Near-Infrared Interference Filter Stack With Low Angular Shift
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing near-infrared interference filters face challenges in achieving high transmission and low angular shift, particularly in high numerical aperture optical systems, due to the trade-off between hydrogenation and refractive index changes in amorphous hydrogenated silicon (a-Si:H) layers.
Innovation Solution
Incorporating a controlled amount of nitrogen into the a-Si:H layers to form a-Si:H,N, and using higher refractive index dielectric materials like silicon nitride (Si3N4) for low index layers to improve transmission and control angular shift in the 800-1100 nm range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If hydrogenated amorphous silicon (a-Si:H) layers are used to extend operation into the near infrared, then material loss is reduced and transmission is improved, but the refractive index is reduced causing increased angular shift
Solution Approach 1:
The patent changes the chemical composition parameters of the silicon layers by incorporating nitrogen to form a-Si:H,N alloy. This parameter change allows simultaneous optimization of both refractive index and material loss characteristics, resolving the contradiction between reduced material loss and controlled angular shift in the near-infrared range.
Solution Approach 2:
The patent uses composite material structure by creating nitrogen-containing amorphous silicon hydrogenated alloy (a-Si:H,N) that combines the advantages of hydrogenation (reduced absorption) with nitrogen addition (maintained or enhanced refractive index). This composite approach allows both improved transmission and controlled angular dependence.
2Ease of operation
If higher refractive index materials are used to reduce angular shift, then angular shift is reduced, but the passband width becomes narrower
Solution Approach 1:
By adjusting the nitrogen concentration in the a-Si:H,N layers, the patent enables independent optimization of refractive index and absorption characteristics. This parameter control allows achieving low angular shift while maintaining adequate passband width, resolving the trade-off between these two parameters.
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 enables the fabrication of near-infrared interference filters with improved transmission and reduced angular shift, extending the practical operational range to 750 nm and enhancing performance in applications like optical data communications.
Implementation Method 1
A known transmission interference filter employs a stack of alternating silicon and silicon dioxide (SiO2) layers
Implementation Method 2
The spectral profile of an optical interference filter is, among other things, dependent on the angle of illumination. As the angles increase the filters shift to shorter wavelength. This angular shift is dependent on the materials used and the distribution of those materials. Higher refractive index results in less angle shift.
Implementation Method 3
The method comprises sputtering silicon from a silicon target onto a filter substrate and, during the sputtering, alternating between (i) a process gas including hydrogen and nitrogen in order to deposit a Si:H,N and (ii) a process gas including oxygen in order to deposit SiOx
Data Source
AI summary
An interference filter includes a layers stack comprising a plurality of layers of at least: layers of amorphous hydrogenated silicon with added nitrogen (a-Si:H,N) and layers of one or more dielectric materials, such as SiO2, SiOx, SiOxNy, a dielectric material with a higher refractive index in the range 1.9 to 2.7 inclusive, or so forth. The interference filter is designed to have a passband center wavelength in the range 750-1000 nm inclusive. Added nitrogen in the a-Si:H,N layers provides improved transmission in the passband without a large decrease in refractive index observed in a-Si:H with comparable transmission. Layers of a dielectric material with a higher refractive index in the range 1.9 to 2.7 inclusive provide a smaller angle shift compared with a similar interference filter using SiO2 as the low index layers.