Diffractive Optical Element With Inter-Gap Thin Film
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Solution Overview
Problem
Existing diffractive optical elements face challenges in reducing unnecessary light, particularly at obliquely incident angles, while maintaining high diffraction efficiency and desired wavelength and polarization characteristics.
Innovation Solution
A diffractive optical element is designed with a pair of diffraction gratings and a thin film between them, where the thin film has a low extinction coefficient and specific refractive index differences, optimizing the relative refractive index difference and film width to improve diffraction efficiency and reduce unwanted light, while minimizing wavelength and polarization dependencies.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-generated harmful factors
If a thin film is provided on the grating wall surface to reduce unnecessary light from obliquely incident angles, then the diffraction efficiency of ±1st order is reduced, but the wavelength characteristics and polarization characteristics deteriorate due to high wavelength and polarization dependence
Solution Approach 1:
The invention changes the critical parameter of the thin film from its conventional position on the grating wall surface to a new position in the space between the first and second diffraction gratings. This parameter change in position, combined with optimizing the film thickness and refractive index, reduces wavelength and polarization dependence while maintaining the ability to suppress unnecessary light.
Solution Approach 2:
The thin film acts as an intermediary element positioned between two diffraction gratings rather than directly on the grating wall surface. This intermediary positioning allows the film to control light propagation in the gap region, reducing unnecessary light from oblique angles while minimizing adverse effects on wavelength and polarization characteristics through proper thickness and refractive index selection.
2Productivity
If the thin film is positioned on the grating wall surface to improve diffraction efficiency of designed order, then unnecessary light is reduced, but the structure becomes more complex and manufacturing becomes more difficult
Solution Approach 1:
The invention moves the thin film from a two-dimensional surface position (on the grating wall) to a three-dimensional space between gratings. This dimensional change allows the film to function effectively in the optical path without requiring direct contact with the grating surface, simplifying the overall structure and potentially easing manufacturing while maintaining high diffraction efficiency.
3Object-generated harmful factors
If the extinction coefficient of the thin film is increased to reduce unnecessary light, then the diffraction efficiency decreases and wavelength characteristics deteriorate
Solution Approach 1:
The invention optimizes the extinction coefficient parameter by selecting materials with very low extinction coefficients (k ≤ 0.0005). This parameter change, combined with positioning the film in the gap between gratings and optimizing its thickness, allows the film to reduce unnecessary light through interference effects rather than absorption, thereby maintaining high diffraction efficiency and good wavelength characteristics.
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 effectively reduces unnecessary light reaching the imaging plane, enhances diffraction efficiency for designed orders, and achieves desired wavelength and polarization characteristics across a wide wavelength band.
Implementation Method 1
a thin film provided between a first grating wall surface and a second grating wall surface... with respect to the wavelength λ, refractive indices of materials of the thin film, the first diffraction grating, and the second diffraction grating, a relative diffractive index difference of the thin film and the first diffraction grating
Implementation Method 2
diffractive optical element... first diffraction grating including a first grating surface and a first grating wall surface, a second diffraction grating including a second grating surface and a second grating wall surface
Data Source
AI summary
A diffractive optical element (1) includes a first diffraction grating (21) including a first grating surface (21a) and a first grating wall surface (21b), a second diffraction grating (31) including a second grating surface (31a) and a second grating wall surface (31b), and a thin film (11) provided between the first grating wall surface and the second grating wall surface and being in contact with both of the first and second grating wall surfaces, an extinction coefficient of the thin film with respect to a wavelength λ in a use wavelength band is not greater than 0.0005, and with respect to the wavelength λ, refractive indices of materials of the thin film, the first diffraction grating, and the second diffraction grating, a relative diffractive index difference of the thin film and the first diffraction grating, and a width of the thin film satisfy predetermined conditions.