Variable Period Volume Phase Gratings for HMD Waveguides
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Solution Overview
Problem
Head-mounted displays used for virtual and augmented reality often face issues with optical performance, such as undesirable color shifts, efficiency losses, and brightness variations due to the dependence of diffraction efficiency on incident angle, which can limit the field of view and overall image quality.
Innovation Solution
The use of volume phase holographic gratings with constant pitch and variable period in the optical system of head-mounted displays, where the period is tailored to maximize diffraction efficiency for different field angles, ensuring high efficiency across a range of angles and wavelengths, and incorporating additional gratings for vertical field of view expansion to redirect out-of-view light towards the user's eyes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional diffraction gratings with constant period are used, then the device complexity is low, but the diffraction efficiency varies significantly with incident angle causing color shifts and brightness variations
Solution Approach 1:
The patent applies local quality by varying the grating period locally across different spatial regions. Each region of the grating is designed with a specific period optimized for its corresponding field angle, allowing the grating to maintain high diffraction efficiency across the entire field of view while managing complexity through a systematic gradient structure.
Solution Approach 2:
The patent implements parameter changes by continuously varying the grating period parameter as a function of position. This allows the grating to adapt its diffraction characteristics to different incident angles, maintaining uniform diffraction efficiency and preventing color shifts across the field of view.
2Adaptability or versatility
If the field of view is expanded to accommodate broader viewing angles, then the adaptability improves, but the diffraction efficiency decreases due to angle dependence
Solution Approach 1:
The patent uses local quality to optimize each region of the grating for its specific angular range. By tailoring the grating period to match the incident angles of different field regions, the system achieves high diffraction efficiency across the entire expanded field of view rather than optimizing for a single angle.
Solution Approach 2:
The patent applies dynamics by making the grating structure adaptable to different incident angles through its variable period design. The grating dynamically adjusts its diffraction characteristics across its surface to maintain efficiency as light enters from various angles within the expanded field of view.
3Ease of manufacture
If the grating period is optimized for a specific wavelength, then the manufacturing precision is simplified, but the performance degrades for broadband illumination sources
Solution Approach 1:
The patent applies parameter changes by varying the grating period across its surface rather than using a constant period. This allows the grating to maintain optimal diffraction conditions across a broader wavelength range by compensating for wavelength-dependent effects through spatial variation of the period parameter.
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 maintains high diffraction efficiency and uniform image quality across different angles of incidence, preventing color shifts and brightness variations, and allows for both broadband and narrow spectrum illumination sources, enhancing the overall optical performance and user experience.
Implementation Method 1
Holographic gratings in the waveguide may have fringes with constant pitch and variable period. The period at a given portion of the grating may be Bragg-matched to maximize diffraction efficiency for light of a given wavelength and incident angle.
Implementation Method 2
The period at a given portion of the grating may be Bragg-matched to maximize diffraction efficiency for light of a given wavelength and incident angle.
Implementation Method 3
An optical system with one or more waveguides and input and output coupler systems may be used to distribute the images to a user.
Data Source
AI summary
An electronic device may have a display system that produces images. The display system may have one or more pixel arrays such as liquid-crystal-on-silicon pixel arrays. Images from the display system may be coupled into a waveguide by an input coupler and may be coupled out of the waveguide using an output coupler. The input and output couplers may be formed from volume phase holographic gratings. An additional grating may be used to shift light that would otherwise pass above or below the user's field of view towards the viewer. Holographic gratings in the waveguide may have fringes with constant pitch and variable period. The period at a given portion of the grating may be Bragg-matched to maximize diffraction efficiency for light of a given incident angle.