Selective Overcoat for Waveguide Output Grating

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Solution Overview

Problem

Diffractive waveguides in head-mounted displays and helmet-mounted displays are susceptible to environmental conditions and physical damage, leading to performance issues such as rainbow effects and reduced optical throughput, due to the sensitivity of gratings to humidity, contaminants, and physical stress.

Innovation Solution

A surface-relief diffraction grating assembly with a substrate and photoresist gratings, where a cross-linkable overcoat material is applied only on the output grating, providing protection without affecting the input grating, and characterized by low absorption, haze, and refractive index within specific ranges to maintain optical clarity and durability.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If an overcoat is applied to protect the gratings from environmental conditions and physical damage, then the durability and optical throughput are improved, but the manufacturing complexity and process difficulty increase

Engineering Contradiction:

Improvegrating protectionVSAvoidmanufacturing process

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The patent applies overcoat material selectively only to the output grating region while leaving the input grating uncovered. This localized application protects the output grating from environmental damage and rainbow effects without affecting the input grating's optical coupling efficiency, thereby resolving the contradiction between protection needs and manufacturing complexity

Inventive Principle:

Principle #3Local quality

Solution Approach 2:

The waveguide surface is divided into distinct regions (input grating area and output grating area) with different overcoat requirements. The input grating remains uncovered to maintain optimal light coupling, while the output grating receives protective overcoat coverage, segmenting the protection strategy according to functional needs

Inventive Principle:

Principle #1Segmentation

2Object-affected harmful factors

If an overcoat is applied to reduce rainbow effects and improve optical uniformity, then the visual quality is improved, but the absorption and haze increase

Engineering Contradiction:

Improverainbow effectsVSAvoidoptical absorption

Core Design Contradiction:

Object-affected harmful factorsVSLoss of energy

Solution Approach 1:

The patent carefully controls the optical parameters of the overcoat material, specifically selecting materials with refractive indices between 1.30-1.70 and minimizing absorption coefficients. By optimizing these parameters, the overcoat reduces rainbow effects through improved optical uniformity while maintaining sufficient light transmission, thus resolving the contradiction between visual quality and energy loss

Inventive Principle:

Principle #35Parameter changes

3Use of energy by moving object

If the overcoat material has high refractive index to improve optical coupling, then the light extraction efficiency is improved, but the absorption increases

Engineering Contradiction:

Improvelight extraction efficiencyVSAvoidmaterial absorption

Core Design Contradiction:

Use of energy by moving objectVSLoss of energy

Solution Approach 1:

The patent identifies and specifies an optimal refractive index range of 1.30-1.70 for the overcoat material that balances two competing requirements: achieving sufficient light extraction efficiency from the waveguide while minimizing absorption losses. This parameter optimization resolves the contradiction between coupling efficiency and energy loss

Inventive Principle:

Principle #35Parameter changes

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 solution enhances the durability and optical performance of waveguides by protecting the output grating while maintaining the functionality of the input grating, reducing rainbow effects and improving overall display quality in augmented and virtual reality systems.

Implementation Method 1

a substrate for guiding image light therein by total internal reflection (TIR)

Methodology Applied

Scientific EffectTotal internal reflection: Total Internal Reflection

Implementation Method 2

the overcoat precursor material being a photopolymerizable material; selectively exposing the overcoat precursor material on the output grating to UV light to polymerize the overcoat precursor material

Methodology Applied

Scientific EffectPhotopolymerization: Photopolymerisation

Data Source

PatentUS11105982B2Imageable overcoat for an optical waveguide and process for making the same

Publication Date: 2021.08.31 META PLATFORMS TECHNOLOGIES LLC

AI summary

An imaging waveguide for a visual display includes a substrate for guiding image light therein by total internal reflection (TIR). An input grating is supported by the substrate for coupling the image light into the imaging waveguide. An output grating is supported by the substrate and spaced apart from the input grating for coupling the image light guided in the substrate out of the imaging waveguide for observation by a user. A gap filling overcoat is formed on and within the output grating, but not on or within the input grating. The material is characterized by a refractive index between 1.40 and 1.80 at 500 nm, absorption between 0% and 1% in the visible region of the electromagnetic spectrum, and % haze between 0% and 0.2% in the visible region of the electromagnetic spectrum.