Optical Modulator Resin Buffer for Stress Relief
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Solution Overview
Problem
Optical modulators using lithium niobate substrates with metal modulation electrodes face issues of stress-induced damage and refractive index changes due to temperature variations, leading to substrate cracking and modulation efficiency fluctuations, which conventional buffer layers fail to adequately address, especially in thin substrates.
Innovation Solution
Incorporating a resin with lower rigidity between the modulation electrode and the substrate to act as a stress-relief cushion, reducing the impact of linear expansion coefficient differences and maintaining a thicker film thickness compared to conventional sputtering methods, while ensuring effective electric field application and modulation efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the substrate is thinned to reduce thickness for speed matching between microwave and optical wave, then the modulation speed is improved, but the substrate becomes more susceptible to stress damage and cracking
Solution Approach 1:
A buffer layer is formed between the modulation electrode and the substrate before the electrode is attached. This buffer layer acts as a cushion that absorbs and distributes the stress generated by thermal expansion differences between the metal electrode and the thin LN substrate, preventing stress concentration that would cause cracking in thinned substrates.
Solution Approach 2:
The structure combines multiple materials with different properties: the LN substrate, the buffer layer material (such as SiO2 or SiN), and the metal modulation electrode. This composite structure allows each material to contribute its favorable properties - the LN provides electro-optic effect, the buffer layer provides stress relief, and the metal electrode provides electrical functionality.
2Strength
If a conventional sputtering buffer layer is formed to relieve stress, then some stress relief is achieved, but the film thickness is too thin to sufficiently relieve stress on thin substrates
Solution Approach 1:
The buffer layer thickness is increased from conventional thin sputtering films to a thicker range (e.g., 1-10 μm or more), which provides sufficient stress relief capability for thin substrates while maintaining manufacturability through established deposition techniques.
3Reliability
If stress is generated on the substrate from modulation electrodes, then the linear expansion coefficient difference causes internal stress, but this stress changes the refractive index due to photoelastic effect, causing phase difference and property deterioration
Solution Approach 1:
The buffer layer is positioned between the modulation electrode and substrate to cushion and distribute the stress before it reaches the substrate. This prevents stress-induced refractive index changes through the photoelastic effect, maintaining stable modulation performance and preventing phase differences in Mach-Zehnder structures.
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 resin effectively reduces stress on the substrate, preventing damage and deterioration of the modulator's properties, enhancing modulation efficiency and maintaining substrate integrity even in thin configurations.
Implementation Method 1
a resin is arranged between a part of a bottom surface of the modulation electrode and the substrate facing the part of the bottom surface of the modulation electrode
Implementation Method 2
the linear expansion coefficient differs between the substrate and the modulation electrode. Therefore, due to the difference in linear expansion coefficient caused by the temperature change, internal stress is generated in the substrate
Implementation Method 3
the substrate is made of a material having an electro-optic effect such as LN, and light modulation is performed by applying electricity to change the refractive index
Implementation Method 4
when the stress is generated on the substrate, the refractive index of the substrate changes due to a photoelastic effect, so that the propagation speed of the light wave changes
Data Source
AI summary
In order to provide an optical modulator capable of preventing damage to the substrate and preventing deterioration of the properties of the modulator by reducing the stress on the substrate generated by the modulation electrode, there is provided an optical modulator 1, including: a substrate 5 having an electro-optic effect; an optical waveguide 10 formed on the substrate 5; and a modulation electrode (signal electrode S and ground electrode G) provided on the substrate 5 and modulating a light wave propagating in the optical waveguide 10, wherein a resin 8 to reduce the stress on the substrate 5 generated by the modulation electrode is arranged between a part of the bottom surface of the modulation electrode and the substrate 5 facing a part of the bottom surface of the modulation electrode.