Variable Optical Retarder With Sub-Wavelength Grating
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Liquid crystal variable optical retarders face issues with polarization sensitivity and excessive optical loss due to the placement of quarter-wave plates, which increase beam divergence and require higher driving voltages.
Innovation Solution
Incorporating a sub-wavelength grating between the electrode and the liquid crystal layer, acting as a quarter-wave plate, reduces polarization sensitivity and optical loss by minimizing the driving voltage penalty and using a dielectric or semiconductor material to avoid metal reflectors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a quarter-wave plate is placed between the liquid crystal cell and the mirror to achieve polarization insensitivity, then the variable optical retarder becomes insensitive to the state of polarization of the incoming beam, but the distance between the mirror and the liquid crystal cell increases, causing beam divergence and reduced spatial resolution
Solution Approach 1:
The quarter-wave plate function is extracted from a separate optical element and integrated directly into the liquid crystal cell structure. The liquid crystal layer itself is configured to provide the quarter-wave retardation, eliminating the need for a separate quarter-wave plate between the liquid crystal cell and mirror, thereby maintaining compact geometry and spatial resolution while achieving polarization insensitivity
Solution Approach 2:
The quarter-wave plate function is merged with the liquid crystal cell by configuring the liquid crystal layer to provide the necessary retardation. This combines multiple functions (liquid crystal modulation and quarter-wave plate) into a single integrated structure, reducing the number of separate optical elements and maintaining compact beam geometry
2Adaptability or versatility
If a quarter-wave plate is placed between the liquid crystal cell and the mirror to achieve polarization insensitivity, then the variable optical retarder becomes insensitive to the state of polarization of the incoming beam, but optical loss increases due to the additional optical elements and longer optical path
Solution Approach 1:
The quarter-wave plate function is merged with the liquid crystal cell by configuring the liquid crystal layer to provide the necessary retardation. This combines multiple functions (liquid crystal modulation and quarter-wave plate) into a single integrated structure, reducing the number of separate optical elements and minimizing optical loss at interfaces
Solution Approach 2:
The separate quarter-wave plate element is extracted from the optical path and its function is realized through the liquid crystal layer configuration itself, eliminating additional optical interfaces and reducing overall optical loss
3Adaptability or versatility
If the liquid crystal cell is made transmissive to accommodate an external quarter-wave plate, then the quarter-wave plate can be placed outside the cell, but optical loss increases due to light passing twice through transparent electrodes
Solution Approach 1:
The quarter-wave plate function is merged into the liquid crystal cell structure, allowing the use of a reflective liquid crystal cell configuration. This eliminates the need for light to pass twice through separate transparent electrodes for both liquid crystal modulation and quarter-wave plate functions, reducing optical loss
Solution Approach 2:
The liquid crystal layer parameters (thickness, birefringence) are optimized to provide quarter-wave retardation at the operating wavelength, enabling the liquid crystal cell to function as both the modulation element and the quarter-wave plate, thereby reducing optical loss
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 reduces polarization sensitivity and optical loss, maintaining spatial resolution and efficiency while minimizing the driving voltage required, thus enhancing the performance of variable optical retarders.
Implementation Method 1
a sub-wavelength grating disposed between the liquid crystal layer and the first electrode and having grating lines at an acute angle, preferably 45 degrees, to the director
Implementation Method 2
When a voltage is applied to the electrodes, an electric field between the electrodes orients liquid crystal molecules, which are highly anisotropic. Field-induced orientation of the liquid crystal molecules changes an effective index of refraction of the liquid crystal layer
Implementation Method 3
liquid crystal molecules, which are highly anisotropic
Implementation Method 4
a mirror 13. In operation, an incoming vertically linearly polarized (V-LP) optical beam 14 propagates through the liquid crystal cell 12, the quarter-wave plate 11, and is reflected by the mirror 13
Data Source
AI summary
A sub-wavelength grating is placed inside a liquid crystal variable optical retarder to reduce polarization dependence of the optical retardation generated by the variable optical retarder. A small thickness of the sub-wavelength grating, as compared to a conventional waveplate, reduces the driving voltage penalty due to the in-cell placement of the sub-wavelength grating.