Lithium Niobate Waveguide Slab Thickness for Low Loss
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Solution Overview
Problem
Conventional ridge-shaped optical waveguide elements suffer from high propagation loss in the TM fundamental mode due to coupling with TE higher-order modes, and this loss increases with variations in ridge width and manufacturing processes, making it difficult to maintain low loss across different dimensions.
Innovation Solution
The optical waveguide element is designed with a lithium niobate substrate and waveguide layer featuring a slab part and a ridge part, where the slab part's thickness is optimized to be between 0.05 and 0.4 times the wavelength, and the ridge part's width is between 0.1 and 1.0 times the wavelength, with inclined sides at 70° or more, to minimize coupling between TM fundamental and TE slab modes, thereby reducing propagation loss.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Volume of moving object
If the ridge width is decreased to reduce the device size, then the propagation loss in TM fundamental mode increases due to enhanced coupling with TE higher-order modes
Solution Approach 1:
The patent applies local quality by creating a non-uniform thickness distribution in the waveguide layer, with a thicker region at the ridge part and a thinner region at the slab part. This local variation in thickness allows independent optimization of different regions: the thicker ridge region confines TM fundamental mode while the thinner slab region suppresses TE higher-order modes, thereby reducing propagation loss even as the overall device size is reduced.
Solution Approach 2:
The patent introduces a new dimension of control by varying the thickness of the waveguide layer in different regions (ridge part vs. slab part). This thickness dimension provides an additional degree of freedom to independently control mode coupling, allowing the designer to suppress TE higher-order modes without increasing the ridge width, thus maintaining compact device size while reducing propagation loss.
2Speed
If the ridge width is decreased to achieve high-speed modulation, then the propagation loss increases due to manufacturing variations
Solution Approach 1:
The patent uses local quality by designing different thickness regions within the waveguide layer. The thicker ridge region provides strong confinement for high-speed modulation, while the thinner slab region acts as a buffer that is less sensitive to manufacturing variations. This local differentiation ensures that propagation loss remains stable even when ridge width is decreased for high-speed operation.
Solution Approach 2:
The thinner slab part can be considered a cushioning region that compensates for manufacturing variations. By having this softer, more compliant region at the slab part, the waveguide structure becomes less sensitive to dimensional variations during manufacturing, thereby maintaining stable propagation loss even when the ridge width is optimized for high-speed modulation.
3Loss of energy
If the slab part thickness is increased to suppress TE higher-order modes, then the coupling to TE slab modes increases
Solution Approach 1:
The patent applies local quality by creating a non-uniform thickness distribution where the ridge part is thicker and the slab part is thinner. This local differentiation allows the ridge region to provide strong confinement for TM fundamental mode while the thinner slab region naturally suppresses TE higher-order modes, achieving low propagation loss without enhancing coupling to TE slab modes.
Solution Approach 2:
Instead of increasing the slab part thickness to suppress TE modes (the conventional approach), the patent inverts the logic by making the slab part thinner than the ridge part. This inverted thickness relationship creates a natural barrier that suppresses TE higher-order modes through evanescent field decay, achieving the same goal with opposite reasoning.
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 design effectively suppresses the coupling of TM fundamental mode to TE higher-order modes, resulting in a stable low propagation loss that is not significantly affected by dimensional changes during manufacturing, leading to a high-performance optical modulator with low insertion loss and a large extinction ratio.
Implementation Method 1
an optical waveguide element according to one embodiment of the invention guides light in a TM fundamental mode through a ridge part 3 of a waveguide layer 2
Implementation Method 2
a waveguide layer 2 made of lithium niobate and formed on the substrate 1
Data Source
AI summary
Disclosed herein is an optical waveguide element that includes a substrate and a waveguide layer formed on the substrate and comprising lithium niobate. The waveguide layer has a slab part having a predetermined thickness and a ridge part protruding from the slab part. The maximum thickness of the slab part is 0.05 times or more and less than 0.4 times a wavelength of a light propagating in the ridge part.