The optical combiner is often the most complex and most costly optical element in the entire MR display architecture: it is the one component seen directly by the user and the one seen directly by the world. It often defines the size and aspect ratio of the entire headset. It is the critical optical element that reduces the quality of the see-through and the one that defines the eyebox size (and in many cases, also the FOV).
There are three main types of optical combiners used in most MR/AR/smart glasses today:
- Free-space optical combiners6,
- TIR prism optical combiners7 (and compensators), and
When optimizing an HMD display system, the optical engine must be optimized in concert with the combiner engine11. Usually, a team that designs an optical engine without fully understanding the limitations and specifics of a combiner engine designed by another team, and vice versa, can result in a suboptimal system or even a failed optical architecture, no matter how well the individual optical building blocks might be designed.
Freeform TIR prism combiners are at the interface between free space and waveguide combiners. When the number of TIR bounces increases, one might refer to them as waveguide combiners. Waveguide combiner architectures are the topic of this review paper.
Waveguide combiners are based on TIR propagation of the entire field in an optical guide, essentially acting as a transparent periscope with a single entrance pupil and often many exit pupils12.
The primary functional components of a waveguide combiner consists of the input and output couplers. These can be either simple prisms, micro-prism arrays, embedded mirror arrays, surface relief gratings, thin or thick analog holographic gratings, metasurfaces, or resonant waveguide gratings. All of these have their specific advantages and limitations, which will be discussed here. Waveguide combiners have been used historically or tasks very different from AR combiner, such as planar optical interconnections13 and LCD backlights14,15.
Waveguide combiners are an old concept, some of the earliest IP dates back to 1976 and applied to HUDs. Fig. 13a shows a patent by Juris Upatnieks dating back 1987, a Latvian/American scientist and one of the pioneers of modern holography16, implemented in a di-chromated gelatin (DCG) holographic media. A few years later, one-dimensional eyebox expansion (1D EPE) architectures were proposed as well as a variety of alternatives for in- and out- couplers technologies, such as surface relief grating couplers by Thomson CSF (Fig. 13b). Fig. 13c shows the original 1991 patent for a waveguide-embedded partial mirror combiner and exit pupil replication (All of these original waveguide combiner patents have been in the public domain for nearly a decade17-19).
One can take the concept of a flat waveguide with a single curved extractor mirror (Epson Moverio BT300) or freeform prism combiner, or a curved waveguide with curved mirror extractor, to the next level by multiplying the mirrors to increase the eyebox (see the Lumus LOE waveguide combiner) or fracturing metal mirrors into individual pieces (see the Optinvent ORA waveguide combiner or the LetinAR waveguide combiner using fractured mirrors instead of cascaded partial mirrors20-23).
While fracturing the same mirror into individual pieces can increase see-through and depth of focus, the use of more mirrors to replicate the pupil is a bit more complicated, especially in a curved waveguide where the two exit pupils need to be spatially de-multiplexed to provide a specific mirror curvature to each pupil to correct for image position: this limits the FOV in one direction so that such overlap does not happen.
Fig. 14 summarizes some of the possible design configurations with such waveguide mirror architectures. Note that the grating- or holographic-based waveguide combiners are not listed here; they are the subject of the next sections.
Fig. 14 shows that many of the waveguide combiner architectures mentioned in this section can be listed in this table24,25. Mirrors can be half-tone (Google Glass, Epson Moverio), dielectric (Lumus LOE), have volume holographic reflectors (Luminit or Konica Minolta), or the lens can be fractured into a Fresnel element (Zeiss Tooz Smart Glass). In the Optinvent case, we have a hybrid between fractured metal mirrors and cascaded half-tone mirrors. In one implementation, each micro-prism on the waveguide has one side fully reflective and the other side transparent to allow see-through.
In the LetinAR case, all fractured mirrors are reflective, can be flat or curved, and can be inverted to work with a birdbath reflective lens embedded in the guide.
Even though the waveguide might be flat, when using multiple lensed mirrors, the various lens powers will be different since the display is positioned at different distances from these lensed extractors. When the waveguide is curved, everything becomes more complex, and the extractor mirror lenses need to compensate for the power imprinted on the TIR field at each TIR bounce in the guide. In the case of curved mirrors (either in flat or curved waveguides), the exit pupils over the entire field cannot overlap since the power to be imprinted on each exit pupil (each field position) is different (Moverio BT300 and Zeiss Tooz Smart Glass). This is not the case when the extractors are flat and the field is collimated in the guide (Lumus LOE).
Two-dimensional eyebox expansion is desired (or required) when the input pupil cannot be generated by the optical engine over an aspect ratio tall enough to form the 2D eyebox, due to the FOV (etendue limitations) and related size/weight considerations. A 2D exit pupil expansion (2D EPE) is therefore required (see Fig. 15). 2D expansion can now be performed with either diffractive, holographic or even reflective couplers as in the Lumus Maximus waveguide (2020).
While holographic recording or holographic volume gratings are usually limited to linear gratings, or gratings with slow power (such as off-axis diffractive lenses), surface relief gratings can be either 1D or 2D, linear or quasi arbitrary in shape. Such structures or structure groups can be optimized by iterative algorithms (topological optimization) rather than designed analytically.
Holographic recording, sometimes also called interference lithography when used to create patterns in a resin to be etched in a substrate (such as a nanoimprint master for waveguides) are not limited to linear fringes, but can also produce much more complex structures, such as 2D gratings or even computer generated holograms (CGH) when a digital CGH is used as a reference beam.
The coupler element is the key feature of a waveguide combiner. The TIR angle is dictated by the refractive index of the waveguide, not the refractive index of the coupler nanostructures. Very often, the index of the coupler structure (grating or hologram) prescribes the angular and spectral bandwidth over which this coupler can act, thus impacting the color uniformity over the FOV and eyebox.
Numerous coupler technologies have been used in industry and academia to implement the in-coupler and out-couplers, and they can be defined either as refractive/reflective or diffractive/holographic coupler elements.
A prism is the simplest TIR in-coupler, and can be very efficient. A prism can be bounded on top of the waveguide, or the waveguide itself can be cut at an angle, to allow normal incident light to enter the waveguide and be guided by TIR (depending on the incoming pupil size). Another way uses a reflective prism on the bottom of the waveguide (metal coated). Using a macroscopic prism as an out-coupler is not impossible, and it requires a compensating prism for see-through, with either a reflective coating or a low-index glue line, as done in the Oorym (Israel) lightguide combiner concept.
Cascaded embedded mirrors with partially reflective coatings are used as out-couplers in the Lumus (Israel) Lightguide Optical Element (LOE) waveguide combiner. The input coupler remains a prism. As the LOE is composed of reflective surfaces, it yields good color uniformity over the entire FOV. As with other coupler technologies, intrinsic constraints in the cascaded mirror design of the LOE might limit the FOV26. See-through is very important in AR systems: the Louver effects produced by the cascaded mirrors in earlier versions of LOEs have been reduced recently thanks to better cutting/polishing, coating, and design. Two dimensional pupil expansion in LOEs have been recently succcesfully demonstrated by Lumus in their latest Maximus X-Lens and Z-lens waveguide designs. High MTF, great color uniformity and high efficiency are key attributes of all reflective waveguide combiners.
Micro-prism arrays are used in the Optinvent (France) waveguide as out-couplers20. The in-coupler here is again a prism. Such microprism arrays can be surface relief or index matched to produce an unaltered see-through experience. The micro-prisms can all be coated uniformly with a half-tone mirror layer or can have an alternance of totally reflective and transmissive prism facets, provide a resulting 50% transmission see-through experience. The Optinvent waveguide is the only flat waveguide available today as a plastic guide, thus allowing for a consumer-level cost for the optics. The micro-prism arrays are injection molded in plastic and bounded on top of the guide. Focus lens can also be integrated in a flat waveguide by adding a slight curvature to the extraction prisms. This is possible as such elements are pressure molded in a mold using free-form diamond turned metal inserts rather than cut in glass as embedded cascaded mirrors.
Transparent volume holograms working in reflection mode—as in di-chromated gelatin (DCG), bleached silver halides (Slavic or Ultimate Holography by Yves Gentet), or more recently photopolymers such as Bayfol® photopolymer by Covestro/Bayer, (Germany)27, and photopolymers by DuPont (US), Polygrama (Brazil), or Dai Nippon (Japan)—have been used to implement in- and out-couplers in waveguide combiners. Such photopolymers can be sensitized to work over a specific wavelength or over the entire visible spectrum (panchromatic holograms).
Photopolymer holograms do not need to be developed as DCG, nor do they need to be bleached like silver halides. A full-color hologram based on three phase-multiplexed single-color holograms allows for a single plate waveguide architecture, which can simplify the combiner and reduce weight, size, and costs while increasing yield (no plate alignment required). However, the efficiency of such full-RGB phase-multiplexed holograms are still quite low when compared to single-color photopolymer holograms.
Also, the limited index swing of photopolymer holograms allows them to work more efficiently in reflection mode than in transmission mode (allowing for better confinement of both the wavelength and angular spectrum bandwidths).
Examples of photopolymer couplers include Sony LMX-001 Waveguides for smart glasses and the TrueLife Optics (UK) process of mastering the hologram in silver halide and replicating it in photopolymer.
Replication of the holographic function in photopolymer through a fixed master has proven to be possible in a roll-to-roll operation by Bayer/Covestro (Germany). Typical photopolymer holographic media thicknesses range from 16–70 microns, depending on the required angular and spectral bandwidths.
Covestro photopolymer replication technologies such as roll to plate or roll to roll can be used for volume production of such holographic optics, and used in coupler applications, both for AR, but also for larger areas such as in automotive/avionics HUDs and smart windows.
When the index swing of the volume hologram can be increased, the efficiency gets higher, and the operation in transmission mode becomes possible. This is the case with Digilens’ proprietary holographic polymer dispersed liquid crystal (H-PDLC) hologram material28. Transmission mode requires the hologram to be sandwiched between two plates rather than laminating a layer on top or bottom of the waveguide as with photopolymers, DCG, or silver halides. Digilens’ H-PDLC has the largest index swing today and can therefore produce strong coupling efficiency over a thin layer (typically four microns or less). H-PDLC material can be engineered and recorded to work over a wide range of wavelengths to allow full-color operation.
Increasing the index swing can optimize the efficiency and/or angular and spectral bandwidths of the hologram. However, this is difficult to achieve with most available materials and might also produce parasitic effects such as haze. Increasing the thickness of the hologram is another option, especially when sharp angular or spectral bandwidths are desired, such as in telecom spectral and angular filters. This is not the case for an AR combiner, where both spectral and bandwidths need to be wide (to process a wide FOV over a wide spectral band such as LEDs). However, a thicker hologram layer also allows for phase multiplexing over many different holograms, one on top of another, allowing for multiple Bragg conditions to operate in concert to build a wide synthetic spectral and/or angular bandwidth, as modeled by the Kogelnik theory29. This is the technique used by Akonia, Inc. (a US start-up in Colorado, formerly InPhase Inc., which was originally funded and focused to produce high-density holographic page data-storage media, ruled by the same basic holographic phase-multiplexing principles30).
Thick holographic layers, as thick as 500 microns, work well in transmission and/or reflection modes, but they need to be sandwiched between two glass plates. In some specific operation modes, the light can be guided inside the thick hologram medium, where it is not limited by the TIR angle dictated by the index of the glass plates. As the various hologram bandwidths build the final FOV, one needs to be cautious in developing such phase-multiplexed holograms when using narrow illumination sources such as lasers.
Replication of such thick volume holograms are difficult in roll-to-roll operation, as done with thinner single holograms (Covestro Photopolymers, H-PDLC), and require multiple successive exposures to build the hundreds of phase-multiplexed holograms that compose the final holographic structure. However, this can be relatively easy with highly automated recording set-ups as the ones developed by the now-defunct holographic page data-storage industry (In-Phase Corp., General Electric, etc.).
Note that although the individual holograms acting in slivers of angular and spectral bandwidth spread the incoming spectrum like any other hologram (especially when using LED illumination), the spectral spread over the limited spectral range of the hologram is not wide enough to alter the MTF of the immersive image and thus does not need to be compensated by a symmetric in- and out-coupler as with all other grating or holographic structures31. This feature allows this waveguide architecture to be asymmetric, such as having a strong in-coupler as a simple prism: a strong in-coupler is always a challenge for any grating or holographic waveguide combiner architecture, and a macroscopic prism is the best coupler imaginable.
Fig. 16 shows both thin and thick volume holograms operating in reflection and/or transmission modes. The top part of the figure shows a typical 1D EPE expander with a single transmission volume hologram sandwiched between two plates. When the field traverses the hologram downwards, it is in off/Bragg condition, and when it traverses the volume hologram upwards after a TIR reflection, it is in an on/Bragg condition (or close to it), thereby creating a weak (or strong) diffracted beam that breaks the TIR condition.
A hologram sandwiched between plates might look more complex to produce than a reflective or transmission laminated version, but it has the advantage that it can operate in both transmission and reflection modes at the same time (for example, to increase the pupil replication diversity).
Fig. 17 reviews the various surface-relief gratings used in industry today (blazed, slanted, binary, multilevel, and analog), and how they can be integrated in waveguide combiners as in-coupling and out-coupling elements31-35.
Fig. 17 Surface-relief grating types used as waveguide combiner in-couplers and out-coupler. Solid lines indicate reflective coatings on the grating surface, and dashed lines indicate diffracted orders.
Covering a surface-relief grating with a reflective metallic surface (see Fig. 17) will increase dramatically its efficiency in reflection mode. A transparent grating (no coating) can also work both in transmission and reflection modes, especially as an out-coupler, in which the field has a strong incident angle.
Increasing the number of phase levels from binary to quarternary or even eight or sixteen levels increases its efficiency as predicted by the scalar diffraction theory, for normal incidence. However, for a strong incidence angle and for small periods, this is no longer true. A strong out-coupling can thus be produced in either reflection or transmission mode.
Slanted gratings36,37 are very versatile elements and their spectral and angular bandwidths can be tuned by the slant angles. Front and back slant angles in a same period (or from period to period) can be carefully tuned to achieve the desired angular and spectral operation.
Surface relief gratings have been used as a commodity technology since mastering and mass replication techniques technologies were established and made available in the early 1990s. Typical periods for TIR grating couplers in the visible spectrum are below 500 nm, yielding nanostructures of just a few tens of nanometers if multilevel structures are required. This can be achieved by either direct e-beam write, i-line (or DUV) lithography, or even interference lithography (holographic resist exposure). Surface relief grating structures can be replicated in volumes by nano-imprint, a micro-lithography wafer fabrication technology developed originally for the IC industry38,39. Going from wafer-scale fabrication to panel-scale fabrication will reduce costs, allowing for consumer-grade AR and MR products.
Fig. 18 and Fig. 19 illustrate how some of the surface relief gratings shown in Fig. 17 have been applied to the latest waveguide combiners such as the Microsoft HoloLens 1 and Magic Leap One. Multilevel surface relief gratings have been used by companies such as Dispelix Oy, and quasi-analog surface relief CGHs have been used by others, such as WaveOptics Ltd (now part of Snap Inc).
Fig. 18 Spatially color-multiplexed input pupils with slanted gratings as in- and out-couplers working in transmission and reflection mode (HoloLens 1 MR headset).
Fig. 19 Spatially color-de-multiplexed input pupils with 100% reflective blazed gratings as in-couplers and binary phase gratings as out-couplers (Magic Leap One MR headset).
Fig. 18 shows the waveguide combiner architecture used in the Microsoft HoloLens 1 MR headset (2015). The display engine is located on the opposite side of the eyebox. The single input pupil carries the entire image over the various colors at infinity (here, only two colors and the central field are depicted for clarity), as in a conventional digital projector architecture. The in-couplers have been chosen to be slanted gratings for their ability to act on a specific spectral range while letting the remaining spectrum unaffected in the zero order, to be processed by the next in-coupler area located on the guide below, and to do this for all three colors. Such uncoated slanted gratings work both in transmission and reflection modes but can be optimized to work more efficiently in a specific mode. The out-couplers here are also slanted gratings, which can be tuned to effectively work over a specific incoming angular range (TIR range) and leave the see-through field quasi-unaffected. The part of the see-through field that is indeed diffracted by the out-couplers is trapped by TIR and does not make it to the eyebox. These gratings are modulated in depth to provide a uniform eyebox to the user. Note the symmetric in- and out-coupler configuration compensating the spectral spread over the three LEDs bands.
The redirection gratings are not shown here. Input and output grating slants are set close to 45 deg and the redirection grating slants at half this angle. The periods of the gratings are tuned in each guide to produce the right TIR angle for the entire FOV for that specific color (thus the same central diffraction angle in each guide for each RGB LED color band).
Fig. 19 depicts the waveguide combiner architecture used in the Magic Leap One MR headset (2018). The display engine is located on the same side as the eyebox. The input pupils are spatially color-demultiplexed, carrying the entire FOV at infinity (here again, only two colors and the central field are depicted for clarity).
Today, typical yields for slanted gratings produced by Nano-Imprint Lithography (NIL) are typically in the 90% for indices lower than 1.8, for which the process is well defined. For refractive indices of 1.9 and 2.0, the yields are quite lower today. Various contract manufacturers are today using NIL equipment (EVG, Suss, Canon, etc…) to produce volume quantities of waveguides on either plates or wafers.
Spatial color de-multiplexing can be done conveniently with a color sequential LCoS display mode for which the illumination LEDs are also spatially de-multiplexed. In this configuration, the input grating couplers are strong blazed gratings, coated with a reflective metal (such as Al). They do not need to work over a specific single-color spectral width since the colors are already de-multiplexed. The out-couplers are simple top-down binary gratings, which are also depth modulated to produce a uniform eyebox for the user. These binary gratings are shallow, acting therefore very little on the see-through, but they have much stronger efficiency when working in internal reflection diffraction mode, since the optical path length in this case is longer by a factor of 2ncos(α) than in transmission mode, (where n is the index of the guide, and α is the angle if there is incidence in the guide). As in the HoloLens 1, most of the see-through field diffracted by the out-couplers is trapped by TIR.
The redirection gratings (not shown here) are also composed of binary top-down structures. The periods of the gratings are tuned in each guide to produce the right TIR angle for the entire FOV for that specific color (same central diffraction angles for each RGB LED color band).
Other companies use multilevel and/or quasi-analog surface relief diffractive structures to implement in- and out-couplers (see Fig. 17, Fig. 15). This choice is mainly driven by the complexity of the extraction gratings, acting both as redirection gratings and out-coupler gratings, making them therefore more complex than linear or slightly curved (powered) gratings, similar to iteratively optimized CGHs39. Allowing multilevel or quasi-analog surface relief diffractive structures increases the space bandwidth product of the element to allow more complex optical functionalities to be encoded with relatively high efficiency.
Resonant waveguide gratings (RWGs), also known as guided mode resonant (GMR) gratings or waveguide-mode resonant gratings40,41, are dielectric structures where these resonant diffractive elements benefit from lateral leaky guided modes. A broad range of optical effects are obtained using RWGs such as waveguide coupling, filtering, focusing, field enhancement and nonlinear effects, magneto-optical Kerr effect, or electromagnetically induced transparency. Thanks to their high degree of optical tuning (wavelength, phase, polarization, intensity) and the variety of fabrication processes and materials available, RWGs have been implemented in a broad scope of applications in research and industry. RWGs can therefore also be applied as in- and out-couplers for waveguide gratings42.
Fig. 20 shows an RWG on top of a lightguide (referred often incorrectly through the popular AR lingo as a “waveguide”), acting as the in- and out-couplers.
Roll-to-roll replication of such grating structures can help bring down overall waveguide combiner costs. The CSEM research center in Switzerland developed the RWG concept back in the 1980s, companies are now actively developing such technologies43.
Metasurfaces are a hot topic in research44: they can implement various optical element functionality in an ultra-flat form factor by imprinting a specific phase function over the incoming wavefront in reflection or transmission (or both) so that the resulting effect is refractive, reflective, or diffractive, or a combination of them. This phase imprint can be done through a traditional optical-path-difference phase jump or through Pancharatnam-Berry (PB) phase gratings/holograms.
Due to their large design space, low track length, and ability to render unconventional optical functions, metalenses could grow out of the lab to become a unique item in the engineer’s bag of tools. If one can implement in a fabricable metasurface an optical functionality that cannot be implemented by any other known optical element (diffractives, holographics, or Fresnels), it is particularly interesting. For example, having a true achromatic optical element is very desirable not only in imaging but also in many other tasks such as waveguide coupling. Another example is ultra-low track length focal stack for IR cameras from Metalenz Corp. Additionally, if one can simplify the fabrication and replication process by using metasurfaces, the design for manufacturing (DFM) can be compelling. However, optical efficiencies, design tools, and large scale fabrication will need to continue to improve and find their way into product.
Waveguide combiners could benefit greatly from a true achromatic coupler functionality: in- and/or out-coupling RGB FOVs, and matching each color FOV to the maximum angular range (FOV) dictated by the waveguide TIR condition. This would reduce the complexity of multiple waveguide stacks for RGB operation.
When it comes to implementing a waveguide coupler as a true achromatic grating coupler, one can either use embedded partial mirror arrays (as in the Lumus LOE combiner), design a complex hybrid refractive/diffractive prism array, or even record phase-multiplexed volume holograms in a single holographic material. However, in the first case, the 2D exit pupil expansion implementation remains complex, in the second case, the microstructures can get very complex and thick, and in the third case the diffraction efficiency can drop dramatically (as in the Konica Minolta or Sony RGB photopolymers combiners, or in the thick Akonia holographic dual photopolymer combiner, now part of Apple, Inc.).
It has been recently demonstrated in literature that metasurfaces can be engineered to provide a true achromatic behavior in a very thin surface with only binary nanostructures. It is easier to fabricate binary nanostructures than complex analog surface relief diffractives, and it is also easier to replicate them by nanoimprint lithography (NIL) or soft lithography and still implement a true analog diffraction function as a lens or a grating. The high index contrast required for such nanostructures can be generated by either direct imprint in high index inorganic spin-on glass or by NIL resist lift-off after an atomic layer deposition (ALD) process. Direct dry etching of nanostructured remains a costly option for a product.
It is important to remember that metasurfaces or thick volume holograms are not inherently achromatic elements, and never will be. However, when many narrow band diffraction effects are spatially or phase multiplexed in a metasurface or a thick volume hologram, their overall behavior over a much larger spectral bandwidth can effectively lead the viewer to think they are indeed achromatic: although each single hologram or metasurface operation are strongly dispersive, their cascaded contributions may result in a broadband operation which looks achromatic to the human eye (e.g. the remaining dispersion of each individual hologram or metasurface effect affecting a spectral spread that is below human visual acuity-one arcmin or smaller). It is also possible to phase multiplex surface relief holograms to produce achromatic effects, but more difficult than with thick volume holograms or thin metasurfaces.
Mirrors are of course perfect achromatic elements and will therefore produce the best polychromatic MTF (such as with Lumus LOE combiners or LetinAR pin mirror waveguides).
As we have reviewed in this section, world side leakage (also commonly referred to as “eye glow”) is a parasitic display feature that in some instances can block the eye contact when the display is on, and furthermore provide a very unnatural glow replacing the headset wearer’s eyes. Not all waveguides have same world side leakage. We classify in order the various waveguide combiner technologies below.
Waveguide technologies classification for world side leakage (and world side display rainbow effect):
- Surface relief gratings with binary profiles (about 50% of the eye side leakage).
- Surface relief gratings with slanted profiled (about 40% of the eye side leakage)
- Holographic grating couplers (about 8-10% of the eye side leakage).
- Reflective waveguides (5% without AR coating and <1% with AR coating)
Table 1 summarizes the various waveguide coupler technologies reviewed here, along with their specifics and limitations.
Operation Reflective coupling Transmission coupling Efficiency modulation Lensed out- coupler Spectral dispersion, Color uniformity Dynamically tunable Polarization maintaining Mass
Reflective Yes No Dielectric coatings No Minimal Good No Yes Slicing, coating, polishing, Lumus Ltd.
DK 50, Maximus 2D EPE
Micro-prisms Reflective Yes No Dielectric coatings Yes Minimal Good No Yes Injection molding Optinvent. ORA Surface relief slanted grating Diffractive Yes Yes Depth, Duty cycle, slant Yes Strong Needs comp. Possible with LC No NIL (wafer, plate) Microsoft HoloLens, Vuzix Inc, Nokia,… Surface relief blazed grating Diffractive Yes No Blaze/antiblaze angles No Strong Needs comp. Possible with LC No NIL (wafer, plate) Magic Leap One, Surface relief binary grating Diffractive Yes Yes Depth, Duty cycle Yes Strong Needs comp. Possible with LC No NIL (wafer, plate) Magic Leap One Multilevel surface relief grating Diffractive Yes Yes Depth, Duty cycle Yes Strong Needs comp. Possible with LC Possible, but difficult NIL (wafer,plate) WaveOptics Ltd, BAE. Dispelix. Thin photopolymer hologram Diffractive Yes Yes Index swing Yes, but difficult Strong Needs comp. Possible with shear No Contact print roll to roll Sony Ltd, TruelifeOptics Ltd, H-PDLC volume holographic Diffractive No Yes Index swing Yes, but difficult Strong Need comp. Yes
No Exposure Digilens Corp. (MonoHUD) Thick photopolymer hologram Diffractive Yes Yes Index swing Yes, but difficult Minimal Need com;. No No Multiple exposure Akonia Corp (now Apple Inc.) Resonant Waveguide Grating Diffractive Yes Yes Depth. Duty cycle Yes Can be mitigated NA Possible with LC Possible Roll to roll NIL CSEM / Resonannt Screens Metasurface coupler Mostly diffractive Yes Yes Various Yes Can be mitigated Needs comp. Possible with LC Possible NIL (wafer, plate) Metalenz Corp.
Table 1. Benchmark of various waveguide coupler technologies.
Although this table shows a wide variety of optical couplers, most of today’s AR/MR/smart glass products are based on only a handful of traditional coupler technologies such as thin volume holograms, slanted surface-relief gratings, and embedded half-tone mirrors. The task of the optical designer (or rather the product program manager) is to choose the right balance and the best compromise between coupling efficiency, color uniformity over the eyebox and FOV, mass production costs, and size/weight.
Fig. 21 shows the various coupler elements and waveguide architectures grouped in a single table, including surface relief grating couplers, thin holographic couplers, and thick holographic couplers in three, two, and single flat guides. For geometric waveguide combiners that use embedded mirrors or other reflective/refractive couplers (such as micro-prisms).
Holographic optics in planar optical systems for next generation small form factor mixed reality headsets
- Light: Advanced Manufacturing , Article number: 42 (2022)
- Received: 06 December 2021
- Revised: 27 June 2022
- Accepted: 28 June 2022 Published online: 02 August 2022
Abstract: Helmet Mounted Displays (HMDs), such as in Virtual Reality (VR), Augmented Reality (AR), Mixed reality (MR), and Smart Glasses have the potential to revolutionize the way we live our private and professional lives, as in communicating, working, teaching and learning, shopping and getting entertained. Such HMD devices have to satisfy draconian requirements in weight, size, form factor, power, compute, wireless communication and of course display, imaging and sensing performances. We review in this paper the various optical technologies and architectures that have been developed in the past 10 years to provide adequate solutions for the drastic requirements of consumer HMDs, a market that has yet to become mature in the next years, unlike the existing enterprise and defense markets that have already adopted VR and AR headsets as practical tools to improve greatly effectiveness and productivity. We focus specifically our attention on the optical combiner element, a crucial element in Optical See-Through (OST) HMDs that combines the see-through scene with a world locked digital image. As for the technological platform, we chose optical waveguide combiners, although there is also a considerable effort today dedicated to free-space combiners. Flat and thin optics as in micro-optics, holographics, diffractives, metasurfaces and other nanostructured optical elements are key building blocks to achieve the target form factor.
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