Composite Diffractive Optical Elements with Sub-wavelength Structures
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Solution Overview
Problem
Existing diffractive optical elements face challenges in achieving high diffraction efficiencies at high diffraction angles due to the limitations of surface-relief micro-structure design, which often result in high fabrication difficulties and low aspect ratios, leading to suboptimal performance.
Innovation Solution
A composite optical element comprising two transmission gratings with phase modulation profiles of a common period but different numbers of modulation peaks and troughs, positioned in mutual proximity and orientation, with carefully optimized peak separation and offset to maximize diffraction efficiency and minimize reflection losses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If surface-relief micro-structures are used in DOEs, then diffraction efficiency can be improved, but fabrication difficulty increases and aspect ratio becomes limited
Solution Approach 1:
The patent divides the single DOE into two separate transmission gratings with different numbers of modulation peaks and troughs. Each grating has a simpler micro-structure that is easier to fabricate, while their combination achieves the high diffraction efficiency of a complex single DOE. The first grating has one peak and one trough per period, while the second grating has two peaks and two troughs per period, making each individually manufacturable while collectively achieving superior performance.
2Reliability
If surface-relief micro-structures with high aspect ratios are used, then diffraction efficiency improves, but manufacturing precision becomes more difficult to achieve
Solution Approach 1:
By segmenting the complex micro-structure into two separate gratings with simpler profiles, each grating can be manufactured with lower aspect ratios that are easier to control precisely. The first grating has a simple sawtooth profile with one peak and one trough, while the second grating has two peaks and two troughs per period. This segmentation allows each component to be fabricated with better precision individually, avoiding the extreme aspect ratio challenges of a single complex DOE.
3Device complexity
If a single DOE design is used, then device complexity is low, but diffraction efficiency at high angles becomes suboptimal
Solution Approach 1:
The patent merges two transmission gratings into a single composite optical element that functions as a unified system. The first grating with one peak and one trough per period is combined with the second grating having two peaks and two troughs per period. This merging creates a composite structure that achieves high diffraction efficiency at high angles while maintaining a manageable level of complexity through the systematic combination of two regular patterns.
4Reliability
If modulation peaks are positioned closer together, then diffraction efficiency increases, but fabrication precision requirements become more stringent
Solution Approach 1:
The patent segments the high-density peak arrangement into two separate gratings, each with more spaced-out peaks that are easier to manufacture. The first grating has peaks spaced at the full period distance, while the second grating has peaks spaced at half the period distance. This segmentation allows each grating to be fabricated with relaxed precision requirements, while their combination achieves the equivalent of having peaks spaced at quarter-period distances, thus achieving high diffraction efficiency without extreme fabrication challenges.
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 composite design achieves diffraction efficiencies greater than 80% even at high diffraction angles, while maintaining low aspect ratios for accurate fabrication and minimizing reflection losses, thereby overcoming the limitations of single DOE designs.
Implementation Method 1
Diffractive optical elements (DOEs) are a group of optical elements that use diffractive structures, such as gratings, to modulate the local phase of incident light
Implementation Method 2
The spatial dimensions of these micro-structures vary from a fraction of the operating wavelength to several hundred wavelengths. The period of the phase modulation profile of a DOE, measured in a direction parallel to the surface of the DOE, controls the angles of diffraction
Data Source
AI summary
An optical element includes first and second transmission gratings positioned in mutual proximity and in a mutually-parallel orientation and having respective first and second phase modulation profiles with a common period and different, respective first and second numbers of modulation peaks and troughs in each period.