The development of an ideal optical system to support Mixed Reality and Augmented Reality (AR) applications has raised a lot of interest in the scientific community in the last decades. The perfect device remains an inaccessible target and researchers have to focus on the optimization of some specific behaviors. Several years ago, we introduced a disruptive display concept to push the device integration to the limit, with the suppression of the optical system. This allows the imaging process to be considered in a different way with a specific monitoring of the field of view. With this ‘smart glass’ concept, the glass is the display, and the image is formed directly onto the retina with a combination of refractive and diffractive effects. This conceptual target allowed us to define a technological roadmap to support our development. Technologies involved in this concept concern principally the field of Photonic Integrated Circuits in the visible range, digital/analogic holography and Liquid Crystal devices. We will present the current state of our research with a particular focus on the holographic display element. Recent results related to analogic pixelated hologram recording validate and question both our technological and conceptual approach. We will show images formed by sparse holographic pixel distributions with controlled angular characteristics that demonstrate the mix of refractive and diffractive effects. The transmission behavior of this holographic device will also be analyzed.
KEYWORDS: Retroreflectors, Retroreflector prisms, Augmented reality, Automotive front vision, Heads up displays, Diffusers, Deep reactive ion etching, Projection systems
The superposition of digital information in the Field of View (FOV) of a user is the basis of the current developments in mixed and augmented reality. Before being studied for near eye device and head mounted display, this application was implemented in Head Up Display (HUD) to help pilots and drivers to manage both the driving stress and the information flow related to the vehicle. Classical optical design of HUD based on the use of a combiner are strongly limited in FOV due to the issues related to pupil management. To overcome this issue head up projection displays have been developed based on the projection of digital image directly on the windshield. To support this approach an efficient projection surface that meets bright reflection and clear transparency has to be developed. We have introduced few years ago an optical approach based on retro-reflective transparent projection surface and a manufacturing process to provide microscopic corner cubes that incorporate an optical diffuser function. We present in this contribution an optimized design that increases the efficiency of the retroreflective structure towards 100%. We also discuss a possible technological process that allows the manufacturing of the master used to replicate the microstructure. This process based on grayscale lithography and on Deep Reactive Ion Etching (DRIE) may guaranty a high retro-reflection efficiency, a high transparency and a realistic draft to allow a molding manufacturing process for the microstructure fabrication.
In our Augmented Reality (AR) project, we are investigating the use of a retinal projection display based on the association of pixelated holograms and a dense distribution of waveguides. We study the use of gratings impregnated with liquid crystal to actively extract light from waveguides. We explore two extraction strategies: tuning the refractive index contrast between the grating teeth and grooves to erase the grating diffraction effect and changing the index of the waveguide cladding to tune the evanescence of the guided mode. Firstly, we present and discuss the measurements of the diffraction efficiency of nano-imprint gratings impregnated with liquid crystal and refractive liquid index. Secondly, we discuss the results of integrated switchable extraction grating of the second strategy.
Holographic based optical elements are key components for many product in Augmented Reality and Virtual Reality. We describe in this work the use of pixelated micrometric holograms to fulfill the role of directive in phase reflector for self-focusing purpose. We present the optical set-up used to record these pixelated holograms as well as a set-up to realize a dynamic addressing on these holograms. First results of dynamic holograms addressing are shown and discussed.
We are developing a non-conventional retinal projector for augmented reality (AR) applications. In our concept, light at λ = 532 nm is guided in silicon-nitride (SiN) photonic integrated circuits (PICs) embedded in the lens of a pair of glasses. We use holographic elements to transmit the emissive points towards the user’s retina without using lenses. Pixels are formed in the eye using the self-focusing effect and the eye lens. The transparency of the device is an absolute requirement for our application. In this work, we present the fabrication and the characterization of our latest SiN PICs on transparent substrate. The device was fabricated by transferring the SiN PICs from a silicon to a glass substrate. We characterized the PICs and the free-space optical transmission properties of our device using in-house goniometers and a Modulation Transfer Function (MTF) setup. We found a 76% transparency at our wavelength and no image alteration. However, we measured significant waveguide propagation losses; solutions are discussed to tackle this problem. Our glass-substrate device is a major step towards a future prototype for our AR retinal projector.
Liquid Crystals are birefringent materials, which address many applications such as visualization with Liquid Crystal Display (LCD) or beam shaping with Liquid Crystal on Silicon devices (LCoS). Recently, several research teams proposed using liquid crystals in photonics devices applied to new kinds of projection displays. Augmented Reality (AR) is one of the domains, which could benefit from these developments, thanks to the necessity to create active and transparent optical function. In this contribution, we present recent works at CEA Leti to develop a switchable photonic extraction grating adapted to a specific near-eye device. Two different technics are detailed and studied with FDTD simulations. We also show first experimental characterization of an impregnated diffraction grating used in a free space optical set-up
We found Holographic materials in a widespread field of applications and particularly in Augmented Reality (AR) area, which has been attracting attention for several years. Scientists have developed various complex holographic optical design for displaying clear and bright images in transparent devices. Holographic materials developed for this technology necessitate stringent optical properties such as photosensitivity, transparency, low cost and robustness. Photopolymer materials offer a reliable solution for these requirements. Our research team has recently presented a unique concept for AR applications that requires evaluating different photopolymers system in order to support our development. Among the photomaterials under test, we have studied in particular a photopolymer formulated in our own laboratory based on du Pont patents that contains N-Phenylmethacrylamide as monomer. This solution is not commercially available and has the advantage of a good transparency and wet chemistry. Another photopolymer under test, based on a different photochemistry mechanism, is the commercial product Bayfol® HX from Covestro available as laminated layers on triacetate cellulose film substrates.
Our AR optical concept requires the use of pixelated holograms with a complex recording process that strongly depends on the inhomogeneous properties response of the photomaterial. In this paper, we describe the both photopolymer materials behavior during this holographic recording step. Then, we discuss about writing strategies implemented to improve the hologram homogeneity. In a second part, we evaluate the robustness of the holograms written in our photomaterial and we mesure their spectral stability under thermal stresses in order to extrapolate their natural aging. A comparison is made with the commercial product, it underlines that the robustness strongly depends on the nature of the polymer chemical formulation.
We present our first results on the recording of pixelated holograms. This specific recording process is dedicated to an unconventional approach of smart glass design. Due to the use on integrated photonics, this concept requires to adjust locally the properties of out-coupling holographic elements with specific angular distribution. We analyze here a simple Lippmann recording configuration that focus on the material behavior regarding the pixelated process. We demonstrate our ability to record distribution of holographic elements, few micrometers in size, and compare our experimental results to first elements of simulation.
We introduce an original approach for an extended Head Up Display solution. This configuration is based on the projection of images directly on the windshield of a vehicle, allowing the display of various information around the user viewing axis, as a peripheral dashboard. We highlight that this solution is only effective if we can manage both directivity and diffusivity of the reflected light. We introduce for that purpose two technological options. A pragmatic one allows us to evaluate at short term the HUD behavior. Another proposes an original approach with a manufacturing process based on the etching of a deep cube corner cavity in a composite silicon wafer that incorporates a diffuser surface. We demonstrate both technological options and give some perspectives for future works.
Backthinning of image sensors is a very well established process for achieving high Quantum Efficiency for high-specification space and science applications. To optimize the QE performance in various spectral bands, the AR coating need to be adjusted. A new multilayer low thickness UV AR coating has been developed by e2v and CEA-Leti with very high transmission at 266 nm and 355 nm laser wavelengths. It is compatible with CCD and CMOS backthinned image sensor process. We describe hereafter the first results obtained on glass and silicon substrates for this AR coating. The manufacturing of backthinned CCD and CMOS image sensors samples is ongoing. This development is supported by Minalogic project (financed by French FUI-DGCIS).
From the first ultra-realistic 3D images in the sixties to the most recent Augmented Reality devices, the field of holography has been involved in display technology for a long time. The spectral selectivity of the hologram reflection together with the very good transparency of the holographic material make it a suitable option for some of the key optical components in smart glasses. However, these devices are still very limited by the overall optical system based on the conventional scheme Display - Optical System - Combiner. Recently, we have proposed an unconventional scheme that puts the hologram at the core of the display device. Due to its 3D nanoscale complexity, the dynamic updatable hologram display is still an unreachable goal. As an alternative, our configuration is based on a concept of switchable static holographic elements. These elements are interleaved on the surface of the display and form various groups of emissive point distributions that are phase-adjusted for given angular directions. The activation of these holographic elements produces angular planar wavefronts in the far field and the display is expected to achieve retinal projection without the help of an optical system. We present our concept and describe the development of the optical set-up used to investigate our holographic configuration. We record phase-adjusted distributions of holographic elements that are multiplexed on the surface of our sample, each distribution targeting a specific angular direction. First recording results on a holographic photopolymer are given.
Ultra-realistic virtual object representation is an old dream of humanity. From the 3D Paleolithic rock painting to the late Michael Jackson holographic shows, humans have investigated display solutions to give life to the abstraction. The incredible opportunities given by the digital revolution have paved the way to the recent development of innovative volumetric displays. These complex solutions are however still limited in the visual experience they can offer to the viewer. In a more concerning manner such devices often appear as empty shells as their effective usefulness is not yet clearly defined. Recently, we have proposed an original volumetric display concept based on a 360° projection configuration. Inspired from the pepper ghost concept and from the praxinoscope design of the end of the XIX century, our display mixes real projection on transparent retroreflective surface and virtual images superimposition. This development has been made in collaboration with a group of live performing artists in France. The 360° display has been used to present an original creation of the artists and the confrontation with the public has highlighted some unexpected properties of this family of displays. We describe here the technological concept of our display and the evolutions we target to improve the visual rendering. The collaboration project with the artists is also presented and we give our analysis on the feedback of the public.
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