Lithium Niobate Optical Waveguide Roughness for Low-Loss Modulation

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Solution Overview

Problem

Existing optical modulators face challenges with high light propagation loss and micro-cracks due to stress from the difference in expansion coefficients between the optical waveguide material and protective layers, limiting their performance and reliability.

Innovation Solution

The optical device and modulator incorporate a roughened optical waveguide with specific surface roughness measurements and a protective layer to reduce stress, using lithium niobate as the electro-optical material, and a ridged shape to minimize light propagation loss and micro-crack occurrence.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If a protective layer is formed adjacent to the lithium niobate film, then the adhesion and protection are improved, but the light propagation loss increases due to stress from different expansion coefficients

Engineering Contradiction:

ImproveadhesionVSAvoidlight propagation loss

Core Design Contradiction:

ReliabilityVSLoss of energy

Solution Approach 1:

A buffer layer is introduced between the protective layer and the lithium niobate film to act as an intermediary that reduces stress transmission. This buffer layer absorbs the expansion coefficient mismatch, protecting the lithium niobate film from stress-induced micro-cracks while maintaining adhesion and reducing light propagation loss.

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

The refractive index of the protective layer is optimized to match or closely approximate the refractive index of the lithium niobate film. By adjusting this parameter, the optical impedance mismatch is reduced, minimizing light propagation loss at the interface while maintaining the protective function.

Inventive Principle:

Principle #35Parameter changes

2Loss of energy

If the surface roughness is reduced to minimize light scattering, then the light propagation loss decreases, but the stress concentration increases leading to micro-cracks

Engineering Contradiction:

Improvelight propagation lossVSAvoidmicro-crack resistance

Core Design Contradiction:

Loss of energyVSReliability

Solution Approach 1:

The surface of the lithium niobate film is given localized roughness characteristics in specific regions to reduce stress concentration, while maintaining smooth surfaces in light propagation regions. This local differentiation allows the film to withstand stress without forming micro-cracks, while preserving low light propagation loss in the optical path.

Inventive Principle:

Principle #3Local quality

3Use of energy by moving object

If the optical waveguide length is increased to achieve sufficient light confinement, then the driving voltage is reduced, but the device size becomes large

Engineering Contradiction:

Improvedriving voltageVSAvoiddevice length

Core Design Contradiction:

Use of energy by moving objectVSLength of moving object

Solution Approach 1:

The refractive index difference between the lithium niobate film and the surrounding materials is optimized to enhance light confinement. By increasing the refractive index contrast, the waveguide achieves stronger optical confinement, allowing for shorter device lengths while maintaining effective light guidance and low driving voltage.

Inventive Principle:

Principle #35Parameter changes

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 effectively reduces light propagation loss and suppresses micro-cracks, enhancing the reliability and performance of optical modulators by optimizing the surface roughness and material properties.

Implementation Method 1

by roughening the optical waveguide, the influence of stress due to the difference in expansion coefficient between the optical waveguide material and the protective layer can be reduced

Methodology Applied

Scientific EffectStress relaxation through surface roughening:

Implementation Method 2

sufficient light confinement effect is obtained so that the operation speed of the electro-optical device is speed up

Methodology Applied

Scientific EffectLight confinement: Waveguide (optics)

Implementation Method 3

an optical modulator using a lithium niobate film... achieves significant reduction in size and has a lower driving voltage

Methodology Applied

Scientific EffectElectro-optical effect: Electro-Optic Effects

Data Source

PatentUS12481100B2Optical device and optical modulator

Publication Date: 2025.11.25 TDK CORP

AI summary

An optical device and an optical modulator, the optical device includes an optical waveguide, at an surface of plate-like or film-like electro-optic material forming the optical waveguide, 3 locations are selected in the extension direction of the optical waveguide, and 2 locations are selected in the width direction of the optical waveguide in a range of a region of 0.1×0.1 μm, and for a total of 6 locations, an surface roughness RMS is measured with Atomic Force Microscope, and an average of the RMS is 5.1 nm or less.