Mid-air display (MAD) technology is attractive to practitioners nowadays. The interest is due to the potential application in consumer products with embedded floating image displays, like smartphones, smartwatches and dock-stations, and as a part of new holographic user interfaces for safe and contactless control. Some of the problems to solve on the way to compact and light efficient MAD include small field of view, small image size, low image resolution, low image contrast, absence of image magnification, low perceived sense of depth, etc. In order to overcome these challenges, the authors propose a MAD based on a DMD pico-projector and a DOE waveguide with a positive Fresnel lens, placed near the out coupling aperture of the DOE waveguide. The developed MAD forms a real image with a positive relief from the display surface so that the viewer perceives this image floating in front of it, at the back focal plane of the Fresnel lens. For mid air image with ≥ 1-inch diagonal with 57 mm image relief horizontal field of view was 35 degrees, with image brightness 100 cd/m2 . The proposed mid-air image display has a compact form factor with dimensions 100 mm × 50 mm × 3 mm, without dimensions of the DMD pico-projector. It can be used in consumer products to provide a new kind of experience including contactless holographic user interaction
We report on a novel state-of-the-art diffraction optical elements (DOE) based waveguide architecture for aug- mented reality (AR) display with increased field of view and method for analytical design of such an architecture. The effectiveness of the architecture results from the multiple usage of the same propagation directions inside the waveguide by different field of view parts. Unlike in previous solutions, where such approach would lead to crosstalk generation, the proposed architecture different field of view parts are propagated in different waveguide locations, separated by the corresponding DOEs. The architecture can be applied either for increasing the verti- cal field of view size or the horizontal field of view size with compensation of chromatic dispersion resulting from the diffraction. The architecture configuration, analytical derivations of the DOEs parameters, and modeling results are discussed. The architecture satisfies market demands for the form-factor, size and weight, as well as allows up to four times increase of the field of view size in comparison with the conventional solutions. For the DOEs refractive index of 1.5, the architecture provides 48x44 degrees white-light field of view within two waveguides and 56x56 degrees white-light field of view within three waveguides. For the DOEs refractive index of 1.9, the architecture provides 58x58 degrees white-light field of view within only one waveguide.
The mismatch between positions of virtual images and a see-through view constitute a serious problem in virtual and augmented reality optical systems with a single projection plane. These issues may lead to a user’s discomfort: eye fatigue, headache and nausea. In order to solve these problems a tunable lens forming several projection planes at different locations can be used. Developed varifocal lens consists of two tunable liquid crystal cells. The first cell for fine adjustment varies optical power from 1 D to 3 D, the second cell for coarse adjustment varies power from 0.25 D to 1 D. The total dioptric range is -4 D … +4 D with an equidistant step of 0.25 D that forms 33 projection planes. Electrode pattern made of indium zinc oxide consists of rings corresponding to Fresnel zones, each zone is divided to multiple subzones. In order to minimize the number of control electrodes (bus lines) and keep high diffraction efficiency, the bus lines shunt together all of the corresponding sub-zones in all of the zones. Developed lens is tested with AR glasses based on a holographic waveguide. Displacement of virtual image from 250 mm to 1 meter is demonstrated.
Augmented reality (AR) systems are of huge interest for last decade since they are predicted to be the next generation of mobile devices for consumers. One of the key parameters in terms of AR systems properties is the field of view. The best performance in this regard is shown by DOE/HOE-based planar-waveguides systems since they can provide the widest field of view among other approaches even with the simplest architecture. However it is still not wide enough for consumers, so more complex architectures are created. In this work, a novel approach for reaching wide field of view is proposed. It is based on the eyebox magnification in two directions by two different waveguides systems. The first system provides magnification along the axis with wider field of view and consists of waveguides inclined along the field of view central beam with HOE-based 1D gratings, providing the TIR diffraction in both +1 and -1 orders. The TIR condition in this case is reached more easily because of inclination, so the wider angular spectrum can be transferred. The second system provides magnification along the axis with narrower field of view and consists of conventional HOE-based periscope system with in-coupling and out-coupling zones. The system working principle, HOEs specifications, main advantages, challenges and solutions are discussed. The proposed system allow 60-degrees diagonal field of view for the white (RGB) color.
We devote this article to the spectral analysis of volume holograms recorded in materials with diffusion-based formation mechanisms. Two media of different natures were examined: polymer material with dye molecules photoattachable to macromolecules (PQ-PMMA) and photochromic additively colored fluorite crystal (CaF2). Differential spectra of holograms refractive index and absorbance were determined by two approaches: numerical approximation of both spectral and angular hologram response (selectivity hypercontour) by coupled-wave theory and processing hologram spectra by Kramers–Kronig dispersion relations. The principles of the two approaches, results of their application, and advantages and disadvantages are discussed. Kramers–Kronig relations can be usefully applied in cases where the nature of holograms formation is well studied, whereas the hypercontour approach is indispensable for obtaining the values of absorbance and refractive index modulation; therefore, a combination of both the techniques is a powerful tool for fundamental investigation of the hologram formation mechanisms.
The article is devoted to the spectral analysis of volume holograms recorded in materials with diffusion-based formation mechanisms. Two media of different nature were examined: polymer material with photosensitive macromolecules (PQ-PMMA) and additively colored solid-state crystal (CaF2). Differential spectra of holograms optical parameters were determined by means of two approaches: numerical approximation of both spectral and angular hologram response (selectivity hypercontour) by Coupled wave theory and processing hologram spectra by Kramers-Kronig dispersion relations. The operation principles, experimental performance, determination results, advantages and disadvantages, as well as validity limits were discussed for both the approaches. Kramers- Kronig relations are operable tool in cases where the nature of holograms formation is well studied, whereas hypercontour approach is indispensable for investigation of hologram formation mechanisms.
Volume holographic gratings, both transmission and reflection-type, may be employed as one-dimensional pho- tonic crystals. More complex two- and three-dimensional holographic photonic-crystalline structures can be recorded using several properly organized beams. As compared to colloidal photonic crystals, their holographic counterparts let minimize distortions caused by multiple inner boundaries of the media. Unfortunately, it’s still hard to analyze spectral response of holographic structures. This work presents the results of thick holographic gratings analysis based on spectral-angular selectivity contours approximation. The gratings were recorded in an additively colored fluorite crystal and a glassy polymer doped with phenanthrenequinone (PQ-PMMA). The two materials known as promising candidates for 3D diffraction optics including photonic crystals, employ diffusion-based mechanisms of grating formation. The surfaces of spectral-angular selectivity were obtained in a single scan using a white-light LED, rotable table and a matrix spectrometer. The data expressed as 3D plots make apparent visual estimation of the grating phase/amplitude nature, noninearity of recording, etc., and provide sufficient information for numerical analysis. The grating recorded in the crystal was found to be a mixed phase-amplitude one, with different contributions of refractive index and absorbance modulation at different wavelengths, and demonstrated three diffraction orders corresponding to its three spatial harmonics originating from intrinsically nonlinear diffusion-drift recording mechanism. Contrastingly, the grating in the polymeric medium appeared purely phase and linearly recorded.
Amplitude and phase contributions to mixed volume holographic gratings were extracted from measured contours of angular selectivity. Holograms for the investigation were recorded in the glassy polymer material with phenan-threnequinone (PQ) using the DPSS CW laser (532 nm) and then self-developed due to molecular diffusion of PQ, reaching diffraction efficiency about 40%. Refractive index and absorbance modulation amplitudes of those holograms were obtained as adjustable parameters from theoretical equations by fitting angular dependencies of zeros and 1st orders diffraction efficiency measured at 450, 473, 532, and 633 nm at the different stages of hologram development. Mixed gratings manifest themselves in asymmetrical transmittance selectivity contours with one minimum and one maximum shifted with respect to the Bragg angle, while symmetrical contours with a minimum or a maximum at the Bragg angle are characteristic of pure phase and amplitude gratings, respectively. In the course of a hologram development, it converts from a predominantly amplitude-mixed to almost purely phase one in the case of readout using a light within the absorption band of PQ and maintains the phase nature besides it. The value of refractive index amplitude is ranging from 5×10-6 to 10-4 and the value of absorbance amplitude is up to 140 m-1.
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