Apodized Volume Grating Refractive Index Profile
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Solution Overview
Problem
Pupil-replicating lightguides in near-eye displays suffer from optical crosstalk and image ghosting due to the narrow angular range of volume Bragg gratings, leading to reduced image contrast and clarity.
Innovation Solution
Apodizing the refractive index profile of volume gratings in the direction of thickness using photochemical or chemical processes, where apodization light reduces the amplitude of periodic refractive index variations, either before, during, or after the grating formation, to minimize optical crosstalk and enhance angular range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If volume Bragg gratings are used in pupil-replicating lightguides, then high diffraction efficiency is achieved, but optical crosstalk and image ghosting occur due to narrow angular range
Solution Approach 1:
The patent applies local quality by varying the refractive index modulation amplitude across the grating thickness. The apodization profile creates different local properties: strong modulation in the center region for high diffraction efficiency, and reduced modulation at the boundaries to minimize angular sensitivity and crosstalk. This spatial variation in grating strength resolves the contradiction between efficiency and angular range.
Solution Approach 2:
The patent introduces dynamic characteristics by creating a continuously varying grating profile rather than a uniform structure. The apodized profile dynamically adjusts the diffraction characteristics across the grating depth, allowing the system to maintain high efficiency for on-axis light while progressively reducing sensitivity to angular deviations, thereby expanding the operational angular range.
2Ease of manufacture
If uniform refractive index profile is used in volume gratings, then manufacturing simplicity is maintained, but optical crosstalk and ghosting reduce image contrast
Solution Approach 1:
The patent changes the refractive index profile parameter from uniform to apodized (non-uniform) distribution. By modifying the amplitude of periodic refractive index variations across the grating thickness, the invention improves image contrast and eliminates ghosting while maintaining compatibility with existing photopolymer fabrication processes, thus resolving the contradiction between manufacturing simplicity and optical quality.
3Productivity
If photopolymerization is performed uniformly throughout the photopolymer layer, then grating formation is efficient, but refractive index contrast at boundaries causes optical artifacts
Solution Approach 1:
The patent applies preliminary action by exposing the photopolymer layer to apodization light before or during the main grating formation exposure. This pre-conditioning creates a spatially varying sensitivity profile that prevents uniform polymerization, instead establishing the desired apodized refractive index profile from the beginning, thereby avoiding boundary artifacts while maintaining overall grating formation efficiency.
Solution Approach 2:
The patent introduces apodization light as an intermediary element that mediates the photopolymerization process. This intermediary illumination pattern modulates the polymerization reaction spatially, creating the apodized profile that eliminates optical crosstalk and ghosting while allowing the main grating formation to proceed efficiently through the photopolymer layer.
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
The apodization of volume gratings reduces optical crosstalk and ghosting, resulting in improved image contrast and clarity by expanding the angular range and preventing incorrect diffraction, thus enhancing the overall performance of pupil-replicating lightguides in near-eye displays.
Implementation Method 1
exposing a photopolymer layer to grating forming light for forming periodic refractive index variations in the photopolymer layer
Implementation Method 2
apodization light reduces the amplitude of periodic refractive index variations, either before, during, or after the grating formation
Data Source
AI summary
A grating coupler may be fabricated by exposing a photopolymer layer to grating forming light for forming periodic refractive index variations in the photopolymer layer. The photopolymer layer may be exposed to apodization light for reducing an amplitude of the periodic refractive index variations in a spatially-selective manner. The apodization may also be achieved or facilitated by subjecting outer surface(s) of the photopolymer layer to a chemically reactive agent that causes the refractive index contrast to be reduced near the surface(s) of application. The apodized refractive index profile of the gratings facilitates the reduction of optical crosstalk between different gratings of the grating coupler.