Compact display system having field-of-view magnification

Abstract

There is provided an optical device, including a first light-transmitting substrate having at least two parallel major surfaces and edges, characterized by a main propagation direction axis parallel to the major surfaces and an orthogonal axis normal to the main propagation direction axis and parallel to the major surfaces, an input aperture, an output aperture, an eye-motion box, a coupling-in element for coupling light waves into the substrate to effect total internal reflection from the major surfaces of the substrate, and a coupling-out element located between the two major surfaces of the light-transmitting substrate for coupling light waves out of the substrate through the output aperture into the eye-motion box, wherein the coupling-in element comprises at least a first diffractive element and the coupling-out element comprises at least a first lat reflecting surface without a diffractive element.

Description

[0001] COMPACT DISPLAY SYSTEM HAVING FIELD-OF-VIEW MAGNIFICATION

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to substrate-based light wave guided optical devices, and particularly to devices which include reflecting surfaces carried by a light-transmissive substrate.

[0004] The invention can be implemented to advantage in many imaging applications, such as head-mounted and head-up displays, cellular phones, compact displays, and 3-D displays.

[0005] BACKGROUND OF THE INVENTION

[0006] One of the important applications for compact optical elements is in headmounted displays (HMDs), wherein an optical module serves both as an imaging lens and a combiner, in which a two-dimensional display is imaged to infinity and reflected into the eye of an observer. The display can be obtained directly from either a spatial light modulator (SLM), such as a cathode ray tube (CRT), a liquid crystal display (LCD), an organic light emitting diode array (OLED), a scanning source and similar devices, indirectly, employing a relay lens, or an optical fiber bundle. The display comprises an array of elements (pixels) imaged to infinity by a collimating lens and transmitted into the eye of the observer using a reflecting or partially reflecting surface acting as a combiner for non- see-through and see- through applications, respectively. Typically, a conventional, free-space optical module is used for these purposes. As the desired field-of-view (FOV) of the system increases, such a conventional optical module becomes larger, heavier, and bulkier, and, therefore, even for a moderate-performance device, is impractical. Therefore, this is a major drawback for all kinds of displays, especially in HMDs, wherein the system should be as light and compact as possible.

[0007] The need for compactness has led to several different complex optical solutions, all of which, on the one hand, are still not sufficiently compact for most practical applications, and on the other hand, suffer major drawbacks in terms of manufacturability, price, and performance.

[0008] The teachings included in International Patent Publication Numbers WO2017 / 141239, W02017 / 141240, WO2017 / 141242, WO2019 / 077601,

[0009] W02020 / 157747, WO2022 / 029764, W02022 / 054047 and IL / 304336 are herein incorporated by reference.

[0010] SUMMARY OF THE INVENTION

[0011] The present invention facilitates the provision of compact substrates for, amongst other applications, HMDs. The invention allows relatively wide FOVs together with relatively large eye-motion box (EMB) values. The resulting optical system offers a large, high-quality image, which also accommodates large eye movements. According to the present invention, the optical system is particularly advantageous because it is substantially more compact than state-of-the-art implementations. Yet, it can be readily incorporated even into optical systems having various specialized configurations.

[0012] A broad object of the present invention is, therefore, to alleviate the drawbacks of state-of-the-art compact optical display devices and to provide other optical components and systems having improved performance according to specific requirements.

[0013] In accordance with the present invention, there is therefore provided an optical device, including a first light-transmitting substrate having at least two parallel major surfaces and edges, characterized by a main propagation direction axis parallel to the major surfaces and an orthogonal axis normal to the main propagation direction axis and parallel to the major surfaces, an input aperture, an output aperture, an eye-motion box, a coupling-in element for coupling light waves into the substrate to effect total internal reflection from the major surfaces of the substrate, and a coupling-out element located between the two major surfaces of the light-transmitting substrate for coupling light waves out of the substrate through the output aperture into the eye-motion box, wherein the coupling-in element comprises at least a first diffractive element and the coupling-out element comprises at least a first flat reflecting surface without a diffractive element

[0014] BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The invention is described in connection with certain preferred embodiments, with reference to the following illustrative figures so that it may be more fully understood.

[0016] With specific reference to the figures in detail, it is stressed that the particulars shown are by way of example and for the purpose of illustrative discussion of the preferred embodiments of the present invention only and are presented to provide what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention. The descriptions taken with the drawings are to serve as direction to those skilled in the art as to how the several forms of the invention may be embodied in practice.

[0017] In the drawings:

[0018] Fig. 1 is a side view of a prior art exemplary light-transmitting substrate;

[0019] Fig. 2 is a side view of another prior art exemplary light-transmitting substrate;

[0020] Fig. 3 is a schematic sectional view of a prior art light-transmitting substrate, wherein the coupling-in, as well as the coupling-out elements, are diffractive optical elements; Figs. 4A and 4B illustrate sectional views of a prior art transparent substrate each having coupling-in and coupling-out surfaces, and a partially reflecting redirecting element;

[0021] Figs. 5A and 5B schematically illustrate other sectional views of a prior art transparent substrate, each having coupling-in and coupling-out surfaces, and a partially reflecting redirecting element;

[0022] Figs. 6 schematically illustrates a substrate-guided system comprising a coupling-in diffractive element and a single coupling-out reflective element, according to the present invention;

[0023] Fig. 7 schematically illustrates a substrate-guided system comprising a coupling-in diffractive element and multiple coupling-out partially reflecting elements, according to the present invention;

[0024] Figs. 8A and 8B schematically illustrate substrate-guided systems comprising a coupling-in diffractive element, two internal reflecting surfaces, and a partially reflecting redirecting element, according to the present invention;

[0025] Figs. 9A and 9B schematically illustrate other substrate-guided systems comprising a coupling-in diffractive element, two internal reflecting surfaces, and a partially reflecting redirecting element, according to the present invention;

[0026] Fig. 10 schematically illustrates substrate-guided systems comprising a coupling-in diffractive element having optical power, two internal reflecting surfaces, and a partially reflecting redirecting element, according to the present invention;

[0027] Fig. 11 schematically illustrates substrate-guided systems comprising a coupling-in diffractive element placed on the back surface of a transparent plate, two internal reflecting surfaces, and a partially reflecting redirecting element, according to the present invention;

[0028] Figs. 12A and 12B illustrate a side and a bottom view of an optical system comprising two substrates for two-dimensional magnification of the projected image's FOV along two orthogonal axes, according to the present invention; Fig. 13 A schematically illustrates a non-hybrid substrate-guided system comprising a coupling-in diffractive element, two internal reflecting surfaces, and a diffractive redirecting element, according to the present invention;

[0029] Fig. 13B schematically illustrates a non-hybrid substrate-guided system comprising a coupling-in reflective element, two internal reflecting surfaces and a partially reflective redirecting element, according to the present invention;

[0030] Figs. 14A, 14B and 14C illustrate side and bottom views, respectively, of an optical system comprising two substrates for two-dimensional expansion of the output aperture along two orthogonal axes using diffractive elements, according to the present invention, and

[0031] Figs. 15A, 15B and 15C illustrate side and bottom views of an optical system comprising two substrates for two-dimensional expansion of the output aperture along two orthogonal axes, using partially reflecting surfaces, according to the present invention.

[0032] DETAILED DESCRIPTION OF EMBODIMENTS

[0033] Fig. 1 illustrates a sectional view of a prior art light-transmitting substrate. The first reflecting surface 16 is illuminated by a collimated light wave 12 emanating from a display source 4 and collimated by a lens 6 located between the source 4 and a substrate 20 of the device. The reflecting surface 16 reflects the incident light from the source such that the light wave is trapped inside the planar substrate 20 by total internal reflection. After several reflections off the major surfaces 26, 27 of substrate 20, the trapped light waves reach a partially reflective element 22, which couples the light out of the substrate into the eye 24, having a pupil 25 of a viewer. Herein, the input aperture 17 of the substrate will be defined as the aperture through which the input light waves enter the substrate, and the output aperture 18 of the substrate will be defined as the aperture through which the trapped light waves exit the substrate. In the case of the substrate illustrated in Fig. 1, both the input and the output apertures coincide with the lower major surface 26. Other configurations are envisioned, however, in which the input and the image light waves from the displace source 4 are located on opposite sides of the substrate or on one of the edges of the substrate. As illustrated, the active areas of the input and output apertures 17, 18, which are approximately the projections of the coupling-in reflecting surface 16 and the coupling-out reflecting elements 22 on the major surface 26, respectively, are similar to each other.

[0034] In HMD systems, the entire area of the EMB should be illuminated by all the light waves that emerge from the display source to enable the viewer's eye to look at the entire FOV of the projected image simultaneously. As a result, the output aperture of the system should be extended accordingly. On the other hand, it is required that the optical module be light and compact. Since the lateral extent of the collimating lens 6 is determined by the lateral dimension of the input aperture of the substrate, it is desired that the input aperture will be as small as possible. In systems, as illustrated in Fig. 1, wherein the lateral dimensions of the input aperture 17 are similar to that of the output aperture 18, there is an inherent contradiction between these two requirements. Most of the systems based on this optical architecture suffer from small EMB and small achievable FOV, as well as from large and cumbersome imaging modules. As a result, a suitable method for lateral expansion of the output aperture as compared with the input aperture is required to yield a practical imaging system.

[0035] A method for solving this problem, at least partially, is illustrated in Fig. 2, wherein the element which couples out the light waves from the substrate is an array of partially reflecting surfaces 22a, 22b, etc. The output aperture of this configuration can be extended by increasing the number of partially reflecting surfaces embedded inside the substrate 20. As a result, it is possible to design and construct an optical module having a small input aperture 17, as well as a large output aperture 18. As can be seen, the trapped rays arrive at the reflecting surfaces from two distinct directions 28, 30. In this particular embodiment, the trapped rays arrive at the partially reflecting surface 22 from one of these directions 28, after an even number of reflections from the substrate major surfaces 26 and 27, wherein the incident angle between the trapped ray and the normal to the reflecting surface is flref. The trapped rays arrive at the partially reflecting surface 22 from the second direction 30 after an odd number of reflections from the substrate surfaces 26 and 27, wherein the incident angle between the trapped ray and the normal to the reflecting surface is / 3ref As further illustrated in Fig. 2, for each reflecting surface, each ray first arrives at the surface from the direction 30, wherein some of the rays again impinge on the surface from direction 28. The reflectance must be negligible for the rays impinging on the surface having the second direction 28 to prevent undesired reflections and ghost images.

[0036] A solution for this requirement that exploits the angular sensitivity of thin film coatings was previously proposed in the Publications referred to above. The desired discrimination between the two incident directions can be achieved if one angle is significantly smaller than the other one. It is possible to provide a coating with very low reflectance at high incident angles and a high reflectance for low incident angles. This property can be exploited to prevent undesired reflections and ghost images by eliminating the reflectance in one of the two directions.

[0037] One of the main problems of the proposed embodiment illustrated in Fig. 2 is that the requested reflectance behavior of the partially reflective surfaces 22, is not conventional. In addition, to keep the low reflectance at the higher angular region, the reflectance at the relevant angular region cannot be higher than 20% - 30%. Furthermore, to achieve a uniform brightness over the entire FOV, it is required that the reflectance of partially reflecting surfaces will gradually be increased toward the edge of the substrate, and hence, the maximum achievable efficiency is comparatively low and usually cannot be more than 10%. Another approach for coupling light waves into and out from a light-guided optical element is by using diffractive elements. As illustrated in Fig. 3, the light rays 34 and 36 are coupled into the transparent substrate 20 by a diffractive element 48. After some total internal reflection from the external surfaces of the substrate, the light rays are coupled out from the substrate by a second diffractive element 50. As illustrated, ray 34 is coupled-out at least twice at two different points 52 and 54, on element 50. Consequently, to achieve uniform output light waves, the diffraction efficiency of element 50 should be increased gradually along the axis. As a result, the overall efficiency of the optical system is even lower than that of the system illustrated in Fig. 2, and it is usually not more than a few percent. That is to say, in the embodiments illustrated in Figs. 2 and 3, the output aperture is extended to be much larger than the input aperture but on account of significantly reducing the brightness efficiency of the optical module as well as complicating the fabricating process of the substrate.

[0038] Figs. 4A and 4B illustrate embodiments for overcoming the above-described problem according to the present invention. Instead of using a single element (22 in Fig. 2 or 50 in Fig. 3), which performs the dual function of coupling the light waves out of the substrate 20, as well as directing the light waves into the user's eye 24, the requested function is divided into two different elements; namely, one element which is embedded inside the substrate couples the light waves out of the substrate, while a second conventional partially reflecting element which is located outside of the substrate, redirects the light waves into the viewer's eye. As illustrated in Fig. 4A, two rays 63 (dashed lines) and two rays 68 (dotted-dashed lines) from a plane light wave emanating from a display source and collimated by a lens (not shown) enter a light transparent substrate 64, having two major parallel surfaces 70 and 72, through the input aperture 86 of the coupling-in element, e.g., a prism 69, at an incident angle with respect to the major surfaces 70, 72 of the substrate. The rays impinge on the reflecting surface 65, which is inclined at an angle aSUri to the major surfaces of the substrate. The reflecting surface 65 reflects the incident light rays such that the light rays are trapped inside a planar substrate 64 by total internal reflection from the major surfaces and then coupled out from the substrate by the coupling-out element 67, which is parallel to surface 65. Since the number of reflections from surface 65 is equal to that from surface 67 (once for rays 63 and twice for rays 68), the inclination angle aout of the coupled-out image is identical to the incident angle

[0039] As further illustrated in Fig. 4A, the inclination angle aOut could be adjusted by adding a partially reflecting surface 79, which is inclined at an angle of arto the surface 72 of the substrate. As shown, the image is reflected and rotated such that it passes again through the substrate substantially normal to the substrate's major surfaces and reaches the viewer's eye 24 through the output aperture 89 of the substrate. To minimize distortion and chromatic aberrations, it is preferred to embed surface 79 in a redirecting prism 80 and to complete the shape of the substrate 64 with a second prism 82, both fabricated of the same material, which should not necessarily be similar to that of the redirecting element 80. There are some options for materializing the partial reflection of surfaces 79. This may be a dielectric, a metallic, or a hybrid beamsplitter. For a polarized source, it may be a polarizing beamsplitter. For a laser-based display source, it may be a dichroic coating reflective only for the specific wavelengths of the source and transparent to the other spectrum. In addition, it may be a multiple reflective phase hologram sensitive only to three narrow-band spectral regions according to the specific colors of the source or even to a single band for a monochromatic display source. To minimize the thickness of the system, it is possible, as illustrated in Fig. 4B, to replace the single reflecting surface 79 with an array of parallel partially reflecting surfaces 79a, 79b, etc., where the number of the partially reflecting surfaces and the specific type of beamsplitter can be determined according to the requirements of the system. Another issue to consider for all the embodiments illustrated above is the maximum achievable FOV of the image projected into the viewer's eye. In most of the substrate-guided-based HMD technologies, either reflective or diffractive, the light waves are coupled out from the guiding substrate substantially normal to the major surfaces of the substrate. Consequently, due to the Snell refraction from the substrate, the external FOV of the image in the air is: (air)_ (s) .Vs(1)

[0040] Henceforth, the superscripts and the subscripts air and s refer to the external and the internal media, respectively.

[0041] In the embodiment of Fig. 2, the substrate is fabricated from a single optical material. As a result, the two marginal rays coupled inside the substrate are refracted into different directions only when passing through the system's input and output surface. As a result, the coupled rays do not experience any refraction when passing through the partially reflecting surfaces. Since the optical rays are refracted only at angles with close proximity to the normal of the entrance and the exit surfaces, the directions of the rays are modified according to the approximated equation: and subsequently, the limitation of Eq. (1) is sustained. Since the substrate's refractive index should be identical to the adhesive applied to partially reflecting surfaces, it is bounded by the maximal available refractive index of the existing optical adhesives. Hence, the upper limit of the substrate's refractive index of this embodiment affects its maximum available FOV.

[0042] Referring to Fig. 3, the basic diffraction equation of a grating is wherein m is the diffraction order, I is the wavelength, and d is the distance between two adjacent grating lines. The angle a'srepresents the direction of the light ray after being refracted into the substrate and before being diffracted by the grating inside the substrate. That is to say sin aair= v ■ sin a's, (4)

[0043] Using the first order of Taylor series expansion yields wherein the subscripts high, low, and mid represent the two marginal and the central rays of the image inside the substrate, respectively. Combining Eqs. (3) and (5) yields

[0044] Combining Eqs. (4) and (6), and considering that usually amidis approximately 60°, yields

[0045] That is to say, for diffractive element Eq. (1) is no longer sustained, and for a given internal FOV, the external FOV has been reduced by a factor of ~2.

[0046] Fig. 5 A illustrates a modified embodiment of Fig. 4B, wherein the apertures 90 and 93 are the interface surfaces between the main substrate 64 and the elements 69 and 80, respectively. The coupling-in and the redirecting elements are now fabricated from the same optical material having a refractive index that has the following optical characteristic wherein vpis the refractive index of the elements. That is to say, the refractive index of the elements 69 and 80 is substantially lower than that of the main substrate 64.

[0047] Fig. 5B illustrates a zoom-in scheme of the encircled area 96 in Fig. 5A. As shown, due to the dissimilarities between the optical material of the substrate and those of the coupling-in and the redirecting elements and the high obliquity of the rays incident to the interface surfaces 90 and 93, the rays experience substantial refraction when passing through the interface surfaces. The fact that the optical rays enter the substrate at oblique angles can be exploited to improve the above limitation. Since elements 69 and 80 have the same optical characteristics, the refractions at interface surfaces for each passing ray will have the same magnitude and the opposite directions, respectively. Therefore, they will be mutually compensated. The angular deviation between two different light rays inside the elements as a function of the deviation inside the substrates can be calculated according to the approximated equation wherein asand apare the median off-axis angles inside the substrate and the elements, respectively. Similarly, the angular deviation between the rays outside of redirecting prism 80 is

[0048] Consequently, the ratio between the angular deviation outside the element and inside the substrate is or

[0049] / 7(air) _ (s) .v■C0S Q:s (12) s cos ap'

[0050] That is to say, by modifying the optical material of the elements, for a given internal cos a

[0051] FOV, it is possible to increase the external FOV by a factor of - which is in the

[0052] COS Up order of 1.5 -1.8.

[0053] Another important issue to consider is the total FOV projected to the viewer's eye. In most optical systems, the projected FOV is one of the most critical parameters and should have at least a required minimal value. It is often advantageous to expand the FOV as much as possible. However, increasing the projected FOV coupled out from the substrate will increase accordingly the input FOV coupled into the substrate. Unfortunately, the collimating mechanism that generates the input image should usually be as compact as possible for near-eye displays. As a result, it is relatively difficult to achieve the required projecting system having compact dimensions and a wide FOV. Moreover, the FOV of laser scanner devices is usually limited by mechanical constraints on the device's structure. Hitherto, the substrate's coupling-in and coupling-out mechanisms were the same in all the embodiments illustrated above. In the embodiments of Figs. 1, 2, and 4 they are reflective elements, while in Fig. 3, they are diffractive elements. Consequently, the FOVs of the input image coupled into the substrate and the output image coupled out from the substrate are the same. That is,

[0054] / 7(out)_ (in) _ (13)

[0055] Henceforth, the superscripts and subscripts out and in refer to the external and internal light waves coupled out from and into the substrate, respectively. Therefore, increasing the required output FOV will correspondingly increase the input FOV in all these embodiments. As a result, the materialization of the projecting system will be complicated and, in many cases, impossible. Therefore, there is usually a contradiction between the required wide output FOV and the available narrower input FOV.

[0056] Seemingly, the limitation shown in Eq. (7) is a severe drawback of the diffractive systems that substantially reduces the available FOVs for these systems. However, as shown herein, it can be exploited to significantly increase the output FOV while maintaining a relatively small input FOV. Henceforth, it should be assumed that the projected image is monochromatic, or alternatively, a combination of three singular RGB wavelengths that can compose the entire required gamut of the projected image.

[0057] Fig. 6 illustrates a modified embodiment of the embodiment illustrated in Fig. (1). Instead of coupling the light waves into the substrate by the reflecting surface 16, the light waves are coupled into substrate 20 by a diffractive element 97.

[0058] Using Eq. (7) and denoting asas the median off-axis angle of the coupled light waves inside the substrate yields the coupled FOV as

[0059] Combining Eqs. (1) and (14) yields (out)

[0060] That is, in the hybrid embodiment of Fig. 6, comprising a diffractive coupling-in element and reflective coupling-out element, the output FOV is magnified by a 1 factor of - in relation to the input FOV. cos as

[0061] Fig. 7 illustrates an embodiment that is a modified embodiment of Fig. 2. Similar to Fig. 6, the light waves are coupled into the substrate by the diffractive element 97. Since the coupling-out mechanism of the two embodiments is similar, that is, both use partially reflecting surfaces 22a, 22b, Eq. (15) is also applicable to Fig. (7).

[0062] Fig. 8 A illustrates a modified embodiment of Fig. 5 A. Here, the coupling-in prism is replaced by a transmission grating 97 embedded inside a thin plate 98 having the same refractive index vpas the redirecting element 80. After being diffracted from grating 97, the FOV inside plate 98 is

[0063] Fr . F (16) (g) _ v,. ■ cos a ’ wherein the superscript g refers to the grating's plate 98 and apis the median off- axis angle of the diffracted light waves inside plate 98. Since the coupling surfaces 65 and 67, and the substrate's major surfaces 70 and 72 are parallel, respectively, the coupled-out light waves from the main substrate 64 are identical to the coupledin light waves. Consequently: (P)=(g) (17)

[0064] Combining Eqs. (10), (16), and (17) yields

[0065] F( ,ou which is similar to Eq. (15). The main difference between the embodiments of Figs. 6 and 7 and that of 8A is that the magnification factor here depends on the off-axis angle of the coupled light waves inside plate 98 and the redirecting element 80, instead of the substrate 64.

[0066] Fig. 8B illustrates a slightly modified embodiment of Fig. 8A. Instead of utilizing a transmission grating, the light waves are coupled into the substrate by a reflection grating 97. As shown, the input light waves pass through substrate 64 and the coupling-in element 65 with no apparent reflection, before being diffracted and reflected into the substrate by the grating 97. In the embodiments of Figs. 8A- 8B, grating 97 and redirecting element 80 are located at opposite surfaces 70, 72 of substrate 64. There are cases, however, where locating these elements at the same surface is preferred.

[0067] Figs 9A and 9B illustrate embodiments wherein a transmission and a reflection grating, respectively, are located at the same surface 72 as the redirecting element. As shown, after being diffracted by the grating 97, the light waves pass through the substrate 64 and the coupling-in surface 65 with no apparent reflection, and are then totally reflected by the upper surface 70 before being reflected by the coupling-in surface 65.

[0068] There are some apparent advantages to the embodiments illustrated in Figs. 6, 8B, 9A and 9B in relation to the embodiments of Figs. 6 and 7, which are a direct consequence of Eq. (8) and the refraction phenomenon illustrated in Fig. 5B. As shown in Eq. (12), the output FOV projected to the viewer's eye can be expanded by a factor of - in relation to the allowed FOV inside the substrate. Similarly, as shown by the difference between Eqs. (15) and (18), the magnification of these embodiments is bigger by the same factor of - - than that of the former embodiments.

[0069] Another possibility for improving the performance of the optical system is to combine the collimating lens 6, or at least part of the collimating module, with the coupling grating 97. As illustrated in Fig. 10, a modified embodiment of Fig. 9B, the grating 97' not only diffracts the input image into the substrate but also collimates the incoming beam before diffracting it into the substrate. Seemingly, this modification will complicate the structure of the grating, however, since the modified grating 97' is a combination of a simple linear grating 97 with an on-axis collimating lens, and since the input FOV is substantially reduced compared to the output FOV, the design and fabrication of this combined grating will be fairly simple.

[0070] As illustrated in Fig. 11, another option for reducing the size of the system is to locate grating 97 at the back surface 99 of plate 98. In addition, the thickness of plate 98 is increased. As shown, due to the high obliquity of the coupled light waves inside the plate, the rays diffracted from grating 97 propagate a significant lateral distance along the axis inside plate 98 before entering the main substrate 64. As a result, the overall active aperture of the coupling grating 97 can be considerably reduced.

[0071] The following notation is defined herein to simplify the explanation. As shown in Fig. 11, the main axis is defined as the propagation direction of the central light wave of the FOV inside the substrate; the orthogonal axis 1} is normal to axis and oriented parallel to the main lateral axis of the FOV; the major surfaces of the substrate are parallel to the axes and IJ, and the normal axis q is perpendicular to these surfaces. As was previously shown in Figs. 9A-9D of Publication W02020 / 157747 referred to hereinabove, it is possible to materialize a substrate-guided element similar to that illustrated in Fig. 5A having a substantially smaller input aperture along the main axis by at least a factor of three as compared with the original input aperture 86, as well as the output aperture 89 of the substrate, while still not attenuating the brightness of the projected image. That is to say, the light transmission efficiency of substrate 64 can be close to 100%, wherein a substantial lateral expansion of the output aperture along the propagation direction of the light inside the substrate has been achieved. Since the internal structure of the main substrate 64 has been preserved in the modified embodiments of Figs. 8 A to Fig. 11, this expansion capability has been maintained, namely, not only is the output FOV significantly magnified as compared with the input FOV along the propagation direction but the output aperture 89 is substantially expanded as compared with the input aperture along that axis. In many applications, however, it is required to expand the aperture and the projected FOV also along the orthogonal axis IJ.

[0072] Fig. 12A illustrates a modified embodiment of Fig. 8B, wherein a second substrate 164 is attached to the lower surface 72 of the main substrate 64. Similar to substrate 64, the second substrate 164 comprises two parallel major surfaces 170 and 172, and two parallel coupling-in and coupling-out surfaces 165 and 167, respectively. Substrate 164 is oriented such that surfaces 165 and 167 are parallel to the axis and the main propagation direction here is along the 1} axis. As shown, the input light waves 68 are diffracted by the grating 197, embedded in plate 198 attached to the lower surface 72 of the main substrate 64, into substrate 164. The light waves pass through substrate 164, are reflected from the upper surface 170, and are coupled into substrate 164 by the coupling-in element 165. Reflecting surface 165 reflects the incident light rays such that the light rays are trapped inside a planar second substrate 164 by total internal reflection from the major surfaces and then coupled out from the substrate by the coupling-out element 167. The light waves are then reflected by the redirecting element 179 embedded inside plate 180, pass through substrates 164 and 64, and are coupled into substrate 64 by the diffractive grating 197.

[0073] Similar to the expansion inside the main substrate 64, the FOV of the coupled image is magnified by substrate 164 along the 1} axis, and the output aperture 189 of the substrate is considerably expanded as compared with the input aperture 186.

[0074] Fig. 12B, which demonstrates a bottom view of the embodiment of Fig. 12A, illustrates how the projected image is expanded, first by substrate 164 along the 1} axis and then by substrate 64 along the axis. As explained in Publication W02020 / 157747, this two-dimensional expansion is without the penalty of attenuating the brightness of the projected image. In addition, the FOV of the projected output image is magnified along the two orthogonal axes 1} and by the embodiment of the double-substrate 164 and 64.

[0075] In all the embodiments illustrated hereinabove in Figs. 8 A, 8B, 12A and 12B, the display system has a hybrid configuration, wherein the coupling-in means is a diffractive grating 97 while the redirecting means is a reflective element 79. There are systems, however, especially those having narrow to medium FOV, wherein it is preferred to redirect the coupled-out light waves into the viewer's eye by a second diffracted element.

[0076] As illustrated in Fig. 13 A, representing a modified embodiment of the embodiment illustrated in Fig. 8B, the redirecting element 80 is replaced by a diffractive element 101, embedded inside plate 100 having the same refractive index as plate 98. The FOV of the projected image is no longer magnified, and the limitation on the internal FOV of Eq. (7) is sustained here. However, still, this embodiment has some advantages as compared with that of Fig. 8B. Since the chromatic dispersion of grating 97 is compensated by that of the second grating 101, the embodiment may also be used for display sources that are not necessarily monochromatic (or a combination of a few monochromatic wavelengths). In addition, the diffractive element 101, especially when it is a thick phase reflective hologram, might be designed to have high reflectance for the spectral bandwidths of the projected image and high transmission for the rest of the photopic range. As a result, the system can be highly efficient for the projected image and highly transmissive for the image from the external scene.

[0077] Another non-hybrid embodiment is illustrated in Fig. 13B, representing a modified embodiment of the embodiment illustrated in Fig. 11. Grating 97 is replaced by an array of flat parallel reflecting surfaces 95 embedded at the external surface 99 of the transparent plate 98. Here, the chromatic dispersion of the redirecting element 80 is compensated by that of the reflecting element 95. Hence, the embodiment may also be used for display sources that are not necessarily monochromatic.

[0078] Fig. 14A illustrates a modified embodiment of Fig. 12 A wherein the substrate 164 is replaced by a pure diffractive structure in a similar manner to the embodiment illustrated in Fig. 3. That is to say, coupling-in and coupling-out diffractive gratings 197, 199 are embedded inside plate 198. The light waves coupled into the plate by grating 197 are laterally expanded in the two orthogonal axes 1} and £ by the second grating 199 and projected into grating 97 that couples it into the main substrate 64. The FOV of the projected image is no longer magnified along the 1} axis, and the limitation on the internal FOV of Eq. (7) is sustained here. However, since the FOV along the 1} axis for eyeglasses configuration is usually not wide, these limitations are not severe. On the other hand, the system might now have an extremely narrow input aperture suitable for laser sources. In addition, since the FOV along the axis is significantly magnified by substrate 64, and the active area of the grating 199 is considerably smaller than the output aperture 89 of the system, the design and fabrication of this embodiment are simpler, and the total efficiency will be considerably higher than that of the pure diffractive system of Fig. 3.

[0079] Fig. 14B, which is a bottom view of the embodiment of Fig. 14A, illustrates how the projected image is expanded, first by the diffractive plate 198 along the h and the x axes and then by substrate 64 along the 1} axis. Combining the gratings 199 and 197 into a single grating will sometimes be advantageous. The light waves will be diffracted from the combined gratings at the required off-axis angle inside substrate 64 to be coupled accordingly.

[0080] As illustrated in Fig. 14C, for systems having narrow to medium FOV, it is sometimes preferred to replace grating 197 with a reflective element. The FOV of the projected image is no longer magnified. Still, the limitation on the internal FOV of Eq. (7) is sustained only on substrate 164 along the IJ-axis and not on substrate 64 along the ^-axis. However, since the chromatic dispersion of grating 97 is compensated by that of the second grating 199, the embodiment can also be used for display sources that are not necessarily monochromatic (or a combination of a few monochromatic wavelengths).

[0081] Figs. 15 A, 15B and 15C illustrate modified embodiments of the embodiments illustrated in Figs. 14 A, 14B and 14C. Instead of utilizing diffractive elements, the lateral expansion is performed by two orthogonal substrates comprising two arrays of partially reflecting surfaces, as described with reference to Fig. 25 of Publication WO2017 / 141239, referred to hereinabove.

[0082] The input light waves 68 are coupled into the first expanding substrate 246 by the coupling element 216, expanded along the 1} axis and coupled out by the array of the partially reflecting surfaces 218. The light waves are then coupled into the second expanding substrate by the coupling element 220, expanded along the axis and coupled out by an array of partially reflecting surfaces 218. The expanded light waves are then coupled into the first substrate 64 by a diffractive grating 97 (Fig. 15 A) or by a reflective element 207 (Fig. 15C).

[0083] Regarding the hybrid structure of the embodiments illustrated here, unlike the structure of Fig. 3, wherein the chromatic dispersion of grating 48 is compensated by that of grating 50, here, the coupling-out element does not comprise any diffractive element. Therefore, except for Figs. 13 and 14C, there is only a single diffractive grating 97 whose chromatic dispersion is not compensated. As a result, these embodiments are appropriate for monochromatic display sources, or alternatively, a combination of three singular RGB wavelengths that can compose the entire required gamut of the projected image. There are two main methods to achieve the required triple-monochromatic displays. The first one is a scanningtype laser projection engine, wherein the projection display system includes a laser source emitting a laser beam (or three RGB laser beams), and a reflecting mirror system for scanning the laser beam to create the required image. The reflecting mirror system may be one or more micro-electro-mechanical systems (MEMS), scanning mirrors that rotate the laser beam over the FOV. As the human eye's response time is slower than the framerate of the video signal, the image can be reconstructed on the retina of the eye. Another method is to use a two-dimensional spatial light modulator (SLM), such as a digital mirror device (DMD), a liquid crystal display (LCD), or a liquid crystal on silicon (LCOS). The RGB lasers are used as the display's back or front light in this configuration.

[0084] Another issue is the implementation of the required grating for an RGB light source. It is possible to achieve this by fabricating it as a multiplexed grating composed of three overlapping gratings, each sensitive to one of the three basic RGB colors and non-sensitive to the other two colors. This multiplexed grating can either comprise three different gratings attached to each other, or a multiplication of three gratings on the same substrate. The grating can be fabricated as a surfacerelief grating or recorded as a thick-phase Bragg hologram. In cases where it is required to have definite discrimination between the spectral region of the projected image and that of the transmitted image from the external scene, a thick phase reflection hologram will be preferred. For the three gratings, the ratio should be maintained, wherein the subscripts R, G, and B refer to the respective colors of the gratings - red, green, and blue.

[0085] Another issue to be considered is the local magnification at each specific point in the FOV. The magnification factors that are given in Eqs. (15) and (18) are only approximations. The more exact magnification for each point is given by the term >Aa°utThat is, by the angular infinitesimal change in the projected output image ^ain as a function of a change in the input image. This term can be expanded to

[0086] Utilizing the relations given hereinabove yields, respectively

[0087] Since the incoming image is substantially on-axis, it can be approximated that cos a'in~1, and hence, the local magnification for each point in the FOV is wherein apis the off-axis angle of the specific light wave inside element 80.

[0088] This value is variable depending on the point in the FOV. As a result, the local magnification varies as a function of the angular location of each specific point in the image. The main outcome of this phenomenon is that the projected image is distorted. In addition, due to the conservation of etendue, the brightness of the projected output image will be reduced accordingly, and the image's brightness will not be uniform. For laser scanner devices, these issues can be solved by appropriately controlling the scanning speed and / or the modulation rate of the video signal. For the SLM-illuminated method, the brightness non-uniformity issue can at least partially be addressed by a differential non-uniform illumination of the display active surface. Brightness fine-tuning and distortion correction can be performed by an appropriate electronic modification of the video signal.

[0089] Yet another issue is the chromatic dispersion due to angular magnification. For most of the FOVs, the output angle of the coupled-out light wave is substantially different than the input angle. This issue is irrelevant for each specific wavelength due to the monochromatism of the light. For the combination of the three colors, however, since the redirecting element 80 has a finite Abbe number, each color will be refracted in a slightly different direction. This issue can also be addressed by an electronic correction of the video signal of each color, namely, for each color, the image will be slightly shifted accordingly, to compensate for the chromatic dispersion.

[0090] Still a further issue is the polarization of the coupled light inside the substrates. Unlike the reflective elements of the prior art embodiments given in Figs. 4A to 5B that are not sensitive to polarization, the operation of diffractive elements strongly depends on the light waves' polarization. Usually, these elements are designed to operate for s polarized light. As illustrated in Figs. 12A, 12B, 14A, 14B and 14C, the two substrates 64 and 164 are oriented at 90° to each other. Accordingly, light waves having s polarization in substrate 164 will have p polarization in relation to the diffractive element of substrate 64. Consequently, inserting a half-wavelength retardation plate between the two substrates is usually desired to validate that the light waves impinging on grating 97, are s polarized.

[0091] Figs. 10 to 14C illustrate various features which can be added to the basic hybrid configuration illustrated in Figs. 8A to 9B, including: adding optical power to the coupling-in grating (Fig. 10); placing the grating at the back surface of a transparent plate (Fig. 11); magnifying the FOV along the two orthogonal axes (Fig. 12); utilizing diffractive redirecting element (Fig. 13A, 13B), and two-dimensional expansion of the output aperture (Figs. 12A, 12B, 14A, 14B and 14C). Eventually, any combination of these features can be added to the basic embodiment illustrated in Figs. 7-10, according to the specific requirements of the optical system. Moreover, a combination of a few embodiments with the same shape, like a cascade of two substrates having the form of Figs. 8A to 9B, can be used to achieve double magnification for extremely wide FOV.

[0092] It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrated embodiments and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Claims

CLAIMS1. An optical device, comprising: a first light-transmitting substrate having at least two parallel major surfaces and edges, characterized by a main propagation direction axis parallel to the major surfaces and an orthogonal axis normal to the main propagation direction axis and parallel to the major surfaces; an input aperture; an output aperture; an eye-motion box; a coupling-in element for coupling light waves into the substrate to effect total internal reflection from the major surfaces of the substrate, and a coupling-out element located between the two major surfaces of the lighttransmitting substrate for coupling light waves out of the substrate through the output aperture into the eye-motion box, wherein the coupling-in element comprises at least a first diffractive element and the coupling-out element comprises at least a first flat reflecting surface without a diffractive element.

2. The optical device according to claim 1, further comprising a display source emitting an image defined by a field-of view, wherein the field-of-view along the main propagation direction axis of the image coupled out from the substrate is substantially wider than the field-of view along the main propagation direction axis of the image coupled into the substrate by the coupling-in element.

3. The optical device according to claim 1 , wherein the light waves pass through the substrate before being diffracted by the first diffractive element into the substrate.

4. The optical device according to claim 1, wherein the first diffractive element has an optical power, the light waves incident on the substrate through the input aperture are diverging waves and are collimated by the first diffractive element.

5. The optical device according to claim 1, wherein the first diffractive element is embedded inside a transparent plate optically attached to one of the major surfaces of the substrate.

6. The optical device according to claim 1, further comprising a second flat reflecting surface located between the two major surfaces of a second lighttransmitting substrate and parallel to the first flat reflecting surface, wherein the light waves diffracted by the first diffractive element are reflected at least once by the second flat reflecting surface before being coupled into the substrate.

7. The optical device according to claim 6, wherein the light waves diffracted by the first diffractive element are totally reflected from one of the major surfaces of the substrate before the first reflection from the second flat reflecting surface.

8. The optical device according to claim 1, further comprising a redirecting optical element having at least two surfaces positioned outside of the substrate for redirecting light waves reflected from the first reflecting surface through the output aperture into the eye-motion-box.

9. The optical device according to claim 8, wherein the redirecting element is a second diffractive element.

10. The optical device according to claim 2, further comprising a second lighttransmitting substrate having at least two parallel major surfaces and edges, comprising a second coupling-in element for coupling light waves into the second substrate to effect total internal reflection from the major surfaces of the second substrate, and a second coupling-out element located between the two major surfaces of the second light-transmitting substrate for coupling light waves out of the second substrate, wherein the second coupling-in element comprises at least a third diffractive element and the coupling-out element comprises at least a third flat reflecting surface without a diffractive element.

11. The optical device according to claim 10, wherein light waves coupled out from the second substrate are coupled into the first substrate by the first reflecting surface.

12. The optical device according to claim 10, wherein the second coupling-out element is a diffractive-less element.

13. The optical device according to claim 10, having a field-of-view along an orthogonal axis normal to the main propagation direction axis, wherein the field-of- view along the orthogonal propagation axis of the image coupled out from the second substrate, is substantially wider than the field-of view along the orthogonal propagation direction axis of the image coupled into the second substrate by the second coupling-in element.

14. The optical device according to claim 10, wherein the output aperture is substantially bigger than the input aperture along the main and the orthogonal axes.

15. The optical device according to claim 10, wherein the second coupling-out element comprises a third diffractive element.

16. The optical device according to claim 15, wherein the first coupling-in element comprises the third diffractive element.

17. The optical device according to claim 2, wherein the optical device is characterized by a field-of-view magnification, and the magnification for each light wave in the field-of-view depends on the off-axis of the wave inside the first substrate.

18. The optical device according to claim 17, wherein the display source is a laser scanner, and the light modulation of the scanned laser is set according to the local magnification of the field-of-view.

19. The optical device according to claim 10, wherein a half-wavelength retardation plate is placed between the first and the second substrates.

20. The optical device according to claim 2, wherein the display source comprises three different colors, and for each color, the image is shifted to compensate for the chromatic dispersion of the device.

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