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October 17, 2022

New Ansys Lumerical Model Helps Bring Sub-Wavelength Gratings to Visible, Human-Scale Applications in Ansys Speos

As optical science continues advancing to exciting new areas of application, companies that design and build optical systems for augmented reality and virtual reality (AR/VR) see high customer demand for appealing devices that preserve high image quality for users. At Ansys, we've looked for ways to apply the nanoscale wave optics simulation capabilities of Ansys Lumerical to macroscale optical simulation and human vision in Ansys Speos, including for AR/VR wearable devices.

In particular, photonics-to-optics translation gains value for optical design in the use of macroscale technologies based on diffractive gratings and other structures that operate at the sub-wavelength scale. Our latest development from the Ansys optics group, the Ansys Lumerical Sub-wavelength Model (LSWM), uses Lumerical to simulate light wave interaction with these structures, then passes the simulation data to Speos, where it can be analyzed further based on how human users will perceive the product.

What is a diffractive grating?

Diffractive gratings in photonics and optics are three-dimensional structures made of a repeated pattern of tiny grooves or bumps with a constant period.

Diffractive grating surface

Three examples of a diffractive grating surface in Ansys Lumerical: arbitrary grating (left), slanted grating (center), and blazed/echelle grating with aluminum coating (right).

When light hits these surfaces, it diffracts the spectrum into different visible colors at different angle directions, producing a rainbow effect. We can observe this effect in everyday life: butterfly wings, highly secure bank notes, shimmering semiconductor wafers — even the dazzle you see when holding a compact disc upside-down and moving it around slightly in your hand. This pleasing characteristic has made diffractive gratings a common element in the aesthetic design of everything from cell phone cases to skyscrapers. Because they are complex to produce, they are considered a sophisticated feature in product designs.

A cell phone case in Ansys Speos as an example cosmetic application of diffractive grating

On the scientific level, diffractive gratings are technically spectrometers, splitting visible light into a set of wavelengths in precise directions. This capability makes diffractive gratings useful for optical design because of the ways the resulting rays can be manipulated in and out from a light guide. Diffractive gratings are solely a feature of nanoscale optics because they must be to the order of wavelength — in other words, very small; diffraction starts from about one micron, or 1/100 of a human hair (100 micrometers, or µm), and continues up to about 200 nanometers (nm).

Two-dimensional diffraction grating

Two-dimensional diffraction grating (2μm period, [-3:3] orders) illuminated by a directive white source that reflects and transmits light off its surface, which separates white light into a rainbow of colors.

Diffractive gratings show real promise in the evolution of AR/VR systems because they help achieve two key functional requirements: portability and precision. Unlike previous wearable designs that rely on complex systems of lenses, polarizers, and mirrors to produce AR/VR imagery, augmented reality glasses that use diffractive gratings are much lighter and potentially cheaper to mass produce. Diffractive gratings also enable precise control of light to produce a beautiful virtual image and enable a compact design that is more visually acceptable when AR glasses are worn by the user.

Bringing Photonics and Optics Together

What if Ansys technology could capture the simplicity of how diffractive gratings interplay with light waves, and use that to build better-quality wearable devices? Achieving this was a matter of easing the interoperability between nanoscale (Lumerical) and macroscale (Speos) design simulation workflows and quality controls.

Optical designers can simulate nanoscale diffractive structures in Lumerical, then export the results to a data file to import into Speos as a surface property in combination with the LSWM. In Speos, they can apply the grating directly to an optical substrate in a 3D computer-aided design (CAD) environment, letting them model an optical product with sub-wavelength structures. By this method, the nanoscale properties of the design can be viewed at a human-visible scale and applied directly to AR/VR wearable devices, with considerations for a variety of illumination environments: in a dark room, in the sunlight, in fog, under artificial lighting, and so on.

LSWM is also fully compatible with the Speos built-in Human Vision model to simulate not only how a person will look wearing the AR glasses, but also how the virtual image overlapping their environment will appear in the different illumination conditions they might encounter. The model integrates eye sensitivity (scotopic, mesopic, and photopic), glare, dark-to-bright eye adaptation, color vision deficiency, observer age, vision acuity, and depth of field LSWM brings together the photonic and optical principles to enable the end-to-end design of AR/VR systems based on nanoscale diffractive grating designs. Using LSWM, optical designers can simulate diffractive grating-based designs that offer precise control of the light rays used for virtual image generation, all while fitting into lightweight mechanical housing that is wearable by human users without appearing bulky or unattractive.

To learn more, read our LSWM white paper or contact your Ansys sales rep. Future blogs will provide more detail on cutting-edge AR/VR design capabilities with the Ansys optical group of applications, so stay tuned for more.

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