Flat Optics for Compact Augmented Reality Systems: Diffractive Optical Elements and Metasurfaces

Abstract

I denne masteroppgaven presenterer vi fire forskjellige flat-optikk (flat optics) implementeringer av to klassiske arkitekturer for utvidet virkelighet (AR): stråledeler-arkitekturen og friform-arkitekturen. Tradisjonelt er disse arkitekturene implementert ved hjelp av geometrisk optikk gjennom bruk av speil og refraktive linser. Som en konsekvens av dette blir systemene store og ubehagelige å bruke. Denne oppgaven vil demonstrere at det er mulig å lage betydelig mer kompakte systemer ved bruk av flat-optikk. Dette kan potensielt muliggjøre bruken av AR i forbruker- og industrimarkeder.

We present four different flat optics implementations of two classical augmented reality (AR) architectures: the beamsplitter architecture and the freeform architecture. Traditionally, these architectures are implemented using geometric optics through mirrors and refractive lenses. In consequence, the systems become bulky and uncomfortable. This thesis will demonstrate that it is possible to make significantly more compact systems using flat optics, potentially enabling AR for large consumer and industrial markets.

Three implementations of the beamsplitter architecture are presented: a blazed grating, an isosceles triangle grating, and a constant phase gradient metasurface with both multiwavelength and polarization control. Moreover, fabrication of the blazed and isosceles gratings is demonstrated using the novel method of grayscale photolithography.

Due to geometric and fabrication constraints, the shortest period for the diffraction gratings is found to be 4.5μm. The number of orders introduced by such a long period limits the FOV for the blazed grating to be less than 8.1°. Further, the real world view distortion is estimated to be 96.8%.

During fabrication of the blazed grating, it is discovered that the proximity effect can be used to fabricate an isosceles triangle grating with a period of only 1μm. This grating achieves an estimated FOV of 22.6° with a distortion of 11.6%. A simple diffraction efficiency measurement qualitatively validates the soundness of the simulations used in these estimates.

Of the three, the constant phase gradient metasurface achieves the best performance with FOV of 9.6° for three RGB wavelengths and an average distortion of only 6.6%. However, the metasurface is based on titanium dioxide, meaning that large-scale fabrication is currently not possible.

By combining the deflection and collimation function needed in an AR system, the freeform architecture becomes significantly more compact than the beamsplitter architecture. With this premise, the optical system achieves a decent resolution of 107 x 81 pixels over a 7.83 mm x 5.91 mm display. Combined with a field of view of 22.2° x 16.8°, the design achieves an angular resolution of 4.8 x 4.9 pixels per degree (PPD).