Light-guiding optics for polarization scrambling

Abstract

A light-guiding optical element (LOE) configured for polarization scrambling is provided. The LOE includes a transparent substrate having a first refractive index, the substrate having a pair of parallel outer surfaces and a plurality of mutually parallel partially reflective inner surfaces, the pair of parallel outer surfaces being configured to propagate light within the LOE by total internal reflection (TIR), the plurality of mutually parallel partially reflective inner surfaces being non-parallel to the pair of parallel outer surfaces and configured to couple the light out to a viewer. The LOE also includes a first coating on at least one outer surface of the substrate, the first coating being a coating material having a second refractive index higher than the first refractive index; the LOE also includes an anti-reflective (AR) coating on at least one outer surface of the substrate over the first coating.

Description

Technical Field

[0001] The presently disclosed subject matter relates to light guiding optical elements, and more particularly to light guiding optical elements configured for polarization scrambling. Background Art

[0002] The present invention relates to light-guide compact collimating optical devices (LCCDs) and optical systems comprising one or more of these devices. The term "light-guide" refers to any light-transmitting body, preferably a light-transmitting solid, also referred to as an optical substrate. One of the important applications of compact optical devices is in the field of head-mounted displays (HMDs), where the optical module is used as both a reflective optical element and a combiner, where a two-dimensional image is imaged to infinity and reflected into the eyes of a viewer. The image can be obtained directly from a spatial light modulator (SLM) (e.g., a cathode ray tube (CRT), a liquid crystal display (LCD), a liquid crystal on silicon (LCOS) module, an organic light-emitting diode array (OLED), a micro-LED, a scanning source or a similar device) or indirectly with the aid of a relay lens or a fiber bundle. The image comprises an array of elements (pixels) that are imaged to infinity by a collimating lens and transmitted to the eyes of a viewer with the aid of a reflective or partially reflective surface that acts as a combiner for non-perspective and perspective applications, respectively. Typically, conventional free-space optical modules are used for these purposes. The optical module will be referred to herein as a light-guide optical element ("LOE").

[0003] Typically, the LOE is located in front of the viewer's eye. A collimated beam of light entering the LOE is reflected between parallel surfaces by total internal reflection (TIR). The coated facets partially reflect the light toward the viewer's eye.

[0004] Typically, the light entering the LOE is either S-polarized or P-polarized. The coating on the facet reflects light with the same polarization state. Since only one polarization state is involved, the TIR reflection will maintain that polarization state. However, in applications where the light entering the LOE is unpolarized, each TIR reflection causes a phase shift that will change the polarization state of some of the light. This in turn will cause a corresponding change in the amount of light reflected by the facet, which is undesirable. Summary of the invention

[0005] According to one aspect of the subject matter of the present disclosure, there is provided a light-guiding optical element (LOE) constructed for polarization scrambling, comprising: a transparent substrate having a first refractive index, the substrate comprising a pair of parallel outer surfaces and a plurality of mutually parallel partially reflective inner surfaces, the pair of parallel outer surfaces being configured to cause light to propagate within the LOE by total internal reflection (TIR), the plurality of mutually parallel partially reflective inner surfaces being non-parallel to the pair of parallel outer surfaces and being configured to couple light out to a viewer; a first coating having a thickness between 100 nanometers and 10 microns on at least one outer surface of the substrate, the first coating comprising a coating material having a second refractive index higher than the first refractive index; and an anti-reflective (AR) coating over the first coating on at least one outer surface of the substrate.

[0006] In some embodiments, the first coating is configured to increase a phase shift between an s-polarization component and a p-polarization component of light incident at an angle above a TIR cutoff angle of the substrate, the light having a wavelength between 400 nm and 1300 nm.

[0007] In some embodiments, the AR coating is configured to reduce or eliminate reflection of light entering the LOE at incident angles between 0° and 50°.

[0008] In some embodiments, the AR coating is configured to reduce reflection of light entering the LOE at a predetermined angle to between 0.3% and 10% reflected light.

[0009] In some embodiments, the first coating includes a high refractive index dielectric material.

[0010] In some embodiments, the first coating includes a material selected from the group consisting of TiO2, Si3N4, and ZnS.

[0011] In some embodiments, the AR coating includes one or more layers of at least one coating material selected from the group consisting of SiO 2 , HfO 2 , TiO 2 , MgF 2 , and Al 2 O 3 .

[0012] In some embodiments, the AR coating includes one or more layers of at least one material having a refractive index in the range of 1.35 to 2.5. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] In order to understand the invention and to see how it may be implemented in practice, an embodiment will be described by way of non-limiting example and with reference to the accompanying drawings, in which:

[0014] Figure 1 shows a general schematic diagram of a LOE according to the prior art;

[0015] Figure 2shows a generalized schematic diagram of a LOE according to certain embodiments of the disclosed subject matter;

[0016] Figure 3 shows a graph showing an example of phase shift performance of one surface of a LOE without a polarization scrambling coating;

[0017] Figure 4a A graph showing high spectral peaks of RGB light is shown;

[0018] Figure 4b A graph showing a high spectral peak for white light is shown;

[0019] Figure 5 shows a graph showing an example of spectral differential phase shifting performance of a polarization scrambling coating applied to a substrate;

[0020] Figure 6 shows a graph showing the reflective properties of an exemplary polarization scrambling coating without an AR coating at normal incidence;

[0021] Figure 7a shows a graph showing an example of a reflected differential phase shift for light reflected from a LOE surface for a light ray in TIR;

[0022] Figure 7b shows a graph showing an example of differential phase shift on reflection for various angles in the TIR region;

[0023] Figure 8 shows a graph showing an example of reflection at normal incidence with a polarization scrambling coating and an AR coating;

[0024] Fig. 9 shows a graph showing an example of reflection of a polarization scrambling coating applied between a substrate and a metal coating;

[0025] Fig.10 An embodiment of a display system having two waveguides is shown; and

[0026] Fig.11 Another embodiment of a LOE according to the present invention is shown. DETAILED DESCRIPTION

[0027] In the detailed description below, many specific details are set forth to provide a thorough understanding of the present invention. However, it will be appreciated by those skilled in the art that the disclosed subject matter may be implemented without these specific details. In other cases, well-known methods, processes and components are not described in detail in order to avoid obscuring the disclosed subject matter.

[0028] As used herein, the term "phase shift" refers to the difference between the phases of S-polarized light and P-polarized light.

[0029] Remember this, pay attention Figure 1 , which shows a generalized schematic diagram of a LOE according to the prior art. The LOE comprises a substrate 34 having a pair of parallel outer surfaces 26, 27, and a non-parallel set of mutually parallel partially reflective inner surfaces ("facets") 22. The partially reflective properties of the facets 22 are achieved by a coating applied to the facets. Light rays 18 representing an image are coupled into the LOE and propagate within the LOE via TIR between the surfaces 26, 27 and are ultimately reflected by the facets 22 out toward the eye 24 of the viewer.

[0030] Figure 2 A generalized schematic diagram of a LOE according to certain embodiments of the presently disclosed subject matter is shown. In this case, the LOE is intended to receive and couple out unpolarized light. As in prior art LOEs, the LOE of the present invention includes a transparent substrate 34 having a refractive index (represented herein as "n") higher than that of air. The substrate includes a pair of parallel outer surfaces 26, 27, which are configured to cause light to propagate within the LOE by TIR. The substrate 34 also includes a plurality of mutually parallel partially reflective inner surfaces (facets) 22, which are configured to couple out light to a viewer. The facets are not parallel to the outer surfaces 26, 27. For example, they can be tilted relative to the pair of outer surfaces.

[0031] The LOE also includes a polarization scrambling coating 42 applied to at least one of the surfaces 26, 27. The coating 42 includes a coating material having a refractive index higher than the first refractive index of the substrate 34. The polarization scrambling coating 42 is applied to at least one of the outer surfaces 26, 27 or a portion thereof at a thickness greater than 100 nm and up to about 10 microns. In some cases, the polarization scrambling coating 42 can be applied to two parallel outer surfaces. The polarization scrambling coating 42 applied to the outer surface of the substrate increases the phase difference slope between S polarized light and P polarized light entering the substrate and reflecting off the interior of the outer coated surface. This is for angles of incidence above the total internal reflection (TIR) ​​angle. The phase difference slope is the amount of change in the phase shift difference for a small change in wavelength.

[0032] The polarization scrambling coating 42 can be selected according to predetermined design requirements, based on the expected wavelength range of light entering the LOE and the expected range of angles at which the light will enter (e.g., angles above 42° in the TIR region for BK7 glass). After several reflections, the S-polarized light and the P-polarized light propagating within the LOE will be significantly phase shifted relative to each other, essentially maintaining the unpolarized state of the light. According to the present invention, the facets 22 within the LOE are also coated with a partially reflective coating designed for unpolarized light. These coated facets reflect the unpolarized light toward the viewer as designed.

[0033] In an exemplary embodiment, the polarization scrambling coating 42 is applied at a thickness between 300 nm and 10000 nm, and more preferably between 300 nm and 5000 nm, and even more preferably between 300 nm and 1000 nm.

[0034] Preferably, the polarization scrambling coating 42 comprises a high refractive index dielectric material, such as TiO2 (n≈2.2-2.45), Si3N4 (n≈2.0), ZnS (n≈2.2-2.4), or a suitable equivalent. "High refractive index" refers to a refractive index higher than that of the substrate, and is preferably at least 2. Preferably, the coating 42 is configured to increase the phase shift slope of light having a wavelength between 400 nm and 1300 nm, and more preferably between 400 nm and 750 nm, when reflected off the outer surface of the substrate.

[0035] The LOE also includes an anti-reflection (AR) coating 44 applied to at least one outer surface of the substrate over the coating 42. The AR coating 44 applied over the polarization scrambling coating 42 reduces or eliminates reflection of light entering the LOE at a given angle, thereby providing high transmittance of these light rays and allowing a viewer to see the outside world through the LOE. In an exemplary embodiment, the AR coating is configured to reduce or eliminate reflection of light that strikes the surface at a substantially "normal" angle of incidence (e.g., an angle in the range of 0° to 50°). Preferably, the AR coating reduces reflection of light entering the LOE at a predetermined angle to between 0.3% and 10% reflected light.

[0036] It should be noted that the polarization scrambling coating 42 increases the reflectance of the substrate at normal incidence from about 5% (for a substrate with n=1.6) to an average of about 18% in the visible spectrum (430nm to 660nm). Therefore, the AR coating 44 is required to be able to reduce the reflection and also maintain the depolarization characteristics caused by the polarization scrambling coating 42. These requirements increase the design complexity of the AR coating 44.

[0037] Preferably, the AR coating 44 includes a layer of a coating material having a refractive index in the range of 1.35 to 2.5. Preferably, the coating includes one or more high refractive index materials, one or more low refractive index materials, and one or more medium (i.e., between the high and low refractive index) refractive index materials. In an exemplary embodiment, the AR coating 44 may include a layer of one or more of SiO2 (n≈1.45-1.47), HfO2 (n≈2.0-2.1), TiO2 (n≈2.2-2.45), MgF2 (n≈1.35-1.37), and / or Al2O3 (n≈1.63-1.68).

[0038] It should be noted that Figure 2 It is not drawn to scale, and in practice the thickness of the substrate 34 is typically several orders of magnitude greater than the thickness of the coatings 42 , 44 .

[0039] Figure 3 A graph showing the phase shift performance of a LOE without a polarization scrambling coating is shown. In this case, the LOE is made of S-TIM8 glass (n≈1.596) and no coating 42 is applied to the outer surface. As shown, the phase shift is nearly constant for all wavelengths in the visible spectrum. This is undesirable, as described below.

[0040] Typically, the light entering the LOE comes from an RGB LED or a white LED. Figure 4a A graph showing high spectral peaks for RGB light is shown where phase shift slope enhancement should preferably occur. Figure 4b A graph showing a high spectral peak for white light is shown, where phase shift slope enhancement should preferably occur.

[0041] Figure 5 A graph showing an example of a phase shift caused by a polarization scrambling coating 42 applied to a substrate with reference to the spectral peaks of RGB LED light is shown. In this case, the coating 42 comprises TiO2 and is applied to a substrate 34 made of S-TIM8 glass. Figure 5 Results are shown for two different thicknesses of the coating 42, namely, 300 nm and 1000 nm, respectively. Figure 5 As shown, as the thickness of the layer increases, the slope of the phase change at visible wavelengths becomes larger. The spectral region where the highest depolarization and slope should occur is represented by the dashed squares and corresponds to the spectral peaks of the RGB LED.

[0042] like Figure 5As shown, small changes in the wavelength of light entering the LOE produce large changes in the phase shift. Compare this to the spectral range of the LED emissions illuminating the system. The polarization scrambling coating 42 causes different degrees of phase shift to portions of the narrow spectral peaks (at ≈480nm, ≈580nm and ≈640nm for the RGB LEDs, and at ≈443nm for the white LED). Since the human eye integrates the intensity of light of similar wavelengths, the effect is equivalent to observing unpolarized light. The depolarization after reflection is not complete. Since the light is reflected multiple times by the outer surface before being reflected by the facet 22, the cumulative depolarization of the light is relatively high.

[0043] Figure 6 A graph showing the reflective properties of an exemplary polarization scrambling coating 42 without an AR coating 44 is shown. In this case, the coating 42 was applied at two different layer thicknesses (300 nm and 1000 nm, respectively). As shown, the coating 42 tends to be highly reflective (average of about 20% on each side in the visible wavelengths), resulting in an undesirable attenuation of the outside world view through the LOE.

[0044] Figure 7a A graph showing an example of the reflected differential phase shift of light entering the LOE of the present invention is shown. In this case, the substrate is coated with a polarization scrambling coating 42 comprising a thick TiO2 layer and an AR coating 44 on top of the polarization scrambling coating. The graph shows the reflected differential phase shift of light entering the LOE at an incident angle of 55 degrees in the visible wavelength for two different thicknesses of the coating 42, i.e., 300 nm and 1000 nm, respectively.

[0045] Figure 7b The diagram shows the effect of using the same coating 42 for various angles in the TIR region when a coating 42 with a thickness of 1000 nm is applied. Figure 7a Plot of the reflected phase shift for the same coating in .

[0046] Figure 8 The different wavelengths in the visible spectrum are shown, including Figure 7a Reflection of the coatings of the layers of coating 42 (TiO2 layer thickness of 300 nm and 1000 nm) and coating 44 at normal incidence. Figure 8 As shown, the coating behaves as an AR coating at small angles, where it is obvious that Figure 6 The reflection is greatly reduced (from an average of about 18% in the visible spectrum to about 4%). Using different designs and layer materials, this reflection can be reduced even lower.

[0047] It should be noted that the thickness of the coating 42 does not have to be the same for both outer surfaces. Instead, different thicknesses of coating 42 may be applied to different surfaces to impart different phase change slopes for any spectral region to each side of the LOE. For example, if on one side the LOE has a small phase change slope for a particular wavelength, the coating on the other side may randomly fall on a large phase change slope, where the cumulative phase shift will be large.

[0048] It goes without saying that if the incident light beam is polarized parallel or perpendicular to the facet 22, the coating 42 will not affect the phase of the light beam.

[0049] As noted above, the polarization scrambling coating 42 may be applied to only a portion of one or both exterior surfaces, rather than to the entire surface.

[0050] In some cases, it may be desirable to coat one of the outer surfaces of the LOE or a portion thereof with a metal coating. In this case, a polarization scrambling coating 42 may be added between the substrate and the metal layer, which will increase the phase shift variation over the spectral region.

[0051] Fig. 9 A graph showing an example of the reflection of a polarization scrambling coating 42 applied between a substrate and a metal coating is shown. The graph shows the reflection of light at an incident angle of 55° at different wavelengths in the visible spectrum. The graph demonstrates the phase shift slope without the polarization scrambling coating 42 and with different thicknesses of the layer 42 (300 nm and 1000 nm).

[0052] It should be noted that in some cases, the light entering the LOE may not be in the same plane of the figure, i.e., enter the LOE at another three-dimensional angle. In this case, even if the beam is polarized, its reference TIR plane will not be polarized. Therefore, it will change the polarization state of the reflected beam and can be treated as unpolarized light.

[0053] Fig.10 A display system with two LOEs is shown, where a light beam is reflected from one LOE to a second LOE at an arbitrary angle. This reflection introduces a phase shift which will therefore change the polarization state of the light beam, making it unpolarized when reflected from the facet. Here again, the coating 42 applied to the LOE surface will ensure that the reflection from the facet will be independent of the polarization state of the light beam.

[0054] Fig.11Another embodiment of a LOE according to the present invention is shown. In this embodiment, the LOE is designed so that the light beam enters at the center of the waveguide and propagates toward each opposite end. In this case, the incident light beam propagates through a converging lens 6. Light rays 64L and 64R pass through the LOE. The light beam reaching the eye will be transmitted through the central facet or will be reflected by the central facet and propagate to the sides and reflected by other facets toward the eye. Propagating from the central facet to the side facets, the light encounters TIR from surface 1 and surface 2. Therefore, surface 1 and surface 2 should also be coated with a polarization scrambling coating 42 (and optionally one or more layers of AR coating 44) to depolarize the light beam, because the next reflective facets are designed for unpolarized light. Here again, the addition of an AR layer will increase the transmittance of surface 1 and surface 2.

[0055] It should be understood that the embodiments of the LOE described herein can be implemented in a wide variety of imaging applications, such as head-mounted displays (HMDs) and heads-up displays (HUDs), cellular telephones, compact displays, 3-D displays, compact beam expanders, as well as in non-imaging applications, such as flat panel indicators, compact illuminators, and scanners.

[0056] It should be understood that the present invention is not limited in its application to the details set forth in the description contained herein or shown in the accompanying drawings. The present invention can have other embodiments and can be implemented and executed in various ways. Therefore, it should be understood that the words and terms used herein are for descriptive purposes and should not be considered as limiting. Similarly, those skilled in the art will understand that the concept on which the present disclosure is based can be easily used as the basis for designing other structures, methods and systems for achieving the several purposes of the disclosed subject matter.

Claims

1. A light guide optical element (LOE) configured for polarization scrambling, the light guide optical element comprising: a transparent substrate having a first refractive index, the substrate comprising a pair of parallel outer surfaces and a plurality of mutually parallel partially reflective inner surfaces, the pair of parallel outer surfaces being configured to cause light to propagate within the LOE by total internal reflection (TIR), the plurality of mutually parallel partially reflective inner surfaces being non-parallel to the pair of parallel outer surfaces and configured to couple the light out to a viewer; a first coating having a thickness between 100 nm and 10 microns on at least one outer surface of the substrate, the first coating comprising a coating material having a second refractive index higher than the first refractive index; as well as an anti-reflective (AR) coating on at least one outer surface of the substrate above the first coating, Wherein, the first coating is configured to increase the phase shift between the s-polarization component and the p-polarization component of light with a wavelength between 400 nm and 1300 nm incident at an angle above TIR.

2. The LOE according to claim 1, wherein The AR coating is configured to reduce or eliminate reflection of light entering the LOE at an incident angle between 0° and 50°.

3. The LOE according to claim 1, wherein: The AR coating is configured to reduce reflection of light entering the LOE at a predetermined angle to between 0.3% and 10% reflected light.

4. The LOE according to claim 1, wherein: The first coating includes a high refractive index dielectric material.

5. The LOE according to claim 4, wherein: The first coating layer includes a material selected from the group consisting of TiO2, Si3N4 and ZnS.

6. The LOE according to claim 1, wherein: The AR coating includes one or more layers of at least one coating material selected from the group consisting of SiO2, HfO2, TiO2, MgF2 and Al2O3.

7. The LOE according to claim 1, wherein: The AR coating comprises one or more layers of at least one material having a refractive index in the range of 1.35 to 2.5.

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