Infrared Wavelength Discrimination Filter Using Tunable Birefringent Elements
Solution Overview
Problem
Conventional optical interference filters face challenges in achieving high transmission ratios and good out-of-band rejection, especially in the infrared spectral region, due to material absorption and reflection issues, which limits their effectiveness in applications like vibrational spectroscopy.
Innovation Solution
The use of a minimum number of rotationally displaced birefringent elements with specific relative rotational orientations and tunable liquid crystals, optimized for reduced absorption and reflection, along with materials that minimize light energy absorption in the infrared, enables a comb filter transmission characteristic with improved tolerance to rotational errors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If conventional optical interference filters are used in the infrared spectral region, then filter functionality is maintained, but transmission ratio is reduced due to material absorption and reflection
Solution Approach 1:
The patent changes the operational parameters by using a minimum number of birefringent elements (two to four) with specific rotational orientations, optimizing the filter design for infrared wavelengths. This reduces the number of interfaces and material layers, thereby minimizing absorption and reflection losses while maintaining filter functionality in the infrared spectral region.
Solution Approach 2:
The patent employs composite material structures combining specific birefringent materials with optimized rotational orientations. The use of liquid crystals for tuning combined with fixed retarders creates a composite system that achieves high transmission ratios in the infrared region by leveraging the complementary properties of different materials.
2Reliability
If multiple birefringent elements are used to improve discrimination, then out-of-band rejection is improved, but device complexity increases
Solution Approach 1:
The patent applies partial action by using only the minimum necessary number of birefringent elements (two to four) to achieve the desired out-of-band rejection. This is insufficient for conventional filters but sufficient for the optimized infrared filter design, reducing complexity while maintaining reliability.
Solution Approach 2:
The patent changes the parameter of rotational orientation angles to non-integer fractions of π radians, which provides effective wavelength discrimination with fewer elements. This parameter optimization allows achieving good in-band versus out-of-band discrimination with minimum birefringent elements, reducing device complexity.
3Ease of manufacture
If standard rotational orientations are used for birefringent elements, then manufacturing is simplified, but discrimination performance deteriorates
Solution Approach 1:
The patent changes the rotational orientation parameters from standard integer fractions of π to non-integer fractions of π radians. This optimization enhances wavelength discrimination performance while maintaining manufacturability through empirical solutions that balance performance with fabrication capabilities.
Solution Approach 2:
The patent replaces precise mechanical alignment systems with a design that is tolerant of rotational alignment errors. By using minimum birefringent elements with optimized orientations, the filter achieves reliable wavelength discrimination without requiring extremely precise mechanical alignment, simplifying the manufacturing process.
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 configuration achieves high transmission ratios and effective out-of-band rejection, allowing for narrow spectral band selection and operation in the infrared region, including the short wavelength infrared range, while being tolerant of rotational alignment errors.
Implementation Method 1
Birefringent elements, by definition, are characterized by different optical refractive indices for vector components of propagating light that are parallel to mutually orthogonal 'fast' and 'slow' axes of the birefringent element. When light propagates through the birefringent element, the vector component that is parallel to the slow axis is delayed, compared to the vector component parallel to the fast axis.
Implementation Method 2
The liquid crystals add to the fixed retarders a thickness and an electrically controllable amount of birefringence.
Implementation Method 3
A particular plane polarization orientation can be selected by the orientation of an output or selection polarizer in an optical interference filter.
Data Source
AI summary
An optical comb filter has polarizers placed before and after two or more birefringent elements, each having a fixed retarder and an aligned tunable liquid crystal. The birefringent elements are equal in birefringence and thickness and are tuned in unison. Wavelength-dependent polarization state changes are contributed by retardation in unequal proportions on the fast and slow axes of the successive birefringent elements. The filter is tolerant of rotational alignment errors. A two element filter uses angles of 22.5 and 67.5°; a three element version has 12.5, 45 and 77.5°; and a four element filter has 7.5, 29.5, 60.5 and 82.5°; all these angles being ±5°.