Diffractive Optical Element with Stacked Gratings
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Solution Overview
Problem
Existing diffractive optical elements face challenges in manufacturing due to the need for precise surface roughness at interfaces, which is difficult to achieve while maintaining low scattering, especially when the refractive index difference between mediums is large.
Innovation Solution
A diffractive optical element is designed with a first optical member having a sawtooth diffraction grating and a second optical member with an inverted diffraction grating, where the surface roughness of the diffraction surface is optimized to satisfy specific expressions relating to the refractive index difference and arithmetical mean roughness, allowing for reduced scattering and simplified manufacturing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If the refractive index difference between mediums at the interface is large, then the diffraction grating can be formed with sufficient depth and contrast, but the scattering at the interface increases significantly
Solution Approach 1:
The patent introduces an intermediate optical member with refractive index between the first and second optical members, creating a gradient of refractive indices at the interface. This intermediary structure reduces the abrupt refractive index difference, thereby minimizing scattering while still allowing sufficient diffraction grating depth and contrast to be achieved through the stacked grating configuration.
Solution Approach 2:
The patent employs a composite structure of multiple optical members with different refractive indices stacked together. The first optical member has a first diffraction grating, the second optical member has a second diffraction grating, and intermediate optical members have refractive indices between them. This composite material arrangement allows the system to achieve both low scattering (through refractive index grading) and high diffraction efficiency (through the stacked grating structure).
2Object-affected harmful factors
If the surface roughness at the interface is reduced to minimize scattering, then the optical performance improves, but the manufacturing difficulty increases significantly
Solution Approach 1:
The patent changes the parameter of refractive index distribution by introducing intermediate optical members with refractive indices gradually between the first and second optical members. This parameter change allows the system to tolerate higher surface roughness values while maintaining low scattering, because the refractive index gradient compensates for the increased surface irregularities. Consequently, the manufacturing difficulty is reduced as less precise surface finishing is required.
Solution Approach 2:
The patent applies different optical properties to different regions of the stacked structure. The intermediate optical members are specifically positioned at the interfaces where scattering occurs, providing localized refractive index grading only where needed, while other regions maintain their original properties. This local quality approach minimizes scattering at critical interfaces without compromising the overall manufacturing precision requirements of the entire device.
3Reliability
If multiple optical members are stacked in close contact to form a multilayer diffractive optical element, then the functional performance improves, but the manufacturing complexity increases
Solution Approach 1:
The patent designs the stacked optical members so that each one serves multiple functions. The first optical member provides both transmission and the first diffraction grating function, the second optical member provides transmission and the second diffraction grating function, and the intermediate optical members provide both transmission and refractive index grading functions. This multi-functionality reduces the need for separate dedicated components, thereby reducing overall device complexity while maintaining high optical performance.
Solution Approach 2:
The patent merges the diffraction grating structure with the optical member bodies themselves. Instead of having separate grating components and separate optical members, the diffraction gratings are formed as integral parts of the optical members through molding or other fabrication processes. This merging of functions simplifies the assembly process and reduces the number of separate manufacturing steps, thereby reducing manufacturing complexity despite the multilayer configuration.
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 configuration facilitates the manufacturing of close-contact type multilayer diffractive optical elements with reduced scattering, improving manufacturing efficiency and maintaining low scattering levels.
Implementation Method 1
a diffraction grating having a sawtooth cross section is formed at an interface between the optical members
Implementation Method 2
an absolute value of a refractive index difference between mediums sandwiching the interface is smaller
Data Source
AI summary
A diffractive optical element includes a first optical member having a first diffraction grating with a sawtooth cross section; and a second optical member which has a second diffraction grating having a sawtooth cross section and which has a refractive index different from that of the first optical member. In the diffractive optical element, the first and second optical members are stacked so that the first and second diffraction gratings closely contact each other. Arithmetical mean roughness of a diffraction surface of the first diffraction grating is set so as to fall within a predetermined range.