Compact beam expanding system for narrow light source

Abstract

This invention provides an optical device, comprising an input aperture; an output aperture; a first light-transmitting substrate including an input surface, at least one major surface, at least one internal surface having a light wave- splitting arrangement, a light wave output surface inclined at an angle to the major surface of the substrate for coupling the light waves out of the light-transmitting substrate through the output aperture, and edges; wherein the input aperture is located next to the input surface and the output aperture is located next to the output surface; at least one optical surface for coupling at least one input light wave into the first light-transmitting substrate through the input aperture by internal reflection from the major surface; the at least one input light wave, having lateral dimensions along a first x-axis and a second y-axis, incident on the input aperture has an initial direction; wherein light waves coupled into the first light-transmitting substrate through the input aperture are partially reflected from the light wave- splitting arrangement and partially pass through it at least once, and are then coupled out from the first light-transmitting substrate through the output surface and the output aperture, the lateral dimension of the coupled-out light wave along the first x-axis is larger than the lateral dimension of input light wave along the first x-axis, and the ratio between the lateral dimensions of the coupled-out and the input light waves is bigger than the ratio between the lateral dimensions of the output and the input apertures along the first x-axis.

Description

[0001] COMPACT BEAM EXPANDING SYSTEM FOR NARROW LIGHT SOURCE

[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 a large number of imaging applications, such as head-mounted and head-up displays, as well as 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, by means of 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 by means of 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. This is a major drawback for all kinds of displays but especially in HMDs, wherein the system should preferably 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, WO2017 / 141240, WO2017 / 141241, WO2017 / 141242,

[0009] WO20 19 / 077601, W 02020 / 157747, WO2022 / 029764, and W02022 / 054047, and Israel Patent Publication Numbers IL / 300336 and IL / 304993 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 movements of the eye. The optical system according to the present invention is particularly advantageous because it is substantially more compact than state-of-the-art implementations, and 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 light wave input aperture, a light wave output aperture, a first lighttransmitting substrate including an input surface, at least one major surface, at least one internal surface having a light wave splitting arrangement, a light wave output surface inclined at an angle to the major surface of the substrate for coupling the light wave out of the light-transmitting substrate through the output aperture, and edges, the input aperture is located next to the input surface and the output aperture is located next to the output surface, at least one optical surface for coupling at least one input light wave into the first light-transmitting substrate through the input aperture by internal reflection from the major surface, the at least one input light wave, having lateral dimensions along a first x-axis and a second y-axis, incident on the input aperture, has an initial direction, wherein light waves coupled into the first light-transmitting substrate through the input aperture are partially reflected from the light wave splitting arrangement and partially pass through it at least once, and are then coupled out from the first lighttransmitting substrate through the output surface and the output aperture, the lateral dimension of the coupled-out light wave along the first x-axis is larger than the lateral dimension of the input light wave along the first x-axis, and the ratio between the lateral dimensions of the coupled-out and the input light waves, is bigger than the ratio between the lateral dimensions of the output and the input apertures along the first x- axis.

[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 description taken with the drawings is 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: Fig. 1 is a side view of a prior art exemplary light-transmitting substrate;

[0018] Fig. 2 is a side view of another prior art exemplary light-transmitting substrate;

[0019] 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;

[0020] Figs. 4A and 4B illustrate sectional views of a prior art transparent substrate having coupling- in and coupling-out surfaces, and a partially reflecting redirecting element;

[0021] Figs. 5 schematically shows a prior-art method to expand the lateral dimensions of an input light wave;

[0022] Fig. 6 is a schematic sectional view of a light-transmitting substrate for expanding light waves, wherein the lateral dimension of the output light wave is substantially wider than that of the input light wave along a single axis, according to the present invention;

[0023] Figs. 7A and 7B are schematic sectional views of two light-transmitting substrates for expanding light waves, wherein the lateral dimensions of the output light wave are substantially wider than that of the input light wave along two orthogonal axes, according to the present invention;

[0024] Fig. 8 is a schematic sectional view of a light-transmitting substrate, having a right-angled isosceles prism shape, for expanding light waves, wherein the lateral dimension of the output light wave is substantially wider than that of the input light wave along a single axis, according to the present invention;

[0025] Figs. 9A and 9B are schematic sectional views of two prism shaped lighttransmitting substrates, having a right-angled isosceles, for expanding light waves, wherein the lateral dimensions of the output light wave are substantially wider than that of the input light wave along two orthogonal axes, according to the present invention;

[0026] Figs. 10A and 10B are schematic sectional views of a light-transmitting substrate for expanding light waves of an image, wherein the lateral dimension of the output light wave is substantially wider than that of the input light wave along a single axis, according to the present invention; Figs. 11 A and 1 IB schematically illustrate the forming of ghost images in a lighttransmitting substrate, for expanding light waves;

[0027] Figs. 12A and 12B schematically illustrate a method to avoid the forming of ghost images in a light-transmitting substrate, for expanding light waves, according to the present invention;

[0028] Figs. 13A and 13B schematically illustrate another method to avoid the forming of ghost images in a light-transmitting substrate, for expanding light waves, according to the present invention;

[0029] Fig. 14 schematically illustrates a combined method to avoid the forming of ghost images in a light-transmitting substrate, for expanding light waves, according to the present invention;

[0030] Figs. 15A and 15B are schematic sectional views of light-transmitting substrates for expanding light waves, wherein the inclination angle between the major reflecting surfaces and the input surface is substantially different than 45°, according to the present invention;

[0031] Fig. 16 is a schematic sectional view of a light- transmitting substrate for expanding light waves, wherein the inclination angle between the major reflecting surfaces and the input surface is substantially smaller than 45°, according to the present invention;

[0032] Fig. 17 is a schematic sectional view of a light-transmitting substrate for combining and expanding three different laser light sources, having red, green, and blue colors, according to the present invention;

[0033] Fig. 18 is a schematic sectional view of a light- transmitting substrate for combining and expanding three different laser light sources, having red, green, and blue colors, including a scanning device, according to the present invention;

[0034] Fig. 19 is a schematic sectional view of a light- transmitting substrate for combining and expanding three different LED illumination sources, having red, green, and blue colors, according to the present invention, and Fig. 20 is a schematic sectional view of a light-transmitting substrate for homogenizing an input light wave containing dark or bright prominent lines, according to the present invention.

[0035] DETAILED DESCRIPTION OF EMBODIMENTS

[0036] Fig. 1 illustrates a sectional view of a prior art device having light-transmitting substrate 20. 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 4 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 the 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 20 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 20 illustrated in Fig. 1, both the input and the output apertures coincide with the lower 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 20 or on one of the edges of the substrate. As illustrated, the active areas of the input and the output apertures, which are approximately the projections of the coupling-in surface 16 and the coupling-out elements 22 on the major surface 26, respectively, are similar to each other.

[0037] In HMD systems, the entire area of the EMB should be illuminated by all the light waves that emerge from the display source 4, 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 also 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, such as those illustrated in Fig. 1, wherein the lateral dimensions of the input aperture are similar to that of the output aperture, there is an inherent contradiction between these two requirements, and most of the systems which are based on this optical architecture suffer from small EMB and a small achievable FOV, as well as from large and cumbersome imaging modules. As a result, a proper method for lateral expansion of the output aperture compared with the input aperture is required to yield a practical imaging system.

[0038] A method for solving this problem, at least partially, is illustrated in Fig. 2, wherein the element that 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 as well as a large output aperture. 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 surfaces 22a, 22b 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 (3ref. 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 [3r,f.

[0039] As further illustrated in Fig. 2, for each reflecting surface, each ray first arrives at the surface from direction 30, wherein some of the rays again impinge on the surface from direction 28. The reflectance must be negligible for the rays imping on the surface having the second direction 28 to prevent undesired reflections and ghost images.

[0040] 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.

[0041] 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. Furthermore, 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 be increased gradually toward the edge of the substrate, and hence, the maximum achievable efficiency is comparatively low and usually cannot be more than 10%.

[0042] 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, 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. This significantly reduces the brightness efficiency of the optical module as well as complicating the fabricating process of the substrate.

[0043] Figs. 4A and 4B illustrate embodiments for overcoming the above-described problem according to the present invention. Instead of using partially reflecting surfaces (22a, 22b in Fig. 2 or 50 in Fig. 3), which perform 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 surfaces; namely, one surface which is embedded inside the substrate couples the light waves out of the substrate, while a second conventional partially reflecting surface, 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) 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 prism 55, at an incident angle of 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 ocSuri 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.

[0044] As illustrated in Fig. 4A, the inclination angle aout of the image can be adjusted by adding a partially reflecting surface 79, which is inclined at an angle of aredto the surface 72 of the substrate. As shown, the image is reflected and rotated such that it again passes 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 of them being fabricated of the same material, which is not necessarily similar to that of the substrate. To minimize the thickness of the system, it is possible, as illustrated in Fig. 4B, to replace the single reflecting surface 79 (Fig. 4a) with an array of parallel partially reflecting surfaces 79a, 79b, etc., where the number of the partially reflecting surfaces can be determined according to the requirements of the system.

[0045] As shown in Figs. 9A to 9D of the Publication WO2020 / 157747 referred to hereinabove, it is possible to materialize a substrate-guided element having a substantially smaller input aperture, by at least a factor of three, compared to the original input aperture 86, as well as the output aperture 89 of the substrate without attenuating the brightness of the projected image. That is to say, the light transmission efficiency of substrate 64 can be close to 100%, wherein the required lateral expansion of the output aperture along the propagation direction of the light inside the substrate is achieved. In many applications, however, it is required to expand the aperture also along the orthogonal axis. As shown in Fig. 30 of Publication WO2020 / 157747 referred to hereinabove, expanding the beam along two axes utilizing a double substrate configuration is possible.

[0046] For systems with wide FOVs and large EMBs, the required lateral dimension of the output aperture is in the order of 25 mm. Therefore, by using the expansion method of the embodiment illustrated in Figs. 4A and 4B, the input aperture can be reduced to 6-8 mm. These dimensions might be sufficient for many display sources, such as OLED or most existing LCOS. There are novel display sources, however, such as laser scanners, DLP, and micro-LED, wherein the diameter of the collimating image that emerges from the display is much smaller, usually around 1-4 mm. Hence, a proper method to match the narrow input beam with the achievable substrate-guided elements should be found.

[0047] Fig. 5 illustrates another method for expanding the output aperture in relation to the input aperture by adding a light wave- splitting arrangement to the substrate-guided element. As is shown in Fig. 22 of Publication WO2017 / 141239 referred to hereinabove, it is possible to design a system wherein the collimating module of the system (not shown) is deliberately selected in such a manner that the input light wave 12 illuminates only a partial surface of aperture 111, which is an internal part of the input aperture 96. That is to say, its lateral dimension along the axis (i.e., the propagation direction of the central light wave inside the substrate) is substantially smaller than the lateral dimension along the axis of input aperture 96. As described there, without utilizing any light wave- splitting arrangement, only part of the output aperture 113 is illuminated by the coupled-out light waves, and the entire extent of the aperture 114 will not be illuminated at all. This non-illuminated aperture 114, however, can be properly illuminated by cementing, a flat transparent plate 116 to one of the major surfaces using an optical adhesive, e.g., the lower surface 26 of the substrate 20, forming a light wave- splitting arrangement at the interface plane 117 between the substrate 20 and the transparent plate 116.

[0048] As illustrated, the leftmost ray 92 of the partial surface of aperture 111 impinges on the interface surface 117 at point 136. While part of ray 92a, is reflected and continues to propagate inside the substrate 20, another part, 92b (dotted line), passes through the interface surface 117 and is coupled out from the substrate at the exit point 138 located inside aperture 114. Similarly, the rightmost ray 106 of the partial aperture 111 impinges on the interface surface 117 at point 140. While part of ray 106a, is reflected and continues to propagate inside the substrate 20, another part, 106b (dashed line), passes through the interface surface 117 and after two reflections from the bottom surface 27 and another reflection from the interface surface 117, it is coupled out from the substrate at the exit point 142, which is also located inside aperture 114. For simplicity, only two rays with few impingements on the interface surface 117 are illustrated here. Still, it is clear that by utilizing a proper design, it is possible to fully illuminate the entire output aperture 113 of the substrate with homogeneous output light waves. The "filling" method is illustrated here for the embodiment shown in Fig. 1, however, it can also eventually be utilized for the other embodiments illustrated in Figs. 2-4, as well as for other possible optical modules.

[0049] The light wave- splitting arrangement can be materialized by Fresnel reflections from the interface plane 117, wherein the refractive index of the optical adhesive is substantially different than the refractive index of the light-transmitting substrate 20. An alternative method is to apply a light wave- splitting coating at the interface plane 117. A combination of these two methods can also be used. The light wave- splitting arrangement is not limited to a single interface plane, and other partially reflecting surfaces can be added by cementing other thin plates to the major surfaces of the substrate 20.

[0050] The light wave-expanding method illustrated in Fig. 5 may represent many potential optical systems, especially for those having moderate FOVs and long substrates. For other systems, however, having wider FOVs and short substrates, some problems prevent proper uniform expansion. To avoid non-uniformity in the expanded beam, it is required that the coupled beams pass through the partially reflecting surfaces at least six times. In addition, the reflecting coefficient of the surfaces should be between 25% and 75%. For many systems, the coupled waves impinge on the external surfaces of the substrate, and hence, on the partially reflecting surfaces, only a few times before being coupled-out from the substrate. Moreover, for systems having a wide FOV the angular range of the impinging waves on the partially reflecting surfaces is consequently wide and it is usually difficult to achieve the required reflectance for the entire FOV.

[0051] Another main drawback of the proposed method is that in order to avoid aberrations and smearing of the image, the direction of the trapped rays inside the substrate should be strictly retained. Therefore, a high degree of parallelism should be maintained for the three reflecting surfaces - the upper major surface 28 of the substrate 20, the lower surface 27 of the plate 116, and the interface plane 117. As a result, the external surfaces of substrate 20 and plate 116 should have high parallelism and be of very good optical quality before connecting them to each other. Applying an optical coating, however, to one of these surfaces will require a complicated coating process, which usually deforms the surfaces of the coated plate, especially for a thin plate wherein its optimal thickness is in the order of 0.2 mm. Another problem is that the light rays 92a, 106a, 138, and 142, reflected from surface 22, intersect the interface surface 117 before being coupled out from the substrate 20. This undesired intersection is an unavoidable result of the required parallelism between the partially reflecting surface 117 and the output surface, which is coalesced with the major surface 27 of the substrate. As a result, a simple reflecting coating cannot easily be applied to surface 117, since this surface should also be transparent to the light-waves that exit the substrate 20 and transparent to the light wave from the external scene for see-through applications. This means that the light waves should pass through plane 117, without substantial reflections at small incident angles, and be partially reflected at higher incident angles. This requirement complicates the coating procedure and increases the probability that the coated plate will be deformed during the coating process. Consequently, since even a minor deformation will deteriorate the imaging system's performance, an alternative mixing arrangement should be applied.

[0052] The conclusion from the above-described drawbacks of the former method is that the new expanding method should have the following characteristics: the number of the reflections of the coupled waves should be at least six; the applied coating (if any) should be as simple as possible; the thickness of the coated plates should be at least 0.3 mm; it is preferred that the same coating should be applied to both major surfaces of the coated plates; to enable a proper and efficient coupling-out from the substrate, and the partially reflecting surfaces should be inclined at an angle, preferably a substantial angle, to the output surface of the substrate.

[0053] Fig. 6 illustrates a different expanding device and method according to the present invention that fulfils all the above-described required characteristics without complications. As shown, the optical device 150 is a transparent substrate having a parallelepiped shape comprising three parallel plates 152, 154, and 156, optically attached at two interface surfaces 164 and 166. The two major external surfaces 158 and 162 are mutually parallel to the interface surfaces, and the input surface 160 is parallel to the output surface 170. The interface surfaces 164 and 166 are coated with a conventional partially reflecting coating, forming a light wave- splitting arrangement, and the external surfaces 158 and 162 are reflecting surfaces. The required reflectance of surfaces 158 and 162 may be achieved by a reflecting coating forming a light wavesplitting arrangement or, for specific substrate material and substrate geometry, by total internal reflectance from the external surfaces 158 and 162. As illustrated, a light wave 167 having a narrow lateral dimension 168 enters element 150 through an input aperture 169. The light wave 167 is split by the partially reflecting surfaces 164, 166 and reflected by the external surfaces 158, 162. Consequently, the light wave is gradually expanded while propagating through the element, and, as a result, it exits the element through the output surface 170. The lateral dimension of the output light wave 175 substantially covers the entire output aperture 172, and it is significantly wider than the lateral dimension 168 of the input light wave 167. The incoming light waves can be coupled into element 150 by a reflection from one of the major external surfaces 158 and 162, or by one of the internal partially reflecting surfaces 164, and 166. Thesejight waves are also coupled out from element 150 by one of the same surfaces. Since all these surfaces are mutually parallel, the output direction is the same as the input direction, and hence, the initial direction of the incoming light wave is retained. Unlike the embodiment illustrated in Fig. 5 above, the reflecting surfaces here are substantially inclined to the input 160 and the output 170 surfaces. As a result, the input surfaces 160 and the output 170 surfaces can be simple, transparent surfaces, and applying only a conventional anti-reflection (AR) coating thereon is required to maximize the optical system's efficiency. In addition, the coupled-out light waves are not required to pass through surfaces 164 and 166 at all. Hence, a simple light wave- splitting coating might be applied to these surfaces.

[0054] As mentioned above, the active area of the input aperture 169 is substantially wider than the lateral dimension 168 of the input wave. Consequently, the input light wave can be located along the input aperture in many places. As a result, there is flexibility for the exact location of the display source in relation to the expanding element 150. In addition, a few different light sources can be located in the input aperture 169, and the output light wave 175 will result in a uniform combination and expansion of the various input beams. On the other hand, the lateral dimension of the output light wave 175 is similar to the output aperture 172. Therefore, the ratio between the lateral dimensions of the coupled-out and the input light waves is substantially bigger than the ratio between the lateral dimensions of the output and the input apertures along the x-axis.

[0055] For simplification, the illustrated element 150 comprises only three plates, however, in general, the element may have a substantially larger number of plates, depending on the expansion ratio between the lateral dimensions of the input 167 and the output light waves 175. In addition, the characteristics of other elements, such as the material of the plates, the inclination angle otinc between the input surface 160 and the external surface 158, the partial reflectance of the interface surfaces, the thickness of the plate, and the element's dimensions are set according to various system's parameters, like the FOV of the expanded image and the required light wave output diameter. In addition, to preserve the parallelism of the plates and avoid plate deformation resulting from the coating process, it is preferred that each plate be either coated by a partially reflecting coating from both sides, or not coated at all. That is to say, element 150 comprises 2n+l plates arranged in an interlaced arrangement, alternately including n-1 double- sided coated plates and n uncoated plates. For example, in the embodiment illustrated in Fig. 6, the internal plate 154 is coated by a partially reflecting coating on both surfaces 164 and 166, while the external plates are uncoated before the cementing process. After the stacking, the external surfaces 158 and 162 are coated by, e.g., a total reflecting coating, if necessary.

[0056] In the embodiment illustrated in Fig. 6, the required lateral expansion of the output aperture along the x-axis is achieved. In many applications, however, it is required to expand the output aperture also along the orthogonal y-axis.

[0057] Figs. 7A and 7B illustrate a method for expanding the input light wave along two orthogonal axes. As shown in Fig. 7A, the input light wave is first expanded along the x-axis by element 150a, similar to the expansion by element 150 illustrated in Fig. 6. The output light wave from element 150a is then expanded by a second element 150b, rotated by 90° in relation to element 150a, along the main propagation direction z-axis. The output aperture 172 of element 150a is the input aperture for the second element 150b, wherein aperture 176 is the output aperture of the entire system. Elements 150a and 150b should not necessarily be identical. They can have different partial reflectance of the interface surfaces, thickness of plates, element dimensions, and inclination angle otinc according to various system parameters. In the case of a polarized input light wave, it is preferred to reflect an s -polarized light. Hence, the input light wave 167 should be oriented accordingly in relation to the expanding element 150a. In addition, to verify that also in the expanding element 150b, the light waves are correctly oriented, a half- wavelength retardation plate 174 should be inserted between elements 150a and 150b.

[0058] The active area of the input aperture 169 is wider than the lateral dimension 168 of the input wave along the x and the y axes. Consequently, the input light wave can be located along the input aperture in many places along these two axes. On the other hand, the lateral dimension of the output light wave 175 is similar to the output aperture 176 of the two expanding elements. Therefore, the ratio between the lateral dimensions of the coupled-out and the input light waves is substantially bigger than the ratio between the lateral dimensions of the output and the input apertures along the x and the y axes.

[0059] In the embodiments illustrated in Figs. 6 and 7A and 7B hereinabove, the expanding elements have parallelepiped shapes, and the output light wave is coupled from the element in the same direction as the input element. There are systems, however, wherein it is required, in addition to the wave expansion, to fold the wave into a different direction. Fig. 8 illustrates an alternative embodiment wherein the expanding element 178 is a right-angled isosceles prism comprising four parallel plates 179, 180, 181, and 182. As shown, the input light wave 193, having a lateral dimension 195, enters element 178 through the input surface 198 and is partially reflected by the parallel surfaces 186, 189, and 191. There is only one major reflecting surface, 183, parallel to the partially reflecting surfaces. Usually, the central input and the output light waves should be normal to the input, and the output surfaces, respectively, or at least have the same off-axis angle about these surfaces. As a result, the surfaces in the x-z plane are preferred to be isosceles triangles, though they shouldn't necessarily be right-angled triangles. The light wave is coupled out through the output surface 196, which is orthogonal to the input surface 194, when the output lateral dimension, covering the aperture 197, is substantially wider than the input lateral dimension 195. Here, the initial direction of the incoming light wave is not retained, and the direction of the coupled-out light wave is rotated by around 90° in relation to the initial direction of the collimated input light wave. If the expanding element is not a right-angled triangle prism, the coupled-out light waves will be rotated at a constant angle, different than 90°, in relation to the respective initial direction of the incoming light waves.

[0060] Figs. 9A and 9B illustrate a method for expanding the input light wave along two orthogonal axes using two right triangular prism-shaped elements. As shown in Fig. 9A, the input light wave is first expanded along the x-axis by element 178a, similar to the expansion by element 178 illustrated in Fig. 8. The output light wave from element 178a is then expanded by a second element 178b, rotated by 90° in relation to element 178a, along the orthogonal z-axis. The output aperture 197 of element 178a is the input aperture for the second element 178b, wherein aperture 199 (Fig 9B) is the output aperture of the entire system. For linearly polarized input light wave, a halfwavelength retardation plate 190 should be inserted between elements 178a and 178b. Elements 178a and 178b should not necessarily be identical. They may have different partial reflectance of the interface surfaces, thickness of plates, elements dimensions, and inclination angle otinc according to various system parameters. In addition, the expansion along the two orthogonal exes should not be performed by the same type of expanding element. For example, the expansion along the x and y axes can be performed by a parallelepiped and an isosceles triangular prism, respectively, or vice versa.

[0061] In all embodiments illustrated in Figs. 6 to 9 referred to hereinabove, the expansion is demonstrated for a single plane wave incident normal to the input surface. Usually, however, the input image has a finite FOV along the x and y axes, namely, a significant part of the expanded image is incident at the input surface with a substantial off-axis angle. Since the coupled waves inside the expanding elements are reflected from parallel surfaces, the output wave should retain the original direction of the input wave, even for off-axis impingement.

[0062] As illustrated in Figs. 10A and 10B, the coupled waves impinge at the input surface with off-axis angles of ao and -a0, respectively, are coupled out in the original directions (it is assumed henceforth that the off-axis angles of the incident and the coupled-out light waves, are inside element 150). Some rays, however, are coupled out in different directions.

[0063] As illustrated in Fig. 11 A, the input ray 167R, having an off-axis angle ao, is partially reflected from surface 164 at point 203. The reflected part 202a is coupled out in the original input direction ao. However, the passing part 202b impinges on the output surface 170 at an incident angle higher than the critical angle. As a result, ray 202b is totally reflected from surface 170 and then is reflected from surface 162 and coupled out at an off-axis angle of -ao. Consequently, part of the expanded light waves creates a ghost image in the wrong direction.

[0064] Similarly, as illustrated in Fig. 1 IB, the input ray 167L, having an off-axis angle -ao, is partially reflected from surface 164 at point 205. The passing part 204a propagates inside element 150 and is coupled out in the original direction -ao. However, the reflected part 204b impinges on the input surface 160 at an incident angle higher than the critical angle. As a result, ray 204b is totally reflected from surface 160 and then, after propagating inside element 150, is coupled out at an off-axis angle of ao. Again, part of the expanded light waves creates a ghost image in the wrong direction.

[0065] Figs. 12A and 12B illustrate a method to solve the ghost image issue. As shown in Fig. 12 A, a flat plate 206, having a refractive index similar to that of element 150, is cemented to the output surface 170. Preferably, the optical cement used to attach parts 150 and 206 has a similar refractive index. If such cement is unavailable, an appropriate anti-reflection coating should be applied to surface 170. The passing ray 202b is not reflected now from surface 170 but passes through it into plate 206, reflected from the upper surface 207, and absorbed in the opaque surface 209. As a result, the undesired ghost image is avoided.

[0066] Similarly, as shown in Fig. 12B, a flat plate 210, having a refractive index similar to that of element 150, is cemented to the input surface 160. Again, the optical cement used to attach parts 150 and 210 has a similar refractive index. The reflected ray 202b is not reflected now from surface 160 but passes through it into plate 210, is reflected from the lower surface 211, and is absorbed in the opaque surface 212. Once more, the undesired ghost image is avoided.

[0067] Figs. 13A and 13B illustrate another method to solve the ghost image issue. As shown in Fig. 13A, a complementary prism 214 with a head angle of ap\ is attached to the upper part of element 150, wherein and amaxi is the maximal positive off-axis angle of the incident light waves on the input surface 160. As a result, the light wave 167R having an incident angle of amaxi doesn't cross the interface surface 215 between prism 214 and element 150, and hence, the undesired ghost images here are avoided. Similarly, as shown in Fig. 13B, to prevent the ghost images resulting from reflections of the input surface 160, a second complementary prism 216 with a head angle of aP2 is attached to the lower part of element 150, wherein and -max2 is the maximal negative off-axis angle of the incident light waves on the input surface 160. As a result, the light wave 167L, having an incident angle of -amax2 is reflected from the reflecting surface 158 parallel to the surface 217, and hence, doesn't cross the interface surface 217 between prism 216 and element 150, and again, the undesired ghost images here are avoided.

[0068] As illustrated in Fig. 14, a combination of the methods described in Figs, can be utilized to decay the undesired ghost images. As shown, the ghost images are decayed at the upper part of element 150 by attaching a flat plate 206 to the output surface 170, and at the lower part by attaching a complementary prism 216 to the interface surface 217.

[0069] In the embodiments illustrated in Figs. 6, 7A, 7B and 9A to 14, the inclination angle atnc of the reflecting surfaces to the input surface 160 was nearly 45°. Consequently, for an incident FOV symmetric around the normal axis to the input surface 160, the ghost image issue is practically the same for the upper and the lower part of element 150. The inclination angle ainc, however, might have other values.

[0070] As illustrated in Fig. 15 A, the inclination angle fulfills the relation ainc- 45°

[0071] As a result, the light wave 167L, having an incident angle of amiix2 on the input surface 160, is reflected from the reflecting surface 158 parallel to the input surface. Hence, the undesired reflections from this surface are avoided. The light wave 167R, having an incident angle of amaxi on the input surface 160, however, is reflected at a much higher off-axis angle from surface 158 than in the embodiment of Fig. 11 A. Therefore, a substantially thicker plate 206 is required now to decay the ghost image at the upper part of element 150.

[0072] Fig, 15B illustrates an embodiment wherein the inclination angle fulfills the relation

[0073] As a result, the light wave 167R, having an incident angle of amaxi on the input surface 160, is reflected from the reflecting surface 158 parallel to the input surface. Hence, the undesired reflections from the output surface 170 are avoided. The light wave 167L, having an incident angle of amax2 on the input surface 160, however, is reflected at a much higher off-axis angle from surface 158 than in the embodiment of Fig. 11B. Therefore, a substantially thicker plate 216 is now required to decay the ghost image at the lower part of element 150.

[0074] In the embodiment illustrated in Fig. 16, the inclination angle ainc fulfills the relation

[0075] 3<Anc < 90 — C-rnaxl -

[0076] As explained in relation to Fig. 15B, since the inclination angle is substantially smaller than 45°, there are no undesired reflections from the output surface 170. In addition, as shown in Fig. 16, the light wave 167R, having an incident angle of amaxi on the input surface 160, is reflected from the reflecting surface 158 at an off-axis angle of amcai + 2ainc. As a direct consequence of relation (5), the ray reflected from point 242 on surface 160 will not impinge on the left side of surface 166 but on the right side of surface 164 at an off-axis angle of amaxi + 3tz(nc. That is to say, light wave 167R is reflected at least twice by the right side of surface 164 before being reflected from the other reflecting surfaces and is totally reflected from the input surface between the first and the second reflectance from surface 164. By applying on the partially reflecting surfaces 164 and 166 an optical cement having a refractive index substantially smaller than that of the optical material of element 150, it is possible to fulfill the relation

[0077] ^maxl T 3ctinc> Ctcr, wherein acris the critical angle of surfaces 164 and 166. Consequently, the light wave 167R will be totally reflected from surface 164. Subsequently, the light wave will be totally reflected from point 219 on surface 166, then totally reflected from point 244 on surface 170, and then partially reflected from point 220 on surface 166. That is to say, light wave 167R is reflected at least twice by the left side of surface 166 before being coupled out from the substrate and is totally reflected from the output surface between the first and the second reflectance from surface 166. The reflected ray 222a will be coupled out at the original direction amax\- As required, the passing ray 222b will be reflected from surface 162 and coupled out in the same original direction. Since the internal reflection of light wave 167 from the input surface 160 is compensated by the reflection from the output surface 170, there is no need to avoid this phenomenon, as is the case in the embodiments of Figs. 12A, 12B, 13A, 13B, 14, 15A and 15B. Another advantage of the embodiment illustrated in Fig. 16 is that since the inclination angle aincis substantially smaller than 45, the thickness of element 150 along the z-axis will be substantially smaller than that of the former embodiments.

[0078] Hitherto, it has been assumed that the light source to be expanded is a single collimated display source. The combining device 150, however, might also be used, besides the beam expanding, to combine a few laterally separated sources. One common application for such a combiner is for red, green, and blue (RGB) laser displays. Presently, the main method to achieve the required combining is to use an x- cube combiner, wherein dichroic coatings are applied on the inner surfaces of the cube, however, this device is usually expensive and complicated to use. An alternative method to materialize the required combiner is illustrated in Fig. 17. As shown, three different light sources, 167r, 167g, and 167b (wherein the subscripts r, g, and b denote the colors red, green, and blue) each having laterally separated lateral dimensions 168r, 168g, and 168b, respectively, illuminate the input surface 160 through the input aperture 169. Each of the input beams is being expanded by element 150. Hence, the output aperture 172 is evenly illuminated by the output light wave 167w, a uniform combination of the three input light waves. The light waves in Fig. 17 are expanded only along the x-axis, however, as exemplified in Figs. 7A and 7B, a second orthogonal expanding element 150b may be added to further expand the output wave 167w along the y-axis, if required.

[0079] The embodiment illustrated in Fig. 17 combines three laterally separated RGB light waves and expands their output aperture. For RGB laser scanners, however, it is required to add a scanning device that enables the formation of the two-dimensional image of the source. For most existing devices, the scanning is performed after the three light RGB waves are combined by the x-cube. Only then are the scanned light waves expanded to the required aperture.

[0080] Fig. 18 illustrates an alternative method based on the embodiment of Fig. 17 that uses a scanning device before the combining process. As shown, a linear scanner device 250 is inserted between the input light waves 167r, 167g, and 167b, and the input surface 160. The scanner device comprises three double-grating elements, each including two gratings, 254i and 256i (i = r, g, and b). The gratings are placed on two parallel surfaces, 254 and 256, respectively. As extensively elaborated in Figs. 1-10 of Publication WO2017 / 141241 referred to hereinabove, the scanning mechanism can be accomplished by either translating surface 256 along the x-axis or by modifying the refractive index of the material placed between surfaces 254 and 256. As a result of the combined scanning and expanding system, the projected image 267w uniformly covers the entire output aperture 172 for the required scanned FOV and photopic region.

[0081] In all the embodiments illustrated in Figs. 6 to 18 above, the expansion method is demonstrated for imaging systems. As a result, it was crucial to retain the original direction of the incoming light waves to avoid ghost images. The expanding method, however, can also be utilized for non-imaging systems. An example of such a system could be an RGB LED utilized as a front illumination source for time- sequential color imaging, in which the color images are generated by sequentially laying down three primary colors of RGB light in a single image frame. Usually, the different sources of the RGB colors are located at the same circuit but not at the exact location. That is, the color sources are slightly laterally shifted relative to each other. As a result, the three colors do not illuminate the image frame uniformly. Consequently, a mixing device is required to combine the colors properly.

[0082] Fig. 19 illustrates a mixing element 150 that combines the three-color sources 167r, 167g, and 167b, located at the surface of source 260, to illuminate the exit aperture 172 uniformly, and from there, it is projected to the image frame 262. Here, the exact direction of the output light waves is not crucial. Hence, extra elements that decay the potential ghost images, as shown in Figs. 12A, 12B, 13A, 13B, 14, 15A and 15B, are not required here. There are systems, however, wherein it is required to reduce the diverging cones of the sources to increase the system brightness. In that case, a collimating lens 264 might be inserted between the light source 260 and the input surface 160. This collimation can be performed by a simple lens and even a Fresnel element.

[0083] The embodiments illustrated above are used as expansion devices for a narrow input light wave. These embodiments, however, can also be utilized as homogenizers for wide non-homogeny input plane waves. There are many optical imaging systems whose output waves suffer from dark or bright lines over the active output aperture. This phenomenon can result from a non-perfect illumination source or a vignetting in the imaging system.

[0084] As illustrated in Fig. 20, a narrow beam 302, having a narrow aperture 168, is coupled into an element, e.g., a substrate 150. This beam can represent a dark or a light segment in a much wider plane wave 306, which is part of the input image. Before being coupled into element 150, the projected image is seen by an external viewer as having a prominent dark or bright line therein. After propagating through element 150, however, the narrow input light wave is coupled out at many locations over the exit aperture 172 of the substrate. Part of the light waves (dashed lines 304a) are coupled out after being reflected from the parallel reflecting surfaces, and part (dotted- dashed lines) is also totally reflected from the input surface 160 and the output surface 170. As a result, the narrow prominent input beam fades over a much wider area of the output wave 175. Consequently, the coupled-out wave will be substantially more homogenized, and the image quality in the viewer’s eye will be considerably better.

[0085] 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

CLAIMS:

1. An optical device, comprising: an input aperture; an output aperture; a first light-transmitting substrate including an input surface, at least one major surface, at least one internal surface having a light wave- splitting arrangement, a light wave output surface inclined at an angle to the major surface of the substrate for coupling the light waves out of the light-transmitting substrate through the output aperture, and edges; the input aperture is located next to the input surface and the output aperture is located next to the output surface; at least one optical surface for coupling at least one input light wave into the first light-transmitting substrate through the input aperture by internal reflection from the major surface; the at least one input light wave, having lateral dimensions along a first x-axis and a second y-axis, incident on the input aperture has an initial direction; wherein light waves coupled into the first light-transmitting substrate through the input aperture are partially reflected from the light wave- splitting arrangement and partially pass through it at least once, and are then coupled out from the first lighttransmitting substrate through the output surface and the output aperture, the lateral dimension of the coupled-out light wave along the first x-axis is larger than the lateral dimension of input light wave along the first x-axis, and the ratio between the lateral dimensions of the coupled-out and the input light waves is bigger than the ratio between the lateral dimensions of the output and the input apertures along the first x-axis.

2. An optical device according to claim 1, further comprising a display source emitting an array of collimated light waves, wherein the light waves are coupled into the first substrate through the input aperture and out from the substrate through the output aperture.

3. The optical device according to claim 1, further comprising a second lighttransmitting substrate having an input aperture, an output aperture, at least one major surface, an output surface and edge, at least one optical element for coupling light waves into the second light-transmitting substrate by internal reflection and at least one element carried by the second light-transmitting substrate for coupling light waves out of the light-transmitting substrate, wherein the input aperture of the second light-transmitting substrate is located at least partially next to the output aperture of the first light-transmitting substrate, and light waves coupled out from the first light-transmitting substrate, are coupled into the second light-transmitting substrate through the input aperture of the second lighttransmitting substrate.

4. An optical device according to claim 3, wherein the lateral dimensions of the input light wave are substantially smaller than the lateral dimensions of the coupled-out light wave from the second substrate along two lateral axes, and the ratio between the lateral dimensions of the coupled-out light wave from the second substrate and the input light waves is substantially bigger than the ratio between the lateral dimensions of the output aperture of the second substrate and the input aperture of the first substrate along the two lateral axes.

5. An optical device according to claim 3, further comprising a display source emitting an array of collimated light waves, wherein the light waves are coupled into the first substrate through the input aperture of the first substrate and out from the second substrate through the output aperture of the second substrate, for each of the light waves the lateral dimension of the coupled-out light along the two lateral axes is larger than the lateral dimension of the coupled-in light wave along the two lateral axes, and the ratio between the lateral dimensions of the coupled-out and the coupled-in light waves is substantially bigger than the ratio between the lateral dimensions of the output aperture of the second substrate and the input aperture of the first substrate along the two lateral axes.

6. An optical device according to claim 2, wherein the first light-transmitting substrate has a parallelepiped shape, and the initial directions of the collimated input light waves are maintained after being coupled out through the output aperture.

7. An optical device according to claim 2, wherein the first light-transmitting substrate has a right-angled isosceles prism shape, and the directions of the coupled- out light waves are rotated by around 90° in relation to the initial direction of the collimated input light waves.

8. An optical device according to claim 2, wherein the first light-transmitting substrate has a non-right-angled isosceles prism shape, and the directions of the coupled-out light waves are rotated by a constant angle in relation to the initial directions of the collimated input light waves.

9. An optical device according to claim 1, wherein the first light-transmitting substrate comprises at least two parallel flat plates.

10. An optical device according to claim 9, wherein the parallel flat plates are arranged in an interlaced arrangement, alternately including double- sided partially reflecting coated and uncoated plates.

11. An optical device according to claim 1, further comprising at least one flat transparent plate optically attached to the input or the output surface of the first lighttransmitting surface.

12. An optical device according to claim 9, further comprising at least one transparent triangle prism.

13. An optical device according to claim 2, further comprising at least two different display sources laterally separated along the input aperture.

14. An optical device according to claim 13, wherein the display sources are three RGB lasers.

15. An optical device according to claim 14, further comprising a scanning device located between the display sources and the input surface of the first light-transmitting substrate.

16. An optical device according to claim 1, further comprising at least two illuminating light sources laterally separated along the input aperture.

17. An optical device according to claim 16, wherein the illuminating light sources are RGB LEDs.

18. An optical device according to claim 17, further comprising a collimating lens located between the LEDs and the input surface of the first light-transmitting substrate.

19. An optical device according to claim 3, wherein one of the light-transmitting substrates has a parallelepiped shape, the other light-transmitting substrate has an isosceles prism shape, and the directions of the coupled-out light waves are rotated by a constant angle in relation to the initial directions of the collimated input light waves.

20. An optical device according to claim 3, further comprising a half-wavelength retardation plate inserted between the first and the second light-transmitting substrate.

21. An optical device according to claim 1, wherein light waves coupled inside the substrate are reflected at least twice by the same side of one of the elements carried by the first light-transmitting substrate before being coupled out from the substrate through the output aperture and are reflected by internal reflection from the input surface between the first and second reflectance from the element.

22. An optical device according to claim 1, wherein light waves coupled inside the substrate are reflected at least twice by the same side of one of the elements carried by the first light-transmitting substrate before being coupled out from the substrate through the output aperture and are reflected by internal reflection from the output surface between the first and second reflectance from the element.

23. An optical device, comprising: an input aperture; an output aperture; a first light-transmitting substrate having an input surface, at least one major surface, at least one internal surface, an output surface and edges; at least one input light wave having lateral dimensions along a first and a second axis incident on the input aperture having an initial direction;at least one optional element for coupling the input light wave into the first lighttransmitting substrate through the input aperture by internal reflection from the major surface; at least one element carried by the first light-transmitting substrate for coupling the light wave out of the light-transmitting substrate through the output aperture, and a light wave- splitting arrangement applied to the internal-surface, wherein light waves coupled into the first light-transmitting substrate through the input aperture are partially reflected from the light wave- splitting arrangement and partially pass through it at least once, and then coupled out from the first lighttransmitting substrate through the output aperture, for at least one segment of an input light wave, the internal dimension of the coupled out segment along the first axis, is larger than the lateral dimension of the segment along the first axis, and the ratio between the lateral dimensions of the coupled-out and the input segments, is bigger than the ratio between the lateral dimensions of the output and the input apertures along the first axis, and wherein the output aperture is located next to the output surface and the input surface is inclined at an angle to the major surface.

Images