Modelling a holographic waveguide for Augmented Reality (AR) systems: part 1

AR systems commonly use holograms to couple light into the waveguides they use to transport light from the display engine to the wearer's eye. This article demonstrates how to use a hologram surface in OpticStudio as an in-coupler to a planar waveguide structure.

Authored By Sean Lin and Michael Cheng

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Introduction

Augmented Reality (AR) is a technology that allows the virtual world on the screen to combine and interact with real-world scenes. This article demonstrates how to set up an optical system for AR in Sequential Mode, using holographic technology.

 

Augmented Reality systems and hologram

Holograms are interference patterns recorded on a high-resolution photosensitive emulsion. Two distinctive stages are associated with holograms: The construction step and the clayback step, pertain respectively to the hologram creation and its use as an optical element. For more detailed content on this topic refer to the article: How to model holograms in OpticStudio.

In common AR systems, light is coupled into the waveguide using a hologram, which is used to transmit relevant information from the display to the eye. The advantage of the waveguide is that it is largely transparent and does not block light from the real world. In this article, we will guide you to set up a simple structure for an AR design using a reflective hologram embedded in PMMA.

 

Specifications and design strategy

We will start from a simple design and then improve the system further. Initial specifications are:

  • Eye relief = 15 mm
  • Pupil diameter = 3 mm
  • FOV = 10 degrees
  • Waveguide thickness = 10 mm

Rays will be coupled into the waveguide using a hologram. The hologram is embedded in PMMA and the exit surface is tilted by 45 degrees.

The system is modeled “in reverse” with respect to how the setup will actually work. In reality (in the physical system), the source for the AR system is the microdisplay and the image plane will be the retina of the human eye (the exit/entrance pupils of the AR system and human eye will be co-located). However, to model this setup accurately and optimize efficiently in OpticStudio, the system is defined in such a way that the exit pupil of the physical system is the entrance pupil as modeled in OpticStudio, and the microdisplay is treated as the “image plane” of the system. Therefore, any rays referenced in this article are described in the way they are modeled in OpticStudio.

 

Initial design

Initial condition settings include :

  • Entrance pupil diameter = 3.0 mm
  • Field points at 0 degrees, 5 degrees and -5 degrees in Y
  •  Wavelength = 0.55 µm

First add two surfaces after the STOP and set up as shown below. Then rotate the Hologram surface by 45 degrees about X axis using the Tilt/Decenter Element tool.

 

 

   

 

Next, we decide how to set up the hologram. We need to define our two construction beams. To achieve the diffracted ray direction from the hologram, construction beam must be collimated and beam 2 converging to a virtual focus. Because we need to use a hologram in reflection, its material must be set to “Mirror”. This explicitly indicates OpticStudio that rays propagate in the opposite direction after hitting the hologram surface.

In accordance with this idea, we set the coordinates (x, y, z) of the source points of the construction beams as follows. Beam 1 is a collimated beam (0, -infinity, -infinity). The construction point of beam 2 is set at (0, 0, -100), so that the beam will focus 100 mm away from the hologram.

 

 

We assume the hologram is constructed by wavelength = 0.55 µm. However, the hologram is embedded inside the material of PMMA during construction. As the hologram is embedded in a non-air material, we will need to scale the wavelength when entering it in parameter “Construct Wave”. Refractive index of PMMA for 0.55 µm is 1.49358. Therefore, construction wavelength is 0.55/1.49358 = 0.3682 µm.

 

   

 

We can use a little trick here to make the layout easier to read. As drawing the edges between two surfaces is not meaningful in our design, go to Surface Properties…Draw and check option “Do Not Draw Edges From This Surface”. Do this for all surfaces from surface 2 to Image plane.

 

 

In order to model the ray propagation in the waveguide, let’s add 5 more surfaces after the hologram surface. The first 4 surfaces are the sides of the waveguide where total internal reflection (TIR) will happen and the last surface exits the waveguide material.

 

    

 

Next, in order to center each mirror on the chief ray as we work, we can use Chief Ray solves. The Chief Ray solve works only on coordinate break surfaces, so we need to add coordinate break before each surface. When placed on a decenter x or y parameter, the solve will automatically set the value to center the real chief ray from the selected field position at the specified wavelength (use zero for primary wavelength) on the surface following the coordinate break. Insert 6 coordinate breaks, as shown in the following image, and place a chief ray solve on the “Decenter Y” parameter of each.

 

  

 

Prepare for optimization

By this point, the preliminary design is almost complete. Let’s start to optimize the system. First, use the optimization wizard to set RMS Spot size as image quality criterion. Then set construct Z2 as variable, so we can quickly modify for the Hologram’s power.

 

 

Result from this first optimization shows the best hologram power needed to achieve the smallest RMS Spot radius on the Image surface. Now we have a basic design to start. Next, we will do more enhancements for this system, including enlarging the FOV, increasing the Entrance Pupil Diameter (equivalent to increasing the eye box), and make waveguide thinner.

 

      

References

1. Konica Minolta Technology Report Vol.1 (2004)

2. OpticStudio help files

Next Article: Modelling a holographic waveguide for Augmented Reality (AR) systems: part 2

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