Optical Modulator Electrode Layout for Balanced High-Frequency Impedance
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Solution Overview
Problem
Optical semiconductor devices face challenges in achieving balanced impedance on the positive and negative phase sides due to significant differences in path lengths between transmission lines and electrodes, particularly when driven by differential signals, which affects high-frequency characteristics.
Innovation Solution
The optical semiconductor device integrates a modulator unit with electrodes arranged on a semiconductor substrate, where one pad electrode is positioned in the waveguide unit, offsetting the other in the traveling direction of the optical path, and trench portions are used to minimize wire length discrepancies, reducing parasitic capacitance and enhancing impedance matching.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Volume of moving object
If the two electrodes are arranged at positions opposed to each other with respect to a ridge waveguide to achieve compact device structure, then the device size is reduced, but the path lengths between transmission lines and electrodes become significantly different, causing impedance mismatch
Solution Approach 1:
The patent applies asymmetry by intentionally designing different path lengths for the positive and negative phase transmission lines. Specifically, one transmission line is routed through the waveguide unit while the other is arranged outside, creating asymmetric path configurations that compensate for the electrode positioning to achieve balanced impedance characteristics
Solution Approach 2:
The patent utilizes spatial dimensionality by arranging transmission lines in different spatial regions - one inside the waveguide unit and another outside. This dimensional separation allows independent optimization of each transmission line path to achieve equal effective lengths despite the compact electrode arrangement
2Reliability
If the path length difference between positive and negative phase transmission lines is reduced to improve impedance matching, then impedance balance is improved, but the device complexity increases due to additional design constraints
Solution Approach 1:
The patent applies local quality by optimizing specific regions of the transmission line paths differently. The transmission line connected to the electrode inside the waveguide unit is designed with a shorter external path, while the other transmission line is routed externally with a longer path, creating locally differentiated characteristics that achieve global impedance balance
Data Source
AI summary
Provided is an optical semiconductor device that has an excellent characteristic. The optical semiconductor device includes: a modulator unit including first and second conductivity type semiconductor layers; a waveguide unit; a first electrode connected to the first conductivity type semiconductor layer; and a second electrode connected to the second conductivity type semiconductor layer. The first electrode includes a first pad electrode. The second electrode includes a second pad electrode. A differential signal is input to the first pad electrode and the second pad electrode. One of at least a part of the first pad electrode or at least a part of the second pad electrode is arranged in the waveguide unit in plan view. The first pad electrode and the second pad electrode are arranged with an offset in positions in a traveling direction of an optical path of the waveguide unit.