This is the first of three articles introducing the concept of k-space and discussing how to plan for an exit pupil expander design.

Augmented Reality (AR) is no longer just for gaming. The accurate simulation of a real-world environment can also be beneficial for multiple work environments, including the medical field. AR enables doctors to better explain a disease or treatment to a patient, as well as plays a role in planning (and in some cases performing) surgeries and therapies. Some are taking it a step further, using 3D visualization of DNA test results or to view patient records for quick analysis and diagnosis. This technology can also be used in the classroom, training medical students through the use of interactive 3D anatomical models. And for medical sales, the use of a virtual experience can demonstrate their equipment quickly.

However, to make an AR device effective, the system must accurately simulate the proposed environment or object, making it as realistic as possible for the user. Zemax OpticStudio offers the tools needed for optical engineers to efficiently and effectively design AR devices.

In this article, we’ll discuss setting up an exit pupil expander (EPE) for an augmented reality (AR) system in OpticStudio, including an overview of the planning of gratings in k-space (optical momentum), and touch on setting up each grating using the RCWA tool.

Exit pupil expander (EPE) explained

For optical engineers creating these devices, exit pupil expansion (EPE) is a common technique used in a waveguide-based AR system to improve optical performance. In an ideal AR system, the design should allow beams from each field to overlap at the exit pupil, or where light exits the device, so that that the human eye can receive the full image better. By properly arranging all the elements in the system, this results in the exit pupil being expanded, and therefore the structure is often called an exit pupil expander (EPE). With an EPE, light from each field can overlap at an area where the eye pupil is supposed to be placed to see the whole image.

Plan the grating period and direction

AR simulates real world experiences in a virtual environment. To accurately create a real-world representation, the AR system needs to have an understanding of the simulated environment, which can be done through the use of a coordinate system in k-space. It’s important to plan the arrangement of gratings, which couple in/out and rotate beams in a waveguide, in the most efficient manner. In k-space, the beam propagation and effect of a grating can be intuitively visualized. This helps designers to understand and better create the image performance for the end user, as well as for grating, which ultimately makes the image visible to the human eye.

Zemax OpticStudio allows users to set real grating shape internally and simulate it in real time. In other solutions, this process usually needs to be done in two software programs and static data exchange is required.

In this example, we use a simple binary grating for the EPE. It could be slanted or any other shape depending on the ability of the fabrication method.

In our specific case, we used three gratings to build the EPE, and we coupled the rays from air to the waveguide, turned the rays’ direction inside the waveguide, and then couple the rays from the waveguide back into air.

When working with k-space, keep in mind it can only describe how the ray’s propagation direction is changed by each grating. It doesn’t describe how the grating should be placed on the waveguide.

• The first grating was used to move the field of view (FOV) to the total internal reflection (TIR) zone where light is reflected into the material.
• The second grating was used for turning the rays’ propagation by 90 degrees.
• The third grating was similar to first one. Its period is same as the first grating, but the orientation should be rotated by 90 degrees relative to the first grating. 

Stay tuned for Part 2 and Part 3 of this series to learn the next steps in the process to building the exit pupil expander system with a waveguide and three gratings. We will discuss how to simulate the footprint diagram and the image. The image simulation is unique to Zemax OpticStudio, which currently the only program that is able to create this view.

To learn more, Zemax customers can access the entirety of this Knowledgebase article and the entire series on MyZemax.com. Otherwise, please reach out to Zemax Sales to learn more about OpticStudio.

Author: 

Michael Cheng
Senior Optical Engineer
Zemax