New design could make augmented reality glasses more power-efficient and practical for everyday wear.
Researchers at the University of Rochester have designed and demonstrated a new optical component that could significantly enhance the brightness and image quality of augmented reality (AR) glasses. The advance brings AR glasses a step closer to becoming as commonplace and useful as today's smartphones.
"Many of today's AR headsets are bulky and have a short battery life with displays that are dim and hard to see, especially outdoors," says research team leader Nickolas Vamivakas, the Marie C. Wilson and Joseph C. Wilson Professor of Optical Physics with URochester's Institute of Optics. "By creating a much more efficient input port for the display, our work could help make AR glasses much brighter and more power-efficient, moving them from being a niche gadget to something as light and comfortable as a regular pair of eyeglasses."
In the journal Optical Materials Express, the researchers describe how they replaced a single waveguide in-coupler-the input port where the image enters the glass-with one featuring three specialized zones, each made of a metasurface material, to achieve improved performance.
"We report the first experimental proof that this complex, multi-zone design works in the real world," says Vamivakas. "While our focus is on AR, this high-efficiency, angle-selective light coupling technology could also be used in other compact optical systems, such as head-up displays for automotive or aerospace applications or in advanced optical sensors."
Metasurface-powered AR
In augmented reality glasses, the waveguide in-coupler injects images from a micro-display into the lenses so that virtual content appears overlaid with the real world. However, the in-couplers used in today's AR glasses tend to reduce image brightness and clarity.
To overcome these problems, the researchers used metasurface technology to create an in-coupler with three specialized zones. Metasurfaces are ultra-thin materials patterned with features thousands of times smaller than a human hair, enabling them to bend, focus or filter light in ways conventional lenses cannot.
"Metasurfaces offer greater design and manufacturing flexibility than traditional optics," says Vamivakas. "This work to improve the in-coupler, a primary source of light loss, is part of a larger project aimed at using metasurfaces to design the entire waveguide system, including the input port, output port and all the optics that guide the light in between."
For the new in-coupler, the researchers designed metasurface patterns that efficiently catch incoming light and dramatically reduce how much light leaks back out. The metasurfaces also preserve the shape of the incoming light, which is essential for maintaining high image quality.
This research builds on earlier theoretical work by the investigators that showed a multi-zone in-coupler offered the best efficiency and image quality. Vamivakas says that advances in metasurface gratings enabled the design flexibility to create three precisely tailored zones while state-of-the-art fabrication methods-including electron-beam lithography and atomic layer deposition-provided the precision needed to build the complex, high-aspect-ratio nanostructures.
"This paper is the first to bridge the gap from that idealized theory to a practical, real-world component," says Vamivakas. "We also developed an optimization process that accounts for realistic factors like material loss and non-ideal efficiency sums, which the theory alone did not."
Three-zone performance test
To demonstrate the new in-coupler, the researchers fabricated and tested each of the three metasurface zones individually using a custom-built optical setup. They then tested the fully assembled three-zone device as a complete system using a similar setup to measure the total coupling efficiency across the entire horizontal field of view from -10 degrees to 10 degrees.
The measurements showed strong agreement with simulations across most of the field of view. The average measured efficiency across the field was 30 percent, which closely matched the simulated average of 31 percent. The one exception was at the very edge of the field of view, at -10 degrees, where the measured efficiency was 17 percent compared to the simulated 25.3 percent. The researchers attribute this to the design's high angular sensitivity at that exact angle as well as potential minor fabrication imperfections.
The researchers are now working to apply the new metasurface design and optimization framework to other components of the waveguide to demonstrate a complete, high-efficiency metasurface-based system. Once this is accomplished, they plan to expand the design from a single color (green) to full-color (RGB) operation and then refine the design to improve fabrication tolerance and minimize the efficiency drop at the edge of the field of view.
The researchers point out that for this technology to be practical enough for commercialization, it will be necessary to demonstrate a fully integrated prototype that pairs the in-coupler with a real micro-display engine and an out-coupler. A robust, high-throughput manufacturing process must also be developed to replicate the complex nanostructures at a low cost.
