Vertically Offset Waveguide Cores for Low-Loss Optical Coupling
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Solution Overview
Problem
Existing optical couplers face challenges in efficiently connecting silicon waveguides with standard single mode fibers due to high refractive index mismatch, leading to high loss and polarization-dependent issues, and require costly and time-consuming wet etching processes.
Innovation Solution
A composite waveguide with silicon nitride etch stop layers is fabricated, allowing for efficient coupling by evanescent waves and reducing substrate loss, integrated into a CMOS process for seamless manufacturing, using tapered waveguide cores to match mode sizes and reduce polarization-dependent loss.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If a single mode silicon waveguide is used to connect optical fibers, then the refractive index is high, but the mode size is sub-micron leading to high loss due to large mode mismatch
Solution Approach 1:
The patent transitions from lateral mode matching to vertical mode matching by offsetting waveguide cores in the vertical dimension. Multiple waveguide cores are positioned at different vertical heights, allowing the optical mode to couple efficiently from the fiber to the silicon waveguide by propagating through the vertical offset structure rather than requiring lateral size transformation.
Solution Approach 2:
The patent employs a composite waveguide structure with multiple cores made of different materials or material configurations. The waveguide includes silicon cores with different heights or positions, creating a composite structure that bridges the mode field diameter mismatch between fiber and standard silicon waveguide through vertical offset coupling.
2Ease of operation
If traditional inverted-taper-based edge couplers are constructed to work with lensed fiber, then the mode size is around 3 μm, but accurate alignment is required which imposes packaging difficulties
Solution Approach 1:
The patent moves the alignment tolerance enhancement from the lateral dimension to the vertical dimension. Instead of using lateral tapers to expand mode size, the invention uses vertical offset between waveguide cores to achieve mode matching, which provides inherent alignment tolerance in the lateral direction and simplifies packaging.
3Loss of energy
If SiO2 waveguides with Si/SiN waveguide inside are built to match 10 μm mode field diameter, then the mode field diameter is matched, but silicon undercut with wet etch is required which is costly, risky and time-consuming
Solution Approach 1:
The patent extracts the problematic wet etching step from the fabrication process. Instead of requiring silicon undercut through wet etch to achieve mode matching, the invention achieves the same coupling efficiency through vertical offset between waveguide cores, eliminating the need for risky and time-consuming wet etching operations.
Solution Approach 2:
The vertical offset structure acts as an intermediary mechanism that provides mode field diameter matching without requiring material removal through wet etching. The offset waveguide cores serve as a mediator to bridge the optical mode between fiber and waveguide, achieving the same result through a different physical mechanism.
4Adaptability or versatility
If grating couplers are used, then the mode profile is large, but the bandwidth is limited and they are usually polarization dependent
Solution Approach 1:
The patent transitions from the grating coupler's planar mode transformation to vertical offset coupling. By positioning waveguide cores at different vertical heights, the invention achieves polarization-independent coupling with broad bandwidth, as the vertical offset mechanism does not rely on the polarization-sensitive diffraction grating structure.
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 provides high coupling efficiency with low polarization-dependent loss and easier packaging, reducing manufacturing costs and time by avoiding substrate loss and aligning with standard 10 μm mode fibers.
Implementation Method 1
efficient coupling by evanescent waves
Data Source
AI summary
A composite optical waveguide is constructed using an array of waveguide cores, in which one core is tapered to a larger dimension, so that all the cores are used as a composite input port, and the one larger core is used as an output port. In addition, transverse couplers can be fabricated in a similar fashion. The waveguide cores are preferably made of SiN. In some cases, a layer of SiN which is provided as an etch stop is used as at least one of the waveguide cores. The waveguide cores can be spaced away from a semiconductor layer so as to minimize loses.