Patent Publication Number: US-2017363799-A1

Title: Method for fabricating a substrate-guided optical device

Description:
FIELD OF THE INVENTION 
     The present invention relates to substrate-guided optical devices, and particularly to devices which include a plurality of reflecting surfaces carried by a common light-transmissive substrate, also referred to as a light-guide element. 
     The invention can be implemented to advantage in a large number of imaging applications, such as portable DVDs, cellular phone, mobile TV receiver, video games, portable media players or any other mobile display devices. 
     BACKGROUND OF THE INVENTION 
     An important application for compact optical elements is in head-mounted displays (HMDs), wherein an optical module serves both as an imaging lens and a combiner, in which a two-dimensional image source is imaged to infinity and reflected into the eye of an observer. The display source can be directly obtained 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 or similar devices, or indirectly, by means of a relay lens or an optical fiber bundle. The display source comprises an array of elements (pixels) imaged to infinity by a collimating lens and transmitted into the eye of the viewer 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, however, 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 and especially in head-mounted applications, wherein the system should necessarily be as light and as compact as possible. 
     The strive 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. Furthermore, the eye-motion-box (EMB) of the optical viewing angles resulting from these designs is usually very small—typically less than 8 mm Hence, the performance of the optical system is very sensitive, even for small movements of the optical system relative to the eye of the viewer, and does not allow sufficient pupil motion for comfortable reading of text from such displays. 
     The teachings included in Publication Nos. WO01/95027, WO03/081320, WO2005/024485, WO2005/024491, WO2005/024969, WO2005/124427, WO2006/013565, WO2006/085309, WO2006/085310, WO2006/087709, WO2007/054928, WO2007/093983, WO2008/023367, WO2008/129539, WO2008/149339, WO2013/175465, IL 232197 and IL 235642, all in the name of Applicant, are herein incorporated by references. 
     SUMMARY OF THE INVENTION 
     The present invention facilitates the exploitation of very compact light-guide optical element (LOE) for, amongst other applications, HMDs. The invention allows relatively wide FOVs together with relatively large EMB values. The resulting optical system offers a large, high-quality image, which also accommodates large movements of the eye. The optical system offered by 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 specialized configurations. 
     A broad object of the present invention is therefore to alleviate the drawbacks of prior art compact optical display devices and to provide other optical components and systems having improved performance, according to specific requirements. 
     The main physical principle of the LOE&#39;s operation is that light waves are trapped inside the substrate by total internal reflections from the external surfaces of the LOE. In addition, the light waves which are trapped inside the LOE are coupled out into the eyes of the viewer by an array of partially reflecting surfaces. Therefore, in order to achieve an undistorted image having good optical quality it is important that the on one hand the quality of the external as well as the partially reflecting surfaces will be with high quality and on the other hand that the fabrication process of the LOE will be as simple and straightforward as possible. 
     The invention therefore provides a method for fabricating an optical device comprising a light waves-transmitting substrate having at least two major surfaces and edges and a plurality of partially reflecting surfaces carried by the substrate, wherein the partially reflecting surfaces are parallel to each other and not parallel to any of the edges of the substrate, the method comprising: providing at least one transparent flat plate and plates having partially reflecting surfaces, optically attaching together the flat plates so as to create a stacked, staggered form, slicing off from the stacked, staggered form at least one segment by cutting across several plates, grinding and polishing the segment to produce the light waves-transmitting substrate, characterized in that the plates are optically attached to each other by an optically adhesive-free process. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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. 
     With specific reference to the figures in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing 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 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. 
    
    
     
       IN THE DRAWINGS 
         FIG. 1  is a side view of an exemplary, prior art, LOE; 
         FIG. 2  is a diagram illustrating steps (a) to (e) of a method for fabricating an array of partially reflecting surfaces, according to the present invention; 
         FIG. 3  is a schematic diagram illustrating steps (a) to (c) of a method to increase the number of LOEs which can be fabricated out of a single slice according to the present invention; 
         FIG. 4  is a diagram illustrating steps (a) to (e) of an embodiment of another method for fabricating an array of partially reflecting surfaces, according to the present invention; 
         FIG. 5  is a diagram illustrating steps (a) and (b) of a method to attach a blank plate at the edge of the LOE; 
         FIG. 6  illustrates a span of optical rays illuminating the input aperture of an LOE, wherein one of the edges of the LOE is slanted at an oblique angle with respect to the major surfaces, in accordance with the present invention; 
         FIG. 7  is a schematic diagram illustrating a system coupling-in input light-waves from a display light source into a substrate, wherein an intermediate prism is attached to the slanted edge of the LOE, in accordance with the present invention, and 
         FIG. 8  is a diagram illustrating steps (a) to (c) of a method for fabricating an LOE having a slanted edge, according to the present invention; 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       FIG. 1  illustrates a sectional view of a prior art substrate  20  and associated components (hereinafter also “an LOE”), utilizable in the present invention. An optical means, e.g., a reflecting surface  16 , is illuminated by a collimated display light waves  18 , emanating from a light source (not shown). The reflecting surface  16  reflects incident light waves from the source, such that the light waves are trapped inside a planar substrate  20  of the LOE, by total internal reflection. After several reflections off the major lower and upper surfaces  26 ,  28  of the substrate  20 , the trapped light waves reach an array of selective reflecting surfaces  22 , which couple the light out of the substrate into an eye  24 , having a pupil  25 , of a viewer. Herein, the input surface of the LOE will be regarded as the surface through which the input light waves enter the LOE and the output surface of the LOE will be regarded as the surface through which the trapped light waves exit the LOE. In the case of the LOE illustrated in  FIG. 1 , both the input and the output surfaces are on the lower surface  26 . Other configurations, however, are envisioned in which the input and the image light waves could be located on opposite sides of the substrate  20 , or when the light waves are coupled into the LOE through a slanted edge of the substrate. 
     As illustrated in  FIG. 1 , the light waves are trapped inside the substrate by total internal reflections from the two major surfaces  26  and  28  of the substrate  20 . In order to maintain the original direction of the coupled light waves to avoid double images, it is crucial that the parallelism between the major surfaces  26  and  28  will be to a high degree. In addition, the light waves which are trapped inside the LOE are coupled out into the eyes of the viewer by an array of partially reflecting surfaces  22 . As a result, the parallelism of these surfaces should also be as high as possible. Furthermore, to achieve an undistorted image having good optical quality and to avoid scattering and optical noise, it is important that the surface quality of the external surfaces of the substrate, as well as the partially reflecting surfaces, will be very high. On the other hand, it is also important that the fabrication process of the LOE will be as simple and straightforward as possible. 
     A possible method to fabricate an LOE is illustrated in  FIG. 2 . (a) A plurality of transparent flat plates  102  coated with required partially reflecting coatings  103  and a non-coated flat plate  104 , are optically attached together so as to create a stacked form  106 , see step (b). A segment  108 , step (c), is then sliced off the stacked form by cutting, grinding and polishing, to create the desired LOE  110  ( d ). Several LOE elements  112  and  114  can be sliced off from the stacked form, as shown in (e). The number of the LOE elements that can be sliced off the stack can be maximized by a proper staggering of the plates in the stack. 
     Another method to increase the number of the final elements is illustrated in  FIGS. 3A to 3C . A top view of the sliced LOE  108  is shown in  FIG. 3( a ) . The slice is then cut along the lines  120  and  122  to create three similar sub-segments  FIG. 3( b ) . These sliced segments are then processed by cutting, grinding and polishing, to create three similar LOEs  126   FIG. 3( c ) . 
     An alternative method to produce the LOE is illustrated in  FIGS. 4( a ) to 4( e ) . Instead of coating the selected partially reflecting coatings on the surfaces of the blank plates  132  the reflecting surfaces are prepared on an array of thin plates  134 . In addition to thin-film dielectric coating, the reflecting mechanism here can be an anisotropic polarizing-sensitive reflection such as from wire-grid array, or DBEF films.  FIG. 4( a )  shows the blank plates  132  and the plates  134  with the reflecting surfaces alternately optically attached together so as to create a stacked form  136  see  FIG. 4( b ) . A segment  138   FIG. 4( c )  is then sliced off the stacked form by cutting, is finished by grinding and polishing, to create the desired LOE  140 , as shown in  FIG. 4( d ) . Several elements  142  and  144  illustrated in  FIG. 4( e )  can be sliced off from this stacked form. 
     In many applications it is required, from optical as well as mechanical reasons, to add a blank flat plate at the major surfaces of the LOE.  FIG. 5  illustrates a method, applicable to each of the fabrication methods described with reference to  FIGS. 2 and 4 ( a ) to  4 ( e ) in which a blank plate  146   FIG. 5( a )  is optically attached to one of the major surfaces of the substrate  110 , so as to form an LOE  150   FIG. 5( b )  with the appropriate active apertures for all of the reflecting surfaces. There are applications in which it is required that the LOE  110  will have a wedge structure, namely, surfaces  151  and  152  are not parallel. In such a case it is strictly required that the two external major surfaces  154  and  155 , of the final LOE  150 , will be parallel to each other. 
     In the embodiment illustrated in  FIG. 1 , the light waves are coupled into the LOE through the major surface  26 . There are configurations, however, wherein it is preferred that the light will be coupled into the LOE through a slanted edge of the LOE.  FIG. 6  illustrates an alternative method of coupling light waves into the substrate through one of its edges. Here, the light waves-transmitting substrate  20  has two major parallel surfaces and edges, wherein at least one edge  160  is oriented at an oblique angle with respect to the major surfaces. Usually the incoming collimated light waves coupled directly from the air or alternatively a collimating module (not shown), can be optically attached to the LOE. As a result, it is advantageous to couple the central wave  162  normal to the slanted surface  162  to minimize chromatic aberrations. From various optical reasons which are extensively explained in Israeli Patent Application 235642, this requirement cannot be fulfilled by coupling the light directly through surface  160 . 
     A method for solving this problem is illustrated in  FIG. 7 . An intermediate prism  164  is inserted between the collimating module (not shown) and the slanted edge  160  of the substrate, wherein one of its surfaces  166  is located next to the said slanted edge  160 . In most cases the refractive index of the intermediate prism should be similar to that of the LOE. Nevertheless, there are cases wherein a different refractive index may be chosen for the prism, for compensating for chromatic aberrations in the system. As described above, the incoming collimated light waves are coupled directly from the air, or alternatively, the collimating module (not shown) can be attached to the intermediate prism  164 . In many cases the refractive index of the collimating module is substantially different than that of the LOE, and accordingly is different than that of the prism. Therefore, In order to minimize the chromatic aberrations, the input surface  168  of the prism  164  should be oriented substantially normal to the central wave  162  ( FIG. 6 ). 
     A method for fabricating the required LOE with the slanted edge is illustrated in  FIG. 8 . Here, one of the side edges of the un-slanted LOE  110 , which was fabricated according to the procedures described with references to  FIGS. 2 and 4  ( a ), is cut to create the required slanted edge  160  ( b ), the new surface is then processed by grinding and polishing to achieved the required optical quality. In a case that a thin layer  172  is optically attached to the upper surface  28 , according to the procedure illustrated in  FIG. 5 , the final LOE  174  assumes the shape illustrated in  FIG. 8( c ) . 
     The apparent method to achieve the optical attachment between the various optical elements in  FIGS. 2, 4 ( a )- 4 ( e ),  5 ( a ) and ( b ) and  7  is by applying an optical adhesive between the plates. However, this method might suffer from some severe drawbacks. First of all, as explained above with reference to  FIG. 1 , the parallelism between the partially reflecting surfaces  22  should be very high. This can be achieved by assuring that the parallelism between the external surfaces of the coated plates  102  ( FIG. 2 a   ) will have the same required degree of parallelism. However, the cement layer between the attached plates might have some degree of wedge that will create a finite angle between two adjacent coated surfaces. This undesired effect can be minimized by pressing together the attached plates during the cementing procedure in order to ensure that the thickness of each cement layer in not more than a few microns, however, even with this procedure the cemented LOE suffers from other drawbacks. The cementing lines which are located at the intersections between the cemented and the external surfaces usually cause scattering and diffraction effects which deteriorate the optical quality of the image. This phenomenon is even more apparent for the cementing line  176  which is located in the slanted edge  160  wherein all the light waves cross while coupling into the LOE. In addition, after the cementing procedure it is not possible to increase the temperature of the LOE over 60-70 degrees centigrade. This prevents, for example, hot coatings of the LOE. Hence, when such AR or hard coatings are needed, it is required to perform a special cold coating procedure, which is much more complicated and limited than the regular hot coating procedure. Furthermore, the refractive index of the adhesive, located between the cemented plates should be with very close proximity to that of the plates, in order to avoid undesired reflections. Since the variation of the refractive index of existing optical adhesive is very limited, especially for relatively high indices, the number of optical glass materials that can be utilized for fabricating LOEs is very limited as well. 
     As a result of the above description it will be advantageous to utilize optical attachment processes to attach the optical elements without utilization of optical adhesives. One of the candidates to materialize the adhesive-free procedure is the an anodic bonding process. Anodic bonding is a method of hermetically and permanently joining glass to glass without the use of adhesives. Using a thin film of Silicon or Silica as the intermedia layer, the intermedia layer is applied on the glass substrate by sputtering or E-beam evaporation. The glass plates are pressed together and heated to a temperature (typically in the range 300-500 degrees centigrade depending on the glass type) at which the alkali-metal ions in the glass become mobile. The components are brought into contact and a high voltage applied across them. This causes the alkali cations to migrate from the interface resulting in a depletion layer with high electric field strength. The resulting electrostatic attraction brings the Silica and glass into intimate contact. Further current flow of the oxygen anions from the glass to the Silica results in an anodic reaction at the interface and the result is that the glass becomes bonded to the Silica layer with a permanent chemical bond. The typical bond strength is between 10 and 20 MPa according to pull tests, higher than the fracture strength of glass. The bonding time varies between few minutes to few hours—depending on bonding area, glass type, glass thickness, and other parameters. The procedure of anodic bonding can be repeated, and hence, it can be utilized in the iterative procedure that creating a stack of glass plates as illustrated in  FIGS. 2 and 4 ( a )-( 4   e ). 
     Since part of the optically attached surfaces is covered with partially reflecting coatings, it is important to validate that the reflectance properties of the partially reflecting surfaces will not be damaged during the anodic bonding procedure. This can be done, for example, by a proper design of the external layer of the thin film coating to ensure that after the Anodic bonding process, which might change the final thickness of this layer, the reflectance properties of the coating will be as required. 
     In addition to solving the problems of the above-described adhesive process, the proposed attaching process allows the chemical strengthening of the outside surfaces of the LOE and hence enabling scratch resistance and hardness of the element (like in gorilla glass). Chemically strengthened glass is a type of glass that has increased strength as a result of a post-production chemical process. When broken, it still shatters in long pointed splinters similar to float glass. For this reason, it is not considered a safety glass and must be laminated if a safety glass is required. However, chemically strengthened glass is typically six to eight times the strength of float glass. The glass is chemically strengthened by a surface finishing process. Glass is submersed in a bath containing a potassium salt (typically potassium nitrate) at 300° C. This causes sodium ions in the glass surface to be replaced by potassium ions from the bath solution. These potassium ions are larger than the sodium ions and therefore wedge into the gaps left by the smaller sodium ions when they migrate to the potassium nitrate solution. This replacement of ions causes the surface of the glass to be in a state of compression and the core in compensating tension. The surface compression of chemically strengthened glass may reach up to 690 MPa. There also exists a more advanced two-stage process for making chemically strengthened glass, in which the glass article is first immersed in a sodium nitrate bath at 450° C., which enriches the surface with sodium ions. This leaves more sodium ions on the glass for the immersion in potassium nitrate to replace with potassium ions. In this way, the use of a sodium nitrate bath increases the potential for surface compression in the finished article. Chemical strengthening results in a strengthening similar to toughened glass. However, the process does not use extreme variations of temperature and therefore chemically strengthened glass has little or no bow or warp, optical distortion or strain pattern. This differs from toughened glass, in which slender pieces can be significantly bowed. An LOE which is fabricated utilizing anodic bonding process and strengthened by a chemically protection procedure will have much better optical, as well as mechanical properties than LOEs which are fabricated with the existing fabrication processes.