A diffractive waveguide-based optical combiner is one of the most commonly used elements to construct head mount AR displays. This paper proposes a design method for a compact and wide field-of-view layout for an AR-applicable diffractive waveguide. The layout design utilizes multiplexed 1D gratings as both the exit pupil expander (EPE) and the out-coupler, combined with minimum-sized 2D grating to improve image uniformity and deliver a better user experience. The design outcome is optimized and simulated in LightTools and RSoft. The result has indicated the feasibility of a compact and 60 degrees FOV design of a diffractive waveguide AR display with a 12 mm × 10 mm eye box.
For a diffractive waveguide-based augmented reality display, we demonstrate a rapid and straightforward method to define the most compact layout of an exit pupil expander (EPE) from an analysis of diffractive optical elements in kspace. The idea is to consider all k-vectors coming from in-coupling to out-coupling areas according to the grating theory, and with the geometrical tracing of the EPE region to find the optimal layout. For a working wavelength of 532 nm, 40 degrees field of view display, our method is able to fit the layout into a circular shaped area with a minimal radius or to optimize for the minimal EPE area. Furthermore, a web-based interactive application demonstrating the EPE layout calculation is provided as a layout design tool.
Diffractive gratings, one of the most promising near-eye display designs, has been challenging to satisfy the essential features without making sacrifices in output efficiency or the direction of incidence due to the low diffraction efficiency of higher modes at normal incidence. Here, we propose dielectric metagratings that support light deflect to a larger angle with high efficiency within the field of view of 54 degrees. In this paper, through the proposed model of the eye-imaging system, we present optimal designs of metagratings for diffractive total internal reflection combiner or diffractive exit pupil expanders.
The paper demonstrates spatial filtering in reflection based on meta-mirrors, composed of periodic subwavelength gratings. The periodic modulation of the refraction index on the sub-micron scale exhibits optical beam shaping with transverse invariance. The paper starts with a theoretical model for the proposed metamirrors based on multiple scattering theory. The results from the proposed analytical model coincide with the that from FDTD simulations. The analytical studies in the paper show that the filtering performance is enhanced by structured cavities where Mie resonances occur. Observations of different Mie resonances for varying units of the meta-mirrors are also presented in this paper. The metamirror may serve as a versatile tool for narrowing beam with high efficiency and transverse invariance.
Recently, the principle of flat focusing based on one-dimensionally chirped dielectric mirrors has been proposed and experimentally demonstrated. The flat chirped mirror causes anomalous diffraction of the beam during reflection, opposite to the normal diffraction in free space propagation. The anomalous diffraction compensates the normal diffraction of the beam resulting in focusing after reflection. For a better focusing performance and for a larger near-field focal distance, a stronger anomalous diffraction is required. We show that the anomalous diffraction can be enhanced by introducing a defect layer in the chirped mirror, as the structure becomes similar to a Gires–Tournois interferometer. The focal distance can be substantially increased due to the defect layer. In our specific structure, the focal distance shows an increase from 19 to 39 μm, numerically.
A negative Goos–Hänchen (GH) shift of light beam reflecting from planar microphotonic structures consisting of two parallel dielectric subwavelength diffraction gratings is predicted. The phenomenon occurs because the energy flow of the mode guided in the subwavelength structure is predominantly directed in the opposite direction to the energy flow of the incident beam. The numerical calculations show that the negative GH shift can reach approximately six times that of the wavelength in realistic structures, so that the phenomenon can be experimentally observed. In addition to the negative lateral shift, the focusing of the beam in reflection from the grating is also observed as a secondary effect.
We propose and demonstrate a novel functionality of chirped mirror for monochromatic light beams: a diffraction control in reflections resulting in focusing or imaging of beams. The chirped mirrors, commonly used for manipulating temporal profiles of pulses, here are applied for manipulating the spatial dispersion of a monochromatic beam. By penetrating into dielectric layers of chirped mirror, the monochromatic beam experiences the negative diffraction, therefore the beam diverge propagating in front and behind the structure in normal diffraction region can be compensated inside this structure with negative diffraction. The result is focusing or imaging of the reflected beam from a flat interface of chirped mirror without optical axis.
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