Low-Loss Waveguide With Undercut Substrate
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Solution Overview
Problem
Waveguides in optical components experience high leakage of electromagnetic energy into the substrate due to a low effective index relative to the surrounding material, especially when the oxide layer is thin, leading to increased waveguide loss and reduced confinement of the optical mode.
Innovation Solution
Forming an undercut in the silicon substrate to increase the distance between the waveguide and the substrate, thereby reducing leakage and achieving low-loss waveguides, even with a thin oxide layer, by using anisotropic etching to create a long, shallow trench beneath the waveguide.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If the oxide layer is made thin to maintain thermal conductivity and efficient laser operation, then thermal performance is improved, but waveguide loss increases due to higher electromagnetic energy leakage into the substrate
Solution Approach 1:
The patent introduces a third dimension by creating an undercut structure that extends laterally beneath the waveguide. This lateral extension increases the separation distance between the waveguide and substrate without requiring a thicker oxide layer, thus maintaining thermal conductivity while reducing electromagnetic energy leakage into the substrate.
Solution Approach 2:
The undercut structure acts as an intermediary region between the waveguide and substrate. By introducing this intermediate space filled with low-refractive-index material (air or oxide), the patent reduces the direct coupling between the waveguide mode and substrate, thereby decreasing energy leakage while preserving thermal pathways through the oxide layer.
2Loss of energy
If the distance between the waveguide and substrate is increased to reduce leakage, then waveguide loss is reduced, but the oxide layer thickness must be increased which degrades thermal conductivity
Solution Approach 1:
Instead of increasing vertical distance by thickening the oxide layer, the patent exploits the lateral dimension by creating an undercut that extends horizontally beneath the waveguide. This approach increases the effective separation distance for electromagnetic field confinement while maintaining a thin oxide layer for thermal management.
Solution Approach 2:
The patent segments the substrate region beneath the waveguide by creating a localized undercut structure. This segmentation isolates the region directly beneath the waveguide from the bulk substrate, reducing electromagnetic coupling in the critical area while preserving thermal pathways through the surrounding oxide layer and substrate.
3Device complexity
If conventional waveguide structures are used on thin oxide layers, then device complexity is low, but electromagnetic energy leakage into the substrate causes high waveguide loss
Solution Approach 1:
The undercut structure is formed prior to waveguide fabrication, preparing the substrate topology in advance. This preliminary action creates the low-refractive-index region beneath the waveguide before the waveguide itself is formed, enabling reduced energy leakage without requiring complex modifications to the waveguide structure or multiple fabrication steps.
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 undercut significantly reduces waveguide loss, allowing for the integration of optical transceivers with integrated lasers and demultiplexers on a silicon on insulator wafer with a thin oxide layer, while maintaining thermal conductivity and efficient laser operation.
Implementation Method 1
Waveguides in optical components experience high leakage of electromagnetic energy into the substrate due to a low effective index relative to the surrounding material
Implementation Method 2
by using anisotropic etching to create a long, shallow trench beneath the waveguide
Implementation Method 3
when the effective index of a waveguide is relatively low and only a thin layer of oxide separates the waveguide from the substrate, the mode of the waveguide may expand and the leakage from the waveguide into the substrate may reach undesirable levels
Data Source
AI summary
An apparatus comprising a substrate; a waveguide above the substrate; and an undercut into the substrate, the undercut beneath at least a portion of the waveguide, wherein a magnitude of a maximum length of the undercut is lower than a magnitude of a maximum depth of the undercut.