LiDAR sensors are key to ensure safety and efficiency of autonomous vehicles and mobile robotic systems. But current commercial LiDAR technologies cannot face the issues in terms of costs, reliability and form factors that are hindering widespread adoption. Steerlight, a CEA-Leti spin-off, develops a new generation of on-chip LiDARs that relies on Frequency Modulated Continuous Wave (FMCW) and Silicon Photonics. FMCW provides instantaneous depth and velocity acquisition while Silicon Photonics enables the integration of the optical and electronics functionalities on a single chip. This paper will first describe the proposed silicon photonics based architecture. Subsequently, various demonstrations will illustrate the roadmap towards miniature and scalable LiDARs. Such features will contribute to making a wide range of devices smarter and safer, beyond the automotive and robotics markets.
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.
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.
Our team works on a disruptive concept of Near Eye Display for Augmented Reality (AR) applications. This device requires distributions of holographic elements described as Emissive Points Distributions (EPDs) to create a composite planar wavefront emitted towards the eye. The crystalline lens focuses this signal onto the retina in a mix of diffraction and refraction processes, to form the pixels of an image. We experimentally recorded an image of the letter “R” with pixelated holograms. At the reading of this image, we observe speckle that partially alters the image. Using image processing on the experimental results, we can suppress this speckle and recover the initial “R”, which validates our concept. We develop a simulation tool based on Fourier optics to better understand the emergence of this speckle noise. With the knowledge of the recording process and the form of the hologram given by microscopy, we simulate the electric field 𝐸𝑛 reflected by the different holographic elements from a unique collimated laser. Each field 𝐸𝑛 encodes an angular pixel of the recorded image. The sum of these optical beams in field and/or in intensity allows us to analyze the role of the different optical elements in the generation of a speckle. In particular, the role of the cross interferences between different EPDs is questioned. The experimental analysis is brought for periodic EPDs but can be extended to the case of random EPDs. It gives some insights into some possible evolutions of our concept in terms of optical implementation.
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.
One of the big challenges of Augmented Reality (AR) is to create ergonomic smart glasses. Our laboratory proposes an unconventional concept of AR glasses based on self-focusing of multiple beams into the eye. The device comprises a dense electrode network that allows extracting light from a dense waveguide network at the intersections points. Each beam emitted at these points is reflected by a holographic element that directs light towards the eye with a proper angular direction. The image pixels are created on the retina by the interferences of various beam distributions. This paper presents a method to optimize the design of waveguides and electrodes to increase the number of pixels on the final image. Our method considers a first waveguide (resp. electrode) as a curve described by a succession of segments with a unique absolute angle crossing a horizontal (resp. vertical) axe. The other waveguides (resp. electrodes) are created by the translation of the curves so that the minimal distance between two curves is equal to a fixed value. We use the B-Splines mathematical model to approximate the succession of segments. An iterative method adapted to B-Splines allows calculating the intersections between waveguides and electrodes. An Emissive Point Distribution (EPD) is obtained by selecting random groups of waveguides and electrodes. Each EPD forms one pixel onto the retina. The Point Spread Function (PSF) of this EPD characterizes the self-focusing efficiency. We calculate the Signal-to-Noise-Ratio of each EPD to evaluate the quality of the whole self-focusing process and we compare it to the Airy function. Our new mathematical model improves by a factor 3.5 the number of pixels for an equivalent SNR in comparison to the model we previously used.
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.
Augmented Reality (AR) is now subject to a great technological acceleration. Different actors of the field develop many different projection technics. However, with each projection technic comes display strategies to improve the comfort and immersion of the user and the quality of the image displayed. Some recent works propose to use new principle of projection based on unconventional optics to create new products aiming to reduce many of the current problems related with the conventional projection principle of AR devices. CEA Leti recently proposed a disruptive optical concept for AR display[1], which is based on the interaction of a grating of monomode linear waveguides with pixelated holograms. This work uses simulations to analyze optical issues associated with this new optical concept. The main topic covered is the study of the SNR depending on the resolution criteria chosen.
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