Tunable Liquid Crystal Birefringent Filter for Narrow Stop Band
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Solution Overview
Problem
Existing band stop filters face challenges in achieving high rejection ratios and narrow spectral bandwidths, with cascaded stages often resulting in broad stop bands and low free spectral range, making it difficult to selectively block unwanted wavelengths while allowing other wavelengths to pass with minimal attenuation.
Innovation Solution
A band stop filter is designed using a succession of tunable birefringent elements with fixed and liquid crystal retarders, aligned to impart a distinct polarization state to specific wavelengths, allowing for selective blocking of the stop band while transmitting other wavelengths with high transmission ratios, using a combination of birefringence, thickness, and rotational orientation to achieve high discrimination and rejection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If cascaded filter stages are used to achieve high rejection ratios, then the rejection ratio is improved, but the stop band becomes broader and the free spectral range decreases
Solution Approach 1:
The patent employs tunable liquid crystal retarders that can dynamically adjust the stop band wavelength and width. By changing the liquid crystal orientation and birefringence properties through applied voltage, the filter can selectively narrow or broaden the stop band as needed, rather than being fixed by cascaded stage design
Solution Approach 2:
The invention changes the optical parameters of the filter by adjusting the liquid crystal retardation values. By controlling the liquid crystal molecules' orientation and birefringence through electrical fields, the filter transmission characteristics (stop band position, width, and rejection ratio) can be dynamically tuned without physical reconfiguration
2Measurement precision
If the stop band is narrowed to achieve high discrimination, then the spectral bandwidth is improved, but the free spectral range also decreases
Solution Approach 1:
The liquid crystal retarders provide dynamic control over the stop band width. By adjusting the liquid crystal birefringence and thickness parameters through applied voltage, the stop band can be narrowed for high discrimination when needed, while the free spectral range is preserved through independent tuning of each retarder stage
Solution Approach 2:
The filter is divided into multiple independent liquid crystal retarder stages, each contributing to the overall stop band characteristics. This segmentation allows independent optimization of each stage's parameters to achieve both narrow stop band width and adequate free spectral range simultaneously
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 filter achieves a high rejection ratio and narrow spectral bandwidth, effectively blocking the stop band while allowing other wavelengths to pass with minimal attenuation, enabling applications such as protecting imaging systems from laser radiation and maintaining system functionality under attack.
Implementation Method 1
birefringent elements in a series, each having a fixed birefringence and a tunable birefringence
Implementation Method 2
a fixed birefringent retarder, each abutting a tunable liquid crystal retarder
Data Source
AI summary
An optical wavelength filter uses birefringence and polarization to discriminate and block a wavelength stop band. Birefringent elements, each having a fixed retarder and a liquid crystal, are placed along the signal path with the fast and slow axes in each pair being aligned and rotationally displaced from other pairs. The birefringence parts are arranged by their thickness, birefringence and progressive rotational orientation so that each pair contributes to achieving a specific polarization state for specific periodic wavelength discrimination bands. A selection polarizer at the output blocks bands that have this polarization state. The birefringence elements are tuned in a coordinated manner and thereby to select one or more wavelengths as the band stop discrimination band.