New Diffraction Grating from NC State Promises A Higher Field-of-View for AR Headsets
Andrew Wheeler posted on July 02, 2018 |

Diffraction gratings are a type of Diffractive Optical Element (DOE) with a series of parallel linear structures that split light into multiple beams, resulting in a structural coloration. The light beams in structural colorations of diffraction gratings move in different directions, depending on the spacing and form of grating relative to the light’s wavelength. Commonly used in holography, electronic displays and fiber-optic communication technologies, these optical components generally have an acceptance range of 20 degrees. If this sounds overly complicated, it is not. It just means that the light source must be directed into the diffraction grating within an arc of 20 degrees.

The image above shows a one-inch diameter Bragg polarisation diffraction grating bending the white light of an LED flashlight onto a screen. (Image courtesy of Michael Escuti, NC State.)
The image above shows a one-inch diameter Bragg polarisation diffraction grating bending the white light of an LED flashlight onto a screen. (Image courtesy of Michael Escuti, NC State.)

For augmented reality or “mixed reality” headsets like the Microsoft HoloLens, diffraction gratings are one key component of a complex assembly. HoloLens uses a diffraction grating in concert with an optical wave guide, a fascinating component that takes advantage of the fact that once light enters glass or clear plastics at a specific angle and lower, it will totally reflect. This is known as Total Internal Reflection (TIR). For typical glass and plastics, the TIR critical angle is around 45 degrees.

HoloLens uses a diffraction grating to bend the light to 45 degrees. The light then reflects off of flat surfaces within the glass and bounces off a triangular “fold zone”, which redirects the light 90 degrees into an exit zone DOE. The exit zone DOE reduces the angle of the light, canceling out the TIR effect, and allows the light to exit the glass toward the user’s eyeball. (Image courtesy of HoloLens.)
HoloLens uses a diffraction grating to bend the light to 45 degrees. The light then reflects off of flat surfaces within the glass and bounces off a triangular “fold zone”, which redirects the light 90 degrees into an exit zone DOE. The exit zone DOE reduces the angle of the light, canceling out the TIR effect, and allows the light to exit the glass toward the user’s eyeball. (Image courtesy of HoloLens.)

So, by bending light, diffraction gratings produce two side-effects: splitting the light according to wavelength and affecting the polarization of the light. Expanding the acceptance range higher than 20 degrees would allow the light to enter the diffraction grating from a wider range of input angles, yielding a valuable effect for augmented reality hardware—increasing the Field-of-View (FoV).

A professor of electrical and computer engineering at North Carolina State University named Michael Escuti has written a paper detailing the development of a new diffraction grating that expands the acceptance range to 40 degrees.

Escuti is also the chief science officer of ImagineOptix Corp, which funded the research and has now licensed the technology. The new diffusion grating bends 75 percent of light in the right direction, which is more than twice the average (30 percent).

To read about this in more detail and understand the mathematics behind it, the paper Bragg polarization gratings for wide angular bandwidth and high efficiency at steep deflection angles, is published and available to read.


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