Framework for optimizing AR waveguide in-coupler architectures.

Waveguide displays have been shown to exhibit multiple interactions of light at the in-coupler diffractive surface, leading to light loss. Any losses at the in-coupler set a fundamental upper limit on the full-system efficiency. Furthermore, these losses vary spatially across the beam for each field, significantly decreasing the displayed image quality. We present a framework for alleviating the losses based on irradiance, efficiency, and MTF maps. We then derive and quantify the innate tradeoff between the in-coupling efficiency and the achievable modulation transfer function (MTF) characterizing image quality. Applying the framework, we show a new in-coupler architecture that mitigates the efficiency vs image quality tradeoff. In the example architecture, we demonstrate a computation speed that is 2,000 times faster than that of a commercial non-sequential ray tracer, enabling faster optimization and more thorough exploration of the parameter space. Results show that with this architecture, the in-coupling efficiency still meets the fundamental limit, while the MTF achieves the diffraction limit up to and including 30 cycles/deg, equivalent to 20/20 vision.

Waveguide-based augmented reality displays: a highlight.

Augmented reality (AR), which emerged in the 1960s, remains a focal point of interest given its capacity to overlay the real world with digitally presented information through optical combiners. The prevalent combiner, commonly known as the waveguide in the AR literature, is prized for its compact design and generous eyebox-essential elements in human-centric technology. Nonetheless, these combiners encounter unique challenges in meeting various other requirements of the human visual system. This paper highlights a recent review of technological advancements and presents a forward-looking perspective on the future of AR technology.

Metagrating meets the geometry-based efficiency limit for AR waveguide in-couplers.

Recently, augmented reality (AR) displays have attracted considerable attention due to the highly immersive and realistic viewer experience they can provide. One key challenge of AR displays is the fundamental trade-off between the extent of the field-of-view (FOV) and the size of the eyebox, set by the conservation of etendue sets this trade-off. Exit-pupil expansion (EPE) is one possible solution to this problem. However, it comes at the cost of distributing light over a larger area, decreasing the overall system's brightness. In this work, we show that the geometry of the waveguide and the in-coupler sets a fundamental limit on how efficient the combiner can be for a given FOV. This limit can be used as a tool for waveguide designers to benchmark the in-coupling efficiency of their in-coupler gratings. We design a metasurface-based grating (metagrating) and a commonly used SRG as in-couplers using the derived limit to guide optimization. We then compare the diffractive efficiencies of the two types of in-couplers to the theoretical efficiency limit. For our chosen waveguide geometry, the metagrating's 28% efficiency surpasses the SRG's 20% efficiency and nearly matches the geometry-based limit of 29% due to the superior angular response control of metasurfaces compared to SRGs. This work provides new insight into the efficiency limit of waveguide-based combiners and paves a novel path toward implementing metasurfaces in efficient waveguide AR displays.

Method for minimizing lens breathing with one moving group.

Lens breathing in movie cameras is the change in the overall content of a scene while bringing subjects located at different depths into focus. This paper presents a method for minimizing lens breathing or changing angular field-of-view while maintaining perspective by moving only one lens group. To maintain perspective, the stop is placed in a fixed position where no elements between the scene and the stop can move, thus fixing the entrance pupil in one location relative to the object fields. The result is perspective invariance while refocusing the lens. Using paraxial optics, we solve for the moving group's position to focus on every object position and eliminate breathing between the minimum and maximum object distances. We investigate the solution space for optical systems with two positive groups or a positive and a negative group (i.e., retrofocus and telephoto systems). We explain how to apply this paraxial solution to existing systems to minimize breathing. The results for two systems altered using this method are presented. Breathing improved by two orders of magnitude in both cases, and performance specifications were still met when compared to the initial systems.