Variable Optical Filter Materials for Thin Low-Stress Narrowband Sensing
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Solution Overview
Problem
Existing variable optical filters face a tradeoff between optical performance and thickness, leading to issues such as increased internal stresses and delamination due to the need for numerous layers for effective blocking and narrow passband requirements.
Innovation Solution
The use of different materials in the bandpass and blocking regions, where high index contrast is prioritized in the blocking region and low loss materials in the bandpass region, along with a three-material system that includes silicon dioxide, tantalum pentoxide, and silicon, allows for a thin, low-stress filter with low optical loss and narrowband transmission.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the number of layers in the thin film stack is increased to ensure good blocking of wavelengths other than λT and narrow passband around λT, then the optical performance is improved, but the overall thickness increases resulting in increased internal stresses causing the filter to break and/or delaminate
Solution Approach 1:
The patent applies local quality by using different material combinations in different regions of the filter. The blocking region uses high-index materials (such as silicon, titanium dioxide, or tantalum pentoxide) with low-index materials to achieve strong wavelength blocking with fewer layers. The bandpass region uses low-loss materials (such as silicon dioxide or magnesium fluoride) with moderate index to maintain narrow passband transmission. This spatial differentiation of material properties allows the filter to achieve both high optical performance and reduced mechanical stress by minimizing the total number of layers while maintaining functional effectiveness in each region.
2Device complexity
If high-index materials are used to reduce the overall number of layers, then the device complexity is reduced, but the optical loss of the LVF increases
Solution Approach 1:
The patent resolves this contradiction by applying local quality through regional material optimization. High-index materials are strategically placed only in the blocking region where they provide strong reflection and wavelength selection with minimal layers. The bandpass region uses low-loss materials that allow efficient light transmission with minimal absorption. This localized material assignment reduces the total layer count (lowering device complexity) while maintaining low optical loss in the transmission band, as high-index materials are not unnecessarily present in regions where they would cause excessive absorption.
3Strength
If the thin film stack thickness is reduced to decrease internal stresses, then the mechanical reliability is improved, but the blocking performance and passband narrowness deteriorate
Solution Approach 1:
The patent employs composite materials by combining different material systems with complementary properties in a single filter structure. The blocking region uses high-index/low-index material pairs (such as silicon/silicon dioxide or titanium dioxide/magnesium fluoride) that provide strong optical contrast for effective wavelength blocking. The bandpass region uses low-loss material combinations that maintain narrow transmission bands. This composite material approach allows the thin film stack to be sufficiently thin to reduce mechanical stress while achieving both effective blocking and narrow passband transmission through the synergistic properties of different material combinations in different regions.
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 approach results in a compact, reliable optical filter with improved spectral resolution and reduced mechanical stress, achieving low optical loss and strong out-of-band rejection while maintaining a thin profile.
Implementation Method 1
The blocking region 121 includes quarter-wave stacks for blocking wavelengths other than λT
Implementation Method 2
the bandpass region 122 half-wave stacks for transmitting the narrow passband centered around λT
Data Source
AI summary
A variable optical filter is disclosed including a bandpass filter and a blocking filter. The bandpass filter includes a stack of alternating first and second layers, and the blocking filter includes a stack of alternating third and fourth layers. The first, second and fourth materials each comprise different materials, so that a refractive index of the first material is smaller than a refractive index of the second material, which is smaller than a refractive index of the fourth material; while an absorption coefficient of the second material is smaller than an absorption coefficient of the fourth material. The materials can be selected to ensure high index contrast in the blocking filter and low optical losses in the bandpass filter. The first to fourth layers can be deposited directly on a photodetector array.