Athermalized Birefringent Filter Thermal Drift Compensation
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Tunable filters exhibit thermal artifacts and discontinuities in bandwidth due to the stepwise nature of N(λ) and temperature-dependent changes, affecting center wavelength accuracy and signal strength in multispectral imaging systems.
Innovation Solution
A control circuit or processor manages variable retarder settings to maintain constant N(λ) and B(λ) over temperature, ensuring athermal filter response by adjusting the variable retarder to compensate for thermal drift in fixed retarders, thereby eliminating discontinuities and maintaining peak transmission and bandwidth independence from temperature.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the variable retarder is adjusted to compensate for thermal drift in fixed retarders, then center wavelength accuracy is improved, but discontinuities in N(λ) cause thermal artifacts and bandwidth variations
Solution Approach 1:
The patent applies parameter changes by dynamically adjusting the variable retarder settings based on temperature measurements. The system modifies the retarder parameters (retardance values) to compensate for thermal drift in fixed retarders, thereby maintaining accurate center wavelength across temperature variations while managing the discontinuities in N(λ) to prevent bandwidth instability
2Device complexity
If N(λ) is maintained as stepwise to simplify control, then device complexity is reduced, but filter bandwidth exhibits discontinuities affecting spectral response
Solution Approach 1:
The patent implements feedback mechanisms where temperature sensors monitor thermal conditions and feed this information back to the control circuit. The control circuit then adjusts the variable retarder settings based on this feedback, enabling the system to maintain continuous bandwidth response despite the inherently stepwise nature of N(λ), while keeping the control logic manageable through temperature-based decision rules
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 achieves stable and accurate spectral properties across temperature changes, ensuring consistent signal strength and improved accuracy in multispectral imaging systems by maintaining constant bandwidth and center wavelength, reducing thermal variations in optical flux levels.
Implementation Method 1
Filters based on liquid crystal variable retarder elements are described in US Patent 5,247,378
Implementation Method 2
produce a desired optical retardance, based on its electrical capacitance properties
Implementation Method 3
the combination of the fixed retarder element and the variable retarder element produce a net retardance of Nλ; or of (N + 1⁄2)λ; or of (N + 1⁄4)λ or (N + 3⁄4)λ
Implementation Method 4
temperature changes may occur. It is desirable that a tunable filter exhibit athermal response, meaning that none of its properties are altered when the temperature changes
Data Source
Figure 1A
Figure 1B
Figure 2
AI summary
An athermalized birefringent filter (102) for shifts in center wavelength and in bandwidth incorporates fixed retarder elements such as quartz or film retarders, along with electrically-variable retarder elements such as liquid crystal variable retarder cells. A control mechanism determines the amount of thermal drift in the fixed retarder element and produces an equal change in the variable retarder element. The sign of the change depends on whether the variable retarder element adds its retardance with that of the fixed retarder element, or opposes it. This change compensates for the thermal drift of the fixed retarder element. Further, the variable retarder element is constructed to provide the necessary range of retardance adjustment for spectral tuning and thermal compensation over a target thermal range. The control mechanism ensures that, for any specified wavelength, the birefringent filter operates in the same order over the full target thermal range. Multispectral imaging systems (100) are provided based on these filters which provide athermalized response.