We present the design of a dense silicon-nitride photonic integrated circuit applied to a novel concept of unconventional augmented reality display combining holography with integrated photonic circuits embedded in the glass of the display. An integrated projector made of a grid of waveguides and electrodes generates randomly distributed emissive points at the wavelength of 532 nm, from crossings between waveguides and electrodes. These emissive points then interact with a hologram to form a composite plane wave focused on the user’s retina using the self-focusing effect. In this work, we present the design of the entire photonic circuit comprising the projector and the routing architecture to address the emissive points. We define both shapes of waveguides and electrodes by a succession of segments that we approximate by BSplines. We designed the photonic circuits using Matlab, Lumerical and KLayout softwares. We engineer the waveguide architecture on two levels in order to limit optical losses due to the high number of waveguide crossings required for the random addressing. In this contribution, we present the mathematical model used to elaborate our design as well as the technical details. We also evaluate it for our retinal projection project.
Near-eye displays have become a technology of high interest for Augmented, Virtual and Mixed reality due to the unique immersive experience they provide to the user. The majority of these devices use macroscopic optical elements that make them bulky and heavy. Our team has proposed a disruptive near-eye display concept that uses the self-focusing effect to project an image to the user’s retina. To form an image, emissive points are generated from a dense photonic integrated circuit embedded within the lens of a pair of smart glasses. In this work, we present the design of a dense routing architecture that addresses thousands of randomly distributed emissive points from a few hundred inputs. The circuit combines unbalanced waveguide splitter trees with a non-periodical addressing onto a dense waveguide network. We present the design optimization through numerical simulations and estimate the overall device performance based on simulation results. A waveguide interlayer crossing simulation indicates losses better than 0.003 dB/crossing, which guarantees low optical losses over thousands of crossings. By unbalancing correctly the splitter trees, we can obtain homogeneous power profiles over an emissive point distribution. The experimental validation of our design will be a major step towards the elaboration of a first prototype.
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.
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
Our team is currently developing an innovative and compact retinal projection display concept for augmented reality applications. This concept intends to break with conventional optics using a device based on an integrated photonics architecture. Designs and simulations of test structures have been presented in previous works. Here, we present experimental results obtained on stoichiometric Si3N4 integrated photonic circuits designed for λ = 532nm. The samples combine several building blocks such as grating couplers, Multi-Mode Interferometers (MMI) and dense waveguide arrays with several emissive areas. We used a goniometric characterization bench to perform far field measurements. For near field characterization, an imaging setup using a 4.5mm-focal lens system and a high-resolution CMOS camera was mounted on the goniometer. The power profiles of emissive points were measured in the near and far field. Regarding the dense waveguide arrays, their angular characterization confirms our simulations and allows us to emphasize the impact of trapezoidal-shaped waveguides on the emission angle. We measured a homogeneously distributed power profile across the waveguide distribution on the emissive areas. The power ratio of each emissive area remains relatively uniform. This is a promising result for our future work where light must be equally and uniformly distributed over the surface of the display.
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