November 21, 2019

Holographic optical elements for alternate reality and other systems

Holographic optical elements for alternate reality and other systems

A holographic optical element is a thin sheet of photosensitive material produced using holographic imaging processes that can mimic the functions of an optical component. Common uses for holographic optical elements include laser diodes, lidar, heads-up displays, smart glasses, AR/VR headsets, and other technological innovations requiring thinner and lighter optical designs. 


How does a hologram work? 

A hologram is an interference pattern recorded on a substrate carrying a photosensitive emulsion. The hologram is developed and re-illuminated by a reconstruction beam, which reveals the final, reconstructed image. A hologram is created in the construction step, when “fringe planes” are formed and recorded on holographic film, then the hologram effectively becomes a diffraction grating in the playback step. 




Spotlight: Augmented reality systems and holograms

AR systems have begun using holograms to couple light into waveguides which relay light from a display to the eye. Waveguides are useful because they are largely transparent and don’t block light from the real world. Read how our customer DigiLens is revolutionizing waveguide optics here.

Most AR waveguides rely on volume holograms where many fringe planes are recorded, which extinguish undesired orders and ray angles. 

Advantages include:  

  • Selectivity: Only light with the desired wavelength and incident angle is reflected, so it’s good for see-through designs. 
  • Efficiency: Light is largely directed into a single diffraction order.
  • Compactness: Holograms work like a lens system but are thin as a plane.

Challenges include: 

  • Highly dispersive: RGB is diffracted at significantly different angles.
  • Manufacturability: Diffraction efficiency depends on the resolution of the recording film, laser stability, etc.
  • Reliability: Hologram materials are sensitive to temperature and humidity. 
  • Swelling and shrinking: Spacing of the recorded fringes might change during the hologram manufacturing process. 

The ability of volume holograms to diffract rays to any desired angle, and their wavelength and angular selectivity allows the creation of optical systems that are more compact and lighter than traditional designs. However, accurately accounting for the characteristics of volume holograms makes the optical design significantly more challenging as factors such as diffraction efficiency and material shrinkage are usually analyzed separately and not directly integrated with a parametric ray-tracing model. Therefore, advanced analyses like image simulation and comprehensive optimization, including the volume hologram’s properties cannot be effectively performed for the volume hologram designs.


Modeling holograms in OpticStudio

OpticStudio allows engineers to fully simulate holographical imaging systems and model several types of holograms in OpticStudio in different shapes including plane, spherical, conical, toroidal, or aspheric. All holograms have specific parameters defining construction beams, construction wavelength, shape of the substrate, and diffraction order.

OpticStudio treats holograms as infinitely thin surfaces that alter the phase of the ray. Rays landing on the hologram diffract per the hologram equation below: 

It’s important to note that the larger the angle between construction beams, the smaller the spacing. The direction of beam propagation matters, since if engineers reverse one beam, fringe direction changes and the spacing becomes smaller. 

Traditionally, when designing an imaging system using volume hologram elements, careful pre-analysis is often required to make sure light from the designed field of view can be diffracted with high efficiency. The result of full images usually can only be observed from experiments in past. However, with OpticStudio, we are able to simulate the resulted image.

For example, the images below show image simulation for a simple AR waveguide design with a holographic optical element. Worse image quality is observed when considering the diffraction efficiency.

Figure 1: The image simulation with and without considering the diffraction efficiency of the hologram surface for a system that is optimized without considering diffraction efficiency.

The diffraction efficiency can be affected by several factors, such as the thickness of the hologram and the polarization state of the incident beam. 

Including diffraction efficiency in the ray-tracing process enables image simulation and provides the ability to optimize the system with all its complicated considerations. In OpticStudio, this means engineers only need to add more targets in the optimization merit function for the transmission of the chief ray through the whole system for each field and add one more variable, hologram thickness.

 

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