In augmented reality glasses, the image produced by a small screen must reach the eye without hiding the real world. One solution is to inject light into a transparent plate. It then travels inside this plate through successive reflections, like in an optical fiber, before being directed toward the eye.
The problem notably comes from the colors. A complete image combines red, green, and blue, but each color does not react in exactly the same way. The researchers therefore fabricated three distinct holographic gratings, each intended for one of these colors. These microscopic structures deflect light in a chosen direction.
The three gratings are stacked in the same waveguide. Each receives the color for which it was designed, then sends it into the plate. The three components can therefore travel together and recreate a color image.
To understand the significance, it is also necessary to examine how it is made. Stacking several optical layers normally requires very precise positioning. A slight offset can disrupt the paths of the different colors. The team used a hologram-recording method that forms the gratings directly in their intended positions, without having to align each layer separately after fabrication.
Another special feature is that this fabrication does not require a prism during hologram recording. The researchers say they produced the complete waveguide using only two lasers. The process relies on an optical configuration that makes it possible to record the three red, green, and blue gratings successively while preserving their alignment.
The laboratory prototype then transmitted color information through the waveguide. The experiment therefore confirms that the three layers can work together in a single system. The study primarily highlights high diffraction efficiency, meaning that the gratings are effective at sending light in the desired direction rather than losing it.
In practice, more efficient use of light could help produce a visible image without increasing the source's power. This matters for battery-powered glasses, where power consumption, size, and brightness are directly connected.
The next step will therefore be to integrate this architecture into more advanced near-eye displays. The researchers already have a color prototype, but turning it into a wearable product will require taking into account the field of view, image quality, and manufacturing constraints at larger scale.