Lithium Niobate Waveguide Structure for Polarization Rotation Suppression
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Solution Overview
Problem
Existing optical waveguide elements using lithium niobate suffer from polarization rotation, leading to mode conversion, reduced modulation efficiency, wavelength dispersion, and inter-polarization crosstalk due to the use of anisotropic materials.
Innovation Solution
The optical waveguide element employs X-cut lithium niobate or lithium tantalate with a waveguide layer thickness less than 0.7 µm, featuring a cross-sectional shape that maintains a higher effective refractive index for the TE mode compared to the TM mode, and includes electrodes to apply an electric field, suppressing polarization rotation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a rib-type optical waveguide with strong optical confinement is used to enable small bending radius, then light propagation with low loss is achieved, but polarization rotation occurs causing mode conversion and performance deterioration
Solution Approach 1:
The patent changes the propagation direction parameter from the conventional Y-axis direction to a direction forming a specific angle (30-60 degrees) with the Y-axis. This parameter change in propagation direction, combined with controlling waveguide layer thickness (0.1-0.5 μm) and rib portion dimensions, modifies the optical field distribution to suppress polarization rotation while maintaining strong optical confinement for small bending radius propagation.
2Object-generated harmful factors
If the waveguide layer thickness is reduced to suppress polarization rotation, then mode conversion is reduced, but optical confinement strength decreases
Solution Approach 1:
The patent employs a composite structure consisting of the waveguide layer (X-cut lithium niobate or lithium tantalate), substrate, and rib portion material with different refractive indices. This composite structure enables strong optical confinement through refractive index contrast while the specific thickness control (0.1-0.5 μm) and propagation direction adjustment work together to suppress polarization rotation, resolving the contradiction between confinement strength and mode conversion suppression.
3Adaptability or versatility
If X-cut lithium niobate is used as the waveguide layer material, then electro-optic effect is achieved for modulation, but anisotropic refractive index causes polarization rotation
Solution Approach 1:
The patent applies local quality control by specifying the waveguide layer thickness (0.1-0.5 μm) and rib portion dimensions (width 0.5-2.0 μm, height 0.1-0.5 μm) in the regions where light propagation occurs. This localized dimensional control, combined with the specific propagation direction (30-60 degrees from Y-axis), creates optimal optical field distribution that suppresses polarization rotation while preserving the electro-optic modulation capability of X-cut lithium niobate.
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 configuration effectively suppresses polarization rotation, maintaining high modulation efficiency and reducing wavelength dispersion and inter-polarization crosstalk, while allowing for small bending radii and efficient light propagation.
Implementation Method 1
a material having an electro-optic effect, such as lithium niobate (LN), is used for the waveguide layer
Data Source
Figure 1A~1B
Figure 2~3
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AI summary
[Task] Provided is an optical waveguide element which suppresses polarization rotation. [Means for Resolution] An optical waveguide element includes a substrate (10) and an optical waveguide (2) that is disposed on the substrate (10). The optical waveguide (2) has an effective refractive index change portion in which an effective refractive index of the optical waveguide (2) related to a fundamental mode A parallel to a plane of polarization of a light wave propagated through the optical waveguide (2) changes according to propagation of the light wave. In the effective refractive index change portion, a cross-sectional shape of the optical waveguide (2) which is perpendicular to a propagation direction of the light wave is set such that the effective refractive index of the optical waveguide (2) related to the fundamental mode A is higher than an effective refractive index of the optical waveguide (2) related to another fundamental mode B perpendicular to the fundamental mode A.