Combined integrated optics and holography in AR technology

TVs and smartphones that project digital images emit light all around them, as quasi-isotropic sources. Because the images are projected generally over the air without directivity, many viewers see the same image. In typical AR glasses, images are transmitted close to the eyes (high directivity) by a microdisplay that includes an optical system and an optical combiner.

These microdisplays create a small near-to-eye image, which is transformed by the optical system, enabling the user to see it despite the short focusing distance. The combiner superimposes the digital image to the viewers’ vision of the real environment.

CEA-Leti’s innovation is a transparent retinal-projection device that projects various light waves to the eyes from a glass surface. Images are formed in the retina by the interference of light waves, which eliminates the need for optical systems or combiners. The light propagating in the air doesn’t form an image until it interferes precisely in the retina.

CEA-Leti presented its results February 6th at SPIE Photonics West 2019 in a paper titled ‘Integrated Optical Network Design for a Retinal Projection Concept Based on Single-Mode Si3N4 Waveguides at 532nm’.

The project focused on the design and numerical simulations of integrated Si3N4 optical components and the optical circuit at λ = 532 nm. It required building blocks for designing an optical integrated circuit capable of creating an array of emissive points. Starting with single-mode waveguides to efficiently transport light around the circuit, many other components were designed to manipulate light in different locations.

Components for extracting the light, such as diffraction gratings, were also designed and simulated. The team minimised losses of different parts of the circuit, such as waveguide-bending areas, to increase energy efficiency of the system. 

CEA-Leti’s integration of the device and its use of a holographic layer also allow creation of compact AR glasses with a larger field-of-view than existing systems, while the transparent retinal projection device allows ambient light to pass through the device for enhanced AR applications.

“Combining integrated optics and holography is a new research area for the scientific community developing display applications,” said Basile Meynard, a Ph.D. student and lead author of the paper. “It is also a way to imagine a display device that works more as a data transfer system than as an imaging system.”

The novel approach will require further development before it reaches the commercialisation stage. In the medium to long term, the retinal projection concept is expected to support more compact and higher virtual-image quality applications similar to existing AR glasses.

This research project builds on CEA-Leti’s many years of development of micro-displays for near-to-eye displays, such as organic LED technologies (OLED) and liquid crystal devices (LCD). More recently, the institute has made significant strides in the field of inorganic LED display manufacturing. 

Christophe Martinez, Optical Senior Scientist and Project Leader in Leti, added: “Our teams are continuously looking for potential disruptive technologies that could pave the way to new families of display devices down the road. The investigation on retinal displays is part of this exploration of future optical solutions.”