Impact of secondary interactions on in-coupler designs for thin waveguide combiners

The waveguide combiner architecture has become a popular choice for augmented reality systems. Comfort and social acceptability factors drive the desire for weight reduction in the next generation platforms, which in turn requires decreasing the thickness of the glass lightguide. This work focuses on optimizing the in-coupling efficiency of diffractive waveguide combiners for various lightguide thicknesses based on a closed-form expression that includes losses from secondary or back-coupling interactions at the grating. As the lightguide thickness decreases, these secondary interactions lead to a lower theoretical limit of in-coupling efficiency. On the other hand, we find that achieving this limit for thin waveguides does not necessarily require gratings with very high first-order diffraction efficiency, which is commonly a challenge for conventional thick lightguides. Instead, the best possible in-coupling efficiency can still be achieved as long as the grating exhibits high specular reflection (deflection into the zeroth diffraction order following TIR at the grating interface), even if the grating's first order diffraction efficiency is relatively low. We demonstrate that exploitation of this effect opens a larger design space compared to thick lightguides, which depend more heavily on the first-order diffraction efficiency. As a result, it is easier to operate near the theoretical limit for thin waveguides, partially offsetting the overall efficiency reduction. We illustrate our results with optimized slanted surface relief gratings that operate near the system efficiency limit. Finally, we explore structures with vertical sidewalls that might be simpler to fabricate, and through both direct and inverse design techniques, we demonstrate comparable performance to slanted gratings.