Optical Subassembly Transmission Line for Modulator Impedance Matching
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Solution Overview
Problem
High-frequency electrical signals deteriorate due to impedance mismatch between the 50Ω termination system and the optical modulator in semiconductor light emitting devices, leading to degraded optical signal quality.
Innovation Solution
A semiconductor light emitting device design featuring a sub-mount with a conductive pattern, an optical modulator with electrodes, a termination resistor, and a microstrip substrate with a single-ended transmission line having different characteristic impedances in its sections to suppress impedance mismatch effects, including a load circuit with the optical modulator, termination resistor, and a wire connected between the second end portion and the conductive pattern, with the load circuit having a characteristic impedance equal to or lower than the second section of the transmission line.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a 50Ω termination system is used for high-frequency electrical signals, then the transmission line can be simplified, but impedance mismatch with the optical modulator causes signal reflection and degrades signal quality
Solution Approach 1:
The patent applies local quality by creating different characteristic impedance sections (first section with 50Ω, second section with lower impedance) within the transmission line. Each section has optimized impedance characteristics matched to its specific function: the 50Ω section interfaces with the termination system while the lower impedance section interfaces with the optical modulator, eliminating the need for complex impedance matching networks throughout the entire transmission line.
Solution Approach 2:
The patent changes the characteristic impedance parameter along the transmission line by introducing a second section with lower characteristic impedance than the first section. This parameter change allows the transmission line to adapt to the impedance characteristics of the optical modulator while maintaining compatibility with the 50Ω termination system, thereby reducing signal reflection and improving signal quality.
2Reliability
If the characteristic impedance of the transmission line is matched to the optical modulator, then signal reflection is reduced, but the termination system impedance (50Ω) cannot be maintained
Solution Approach 1:
The patent segments the transmission line into two distinct sections: a first section with 50Ω characteristic impedance for compatibility with the termination system, and a second section with lower characteristic impedance for optimal interfacing with the optical modulator. This segmentation allows each section to be optimized for its specific interface requirement, achieving both termination system compatibility and modulator-matched signal transmission.
Solution Approach 2:
The second section of the transmission line with lower characteristic impedance acts as an intermediary between the 50Ω termination system and the optical modulator. It serves as an impedance transition zone that bridges the impedance mismatch, allowing efficient signal transfer from the 50Ω system to the modulator while minimizing reflections at both interfaces.
3Ease of operation
If a long bonding wire is used to connect the transmission line to the optical modulator, then the connection is more flexible, but high-frequency signal quality deteriorates
Solution Approach 1:
The patent extracts the problematic long bonding wire from the signal path and replaces it with an integrated transmission line structure formed on the substrate. The transmission line is directly formed from the substrate material and connected to the optical modulator through controlled impedance pathways, eliminating the need for external bonding wires that would introduce inductance and signal degradation at high frequencies.
Data Source
AI summary
A semiconductor light emitting device includes a microstrip substrate with a single-ended transmission line on a top surface, wherein the single-ended transmission line extends from a first end portion to a second end portion, the microstrip substrate has a ground plane on a bottom surface, and the ground plane is opposed and bonded to the conductive pattern. The single-ended transmission line includes a first section and a second section, wherein the second section extends from the first section and includes the second end portion. The second section is lower in characteristic impedance than the first section. A load circuit that includes the wire, the optical modulator, and the termination resistor is electrically connected between the second end portion and the conductive pattern. The load circuit is equal to or lower in the characteristic impedance than the second section.