Optical system

Abstract

The optical system includes a light guide and an image projection arrangement. The image projection arrangement includes a polarizing beam splitter prism having a diagonal polarizing beam splitter surface that reflects light from the image generation matrix to the reflective collimating optics. A coupling prism is disposed between the polarizing beam splitter surface and the light guide inlet, providing a coupling surface coplanar or parallel to one of the parallel major surfaces of the light guide. The reference length RL is defined as a distance along the optical axis from the main plane of the collimating optics to the surface of the polarizing beam splitter. Both a first optical path from the image plane to the main plane and a second optical path from the main plane to the light guide entrance have a length less than 3 * RL.

Description

[0001] Technical Field and Background Art

[0002] The present invention relates to an optical system, and in particular, to a compact image projector integrated with an optical waveguide.

[0003] U.S. Patent No. 10,546,417 discloses an advantageous compact configuration of integrating an image projector with an optical waveguide. Figures 16 and 17 of this patent are reproduced here with their original reference numerals as Figures 1A and 1B, respectively. These figures show an image projector including two polarizing beam splitter (PBS) prisms. The first PBS prism 500 receives an s-polarized input illumination 505, which is reflected by the PBS surface 507 towards a reflective polarization modulation spatial light modulator such as an LCOS chip 509. The selectively modulated p-polarized divergent image light 511 passes through the PBS surface 507 and is converted to s-polarized light by a half-wave retarder (not assigned a reference numeral) when entering the second PBS prism 526, and is reflected towards a collimating reflective optical device 515 with a quarter-wave retarder (not assigned a reference numeral) at the second PBS surface 513. The reflective optical device 515 collimates the image light into a collimated image, which has a field of view 517 extending from the steepest angle ray 518a to the shallowest angle ray 518b, has p-polarization, and this collimated image passes through the PBS surface 513 to be coupled into the optical waveguide 503. The coupling into the optical waveguide 503 is achieved in part by reflection at a surface 528 provided at a lower portion that is a continuation of one of the surfaces forming the optical waveguide of the PBS prism 526. Figures 1A and 1B differ in the angular range for coupling the image, where Figure 1A presents an image at a relatively high angle, while Figure 1B shows an injected image at a shallower angle. A description of any reference numerals in Figures 1A and 1B not mentioned above can be found in the '417 patent itself. Summary of the Invention

[0004] The present invention is an optical system.

[0005] In accordance with the teachings of embodiments of the present invention, an optical system is provided that includes: (a) an optical waveguide having a pair of parallel major surfaces that support the propagation of image light by total internal reflection at the major surfaces, the optical waveguide having an optical waveguide entrance; (b) an image projection arrangement for generating a collimated image to be introduced into the optical waveguide, the image projection arrangement including: (i) a polarization beam splitter prism having a first face, a second face, and a diagonal polarization beam splitter surface, (ii) an image generation matrix associated with the first face that defines an image plane, and (iii) a reflective collimating optical device associated with the second face that is deployed to collimate image light from the image plane reflected by the polarization beam splitter surface, the reflective collimating optical device having a principal plane and an optical axis; and (c) a coupling prism between the polarization beam splitter surface and the optical waveguide entrance that provides a coupling surface coplanar or parallel with one of the parallel major surfaces, wherein the optical waveguide and the coupling surface are tilted with respect to the optical axis such that the collimated image passing through the polarization beam splitter surface from the reflective collimating optical device enters the optical waveguide entrance at an angle that undergoes total internal reflection within the optical waveguide, partially directly and partially after reflection from the coupling surface, and wherein a reference length RL is defined as the distance along the optical axis from the principal plane to the polarization beam splitter surface, a first optical path from the image plane to the principal plane has a length less than 3×RL, and a second optical path from the principal plane to the optical waveguide entrance has a length less than 3×RL.

[0006] In accordance with another feature of embodiments of the present invention, the second optical path from the principal plane to the optical waveguide entrance has a length less than 2×RL.

[0007] In accordance with another feature of embodiments of the present invention, the rays of the collimated image entering the optical waveguide entrance span an angular field of view, and wherein the angular field of view is provided by image light from the image plane that reaches the reflective collimating optical device after reflection from an effective area of the polarization beam splitter surface, the effective area extending on both sides of the plane of the coupling surface.

[0008] In accordance with another feature of embodiments of the present invention, the entrance of the optical waveguide is defined by an optical cut edge between the optical waveguide and the coupling prism, and wherein a plane passing through the optical cut edge perpendicular to the major surface intersects the effective area of the polarization beam splitter surface.

[0009] In accordance with another feature of embodiments of the present invention, the image generation matrix is a micro-LED array.

[0010] In accordance with another feature of embodiments of the present invention, a field lens arrangement is also provided that includes at least one lens, the field lens arrangement being between the micro-LED array and the first face of the polarization beam splitter prism.

[0011] According to another feature of an embodiment of the present invention, at least one lens of the field lens arrangement is integrated with the micro-LED array.

[0012] According to another feature of an embodiment of the present invention, the image generation matrix is a reflective spatial light modulator (SLM), and the optical system further includes an illumination arrangement between the SLM and the first surface of the polarization beam splitter prism. The illumination arrangement includes an illumination light guide having two mutually parallel surfaces for guiding illumination across the SLM by internal reflection within the illumination light guide. The illumination light guide includes a set of internal partially reflective surfaces for gradually redirecting the illumination out of the illumination light guide towards the SLM.

[0013] According to another feature of an embodiment of the present invention, a field lens arrangement is also provided. The field lens arrangement includes at least one lens and is between the SLM and the first surface of the polarization beam splitter prism.

[0014] According to another feature of an embodiment of the present invention, at least one lens of the field lens arrangement is integrated with the SLM.

[0015] In accordance with the teachings of embodiments of the present invention, an optical system is also provided, the optical system comprising: (a) a light guide having a pair of parallel major surfaces that support the propagation of image light by internal reflection at the major surfaces, the light guide having a light guide entrance; (b) an image projection arrangement for generating a collimated image to be introduced into the light guide, the image projection arrangement comprising: (i) a first micro-LED array, a second micro-LED array, and a third micro-LED array, which are respectively configured to generate images of a first color, a second color, and a third color, (ii) a dichroic combiner having a first input surface, a second input surface, and a third input surface that respectively support the first micro-LED array, the second micro-LED array, and the third micro-LED array, the dichroic combiner including a first diagonally deployed dichroic reflector and a second diagonally deployed dichroic reflector, the first diagonally deployed dichroic reflector selectively reflecting the first color and transmitting the second and third colors, the second diagonally deployed dichroic reflector selectively reflecting the third color and transmitting the second color, (iii) a polarization beam splitter prism associated with the dichroic combiner, having a diagonal polarization beam splitter surface, and (iv) reflective collimating optics associated with a face of the polarization beam splitter prism and deployed to collimate the image light from the first micro-LED array, the second micro-LED array, and the third micro-LED array, the image light being combined by the dichroic combiner and reflected by the polarization beam splitter surface, the reflective collimating optics having a principal plane and an optical axis; and (c) a coupling prism between the polarization beam splitter surface and the light guide entrance, the coupling prism providing a coupling surface coplanar with or parallel to one of the parallel major surfaces, wherein the light guide and the coupling surface are tilted with respect to the optical axis such that the collimated image passing through the polarization beam splitter surface from the reflective collimating optics enters the light guide entrance partially directly and partially after reflection from the coupling surface at an angle to undergo internal reflection within the light guide, and wherein a reference length RL is defined as the distance along the optical axis from the principal plane to the polarization beam splitter surface, and the optical path from the principal plane to the light guide entrance has a length less than 3×RL and preferably less than 2×RL.

[0016] According to another feature of embodiments of the present invention, the second dichroic reflector is transparent to the first color, and wherein the second dichroic reflector is deployed non-parallel to the first dichroic reflector to intersect the first dichroic reflector. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The present invention is described herein by way of example only with reference to the drawings, in which:

[0018] The above FIGS. 1A and 1B respectively correspond to FIGS. 16 and 17 of U.S. Patent No. 10,546,417;

[0019] Figure 2A is a schematic isometric view of an optical system, Figure 2A showing the propagation of an image from a projector through a two-dimensional aperture expanding light guide;

[0020] Figure 2B is Figure 2A a schematic front view of the optical system of

[0021] Figure 3 a schematic side view of an optical system constructed and operated in accordance with the teachings of an embodiment of the present invention, the optical system including a compact image projector integrated with a light guide;

[0022] Figure 4A is a schematic side view of another optical system constructed and operated in accordance with the teachings of an embodiment of the present invention, the optical system including a compact image projector that employs a dichroic combiner arrangement and a micro LED array and is integrated with a light guide;

[0023] Figure 4B is suitable for use in Figure 4A a schematic side view of an alternative implementation of a dichroic combiner arrangement for an optical system;

[0024] Figure 5A is a schematic side view of another optical system constructed and operated in accordance with the teachings of an embodiment of the present invention, the optical system including a compact image projector that employs a color micro LED array and is integrated with a light guide;

[0025] Figure 5B is Figure 5A a schematic side view of another optical system similar to the system of

[0026] Figure 6 is Figure 5A a schematic side view of another optical system similar to the system of Figure 6 showing a further reduction in the size of the coupling prism to enable a reduction in the optical path length from the collimating optics to the light guide entrance;

[0027] Figure 7A is Figure 6 a similar schematic side view, Figure 7A showing an implementation having a coupling surface that is not coplanar with the surface of the light guide;

[0028] Figure 7B is Figure 7A an enlarged view of the region denoted VII in

[0029] Figure 8A isFigure 6 A schematic side view of another similar optical system, but employing a reflective polarization modulation spatial light modulator illuminated via an illumination light guide; and

[0030] Figure 8B is similar to Figure 8A a similar view Figure 8B showing a variant implementation of the illumination light guide. Detailed Description

[0031] The present invention provides an optical system for a compact image projector integrated with an optical waveguide.

[0032] The principles and operation of the optical system according to the present invention can be better understood with reference to the accompanying drawings and the following description.

[0033] By way of introduction, the present invention relates to various improvements to the compact optical system described above with reference to FIGS. 1A and 1B, which improvements employ a compact image projector integrated with an optical waveguide for delivering an image to a viewer's eye in the context of what is typically an augmented reality display. In some most preferred implementations, the optical system includes a two-dimensional optical aperture expanding light guide. Figure 2A and Figure 2B schematically shows the overall architecture of such an arrangement.

[0034] Figure 2A and Figure 2B show a schematic isometric view and a side view of an image projector 2 attached to an optical waveguide 10 having a front parallel face 12a and a rear parallel face 12b. The optical waveguide 10 includes a first set of partially reflective elements (also referred to as "facets") 14 perpendicular to the waveguide faces 12a and 12b. A second set of parallel partially reflective elements (facets) 16 are angled with respect to the waveguide faces.

[0035] A beam 18a schematically represents a collimated image from the projector 2 having a polarization orientation 20a perpendicular to the faces 12a and 12b (which can be referred to as P polarization with respect to these faces). This beam propagates within the optical waveguide 10 (represented by 18b) while maintaining its polarization. Although schematically shown as a single arrow, the light is at an angle to propagate by total internal reflection from the faces 12a and 12b. The beam 18b impinges on the facets 14. Since these facets 14 are perpendicular to the faces 12a and 12b, the polarization 20b of the impinging beam 18b is parallel to the surface of the facets 14, corresponding to S polarization with respect to these facets.

[0036] The partial reflection beam 18c (shown as coming from one facet, but there are partial reflection beams from each of the facets) impinges on the facet 16. Due to the different orientations of the facets 16, the impinging beam has P polarization with respect to these facets.

[0037] In some cases, it may be easier to design a multilayer dielectric coating to provide the desired partial reflectivity and angular dependence for the facets 14 and / or 16 for S polarization (with respect to the facets) than for P polarization. Thus, the following projector arrangement may be advantageous: the projector arrangement introduces P polarization into the light guide 10 such that the image is inherently S polarized with respect to the facets 14. Certain embodiments of the present invention described below can achieve such P polarization injection. Optionally, a half-wave retarder plate can be included within the light guide 10, between the two sets of facets, to convert the light reaching the facets 16 into S-polarized light with respect to these facets.

[0038] Now turning to Figure 3 , Figure 3 An optical system according to a first aspect of the present invention is shown. The structure and function of this optical system are substantially similar to those in FIG. 1A, but the structure is simplified by implementing the portion of the PBS prism between the PBS surfaces 510A and 510B as a single block. This facilitates the production of integrating the PBS surfaces 510A and 510B preferably as dielectric coatings on opposite faces of an edge-parallel prism. This structure is achieved by repositioning the reflective polarization-modifying spatial light modulator (SLM) such that no polarization rotation element is required between the PBS surfaces. The combination of the above double PBS with the surface 528 of the lower portion of the PBS prism (coupling prism) (which is an extension of or parallel to the surface of the light guide) enables a compact and efficient implementation of the optical system.

[0039] All other features of this construction are similar in structure and function to those in FIG. 1A above and are labeled with the same reference numerals. This construction can also be achieved with a shallower injection angle (similar to FIG. 1B).

[0040] Now turning to Figure 4A , an optical system according to the teachings of an embodiment of the present invention is adapted to employ an active matrix image generator (such as an OLED array or more preferably a micro-LED array) to generate an image. In this implementation, a set of three separate arrays (denoted 605R, 605G, and 605B) each provide a different color component of the image, shown as solid lines, dashed lines, and dotted lines respectively.

[0041] Accordingly, in addition to the light guide 10 having a pair of parallel main surfaces 12a and 12b, the optical system further includes an image projection arrangement 2 for generating a collimated image to be introduced into the light guide, the image projection arrangement including a first micro-LED array 605R, a second micro-LED array 605G, and a third micro-LED array 605B that are configured to generate images of a first color, a second color, and a third color (e.g., red, green, and blue for full-color image generation), respectively. The dichroic combiner 606 has a first input surface, a second input surface, and a third input surface that support the first micro-LED array 605R, the second micro-LED array 605G, and the third micro-LED array 605B, respectively. The dichroic combiner 606 includes: a first diagonally deployed dichroic reflector 600A that selectively reflects the first color and transmits the second and third colors; and a second diagonally deployed dichroic reflector 600B that selectively reflects the third color and transmits the second color.

[0042] As previously described, the remainder of the image projection arrangement includes: a polarization beam splitter prism 526 associated with the dichroic combiner, the polarization beam splitter prism 526 having a diagonal polarization beam splitter surface 510B; and a reflective collimation optical device 515 associated with a face of the polarization beam splitter prism 526 and deployed to collimate image light from the first micro-LED array, the second micro-LED array, and the third micro-LED array that is combined by the dichroic combiner 606 and reflected by the polarization beam splitter surface 510B. The reflective collimation optical device 515 has a principal plane PP and an optical axis OA.

[0043] The coupling prism can be the entire prism between the polarization beam splitter surface 510B and the light guide entrance, and the coupling prism provides a coupling surface 528 that is coplanar or parallel with one of the parallel main surfaces of the light guide 10.

[0044] The light guide 10 and the coupling surface 528 are tilted with respect to the optical axis OA such that the collimated image passing through the polarization beam splitter surface 510B from the reflective collimation optical device 515 enters the light guide entrance partially directly and partially after reflection from the coupling surface 528 at an angle that undergoes total internal reflection within the light guide 10.

[0045] The result of this structure is a favorably compact optical arrangement. In this document, the compactness of various configurations is quantified by reference to a "reference length" RL, which is defined as the distance along the optical axis OA from the principal plane PP to the surface of the polarization beam splitter (i.e., where OA intersects the plane of PBS surface 510B). In this implementation, the optical path from the principal plane to the light guide entrance preferably has a length of less than 3×RL (and in some particularly preferred cases less than 2×RL). A coupling prism configuration (as will be described below with reference to Figure 6 is used) to achieve an optimal reduction in the distance from the collimating optics to the light guide entrance. The optical path from the image generation plane (micro LED array) to the principal plane of the reflective collimating optics (corresponding to the focal length of the collimating optics) is preferably no greater than 4×RL.

[0046] In Figure 4A the embodiment, the dichroic combiner 606 (which may be referred to as a "trichroic combiner" since it combines three different color sources) is shown as an "X-cube" in which the first dichroic reflector 600A and the second dichroic reflector 600B intersect each other. In this case, the second dichroic reflector 600B is implemented to be transparent to the first color as well, such that it does not interfere with the first color reaching the entire first dichroic reflector 600A. In some cases, a preferred alternative is a trichroic prism configuration, such as the trichroic combiner prism shown in Figure 4B , which corresponds to the prism structure common in 3CCD camera devices. In this case, light of the first color does not reach the second dichroic reflector, thereby relaxing the spectral requirements for the second dichroic reflector.

[0047] Although two combiner prism configurations are shown here in which the first color image and the third color image are input from opposite sides of the prism, (and thus all principal rays are visible in a single cross-section), the orientation of the dichroic reflectors can alternatively be selected such that the first color image and the third color image are input on adjacent faces of the prism, for example where one color image is introduced from the direction into the page. Additionally, the entire illumination prism can be rotated 90 degrees such that the first image and the third image enter and exit the page simultaneously.

[0048] If the active matrix image source produces unpolarized light, the PBS surface 510B can be relied upon to select the S-polarized light, which is transmitted to the collimating optics. In this case, the uncollimated P-polarized light passes directly through the PBS surface, continues to the lower surface of the coupling prism, where it escapes (since it is not at the angle of total internal reflection), and is absorbed by an external absorbing material (not shown). Alternatively, a polarizer can be included at the surface associated with the active matrices 605A, 605B, 605C, or a single such polarizer can be positioned between the dichroic combiner prism 606 and the PBS surface 510B to filter out the P-polarization before it reaches the PBS surface.

[0049] All other features of this configuration are similar in structure and function to those in FIG. 1A above and are labeled with the same reference numerals. This configuration can also be implemented with a shallower injection image angle (similar to FIG. 1B).

[0050] Turning now to the remaining Figures 5A to 8B , Figures 5A to 8B A series of implementations of an optical system in accordance with the teachings of embodiments of the present invention are shown, in which the image plane of the image generator is significantly reduced compared to previous embodiments, thereby allowing the use of collimating optics with a focal length similar to the distance from the collimating optics to the light guide entrance (generally within about + / - 50%). As in the previous embodiments, positioning the collimating optics close to the light guide entrance enables reduction of the size of the optics for a given field of view (FOV). The reduction in the distance from the image plane of the image generator to the collimating optics results in a corresponding reduction in the focal length of the collimating optics, thereby increasing the light collection efficiency per pixel and enabling a larger FOV for an image matrix of a given size. In addition, by making the focal length similar to the distance from the optics to the light guide entrance, optical aberrations are reduced and the optics required to correct the aberrations are simplified.

[0051] Generally speaking, Figures 5A to 8BThe optical system includes an optical waveguide 10 having a pair of parallel major surfaces 12a and 12b that support the propagation of image light by total internal reflection at the major surfaces. The optical waveguide has an optical waveguide entrance that is bounded on one side by a cut edge 523. The optical system also includes an image projection arrangement 2 for generating a collimated image to introduce into the optical waveguide. The image projection arrangement 2 includes a polarization beam splitter prism 536 having a first face 630, a second face 632, and a diagonal polarization beam splitter surface 610. An image generation matrix 611 or 612 (discussed further below) is associated with the first face 630 and defines an image plane. Reflective collimating optics 615 associated with the second face 632 are deployed to collimate image light reflected by the polarization beam splitter surface 610 from the image plane. The reflective collimating optics 615 have a principal plane PP and an optical axis OA.

[0052] The optical system also includes a coupling prism 637 between the polarization beam splitter surface 610 and the entrance of the optical waveguide 10. The coupling prism 637 provides a coupling surface 638 that is coplanar or parallel to one of the parallel major surfaces 12b of the optical waveguide 10. The optical waveguide 10 and the coupling surface 638 are tilted with respect to the optical axis OA such that the collimated image passing through the polarization beam splitter surface 610 from the reflective collimating optics 615 enters the optical waveguide entrance partly directly and partly after reflection from the coupling surface 638 at an angle to undergo total internal reflection within the optical waveguide 10.

[0053] A feature of a group of embodiments of the present invention is that the first optical path from the image plane to the principal plane is similar in size to the second optical path from the principal plane to the optical waveguide entrance, and both optical paths are relatively short. Quantitatively, again using a reference length RL, which is defined as the distance along the optical axis OA from the principal plane PP to the polarization beam splitter surface 610. In terms of this reference length, the first optical path from the image plane to the principal plane preferably has a length less than 3×RL, and the second optical path from the principal plane to the optical waveguide entrance preferably also has a length less than 3×RL. In some cases, the second optical path from the principal plane to the optical waveguide entrance has a length less than 2×RL. This results in a particularly compact and efficient optical system. Some specific implementations of such an optical system will now be discussed.

[0054] In Figures 5A to 7A an implementation, the image generation matrix is an active matrix image source, which can be an OLED display or, more preferably, a micro-LED array 611. Most preferably, the micro-LED array is a color display including closely spaced or otherwise combined pixels of the three primary colors. A monolithic micro-LED color display can be commercially obtained from Jade Bird Display Co., Ltd. in Shanghai, China (JDB), such as PHOENIX TMSeries.

[0055] In this configuration, there is no external illumination and light from the active matrix image source 611 directly enters the PBS prism 636. In some configurations, the field lens 616 can be implemented on the surface of the active matrix image source 611 and / or on the surface 630 of the PBS prism 636. Since no separate illumination prism is required, this configuration achieves a shorter effective focal length of the collimating optics 615, resulting in a larger illumination field and better system light collection. The shorter distance from the reflective collimating optics 615 to the light guide entrance 523 enables a small and compact optics for a given FOV.

[0056] Figure 5A This configuration for a relatively steep image injection angle is shown, while Figure 5B This configuration for a shallow image injection angle into the light guide is shown. In the latter case, the shallowest part of the field of view labeled 518b includes rays that essentially originate from the edge of the focusing optics, and thus a relatively long coupling surface 638 is required. The coupling surface 638 extends from just below the PBS surface 610 and through the supplementary coupling prism 535.

[0057] It is possible to use Figure 6 The configuration shown to further reduce the distance between the collimating optics 615 and the entrance of the light guide. Figure 6 The case where the required size of the PBS surface 610 is larger than the coupling prism entrance size is shown. This applies to the case of projecting a large field of view, where complex and wide optics are required. Here, the light projected from the image generator 611 passes through a field lens arrangement that includes a field lens 622A applied to the surface of the active matrix image source 611 and another field lens 622B attached to the PBS prism surface 630. As shown, the sample light path from the image source 611 reaches the reflective collimating optics 615 after reflection from the PBS surface 610. This sample light path requires the entire area of the PBS surface 610 (referred to as the "effective area" of the PBS surface) as shown to fill the light guide entrance 523 with the entire desired FOV. At the same time, the light path from the reflective collimating optics 615 to the light guide entrance 523 only passes through a sub-region of the PBS surface 610. This enables the coupling prism 637 to only contact the relevant sub-region of the PBS surface and allows the light guide entrance to be closer to the collimating optics.

[0058] This configuration satisfies one or more of a number of unique geometric constraints. First, it can be seen that the effective area of the PBS surface 610 extends to both sides of the plane of the coupling surface 638. Additionally, as defined above, the entrance of the optical waveguide 10 is defined by the optical cut-off edge 523 between the optical waveguide and the coupling prism 637. In this case, the plane passing through the optical cut-off edge 523 perpendicular to the main surfaces 12a and 12b intersects the effective area of the polarizing beam splitter surface 610.

[0059] Another geometric constraint that places the optical waveguide entrance close to the reflective collimating optics is that the optical waveguide entrance is preferably located within a virtual cube that is constructed by providing a mirror image of the upper PBS prism 636 that is also below the PBS surface 610, and this virtual cube is represented by the phantom dashed outline 639.

[0060] In any case, the image light collimated by the optics 615 preferably fills the optical waveguide aperture with rays corresponding to all parts of the FOV - whether direct rays (downward propagating rays) or rays after reflection in the coupling surface 638 (upward propagating rays).

[0061] Here, in a preferred implementation, the reflective collimating optics 615 is shown as a compound refractive - reflective lens that includes a doublet 618 in front of the reflective surface. The presence of the doublet 618 provides design flexibility to correct for chromatic aberration that may be introduced by other parts of the optical system, which includes but is not limited to the field lens arrangements 622A, 622B and the coupler - out arrangement for coupling the image towards the viewer's eye. The main collimating optical power is typically provided by the reflective surface of the optics 615 itself, which is achromatic.

[0062] The "principal plane" PP of the reflective collimating optics 615 is defined in a conventional manner and corresponds to the plane at which parallel rays entering from one side of the optical system intersect the corresponding converging rays on the other side of the optical system, while ignoring the details of the ray paths within the lens arrangement. The system of lenses has a principal image plane and a principal object plane, but due to the symmetry of the reflective lens system, these two planes typically coincide. As described above, if the main optical power of the collimating arrangement is in the reflective surface, the principal plane is typically close to that surface.

[0063] As described above, the coupling reflector 638 can be coplanar or parallel with the main optical waveguide surface 12b. Now reference will be made to Figure 7A and Figure 7B to describe the special significance of implementing the coupling surface 638 parallel but slightly offset from the main surface 12b.

[0064] In practice, there are engineering challenges associated with the attachment between the light guide 10 and the coupling prism 637. Specifically, in the case where the coupling prism 637 is attached to the light guide 10 by a refractive index-matching optical adhesive, it is challenging to achieve a high-quality continuous surface from the coupling surface 638 across the adhesive boundary to the light guide surface 12b. Any imperfections in the surface at this boundary can lead to scattering, which will propagate in the light guide and degrade the image quality. This problem becomes even more prominent in the design of adding additional optical elements (such as wave plates, depolarizers, or other elements) at the interface between the coupling prism and the light guide, resulting in additional transitions between different optical materials with different physical properties, and thus further hindering the attempt to achieve a continuous high-optical-quality surface.

[0065] Figure 7A and Figure 7B illustrates how, at the junction between the coupling prism 637 and the light guide 10, even in the case of an additional inserted optical element 700, small steps between the elements can eliminate or at least reduce the amount of scattered light that enters and is guided within the light guide 10.

[0066] In the example shown here, the surface 638 of the coupling prism 637 is offset downward (outward) relative to the parallel surface 12b of the light guide 10 such that not all of the light impinging on the interface will enter the light guide. The degree of displacement between 638 and 12b is preferably minimal and is defined such that: prior to the interference at the boundary (whether with the optical element 700 or with the light guide 10), the last ray 702a impinging on the edge of the surface 638 will be reflected as ray 702b to enter at the entrance edge of 12b, while the rays that impinge on the interference (i.e., at or just beyond the interface boundary) and are scattered (dashed arrows) will not enter the light guide. This condition should be met for the steepest rays entering the light guide and will thus also be met for shallower rays.

[0067] Now turning to Figures 8A to 8B, such a particularly compact optical system can also be implemented using an image generation matrix implemented as a reflective spatial light modulator (SLM), such as a liquid-crystal-on-silicon (LCOS) modulator 612. To reduce the optical path from the SLM to the collimating optics to less than 3×RL, the optical system preferably employs a light guide-based illumination arrangement that is interposed between the SLM 612 and the first face 630 of the polarization beam splitter prism 636. The illumination arrangement employs illumination light guides 624A, 624B that have two mutually parallel surfaces for guiding illumination across the SLM by total internal reflection within the illumination light guide, and the illumination light guide has a set of internal partially reflective surfaces 626 for progressively redirecting S-polarized illumination out of the illumination light guide towards the SLM. The P-polarized reflected image is reflected from the LCOS and passes through the facets 626 to enter the PBS prism 636. To manage polarization, the system can include a polarizer after the light guide (on top of the PBS) to filter out non-image S-polarization.

[0068] The image light entering the PBS prism 636 should typically be S-polarized with respect to the PBS surface 610. This can be achieved by including a half-wave retarder plate between the illumination light guide (or subsequent polarizer) and the PBS prism, or by rotating the illumination arrangement 90 degrees with respect to the PBS prism such that the illumination is injected into the page of the drawing (not shown). This second option results in P-polarized image light with respect to the illumination facets 626 being S-polarized with respect to the PBS surface 610. Optionally, in either of these cases, the PBS surface 610 itself can be used as a filter for S-polarized image light from the LCOS. In this case, any P-polarized light passing through the PBS surface 610 will escape from the optics because it arrives at the lower surface of the coupling prism at an angle that does not undergo total internal reflection, where it is preferably absorbed by absorbing material external to the optical arrangement.

[0069] It is generally advantageous to include a field lens arrangement including at least one field lens 622A, 622B between the SLM 612 and the first face of the polarization beam splitter prism 630. In Figure 8A the example, the illumination arrangement is directly associated with the SLM, and the field lens is deployed between the illumination arrangement and the PBS prism 636.

[0070] Figure 8B Another preferred option is shown, where at least one lens 622A of the field lens arrangement is integrated with the SLM, and the illumination light guide 624B is placed on the side of the field lens leaving the SLM 612. Another advantage of this architecture is that the illumination light guide 624B is significantly away from the image plane, thereby reducing the risk that the facet pattern may be visible as interference in the image.

[0071] Figure 8A and Figure 8B two configurations show that even when using a reflective SLM, a highly compact image projector can be integrated with the light guide 10.

[0072] In all of the above configurations, the image projector configuration injects P-polarized light into the light guide (unless intentionally further modified). As described above with reference to Figure 2A and Figure 2B this polarization is preferred in many light guide configurations.

[0073] It will be recognized that the above description is intended only as an example, and many other embodiments are possible within the scope of the invention defined by the appended claims.

Claims

1. An optical system, comprising: (a) an optical waveguide having a pair of parallel major surfaces that support the propagation of image light by internal reflection at the major surfaces, the optical waveguide having an optical waveguide entrance; (b) an image projection arrangement for generating a collimated image to be introduced into the optical waveguide, the image projection arrangement comprising: (i) a polarization beam splitter prism having a first face, a second face, and a diagonal polarization beam splitter surface, (ii) an image generation matrix associated with the first face, the image generation matrix defining an image plane, and (iii) a reflective collimating optical device associated with the second face, the reflective collimating optical device being deployed to collimate image light from the image plane reflected by the polarization beam splitter surface, the reflective collimating optical device having a principal plane and an optical axis; and (c) a coupling prism between the polarization beam splitter surface and the optical waveguide entrance, the coupling prism providing a coupling surface coplanar with or parallel to one of the parallel major surfaces, wherein the optical waveguide and the coupling surface are inclined with respect to the optical axis such that the collimated image passing through the polarization beam splitter surface from the reflective collimating optical device enters the optical waveguide entrance partially directly and partially after reflection from the coupling surface at an angle to undergo internal reflection within the optical waveguide, and wherein a reference length RL is defined as the distance along the optical axis from the principal plane to the polarization beam splitter surface, a first optical path from the image plane to the principal plane has a length less than 3×RL, and a second optical path from the principal plane to the optical waveguide entrance has a length less than 3×RL.

2. The optical system according to claim 1, wherein, The second optical path from the principal plane to the optical waveguide entrance has a length less than 2×RL.

3. The optical system according to claim 1, wherein, The rays of the collimated image entering the optical waveguide entrance span an angular field of view, and wherein the angular field of view is provided by image light from the image plane that reaches the reflective collimating optical device after reflection from an effective area of the polarization beam splitter surface, the effective area extending on both sides of the plane of the coupling surface.

4. The optical system according to claim 3, wherein, The entrance of the optical waveguide is defined by an optical cut-off edge between the optical waveguide and the coupling prism, and wherein a plane passing through the optical cut-off edge perpendicular to the major surface intersects the effective area of the polarization beam splitter surface.

5. The optical system according to claim 1, wherein, The image generation matrix is a micro-LED array.

6. The optical system according to claim 5, further comprising a field lens arrangement including at least one lens, the field lens arrangement being between the micro-LED array and the first face of the polarization beam splitter prism.

7. The optical system according to claim 6, wherein, At least one lens of the field lens arrangement is integrated with the micro-LED array.

8. The optical system according to claim 1, wherein, The image generation matrix is a reflective spatial light modulator (SLM), and the optical system further includes an illumination arrangement between the first surface of the SLM and the polarization beam splitter prism. The illumination arrangement includes an illumination light guide having two mutually parallel surfaces for guiding illumination across the SLM by internal reflection within the illumination light guide. The illumination light guide includes a set of internal partially reflective surfaces for gradually redirecting the illumination out of the illumination light guide towards the SLM.

9. The optical system according to claim 8, further including a field lens arrangement including at least one lens, the field lens arrangement being between the SLM and the first surface of the polarization beam splitter prism.

10. The optical system according to claim 9, wherein, At least one lens of the field lens arrangement is integrated with the SLM.

11. An optical system includes: (a) A light guide having a pair of parallel main surfaces that support the propagation of image light by internal reflection at the main surfaces. The light guide has a light guide entrance. (b) An image projection arrangement for generating a collimated image to be introduced into the light guide. The image projection arrangement includes: (i) A first micro-LED array, a second micro-LED array, and a third micro-LED array respectively configured to generate images of a first color, a second color, and a third color. (ii) A dichroic combiner having a first input surface, a second input surface, and a third input surface that respectively support the first micro-LED array, the second micro-LED array, and the third micro-LED array. The dichroic combiner includes a first diagonally deployed dichroic reflector and a second diagonally deployed dichroic reflector. The first diagonally deployed dichroic reflector selectively reflects the first color and transmits the second color and the third color. The second diagonally deployed dichroic reflector selectively reflects the third color and transmits the second color. (iii) A polarization beam splitter prism associated with the dichroic combiner, having a diagonal polarization beam splitter surface, and (iv) Reflective collimating optics associated with the surface of the polarization beam splitter prism and deployed to collimate the image light from the first micro-LED array, the second micro-LED array, and the third micro-LED array. The image light is combined by the dichroic combiner and reflected by the polarization beam splitter surface. The reflective collimating optics has a principal plane and an optical axis; and (c) A coupling prism between the polarization beam splitter surface and the light guide entrance, the coupling prism providing a coupling surface coplanar or parallel with one of the parallel main surfaces. Wherein, the light guide and the coupling surface are inclined with respect to the optical axis such that the collimated image from the reflective collimating optical device passing through the surface of the polarization beam splitter enters the light guide entrance partially directly and partially after reflection from the coupling surface at an angle that undergoes total internal reflection within the light guide. And wherein, a reference length RL is defined as the distance along the optical axis from the main plane to the surface of the polarization beam splitter, and the optical path from the main plane to the light guide entrance has a length less than 3×RL.

12. The optical system according to claim 11, wherein, The optical path from the main plane to the light guide entrance has a length less than 2×RL.

13. The optical system according to claim 11, wherein, The second dichroic reflector is transparent to the first color, and wherein the second dichroic reflector is deployed non-parallel to the first dichroic reflector to intersect the first dichroic reflector.

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