Sub-wavelength Cylinder Array for Waveguide In-coupling
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Solution Overview
Problem
Existing methods for coupling light into a waveguide, such as refractive prisms and surface-relief gratings, face challenges in achieving high efficiency while maintaining compactness and reducing polarization dependence, with conventional diffraction gratings losing a large fraction of optical energy to zero-order and other modes.
Innovation Solution
A coupling layer with a periodic array of cylinders, where the cylinders are perpendicular to the waveguide surface and have dimensions and spacing less than the target wavelength, is used to deflect multiple diffraction orders into guided modes within the waveguide, achieving coupling efficiencies greater than 90%.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If conventional diffraction gratings are used to couple light into a waveguide, then light coupling is achieved, but a large fraction of optical energy is lost to zero-order and other modes, reducing coupling efficiency
Solution Approach 1:
The coupling layer is segmented into a periodic array of discrete cylinders rather than a continuous grating structure. This segmentation allows precise control over diffraction orders, enabling multiple orders to be coupled into the waveguide while minimizing energy loss to unwanted modes including zero-order transmission.
Solution Approach 2:
The cylinders are designed with specific local geometric properties (height, diameter, spacing) that are optimized to control light diffraction. By varying these local dimensions, the structure directs specific diffraction orders into the waveguide while suppressing others, achieving high coupling efficiency with reduced energy loss.
2Productivity
If refractive prisms are used for light coupling, then coupling efficiency can be improved, but the device size increases and compactness is reduced
Solution Approach 1:
The coupling function is transferred from a three-dimensional prism structure to a two-dimensional periodic array of cylinders in a thin coupling layer. This dimensional reduction maintains the light-coupling functionality while dramatically reducing the device volume and achieving compactness suitable for portable applications.
3Productivity
If conventional gratings are used, then light coupling is achieved, but polarization dependence increases, reducing versatility
Solution Approach 1:
The coupling layer uses a composite structure combining a high-index material (such as silicon nitride or titanium dioxide) with the waveguide medium. This composite arrangement with sub-wavelength cylinder dimensions creates effective medium properties that reduce polarization dependence while maintaining high coupling efficiency for both s and p polarized light.
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 approach allows for efficient coupling of light into the waveguide with reduced polarization dependence and simpler fabrication compared to conventional gratings, maintaining compactness while achieving high coupling efficiency.
Implementation Method 1
The coupling layer is patterned to define a periodic array of cylinders... deflect multiple diffraction orders into the waveguide
Implementation Method 2
light at wavelengths within the range propagates within the waveguide by total internal reflection (TIR) between the planar surfaces
Data Source
AI summary
An optical device includes a waveguide including a first medium, which is transparent and has a first index of refraction at a target wavelength and which has mutually-parallel first and second surfaces arranged so that light at the target wavelength propagates within the waveguide by internal reflection between the first and second surfaces. A coupling layer is disposed over the first surface of the waveguide and includes a second medium having a second index of refraction at the target wavelength, which is greater than the first index of refraction, and is patterned to define a periodic array of cylinders, which have respective cylinder axes perpendicular to the first surface and have respective heights and diameters that are smaller than the target wavelength, and which are spaced apart such that a distance between each of the cylinders and a neighboring cylinder in the array is less than the target wavelength.