DigiLens Switchable Bragg Grating Waveguide Optics for Augmented Reality Applications

Room GH

While waveguides are gaining acceptance as a display technology, delivering wide angle color, with a comfortable eye box in an acceptable form factor, remains a challenge. DigiLens’s Switchable Bragg Grating (SBG) based waveguides enable switchable, tuneable and digitally reconfigurable color waveguide displays with a field of view and form factor surpassing those of conventional refractive and competing waveguide technologies. DigiLens waveguides can be laminated to integrate multiple optical functions such as display and eye-tracking, seamlessly and with minimal impact on form factor. Our current color AR HUD reference design achieves a 40° diagonal field of view with launch products currently entering production. DigiLens’s development roadmap is directed at increasing the field of view to 60° and beyond.

Key to the success of DigiLens’s waveguides is the company’s proprietary polymer and liquid crystal mixture which, by virtue of its uniquely high refractive modulation, offers unprecedented scope for fine-tuning the index modulation to provide any required combination of diffraction efficiency and angular bandwidth in a thin waveguide. The material has been further refined to provide high transparency with very low haze.

To address emerging AR design requirements DigiLens has developed a suite of grating capabilities called IP cores based on fold gratings, rolled k-vectors and other fundamental grating forms, which enable wide angle image content to be transmitted with high efficiency with a large eyebox using a minimal number of waveguide layers. Whether configured as an AutoHUD, MotoHUD or AeroHUD, DigiLens waveguides combine two components: an image generation module, essentially a pico projector, and a holographic waveguide for propagating and expanding the image vertically and horizontally before projecting it into the eyebox. Color is provided by a stack of monochrome red, green and blue waveguides, each further comprising a single passive grating layer capable of addressing the entire field of view. Each grating layer includes an input rolled K-vector grating, a fold grating, and an output grating, the grating layer being sandwiched by a pair of high index substrates to form a waveguide. In our reference designs, the output and fold gratings are passive while the input gratings are switched to eliminated color crosstalk. Rolling the K-vectors of the grating expands the effective angular bandwidth of the waveguide. Fold gratings which have fringes clocked in the waveguide plane at an angle to the input guided beam enable two-dimensional beam expansion in a single waveguide layer, which translates into lower cost, reduced haze, and improved brightness. The design of these complex grating architectures is complicated by their birefringent properties, requiring careful attention to polarization when modelling the propagation of light through the waveguide. This takes the design of DigiLens waveguides well beyond the frontiers of established ray-tracing codes. DigiLens has developed proprietary optical tools that run on the ZEMAX platform together with other models for modelling aspects of waveguide SBGs.

Our paper summarizes the key features of DigiLens waveguide technology and discusses our optical design methodology for designing and simulating the performance of DigiLens waveguides, giving examples from motorcycle, and autoHUD products.

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