Waveguide with embedded mirrors

[0001] This disclosure relates generally to the field of optics, and in particular but not exclusively, relates to near-to-eye optical systems.

[0002] A head mounted display (“HMD”) is a display device worn on or about the head. HMDs usually incorporate some sort of near-to-eye optical system to display an image within a few centimeters of the human eye. Single eye displays are referred to as monocular HMDs while dual eye displays are referred to as binocular HMDs. Some HMDs display only a computer generated image (“CGI”), while other types of HMDs are capable of superimposing CGI over a real-world view. This latter type of HMD is often referred to as augmented reality because the viewer's image of the world is augmented with an overlaying CGI, also referred to as a heads-up display (“HUD”).

[0003] HMDs have numerous practical and leisure applications. Aerospace applications permit a pilot to see vital flight control information without taking their eye off the flight path. Military applications include tactical displays of maps and thermal imaging. Other application fields include video games, transportation, and telecommunications. Due to the infancy of this technology, there is certain to be new found practical and leisure applications as the technology evolves; however, many of these applications are limited due to the cost, size, field of view, and efficiency of conventional optical systems used to implemented existing HMDs.

[0016] Embodiments of an apparatus, system and method for fabrication of a waveguide with embedded mirrors are described herein. In the following description numerous specific details are set forth to provide a thorough understanding of the embodiments. One skilled in the relevant art will recognize, however, that the techniques described herein can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring certain aspects.

[0017] Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

[0018] FIG. 1A illustrates a first conventional near-to-eye optical system 101 using an input lens and two mirrors. An image source 105 outputs an image that is reflected by two mirrors 110 and 115, which form an image near to eye 120. Image source 105 is typically mounted above the head or to the side of the head, while mirrors 110 and 115 bend the image around the front of the viewer's face to their eye 120. Since the human eye is typically incapable of focusing on objects placed within a few centimeters, this system requires a lens 125 interposed between the first mirror 110 and image source 105. Lens 125 creates a virtual image that is displaced further back from the eye than the actual location of mirror 115 by positioning image source 105 inside of the focal point f of lens 125. Optical system 101 suffers from a small field of view (e.g., approximately 20 degrees) limited by the extent of mirrors 110 and 115 and the bulkiness of lens 125. The field of view can be marginally improved by placing mirrors 110 and 115 within a high index material to compress the angles of incidence, but is still very limited.

[0019] FIG. 1B illustrates a second conventional near-to-eye optical system 102 using angle sensitive dichroic mirrors. Optical system 102 includes a single in-coupling mirror 130 and two out-coupling dichroic mirrors 135 disposed within a waveguide 140. This system uses collimated input light from virtual images placed at infinity. In order to produce a useful image at eye 120, each incident angle of input light should correspond to a single output angle of emitted light. Since light can potentially reflect off of output mirrors 135 on either a downward trajectory (ray segments 145) or an upward trajectory (ray segments 150), each input angle can potentially result in multiple output angles, thereby destroying the output image. To overcome this problem, optical system 102 uses angle sensitive dichroic mirrors 135 that pass light with incident sufficiently close to normal while reflecting light having a sufficiently oblique incidence. However, the nature of dichroic mirrors 135 that passes some incident angles while reflecting others limits the field of view optical system 102.

[0020] FIG. 1C illustrates a third conventional near-to-eye optical system 103 using holographic diffraction gratings. Optical system 103 is similar to optical system 102, but uses holographic diffraction gratings 150 in place of mirrors 130 and 135. Diffraction gratings 150 are inefficient reflectors, since they only reflect higher order diffractions while passing the first order diffraction, which contains the largest portion of energy in an optical wave front. In addition to being poor optical reflectors, the input and output diffraction gratings must be precisely tuned to one another, else the output image will suffer from color separation. Achieving a sufficient match between the input and output gratings 150 requires extreme control over manufacturing tolerances, which is difficult and/or costly. Again, optical system 103 suffers from a limited field of view.

[0021] FIGS. 2A and 2B illustrate a waveguide 200 with embedded mirrors for generating a near-to-eye image, in accordance with an embodiment of the invention. FIG. 2A is a top or plan view while FIG. 2B is a side view of the same. The illustrated embodiment of waveguide 200 includes an in-coupling region 205, an out-coupling region 210, and an intermediate region 215. The illustrated embodiment of in-coupling region 205 includes in-coupling mirrors 225. The illustrated embodiment of out-coupling region 220 includes out-coupling mirrors 230 and end cap surface 235.

[0022] Waveguide 200 operates by receiving collimated input light into waveguide 200 at in-coupling region 205. The input light is reflected by in-coupling mirrors 225. In-coupling mirrors 225 are orientated to reflect the input light down waveguide 200 towards out-coupling region 220. The reflected input light is guided towards out-coupling region 220 by intermediate region 215. In-coupling mirrors include reflective surfaces that are angled oblique to light incident surface 240. In one embodiment, in-coupling mirrors 225 are angled relative to the input light and sides of waveguide 200 such that the reflected input light strikes the sides of waveguide 200 with sufficiently oblique angles that it is guided down waveguide 200 via total internal reflection (“TIR”). In one embodiment, one or more metallic reflective layers (not illustrated) are coated along the sides of waveguide 200, including intermediate region 215, to guide the light. A combination of both TIR and metallic reflective coatings may also be used. The guided light eventually reaches out-coupling region 220 where it is once again reflected by out-coupling mirrors 230. Out-coupling mirrors 230 are orientated to reflect the guided light for exit out emission surface 245 within out-coupling region 220 towards eye 120. In one embodiment, each out-coupling mirror 230 includes a reflective surface that is orientated such that the guided light received from intermediate region 215 strikes the side of waveguide 200 with an angle sufficiently close to normal such that the light is output from waveguide 200 towards eye 120.

[0023] Waveguide 200 may be fabricated of a variety of optically transmissive, light guiding materials, such as polymer, quartz, glass, etc. In one embodiment that uses TIR to guide light through intermediate region 215, waveguide 200 is fabricated of a polymer material having an index of refraction of approximately 1.5. The dimensions of waveguide 200 may be adjusted for a particular application; however, example dimensions may include A=1 mm to 15 mm, B=10 mm to 40 mm, and C=20 mm to 70 mm. Of course, other dimensions may be used and may vary based on application, aesthetic design choices, intended user size, etc. It should be appreciated that the figures are not drawn to scale.

[0024] In-coupling region 205 includes a plurality of offset in-coupling mirrors 225 each for reflecting a different portion of the input light image. FIGS. 2A and 2B illustrate each in-coupling mirror 225 as being an elongated structure with a rectangular flat/planar reflective surface orientated obliquely to light incident surface 240. These structures may be disposed along a planar surface, such as front side surface 250. However, it should be appreciated that in-coupling mirrors 225 may be embedded within in-coupling region 205 in a variety of other configurations, as long as, they are offset from one another to each reflect a different portion of the input image. In one embodiment, the in-coupling mirror structures have a substantially triangular cross-section (e.g., see FIG. 3A). However, embodiments of in-coupling mirrors 225 may be implemented with other structures having different cross-sectional shapes and one or more flat reflective surfaces. Furthermore, although FIGS. 2A and 2B illustrate four in-coupling mirrors 225, embodiments may be implemented with fewer or greater numbers of offset in-coupling mirrors 225 (e.g., five in-coupling mirrors).

[0025] Out-coupling region 220 includes a plurality of offset out-coupling mirrors 230 each for reflecting a different portion of the guided light for emission from waveguide 200. FIGS. 2A and 2B illustrate each out-coupling mirror 230 as being an elongated structure with two rectangular flat/planar reflective surfaces: one orientated obliquely to emission surface 245 and the other orientated substantially parallel with emission surface 245. These structures may be disposed along a planar surface, such as front side surface 250. However, it should be appreciated that out-coupling mirrors 230 may be embedded within out-coupling region 220 in a variety of other configurations, as long as, they are offset from one another to each reflect a different portion of the guide light. In one embodiment, the out-coupling mirror structures have a substantially parallelogram cross-section (e.g., see FIG. 4). However, embodiments of out-coupling mirrors 230 may be implemented with other structures having different cross-sectional shapes and one or more flat reflective surfaces. Furthermore, although FIGS. 2A and 2B illustrate four out-coupling mirrors 230, embodiments may be implemented with fewer or greater numbers of offset in-coupling mirrors 225 (e.g., five in-coupling mirrors). In one embodiment, the number of out-coupling mirrors 230 is the same as the number of in-coupling mirrors 225.

[0026] In one embodiment, the offset width between adjacent out-coupling mirrors 230 is selected to permit external light to pass through waveguide 200 between out-coupling mirrors 230 and reach the eye 120. In this regard, the eye 120 is able to see a real world image from the external light passing through waveguide 200 augmented by another image guided through waveguide 200. The other image may typically be CGI for superimposing over a user's real world vision. The degree of transparency may be controlled based on a ratio of areas of out-coupling mirrors 230 and intervening gaps 260 between out-coupling mirrors 230.

[0027] End cap surface 235 inhibits backward reflections propagating towards in-coupling region 205 by discouraging reflections off the right distal end of waveguide 200. In one embodiment, end cap surface 235 is implemented as a light absorbing surface (e.g., a dark matte surface). In one embodiment, end cap surface 235 is implemented as a light scattering surface (e.g., a rough, diffusive surface). In one embodiment, end cap surface 235 is an anti-reflective surface (e.g., AR coating) to permit guided light that reaches the distal end to pass out of the waveguide with reduced back reflections. Of course, end cap surface 235 may include combinations of the above possible embodiments.

[0028] FIG. 3A is a top view illustrating in-coupling mirrors 300, in accordance with an embodiment of the invention. In-coupling mirrors 300 represent one possible implementation of in-coupling mirrors 225. The illustrated embodiment of in-coupling mirrors 300 are structures each having a substantially triangular cross-section with a flat reflective surface 305 orientated obliquely to light incident surface 240 (see FIG. 2A). These structures are disposed along a planar surface 310 and offset from one another to reflect a different portion of input light 315. In one embodiment, reflective surfaces 305 are non-optically-transmissive surfaces. For example, reflective surfaces 305 may be fabricated as a reflective metal film, such as, aluminum, nickel, gold, chromium, tin, or otherwise. The dimensions of in-coupling mirrors 300 may be adjusted for a particular application; however, an example dimension set may include D=70 μm, E=108 μm, F=437 μm, and G=32.98°. Of course, other dimensions/angles may be used. It should be appreciated that the figures are not drawn to scale.

[0029] The inter-mirror separation spacing (dimension F) may be selected for a variety characteristics, including image resolution, image brightness, transparency of in-coupling region, etc. However, an additional consideration is based upon the expected incident angles of input light 315 and angle G. As illustrated in FIG. 3B, when considering the extremes of the incident angles of input light 315 for which waveguide 200 is intended to pass, the inter-mirror separation spacing should be sufficiently large such that these extreme incident angles are not blocked by the backside of an adjacent in-coupling mirror.

[0030] FIG. 4 is a top view illustrating out-coupling mirrors 400, in accordance with an embodiment of the invention. Out-coupling mirrors 400 represent one possible implementation of out-coupling mirrors 230. The illustrated embodiment of out-coupling mirrors 400 are structures each having a parallelogram cross-section with two flat reflective surfaces: surface 405 orientated obliquely to emission surface 245 and surface 407 orientated substantially parallel with emission surface 245. These structures are disposed along a planar surface 410 and offset from one another to reflect a different portion of guided light 415. In one embodiment, reflective surfaces 405 and 407 are non-optically-transmissive surfaces. For example, reflective surfaces 405 and 407 may be fabricated as a reflective metal film, such as, aluminum, nickel, gold, chromium, tin, or otherwise. The dimensions of out-coupling mirrors 400 may be adjusted for a particular application; however, an example dimension set may include H=70 μm, I=108 μm, J=85 μm, and K=100 μm. Of course, other dimensions may be used. It should be appreciated that the figures are not drawn to scale.

[0031] In the illustrated embodiment of out-coupling mirrors 400, reflective surface 407 is provided to increase the field of view and spread the output light over a greater extent of emission surface 245. As guided light 415 propagates down waveguide 200 from intermediate region 215 into out-coupling region 220, some rays will immediately strike one of the oblique reflective surfaces 405 located closer to intermediate region 215. These light rays are reflected with an angle of incidence (angle M) sufficiently large (or sufficiently close to normal with respect to emission surface 245) that they exit waveguide 200 towards eye 120 as the output light. However, other rays of guided light 415 entering into out-coupling region 220 from intermediate region 215 strike one of the substantially parallel reflective surfaces 407. These rays are reflected with an angle of incidence (angle L) sufficiently small that they are reflected off emission surface 245 and continue propagating down out-coupling region 220 until they strike an oblique reflective surface 405 and exit waveguide 200. Thus, the output light is laterally extended over a larger area of emission surface 245. Reflective surfaces 407 operate to increase the output efficiency of waveguide 200 and extend the emission field of view (e.g., potentially achieving a 45 degree field of view). In this manner, oblique reflective surfaces 405 may be thought of as exit mirrors while substantially parallel reflective surfaces 407 may be thought of as propagation mirrors.

[0032] In one embodiment, out-coupling region 220 is partially transparent to pass external light 420 through surfaces 410 and 245 to eye 120. Since reflective surfaces 405 and 407 may be fabricated of non-optically-transmissive material, the degree of transparency of out-coupling region 220 can be adjusted by appropriate selection of dimension J between adjacent out-coupling mirrors 400. By increasing the proportion of unobstructed surface 410 that falls between out-coupling mirrors 400, the transparency of out-coupling region 220 can be increased. In one embodiment, a non-reflective coating may be applied to planar surface 410 in the out-coupling region 220 to reduce reflections of external light 420 when entering waveguide 200.

[0033] FIG. 5 is a flow chart illustrating an example process 500 for fabricating waveguide 200 with embedded mirrors, in accordance with an embodiment of the invention. Process 500 is described with reference to FIGS. 6A-G. The order in which some or all of the process blocks appear in process 500 should not be deemed limiting. Rather, one of ordinary skill in the art having the benefit of the present disclosure will understand that some of the process blocks may be executed in a variety of orders not illustrated.

[0034] In a process block 505, an opaque pattern is formed on a clear substrate 605 (e.g., glass substrate). In one embodiment, the opaque pattern is formed of metal. Once formed, the opaque pattern is covered over with a negative photoresist 610 (process block 510; see FIG. 6B), such as Su8 for example, and flood exposed from the backside through clear substrate 605 (process block 515; see FIG. 6C). After backside flood exposure, the unexposed portions of photoresist 610 protected by the opaque pattern are washed away with developer leaving behind mirror structures 615 (process block 520; see FIG. 6D). In a process block 525, reflective material (e.g., metal) is evaporated or otherwise formed onto the exposed surfaces of mirror structures 615 to form reflective surfaces 620 and 625 (see FIG. 6E). In a process block 530, the reflectively coated mirror structures 615 are embedded into a waveguide material 630, such as clear polymer (see FIG. 6F). Finally, in a process block 535, clear substrate 605 and the opaque pattern may be optionally removed using chemical mechanical polishing (“CMP”) or otherwise (see FIG. 6G). If clear substrate 605 is retained, then a reflective surface (e.g., metal) may be disposed onto clear substrate 605 prior to embedding into waveguide material 630 to provide a reflective interface for guiding light within the waveguide, since clear substrate 605 may affect TIR.

[0035] Process 500 illustrates the fabrication of out-coupling mirrors 400 of FIG. 4. However, it can be readily modified to fabricate in-coupling mirrors 300 or a variety of other shaped in-coupling and/or out-coupling mirror structures. It should be appreciated that process 500 only represents one possible method of fabrication. Other techniques may be used as well. For example, gray-scale lithography may be used in place of the opaque pattern and backside flood exposure.

[0036] FIG. 7 is a top view of a demonstrative near-to-eye imaging system 700 including two waveguides with embedded mirrors, in accordance with an embodiment of the invention. The illustrated embodiment of imaging system 700 includes two waveguides 200, a frame including a nose assembly 705, left ear assembly 710, and right ear assembly 715, and two image sources 720 and 725.

[0037] The two waveguides 200 are secured into an eye glass arrangement that can be worn on a head 730 of a user. The left and right ear assemblies 710 and 715 rest over the user's ears while nose assembly 705 rests over the user's nose. The frame assembly is shaped and sized to position out-coupling region 220 of each waveguide 200 in front of a corresponding eye 120 of the user with the emission surfaces facing eyes 120.

[0038] The illustrated embodiment is capable of displaying an augmented reality to the user. As such, out-coupling region 220 of each waveguide 200 is partially transparent and permits the user to see a real world image via external light 420. Left and right (binocular embodiment) CGIs are generated by image sources 720 and 725, respectively. In one embodiment, image sources 720 and 725 are liquid crystal on silicon (“LCoS”) micro-projectors that use laser lamps and emit collimated light. Of course, other display technology may be used. The CGI output by image sources 720 and 725 is launched into their respective waveguides 200 at in-coupling regions 205, guided through intermediate regions 215, and emitted from out-coupling regions 220 near-to-eyes 120. Since waveguides 200 preserve a one-to-one relation of input light angles to output light angles, the output light is also collimated and therefore virtually projected at infinity. Although the human eye is typically incapable of bring objects within a few centimeters into focus, since the output light is virtually displayed at infinity it is readily in focus. The CGI is seen by the user as a virtual image superimposed over the real world as an augmented reality.

[0039] Although FIG. 7 illustrates image sources 720 and 725 as projecting images normal to waveguides 200, in other embodiments, optical elements (e.g., lenses, mirrors, etc.) maybe placed between image sources 720 and 725 and waveguides 200, such that the CGI light may be launched into in-coupling regions 205 at oblique angles.

[0040] The above description of illustrated embodiments of the invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize.

[0041] These modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.