Refractive-Index Coupling Grating for Laser Stability
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Solution Overview
Problem
Existing semiconductor laser elements with gain-coupling-type diffraction gratings face issues of increased threshold current, reduced slope efficiency, and high power consumption, while refractive-index-coupling-type gratings can suffer from manufacturing variations leading to excessive optical coupling coefficients due to variations in etching depth.
Innovation Solution
A semiconductor optical element with a refractive-index-coupling-type diffraction grating featuring a two-layer structure where the lower portion has a refractive index lower than the upper portion, and an intermediate diffraction grating embedding layer with a refractive index between the two, is used to control the optical coupling coefficient by adjusting the etching rates and refractive indices, thereby embedding the diffraction grating between the substrate and the active layer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a gain-coupling-type diffraction grating is used, then the optical coupling coefficient is enhanced, but the threshold current increases, slope efficiency reduces and power consumption increases
Solution Approach 1:
The diffraction grating is divided into two distinct portions: an upper portion with a first refractive index and a lower portion with a second refractive index lower than the first. This segmentation allows independent optimization of each portion's properties to achieve desired optical coupling without the drawbacks of gain-coupling-type gratings.
Solution Approach 2:
Different portions of the diffraction grating are assigned different refractive indices tailored to their specific functions. The upper portion has higher refractive index for strong optical interaction, while the lower portion has lower refractive index to reduce absorption losses, creating local quality variations that optimize overall performance.
2Manufacturing precision
If the diffraction grating is etched deeper than the design value, then the diffraction grating depth increases, but the optical coupling coefficient becomes larger than the target design value
Solution Approach 1:
The refractive index parameter is changed between the upper and lower portions of the diffraction grating. By setting the lower portion's refractive index lower than the upper portion's, the optical coupling coefficient becomes less sensitive to variations in etching depth, allowing manufacturing tolerances to be accommodated without compromising performance.
Solution Approach 2:
A diffraction grating embedding layer with refractive index lower than the first refractive index and different from the second refractive index is introduced as an intermediary between the diffraction grating and the active layer or optical waveguide layer. This embedding layer acts as a buffer that compensates for depth variations and stabilizes the optical coupling coefficient.
3Use of energy by moving object
If a refractive-index-coupling diffraction grating is used, then power consumption is reduced, but manufacturing variations in etching depth cause optical coupling coefficient variations
Solution Approach 1:
The diffraction grating is constructed as a composite structure with two portions made of materials having different refractive indices. The upper portion uses a material with higher refractive index for strong optical coupling, while the lower portion uses a material with lower refractive index to reduce sensitivity to depth variations, combining the advantages of both material properties.
Solution Approach 2:
The diffraction grating embedding layer serves as an intermediary structure that compensates for manufacturing variations. By positioning this layer between the diffraction grating and the active layer, it provides a reference plane that stabilizes the optical coupling coefficient against etching depth variations.
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 limits variations in the optical coupling coefficient, improves manufacturing efficiency, and reduces in-plane variations, maintaining performance without the drawbacks of gain-coupling-type gratings.
Implementation Method 1
The upper portion has a first refractive index, and the lower portion has a second refractive index lower than the first refractive index
Data Source
AI summary
A semiconductor optical element has a semiconductor substrate, a diffraction grating, a diffraction grating embedding layer, an active layer and a cladding layer. The diffraction grating includes a plurality of grating elements arranged on the semiconductor substrate along a direction (Z direction) in which laser light is emitted. Each grating element has a lower portion and an upper portion provided on the lower portion. The lower portions of the grating elements are connected to each other to form one layer in a lower section of the diffraction grating. The upper portion has a first refractive index and the lower portion has a second refractive index. A refractive index of the diffraction grating embedding layer is an intermediate value between the first and the second refractive index.