Invar Optical Element With Middle Layer For Thermal Stability
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Solution Overview
Problem
Existing image slicer type optical systems in astronomical observations face challenges with surface roughness, thermal expansion issues, and positional precision, leading to problems like cracking and deformation under extreme environments.
Innovation Solution
An optical element with a middle layer between the base material and the reflecting layer, where the thermal expansion coefficients are strategically matched (base material < middle layer < reflecting layer), and using materials like copper or nickel for the middle layer to alleviate thermal stress, combined with precise manufacturing techniques to maintain surface smoothness and positional accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If aluminum base material is used for the optical element, then multiple optical surfaces can be formed with high positional precision, but the high thermal expansion coefficient causes significant thermal expansion and contraction that compromises reliability
Solution Approach 1:
The patent changes the material parameter (thermal expansion coefficient) by selecting Invar as the base material, which has a thermal expansion coefficient of approximately 1.2×10−6, significantly lower than aluminum's 23×10−6. This parameter change resolves the contradiction by maintaining manufacturing precision while dramatically improving reliability under thermal conditions.
Solution Approach 2:
The patent explicitly addresses thermal expansion by choosing a base material (Invar) with extremely low thermal expansion characteristics. The reflecting layer's thermal expansion coefficient is designed to be higher than the base material, creating a controlled gradient that accommodates thermal stresses while maintaining optical surface stability.
2Ease of manufacture
If diamond tool cutting is used on aluminum base material, then manufacturing process is simplified, but surface roughness is limited to approximately 5 nm Rms which is inadequate for visible light observation
Solution Approach 1:
The patent changes the base material from aluminum to Invar, which has superior machinability by diamond tools. This material parameter change enables achieving surface roughness of 1 nm Rms or less while maintaining ease of manufacture through conventional diamond tool cutting processes.
Solution Approach 2:
The patent creates a composite structure with Invar base material and a reflecting layer, where the Invar provides both excellent machinability for ultra-smooth surfaces and low thermal expansion. The combination resolves the contradiction between ease of manufacture and manufacturing precision.
3Manufacturing precision
If glass base material is used and polished to achieve adequate surface roughness, then visible light observation is enabled, but it is difficult to integrally polish the mirror array and assembly precision cannot match integral working
Solution Approach 1:
The patent changes the base material from glass to Invar, which can be integrally machined by diamond tools to achieve ultra-smooth surfaces (1 nm Rms or less) across multiple optical surfaces in a single working process. This eliminates the complexity of separate polishing and assembly operations while maintaining superior surface quality.
Solution Approach 2:
The patent merges multiple optical surfaces into a single integral Invar base material, allowing all surfaces to be machined simultaneously with consistent precision. This combining approach eliminates the need for separate polishing and assembly steps, reducing device complexity while achieving uniform high precision across all surfaces.
4Illumination intensity
If metal reflecting film of Al is applied onto glass base material to enhance reflectance, then optical performance is improved, but stripping and cracking occur due to difference in coefficients of thermal expansion
Solution Approach 1:
The patent changes the base material from glass to Invar, which has a thermal expansion coefficient much closer to that of aluminum reflecting films. This parameter change reduces the thermal expansion mismatch, preventing stripping and cracking of the reflecting film while maintaining high reflectance performance under thermal conditions.
Solution Approach 2:
The patent addresses thermal expansion compatibility by selecting Invar as the base material, whose thermal expansion coefficient (approximately 1.2×10−6) is much closer to aluminum than glass is. This creates a thermally compatible structure where the reflecting film remains stable under temperature variations, resolving the contradiction between optical performance and reliability.
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 optical element achieves high surface roughness and precise positional alignment, reducing thermal-induced cracking and deformation, enabling stable performance in extreme environments while maintaining optical quality.
Implementation Method 1
a middle layer between a base material and a reflecting layer, wherein the base material has a thermal expansion coefficient Tb, the middle layer has a thermal expansion coefficient Ti, the reflecting layer has a thermal expansion coefficient Tr, to meet a relation: Tb<Ti<Tr
Data Source
AI summary
An optical element formed of a plurality of materials includes a middle layer between a base material and a reflecting member so as to suppress stripping, cracking and the like of the optical surface due to the difference in coefficients of thermal expansion among the component materials, in the case where a temperature difference in the service environment or a temperature difference between a manufacturing environment and the service environment is large.