Sub-Diffraction Grating for Second-Order Mode Suppression
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Solution Overview
Problem
Conventional DFB laser diodes face challenges in suppressing the second order transverse mode while maintaining high-speed modulation characteristics, as the coupling coefficient needs to be optimized to achieve single-mode propagation and high-speed modulation at 10 Gb/s or higher, but this is hindered by the conflicting requirements of ridge and active layer width and diffraction grating placement.
Innovation Solution
The introduction of a sub-diffraction grating in conjunction with a main diffraction grating, where the sub-diffraction grating is designed to have a higher refractive index and periodicity different from the main grating, effectively suppresses the second order transverse mode by increasing the coupling coefficient for the fundamental mode and enhancing light confinement, thereby improving optical output and modulation speed.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If the ridge width is increased to improve optical output, then the optical output increases, but the second order transverse mode coupling coefficient increases causing mode oscillation
Solution Approach 1:
The patent applies local quality by creating an asymmetric refractive index distribution within the ridge waveguide structure. Specifically, the lower clad layer has a lower refractive index than the upper clad layer, which locally modifies the optical field confinement characteristics. This asymmetric structure selectively enhances coupling for the fundamental mode while suppressing higher-order modes, allowing increased ridge width for higher optical output without causing second-order mode oscillation.
2Reliability
If the coupling coefficient is increased to suppress higher-order modes, then single-mode propagation is achieved, but the modulation speed decreases due to increased parasitic capacitance
Solution Approach 1:
The patent employs parameter changes by optimizing the refractive index difference between the lower and upper clad layers. By carefully controlling this refractive index parameter, the invention achieves sufficient coupling coefficient to suppress higher-order modes while maintaining acceptable parasitic capacitance levels. The specific parameter optimization allows the ridge width to be increased without excessively increasing the coupling coefficient, thus preserving modulation speed while ensuring single-mode propagation.
3Reliability
If the diffraction grating is placed closer to the ridge center to suppress second-order mode, then mode suppression improves, but manufacturing complexity increases
Solution Approach 1:
The patent extracts the mode suppression function from the diffraction grating positioning and implements it through the refractive index distribution of the clad layers. Instead of relying on precise grating placement near the ridge center, the invention uses the lower refractive index of the lower clad layer to create the necessary asymmetric coupling. This extraction simplifies manufacturing by eliminating the need for precise grating positioning while maintaining effective second-order mode suppression.
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 allows for the suppression of the second order transverse mode, ensuring single-mode propagation and improved high-speed modulation characteristics by optimizing the coupling coefficient and reducing parasitic capacitance, thus enhancing the laser diode's performance.
Implementation Method 1
making light of a fundamental transverse mode propagate in a waveguide by coupling the light with diffraction gratings
Data Source
AI summary
An optical waveguide propagates a laser beam. Main diffraction grating and sub-diffraction grating couple light propagating in the optical waveguide. The main diffraction grating and the sub-diffraction grating couple the light in such a manner that propagation in a second order transverse mode of the light propagating in the optical waveguide when both the main diffraction grating and the sub-diffraction grating are disposed, is suppressed more than propagation in the second order transverse mode of the light propagating in the optical waveguide when only the main diffraction grating is disposed.