At the Frontiers in Optics and Laser Science conference, Dispelix presented an analysis of the effects of laser light in a waveguide. This monochromatic light can enable high-efficiency achromatic gratings, but coherence also causes some unwanted effects. For very narrow bandwidth sources, there can be speckle, but also Newton rings may appear as the light interferes at the many overlapping exit pupil expansion gratings.

The solution that Dispelix suggests includes a thicker waveguide – something that designers would prefer not to have, however. The thicker waveguide creates longer path lengths for the TIR reflections in the waveguide which means less interference, plus fewer interactions with the boundaries (and higher MTF). In addition, developers need to adjust the laser current, or other means to ensure a wide enough bandwidth to avoid Newton rings. A Full Width Half Maximum of at least 2 nm is suggested.

Goertek is developing SiC materials for waveguides for AR. SiC has been used for power electronics, especially in electric vehicles, but it is not the same type of SiC needed for waveguides, which need to have much higher purity and be fully transparent. The high index of refraction of the material, 2.65, means it can support wider field of view designs. Meta’s Orion headset premiered the potential with a 70-degree FOV. But optical grade SiC is very expensive, which is why Goertek is trying to develop solutions that can be sourced for lower cost.

Their paper described some general waveguide improvements like slanting the input grading to reduce reflections from the light engine, which can lead to ghost images. Such structures are a bit difficult in SiC, but Goertek says they can now fabricate them. To reduce diffractive rainbow effects, the grating pitch should be less than 200 nm in SiC. The optical grade of SiC is very important too as impurities and defects lead to absorption of light, reducing efficiency.

The company then presented data on several waveguide designs with 30-degree FOV on SiC; 25- and 22-degree FOVs on glass, and a 25-degree FOV on polymer.

Goertek Specs for a 30-degree FOV SiC Waveguide