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Single-layer achromatic diffractive optical waveguide in AR glasses

Author:

AR glasses seem to have developed rapidly in recent years; many people believe this is the device most likely to replace mobile phones in the future. There are also many consumer market manufacturers in China, such as RayNeo, XREAL (Beijing Unikeen Technology Co., Ltd.), Rokid (Hangzhou Rokid Technology Co., Ltd.), INMO (Sichuan INMO Technology Co., Ltd.), etc.

A core component in AR glasses is the optical waveguide, which plays a crucial role in augmented reality devices. Optical waveguides utilize the principle of total internal reflection in optics to transmit information, providing users with an immersive visual experience.

Key indicators of optical waveguides include the following aspects:

Field of View (FOV) : determines the range of virtual images a user can see; a larger FOV provides a broader visual experience.
Eye Box : refers to the range within which the user's eye can move freely; a larger eye box improves wearing comfort.
Diffraction Efficiency : affects image brightness and clarity; efficient diffractive waveguides reduce light loss and improve display effects.

These indicators collectively determine the performance of diffractive optical waveguides, which in turn affects the overall experience of AR glasses.

The more mainstream implementations of optical waveguides are geometric waveguides and diffractive waveguides . Geometric waveguides have excellent optical performance with virtually no noticeable chromatic aberration issues. However, the manufacturing yield is very low, facing significant mass production challenges ( To achieve eye-tracking expansion, geometric waveguides require many film layers with different reflection/transmission efficiencies inside the waveguide, making the film system very complex ).

Currently, diffractive waveguides are an effective implementation path. Their processing is not as complex as geometric waveguides, but they have noticeable chromatic aberration issues. This is because the inherent dispersion of diffraction itself makes it difficult to achieve ideal achromatic results. Many efforts are now focusing on using materials like SiC, as its high refractive index provides a larger design space and the possibility of stronger dispersion compensation. At the same time, there is much research on non-local structures, which may potentially break through the technical bottlenecks of insufficient dispersion compensation and mode impedance mismatch caused by the limited resonance bandwidth and scattering ability of traditional local structures.

Currently, much research focuses on achieving achromatic information image propagation with highly integrated, thin and light single-layer diffractive optical waveguides. Several similar studies have recently emerged. The author also attempted to design similar structures, with the diffraction efficiency as follows:

I've always had a question about diffraction efficiency: Is it true that current diffractive optical waveguides have low efficiency, or is it because they require eye-tracking expansion, which inherently necessitates lower single-pass output efficiency? Furthermore, in practical use, is the uniformity of diffraction efficiency also highly demanded, otherwise the rainbow effect would be very obvious? Should the priority of three-color efficiency uniformity in design be higher than diffraction efficiency? ？

Is there significant meaning in simply designing achromatic single-layer waveguides with extremely high diffraction efficiency? The author has always considered designing diffractive optical waveguides by setting refractive index discontinuities within periodic structures and optimizing the positions of these discontinuities to find a suitable waveguide structure. However, it's unclear what a proper objective optimization function would be (mainly because the author does not understand the priority of diffractive waveguide performance indicators in practical applications).

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