Diffractive Exit Pupil Expansion for Compact Large-FOV Displays
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Solution Overview
Problem
Existing diffractive lightguides for wearable displays face challenges with large field of view (FOV) due to increased projector size and inefficiencies in beam expansion, particularly for applications requiring FOV greater than 40 degrees, leading to impractical dimensions and reduced efficiency.
Innovation Solution
A cascade of diffractive lightguides is employed, with angled planes to achieve two-dimensional light confinement, using a first lightguide for initial expansion and a second lightguide for further expansion, ensuring compact and efficient beam expansion without image distortions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Shape
If a 1D EPE lightguide is used to expand the exit pupil in one dimension, then the projector aperture stop can be reduced in size, but the in-coupler width becomes large which increases projector size and reduces coupling efficiency
Solution Approach 1:
The patent transitions from 1D exit pupil expansion to 2D expansion by adding a second lightguide element oriented perpendicular to the first. This dimensional expansion allows the in-coupler to remain compact while achieving sufficient exit pupil size for large FOV displays, resolving the contradiction between reduced aperture stop size and manageable in-coupler width
Solution Approach 2:
The patent divides the beam expansion function into two separate lightguide elements, each handling expansion in a different dimension. The first lightguide expands in one dimension while the second lightguide expands in the perpendicular dimension, allowing each component to maintain compact dimensions while collectively achieving large exit pupil
2Adaptability or versatility
If the in-coupler width is increased to support large FOV in 1D lightguides, then the FOV can be increased, but the projector size increases and coupling efficiency decreases
Solution Approach 1:
By implementing 2D exit pupil expansion through two perpendicular lightguide elements, the system achieves large FOV capability without requiring a proportionally large in-coupler width. The second dimension of expansion provides additional exit pupil area, allowing the in-coupler to remain compact while supporting FOV greater than 40 degrees
3Area of stationary object
If a separate lightguide is added in front of the in-coupler for exit pupil expansion, then the main lightguide in-coupler size can be reduced, but the first lightguide becomes too wide for practical applications
Solution Approach 1:
The patent orients the second lightguide element perpendicular to the first, creating 2D expansion. This dimensional change allows the first lightguide to remain narrow in its primary direction while the second lightguide provides expansion in the perpendicular direction, keeping both components within practical size limits for wearable displays
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 approach enables compact, efficient, and lightweight wearable displays with large FOV by confining light propagation, preventing mirror image out-coupling, and maintaining image quality.
Implementation Method 1
The lightguide element is arranged to confine propagation of light laterally in said first plane by reflections
Implementation Method 2
a diffractive exit pupil expander (EPE) that expands the in-coupled light beam in one or two directions
Data Source
Figure 1~2
Figure 3
Figure 4~5
AI summary
The invention relates to a diffractive exit pupil expander arrangement for display applications. The arrangement comprises a first lightguide element (51) comprising an exit pupil expander (53) and arranged in a first plane and a second lightguide element (41) comprising an in-coupler (42) and arranged in a second plane. The in-coupler is optically coupled with the exit pupil expander (53). Further, the first lightguide element (51) is arranged to confine propagation of light laterally in said first plane by reflections, and the first plane and the second plane are arranged at an angle (a) with respect to each other.