Switchable Bragg Grating Waveguide for Compact Eyewear Display
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Solution Overview
Problem
Current wearable displays fail to provide a compact, lightweight, high-brightness, high-contrast information display with a large field of view and transparency, while maintaining a form factor similar to sunglasses, due to trade-offs in eye relief, field of view, and optical design limitations.
Innovation Solution
The use of electrically switchable Bragg gratings (ESBG) integrated with a holographic polymer-dispersed liquid crystal (HPDLC) layer and diffractive optical elements (DOEs) in a wearable display, allowing for adjustable diffraction efficiency and optimized optical design for enhanced transparency and image visibility.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If conventional wearable displays use reflective or diffractive visors with high magnification optics, then image brightness and resolution are improved, but device weight and form factor become cumbersome
Solution Approach 1:
The patent changes the optical parameters by using a waveguide with specific refractive index layers and grating structures that enable light coupling and extraction with high efficiency. This allows bright images to be displayed without requiring heavy conventional optics, as the waveguide structure itself performs the optical functions through carefully controlled refractive indices and grating geometries
Solution Approach 2:
The patent replaces heavy mechanical optics (lenses, mirrors, visors) with a planar waveguide structure that uses total internal reflection and diffraction grating principles. This substitution eliminates the need for bulky optical components while maintaining image brightness through efficient light guidance and extraction mechanisms embedded in a thin transparent substrate
2Measurement precision
If conventional wearable displays use high magnification optics, then image resolution is improved, but field of view and eye relief are reduced
Solution Approach 1:
The patent segments the optical functionality into distinct components within the waveguide: a coupling region for light input, a propagation region for light guidance, and a grating region for light extraction. This segmentation allows each region to be optimized independently, enabling high resolution through precise grating design while maintaining wide field of view through extended coupling areas and multiple extraction points
Solution Approach 2:
The patent transitions from conventional 3D optical paths with large diameter lenses to a planar 2D waveguide structure where light propagation occurs within a thin transparent substrate. This dimensional change enables simultaneous achievement of high resolution (through sub-wavelength grating features) and wide field of view (through extended planar coupling and extraction regions) without the trade-offs inherent in traditional optical systems
3Reliability
If conventional wearable displays use transparent substrates, then transparency is improved, but image contrast and brightness are reduced
Solution Approach 1:
The patent introduces diffractive gratings as intermediary structures within the waveguide that mediate between the transparent substrate and the displayed image. These gratings efficiently extract guided light at specific angles and locations, creating bright visible images while maintaining the inherent transparency of the substrate for non-diffracted light paths, thus achieving both transparency and image brightness simultaneously
Solution Approach 2:
The patent applies local quality by creating spatially varying grating structures within the waveguide that have different properties in different regions. The coupling region has optimized properties for light input, the propagation region maintains low loss, and the extraction region has grating structures optimized for bright image display. This local optimization allows the waveguide to maintain overall transparency while providing localized high-brightness image regions
4Volume of moving object
If wearable displays aim for compact form factor, then device size is reduced, but eye relief and pupil size are compromised
Solution Approach 1:
The patent replaces conventional mechanical optics that require large distances for focus and adjustment with a planar waveguide system where the optical path is defined by the waveguide structure itself. The grating-based light extraction mechanism creates virtual images at comfortable viewing distances without requiring physical eye relief, enabling compact form factor while maintaining ease of operation for various eye positions and pupil sizes
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution enables a compact, high-brightness, high-contrast display with a large field of view and transparency, maintaining a lightweight and non-distracting form factor, suitable for demanding applications like military and sports activities.
Implementation Method 1
Bragg gratings (also commonly termed volume phase grating or holograms), which offer the highest diffraction efficiencies
Implementation Method 2
diffractive optical elements (DOEs) in a wearable display, allowing for adjustable diffraction efficiency
Implementation Method 3
When an electric field is applied to the hologram via the electrodes, the natural orientation of the LC droplets is changed thus reducing the refractive index modulation of the fringes and causing the hologram diffraction efficiency to drop to very low levels
Implementation Method 4
During the recording process, the monomers polymerize and the PDLC mixture undergoes a phase separation, creating regions densely populated by liquid crystal micro-droplets
Implementation Method 5
The optical design benefits of DOEs are well known including unique and efficient form factors and the ability to encode complex optical functions such as optical power and diffusion into thin layers
Data Source
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AI summary
There is provided a wearable display comprising at least one Switchable Bragg Grating (SBG) device recorded in at least one Holographic Polymer Dispersed Liquid Crystal (HPDLC) layer. Each HPDLC layer is sandwiched between first and second transparent plates to which transparent electrodes have been applied. Each SBG device is characterised in that it provides a grating in a separate switchable region and is clear elsewhere. Each SBG device has a diffracting state and a non diffracting state. The transparent plates and HPDLC layers form a laminar structure which functions as a light guide. In one embodiment of the invention the display magnifies and forms a virtual image of information provided by an external image generator. In one embodiment of the invention the display and forms a virtual image of an image of information encoded in the SBG device.