the substrate <NUM>. In <FIG>, the ray <NUM> is transmitted through the facet <NUM> without any notable reflection.

<FIG> is a detailed sectional view of an array of selectively reflective surfaces that couple light into a substrate, and then out into the eye of a viewer. As can be seen, a ray <NUM> from the light source <NUM> impinges on the first partially reflective surface. Part of the ray <NUM> continues with the original direction and is coupled out of the substrate. The other part of the ray <NUM> is coupled into the substrate by total internal reflection. The trapped ray is gradually coupled out from the substrate by the other two partially reflecting surfaces <NUM> at the points <NUM>. The coating characteristics of the first reflecting surface <NUM> should not necessarily be similar to that of the other reflecting surfaces <NUM>, <NUM>. This coating can be a simpler beam-splitter, either metallic, dichroic or hybrid metallic-dichroic. Similarly, in a case of a non-see-through system, the last reflecting surface <NUM> can be a simple mirror.

<FIG> is a detailed sectional view of an apparatus including an array of reflective surfaces wherein the last surface <NUM> is a total reflecting mirror. The extreme left part of the last reflecting surface <NUM> cannot be optically active in such a case, and the marginal rays <NUM> cannot be coupled out from the substrate. Hence, the output aperture of the device will be slightly smaller. However, the optical efficiency can be much higher and fabrication process of the LOE can be much simpler.

It is important to note that, unlike the configuration illustrated in <FIG>, there is a constraint on the orientation of the reflective surfaces <NUM> and <NUM>. In the former configuration all the light is coupled inside the substrate by the reflective surface <NUM>. Hence, surface <NUM> need not be parallel to surfaces <NUM>. Moreover, the reflecting surfaces might be oriented such that the light will be coupled out from the substrate in the opposite direction to that of the input waves. For the configuration illustrated in <FIG>, however, part of the input light is not reflected by surface <NUM>, but continues in an original direction of the input light <NUM> and is immediately coupled-out from the substrate as output light <NUM>. Hence, to ensure that all the rays originating from the same plane wave will have the same output direction, not only should all the reflecting surfaces <NUM> be parallel to each other, but surface <NUM> should be parallel to surfaces <NUM> as well.

Refer again to <FIG> there is shown a system having two reflective surfaces for coupling the light out of the substrate, however, any number of reflective surfaces can be used according to the required output aperture of the optical system and the thickness of the substrate. Naturally, there are cases where only one coupling-out surface is required. In that case, the output aperture will essentially be twice the size of the input aperture of the system: The only required reflecting surfaces for the last configuration are simple beam-splitters and mirrors.

In the apparatus described in the current figure, the light from the display source is coupled into the substrate at the end of the substrate, however, there are systems where having a symmetric system is preferred. That is, the input light should be coupled into the substrate at the central part of the substrate.

<FIG> is a diagram illustrating detailed sectional views of a transverse pupil expansion one-dimensional (1D) lightguide having a symmetrical structure. The current figure illustrates a method to combine two identical substrates, to produce a symmetric optical module. As can be seen, part of the light from the display source <NUM> passes directly through the partially reflecting surfaces out of the substrate. The other parts of the light are coupled into the right side of the substrate 20R and into the left side of the substrate <NUM>, by the partially reflecting surfaces 16R and <NUM>, respectively. The trapped light is then gradually coupled out by the reflecting surfaces 22R and <NUM>, respectively. Apparently, the output aperture is three times the size of the input aperture of the system, the same magnification as described in <FIG>. However, unlike the system there, the system here is symmetric about the cemented surface <NUM> of the right and left substrates.

Referring now to <FIG>, there is shown exemplary implementations of <FIG> on top of a lightguide. The configurations of <FIG> expand the incoming image laterally. The apparatus of <FIG> can be used to implement the first LOE 20a of <FIG>, the apparatus of <FIG> can be used to implement the first LOE 20a' of <FIG>, and the apparatus of <FIG> can be used to implement the second LOE 20b.

<FIG> illustrates an alternative method to expand the beam along two axes utilizing a double LOE configuration. The input wave <NUM> is coupled into the first LOE 20a, which has an asymmetrical structure similar to that illustrated in <FIG>, by the first reflecting surface <NUM> a and then propagates along the η axis. The partially reflecting surfaces 22a couple the light out of first LOE 20a and then the light is coupled into the second asymmetrical LOE 20b by the reflecting surface 16b. The light then propagates along the ξ axis and is then coupled out by the selectively reflecting surfaces 22b. As shown, the original beam <NUM> is expanded along both axes, where the overall expansion is determined by the ratio between the lateral dimensions of the elements 16a and 22b. The configuration given in <FIG> is just an example of a double-LOE setup. Other configurations in which two or more LOEs are combined together to form complicated optical systems are also possible.

Referring now to <FIG>, there is shown a diagram illustrating another method to expand a beam along two axes utilizing a double LOE configuration. Usually, the area where the light is coupled into the second LOE 20b by the surface 16b cannot be transparent to the external light and is not part of the see-through region. Hence, the first LOE 20a need not be transparent. As a result, it is usually possible to design the first LOE 20a to have a symmetric structure, as can be seen in the current figure, even for see-through systems. The second LOE 20b has an asymmetrical structure that enables the user to see the external scene. In this configuration, part of the input beam <NUM> continues along the original direction <NUM> into the coupling-in mirror 16b of the second LOE 20b, while the other part <NUM> is coupled into the first LOE 20a' by the reflecting surfaces 16a, propagates along the η axis and is then coupled into the second LOE 20b by the selectively reflecting surfaces 22a. Both parts are then coupled into the second asymmetrical LOE 20b by the reflecting surface 16b, propagate along the ξ axis, and are then coupled out by the selectively reflecting surfaces 22b.

<FIG> illustrates an example of LOEs 20a/20a' and 20b embedded in a standard eyeglasses frame <NUM>. The display source <NUM>, and the folding and the collimating optics <NUM> are assembled inside the arm portions <NUM> of the eyeglasses frame, just next to LOE 20a/20a', which is located at the edge of the second LOE 20b. For a case in which the display source is an electronic element, such as a small CRT, LCD, or OLED, the driving electronics <NUM> for the display source might be assembled inside the back portion of the arm <NUM>. A power supply and data interface <NUM> is connectable to arm <NUM> by a lead <NUM> or other communication means including radio or optical transmission. Alternatively, a battery and miniature data link electronics can be integrated in the eyeglasses frame. The current figure is an example, and other possible head-mounted displays arrangements can be constructed, including assemblies where the display source is mounted parallel to the LOE plane, or in the upper part of the LOE.

Additional details of this basic technology can be found in <CIT>, and <CIT> which is unpublished and does not constitute prior art to the present invention.

According to the invention there is provided an apparatus for optical aperture expansion according to claim <NUM>. Different embodiments are set out in the dependent claims.

The embodiment is herein described, by way of example only, with reference to the accompanying drawings, wherein:.

For convenience of reference, this section contains a brief list of abbreviations, acronyms, and short definitions used in this document. This section should not be considered limiting. Fuller descriptions can be found below, and in the applicable Standards.

The principles and operation of the apparatus according to a present embodiment may be better understood with reference to the drawings and the accompanying description. A present invention is an optical assembly for optical aperture expansion. The apparatus combines facet reflective technology (reflective components) with diffractive technology (diffractive components). Innovative embodiments with diffractive components use at least two components having opposite optical power (matching), so that chromatic dispersion introduced by a first diffractive component will then be cancelled by a second diffractive component. The two diffractive components are used in combination with a reflective optical component to achieve more efficient aperture expansion (for near eye display), reducing distortions and noise, while also reducing design constraints on the system and individual components, as compared to conventional techniques.

Current, conventional optical aperture expansion uses a single technology for both expansions (lateral and vertical). Current advances in the field are to optimize and improve either one of these technologies. The two main technologies that are used are:.

Using a sequence of reflective and diffractive components, in various quantities and order (one after the other, and vice-versa) eliminates and/or reduces the need for polarization management, while enabling wider field of view. In addition, embodiments can have reduced nonuniformity, as compared to conventional single technology implementations, since the distortion patterns of the two technologies do not correlate (are uncorrelated).

In general, an apparatus for optical aperture expansion includes at least one lightguide and a set of three optical components associated with the at least one lightguide. The set of three optical components includes a pair of matching diffractive optical components, and a reflective optical component. The reflective optical component includes a sequence of a plurality of at least partially reflective, mutually parallel surfaces. The optical components are configured to cooperate to achieve two-dimensional expansion of coupled-out light. In other words, the components cooperate for expanding coupled-in light to coupled-out light. The coupled-in light is light coupled into the at least one lightguide, and the expanding is two-dimensional.

In the context of this description, the term "matching" with regard to diffractive optical components generally refers to the grating and/or the spacing of the grating elements being substantially exactly equal so the optical powers of the diffractive components are equal, and normally opposite. While the overall physical dimensions of the components may be different, the similar gratings result in matching optical powers of the components.

In the context of this description, the term "component" is used for the optical elements, in particular the reflective and diffractive optical elements. Design and production techniques for reflective and optical components are known in the art. Based on the current description, components can be implemented as required in various shapes and sizes of reflective and diffractive optical components, with various operating parameters, including wavelength, power, and angle.

Diffractive optical components, also referred to in the context of this description as "diffractive gratings" and "diffractive patterns", can be embedded within a lightguide, or constructed or mounted on a surface (face) of a lightguide. For example, a diffractive optical component can be implemented as a diffraction grating or holographic elements. Diffractive components are available such as from Horiba Scientific (Kyoto, Japan) and reflective components are available such as OE50 by Lumus (Ness Ziona, Israel).

Referring now to <FIG>, there is shown respective side and front view rough sketches of a diffractive-reflective-diffractive exemplary embodiment, not according to the claimed invention. A combination of different optical components expands light along a different axis. Optical lightguide <NUM> is a two-dimensional (2D) lightguide having a direction of elongation illustrated arbitrarily herein as corresponding to the "x-axis". Lightguide <NUM> is referred to as a 2D waveguide in the sense that lightguide <NUM> guides the injected image in two dimensions by reflection between two sets of parallel faces, as shown in <FIG> by the four arrows inside lightguide <NUM>. A sequence of a plurality of internal partially reflecting surfaces <NUM> at least partially traverse lightguide <NUM> at an oblique angle (i.e., neither parallel nor perpendicular) to the direction of elongation.

Incoming light <NUM> is coupled into lightguide <NUM> by a diffractive component <NUM>. The coupled-in light enters lightguide <NUM> that acts as a first lateral lightguide expander in a first direction. The expanded light 38C from lightguide <NUM> is coupled into lightguide <NUM>. Optical lightguide <NUM> guides light primarily along the "y-axis". The expanded light 38C continues to reflect within lightguide <NUM> expanding in a second direction of expansion (y-axis) as shown by arrows in the side view of <FIG>. The light in lightguide <NUM> is referred to in the context of this document as second expanded light 38D. As the second expanded light 38D encounters the diffractive pattern <NUM>, the second expanded light is coupled-out 38B of the lightguide <NUM> onto the observer <NUM>. A feature of the current embodiment is that the diffractive components can be non-parallel to each other.

In general, a set of three optical components includes a first optical component (diffractive component <NUM>) configured for directing the coupled-in light <NUM> in a first direction of expansion (x-axis) within a first lightguide (lightguide <NUM>), thereby generating first expanded light 38C. A second optical component (sequence of partially reflecting surfaces <NUM>) of the set is configured for coupling the first expanded light 38C into a second lightguide <NUM> in a second direction of expansion (y-axis), thereby generating second expanded light 38D. A third optical component (diffractive component <NUM>) of the set is configured for out-coupling the second expanded light 38D in a third direction as the coupled-out light 38B.

In the context of this description, the term "direction" generally refers to an average direction of propagation within a lightguide, typically along the optical axis (normally the length) of the lightguide. In other words, the course or general way in which light trapped in a lightguide slab by total internal reflection (TIR) advances along the lightguide slab, that is, a course of expansion in the plane of the lightguide slab, the in-plane component of the propagating light rays in the substrate of the lightguide.

The first, second and third directions are non-parallel to each other.

Referring now to <FIG>, there is shown a diagram of the <FIG> diffraction directions of light propagating in the angular domain (angular space). Dashed arrows and solid arrows show two different exemplary wavelengths. The direction area <NUM> is the incidence angle as described in reference to <FIG>. Area <NUM> represents the direction of the light rays (or simply "ray") after lateral expansion and reflection by the sequence of partially reflecting surfaces <NUM>. The partially reflecting surfaces <NUM> divert the direction of the ray to area <NUM>. However, this reflection from area <NUM> to area <NUM> does not introduce extra dispersion, only mirroring the direction of propagation around the mirror direction (shown as dot-dash line <NUM>). The mirror direction <NUM> is determined by the slope of the partially reflecting surfaces <NUM>. The last diffractive element <NUM> diffracts the ray to area <NUM>. As the ray is diffracted in a compensatory fashion to diffractive component <NUM>, then the output direction <NUM> will have no dispersion but need not overlap <NUM>. In this embodiment, dispersion has been eliminated, but the output angle of the coupled-out light 38B does not have to match the input angle of the coupled-in light <NUM>.

Referring now to <FIG>, there is shown a rough sketch of a reflective-diffract exemplary embodiment, not according to the claimed invention.

The current figure is similar to <FIG>, except incoming light <NUM> is coupled into lightguide <NUM> by a slanted prism <NUM> (in place of diffractive component <NUM>). As the current embodiment includes only one diffractive element (diffractive element <NUM>), chromatic dispersion will be significant, as compared to the embodiment of <FIG> that includes two matching diffractive elements (<NUM> and <NUM>). The chromatic dispersion (aberration) can be reduced by using a narrowband light source.

Referring now to <FIG>, there is shown respective side and front views of a rough sketch of a diffractive-diffractive-reflective exemplary embodiment, not according to the claimed invention. Lightguide <NUM> is a 2D lightguide. In the current embodiment, a first optical component of the set is implemented by diffractive component 5A which is configured for directing the coupled-in light <NUM> in a first direction of expansion (x-axis) within lightguide <NUM>, thereby generating first expanded light 38C. A second optical component of the set is implemented by diffractive component <NUM> that is configured for coupling the first expanded light 38C into lightguide <NUM> in a second direction of expansion (y-axis), thereby generating second expanded light 38D. A third optical component of the set is implemented by a sequence of a plurality of partially reflecting surfaces (facets) <NUM>, preferably at least partially traversing lightguide <NUM> at an oblique angle to the faces of lightguide <NUM>, that is configured for out coupling the second expanded light 38D in a third direction as the coupled-out light 38B.

Referring now to <FIG>, there is shown a diagram of the <FIG> diffraction directions of light propagating in the angular domain (angular space). The angular vectors are also shown, <NUM> is the entrance direction, and after the first element 5A the direction is <NUM>. The diffraction element <NUM> has the opposite optical power therefore light will couple from lightguide <NUM> into lightguide <NUM> having the same direction and no chromatic dispersion (overlapping <NUM>). The facets <NUM> reflect the light without dispersion the preferred direction <NUM> with no chromatic dispersion. Some chromatic dispersion may be introduced by the reflective component, and residual diffraction can compensate for this.

Referring now to <FIG>, there is shown respective side and front view rough sketches of a diffractive-reflective exemplary embodiment, not according to the claimed invention. Lightguide <NUM> is a 2D lightguide. The lateral expansion is made by the diffractive component while the vertical expansion is by the reflective facets. The method of coupling into lightguide <NUM> is not depicted. The light propagates within the lightguide <NUM>, impinges on the diffractive surface (component) <NUM>, and is diffracted toward lightguide <NUM>. The diffractive component <NUM> can be at any surface of lightguide <NUM> (in the current figure, depicted on top). As the light propagates within lightguide <NUM>, the light is coupled-out 38B toward the eye <NUM> by facets <NUM>. This configuration does not need polarization management between lightguide <NUM> and lightguide <NUM>. The injected polarization of the light can be oriented to match that required for facets <NUM>.

Referring now to <FIG>, there is shown respective side and front view rough sketches of a diffractive-diffractive-reflective embodiment according to the claimed invention. A non-diffractive optical component <NUM> is configured to direct light into lightguide <NUM> as coupled-in light, shown as light <NUM>. In the current embodiment, a single lightguide <NUM> is used, and two diffractive components have been implemented as parts of lightguide <NUM>. A first diffractive optical component <NUM> is configured for directing the coupled-in light <NUM> in a first direction of expansion (x-axis) within the one lightguide <NUM>, thereby generating first expanded light 38C. The second diffractive optical component <NUM> is configured for expanding the first expanded light 38C in the one lightguide <NUM> in a second direction of expansion (y-axis), thereby generating second expanded light 38D. The reflective optical component (sequence of a plurality of facets <NUM>) is configured for out-coupling the second expanded light 38D in a third direction as the coupled-out light 38B. As in the above embodiments, the first, second and third directions are non-parallel to each other.

A feature of this embodiment is the use of a single, one-dimensional lightguide. The coupling into the lightguide is by non-diffractive component <NUM> and the light is diverted by strong diffracting pattern <NUM>. The light is guided in one dimension, and therefore expands in another dimension while propagating from left to right along diffractive component <NUM>. As the light encounters diffractive pattern <NUM>, the light is also diverted downward. While propagating downward, the light is reflected toward the observer <NUM> by reflective facets <NUM> (depicted in the side view <FIG>). This configuration includes a single lightguide, does not require polarization management (the polarization of the light injected into the lightguide can be suitable for the reflective facets <NUM>). The combination of diffracting pattern <NUM> and diffracting pattern <NUM> has no resulting chromatic dispersion.

Referring now to <FIG>, there is shown a front view rough sketch of overlapping diffractive-reflective-diffractive exemplary embodiment, not according to the claimed invention. Because of the differing technology, the diffractive and reflective elements can be positioned in overlapping relation on the same lightguide. In the current figure, diffraction grating component <NUM> expands coupled-in light <NUM> in a first direction to produce first expanded light 38C. Lateral aperture expansion is implemented by overlapping diagonal facets <NUM> that couple the light back and forth laterally, expanding the light in a second direction 38D, without introducing chromatic aberration. Diffractive pattern <NUM> is used for coupling the light out of the waveguide.

Referring now to <FIG>, there is shown respective side and front views of a rough sketch of a diffractive-reflective exemplary embodiment, not according to the claimed invention. Transverse expansion is based on a one-dimensional lightguide <NUM> (for example, see patent <CIT>). In <FIG>, the coupling into the lightguide <NUM> is performed by a highly reflecting (partially reflecting and reflecting a majority of the energy) internal facet <NUM>, that reflects the majority of the coupled-in light <NUM> to the right and left sides of the lightguide <NUM>, while a portion of the coupled-in light <NUM> passes through the internal facet <NUM> into the lightguide <NUM>. As the current embodiment includes only one diffractive element, chromatic dispersion will be significant, as compared to the below embodiment of <FIG>. The chromatic dispersion (aberration) can be reduced by using a narrowband light source.

Referring now to <FIG>, there is shown a front view of a rough sketch of a diffractive-diffractive-reflective exemplary embodiment, not according to the claimed invention.

In this embodiment, the coupling into lightguide <NUM> is performed by a diffractive component <NUM>, having high efficiency, that reflects the majority of the coupled-in light <NUM> to the right and left sides of the lightguide <NUM>, while a portion of the coupled-in light <NUM> passes through the diffractive component <NUM> into the lightguide <NUM>.

Similar to the description of <FIG>, first expanded light 38C is diffracted in <FIG> by diffraction components <NUM> and in <FIG> by diffractive components <NUM>, to generate second expanded light 38D in lightguide <NUM>.

As can be seen from the exemplary embodiments, the diffractive components can generally be located on any side of the lightguides. As in previous embodiments, by injecting the proper polarization, there is no need for further management along the apparatus.

Different wavelengths of light are deflected by diffractive patterns in different directions. This phenomenon can be used, for example by near eye displays, by implementing a separate lightguide for every wavelength. A typical embodiment is three lightguides, one each for the wavelengths corresponding to red (R), green (G), and blue (B) colored light. Separate diffractive lateral aperture expanders (one for each color) are combined to a single vertical reflective aperture expander.

Referring now to <FIG>, there is shown respective side and front view rough sketches of a diffractive-diffractive-reflective exemplary embodiment, not according to the claimed invention, with separate diffractive lateral expanders. The current embodiment is based on the above-described embodiment with regard to <FIG> lightguide <NUM> is replaced with a set of lightguides <NUM>, <NUM>, and <NUM>. Each lightguide of the set has a first diffractive component (respectively 133R, <NUM>, 133B) configured for a specific wavelength, in this example red, green, and blue. Each lightguide of the set has a second diffractive component (respectively 134R, <NUM>, 134B) matching the first diffractive component. The coupled-in light <NUM> is injected through the first diffractive components. Each of these first diffractive components is wavelength specific, diffracting the specific associated wavelength of light, and passing other wavelengths of light. Wavelength specific diffraction into each lightguide may be improved by adding a set of dichroic reflectors (respectively 133R1, 133G1, 133B1) after each first diffractive component (133R, <NUM>, 133B). The dichroic reflectors can be based on coating or diffractive reflectors, so different wavelengths are coupled to the different respective lightguides (<NUM>, <NUM>, and <NUM>). The light wavelength diffracted by the first diffractive components (133R, <NUM>, 133B) expands and propagates laterally in the respective lightguides (<NUM>, <NUM>, and <NUM>) as respective first expanded light (38CR, 38CG, 38CB). Each lightguide (<NUM>, <NUM>, <NUM>) has a respective second diffractive component (134R, <NUM>, 134B) that diffracts the respective first expanded light ((38CR, 38CG, 38CB) toward lightguide <NUM>. Light from the upper lightguides pass through the lower lightguides with minimal distortion because the second diffractive components (<NUM>, 134B) are wavelength selective or have low diffraction efficiency for other wavelengths. In lightguide <NUM>, the sequence of a plurality of partially reflecting surfaces <NUM> reflects all wavelengths toward the eye <NUM>.

An alternative description of the current embodiment is that the pair of first 133R and second 134R matching diffractive optical components are augmented with <NUM>) a pair of third <NUM> and fourth <NUM> matching diffractive optical components, and <NUM>) a pair of fifth 133B and sixth 134B matching diffractive optical components. Each of the optical components of the matching pairs has a different diffractive spacing from the optical components of other matching pairs. The diffractive spacing is such that each of the optical components of the matching pairs deflects a different wavelength through similar angles from the optical components of other matching pairs. A first lightguide <NUM> includes the pair of first 133R and second 134R matching diffractive optical components. A second lightguide <NUM> includes the pair of third <NUM> and fourth <NUM> matching diffractive optical components. A third lightguide <NUM> includes the pair of fifth 133B and sixth 134B matching diffractive optical components.

In the current configuration, one lightguide can be in front of the eye <NUM>, and optionally have no polarization management between the lightguides (<NUM>, <NUM>, <NUM>, and <NUM>). In this configuration, the lightguides can be placed directly on top of each other (typically, an air gap is used between the lightguides in order to maintain TIR).

Referring now to <FIG>, there is shown respective side and front views of a rough sketch of a diffractive-reflective exemplary embodiment. The current embodiment is similar to the operation described in reference to <FIG>, with lightguide <NUM> replaced/augmented (replaced with three lightguides 160R, <NUM>, and 160B). The coupling-in of light <NUM> to each lightguide (160R, <NUM>, 160B) is by respective highly reflecting internal facets / central splitting mirrors (165R, <NUM>, 165B). Lateral (transverse) expansion is diffractive in each lightguide (160R, <NUM>, 160B), and then the first expanded light 38C is diffracted/diverted into lightguide <NUM> for out-coupling to the user's eye <NUM>.

The current embodiment is similar to the operation described in reference to <FIG>, with diffractive component <NUM> replaced/augmented by a set of diffractive components (133R, <NUM>, 133B) and associated dichroic reflectors (respectively 133R1, 133G1, 133B1) after each first diffractive component (133R, <NUM>, 133B) in the center of each respective lightguide (159R, <NUM>, 159B). Matching diffractive elements (134R, <NUM>, 134B) are replaced with multiple diffractive elements (134R1, 134R2, 134G1, 134G2, 134B1, 134B2) on either side of the central diffractive components (133R, <NUM>, 133B).

Referring now to <FIG>, there is shown respective side, front, and top views of a rough sketch of a reflective-diffractive-diffractive exemplary embodiment, not according to the claimed invention. In the current embodiment, the reflective aperture expander precedes the diffractive expander. Four lightguides are used: reflective component <NUM>, and three diffractive components (<NUM>, <NUM>, and <NUM>). The reflective component <NUM> is a reflective lateral expanding lightguide. This reflective lightguide <NUM> can be a 1D lightguide (similar to lightguide <NUM> in <FIG>) or a 2D light guide (similar to lightguide <NUM> in <FIG>). The light coupling into the reflective lightguide <NUM> includes all wavelengths of the coupled-in light <NUM>, and therefore the reflective lightguide <NUM> can include a reflector (such as reflecting surface <NUM> in <FIG>, or a prism (such as slanted prism <NUM> in <FIG>).

Facets <NUM> (depicted in top view <FIG>) divert the guided light forward and out of lightguide <NUM> into lightguides <NUM>, <NUM> and <NUM>. Each of lightguides <NUM>, <NUM> and <NUM> have respective coupling-in gratings (209R, <NUM>, 209B). These coupling-in gratings (209R, <NUM>, 209B) have a different period in every lightguide, therefore different wavelength will be coupled by each coupling-in grating to each associated lightguide.

The light propagates within the lightguides (<NUM>, <NUM>, <NUM>) and is coupled-out 38B toward the observer <NUM> by respective gratings (25R, <NUM>, 25B) designed according to wavelength within each lightguide, and matched to respective coupling-in gratings (209R, <NUM>, 209B).

In general, the reflective optical component (facets <NUM>) is configured to expand the coupled-in light <NUM> in a first direction of expansion within a first lightguide <NUM>, thereby generating first expanded light 38C. The first 209R, third <NUM>, and fourth 209B diffractive optical components are configured for coupling respective wavelengths of the first expanded light in respective first <NUM>, second <NUM>, and third <NUM> lightguides. The second 25R, fourth <NUM>, and sixth 25B diffractive optical components are configured for expanding and out-coupling the respective light in a third direction as the coupled-out light 38B.

Referring now to <FIG>, there is shown a diagram of the <FIG> diffraction directions of light propagating in the angular domain (angular space). The angular direction front view of a single lightguide shown in <FIG> is shown in <FIG>. The light is coupled in as <NUM> direction and the reflecting mirrors <NUM> divert the rays to direction <NUM> without dispersion. The diffractive coupling-in component (one of 209R, <NUM>, 209B) divert the rays downward with dispersion while diffractive component (one of gratings 25R, <NUM>, 25B) has the opposite optical power, therefore coupling the light out (direction overlapping <NUM>) with no dispersion.

Claim 1:
An apparatus for optical aperture expansion of a light (<NUM>) produced by a display source (<NUM>) towards an eye (<NUM>) of an observer, the apparatus comprising:
- a lightguide (<NUM>) having a pair of mutually-parallel major surfaces;
- a non-diffractive input coupler (<NUM>) configured to directly couple the light (<NUM>) produced by the display source (<NUM>) into the lightguide (<NUM>) so as to propagate within the lightguide (<NUM>) in an input direction by internal reflection at the major surfaces;
- a set of three optical components associated with said lightguide (<NUM>), said set including:
- a pair of first and second matching diffractive optical components (<NUM>, <NUM>); and
- a reflective optical component including a sequence of a plurality of partially reflective, mutually parallel surfaces (<NUM>) angled obliquely to the major surfaces of the lightguide (<NUM>);
- wherein the pair of first and second matching diffractive optical components (<NUM>, <NUM>) forms a diffractive arrangement integrated with the lightguide (<NUM>) and is configured to diffract the light propagating within the lightguide (<NUM>) in the input direction through a first diffraction that redirects the light so as to propagate within the lightguide (<NUM>) in a first direction non-parallel to the input direction by internal reflection at the major surfaces, and through a second diffraction which progressively redirects the light propagating within the lightguide (<NUM>) in the first direction so as to propagate within the lightguide (<NUM>) by internal reflection at the major surfaces in a second direction parallel to the input direction while expanding the light in the first direction, wherein the second diffractive optical component (<NUM>) diffracts light in a compensatory fashion to the first diffractive component (<NUM>), such as to cancel the dispersion introduced by the first diffractive component (<NUM>); and
- wherein the reflective optical component (<NUM>) forms an output coupler progressively coupling the light expanded in the first direction by the diffractive arrangement out of the lightguide (<NUM>) towards the eye (<NUM>) of the observer while expanding the light in the second direction.