Optical Module Bypass Capacitor Overhang Impedance
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Solution Overview
Problem
Optical modules with integrated electro-absorption modulators face a decrease in characteristic impedance due to the presence of a bypass capacitor, which affects waveform quality and power consumption, especially when surface-mounted on substrates with large interconnection patterns.
Innovation Solution
The optical module design includes a bypass capacitor with a lower electrode that has an overhang area overlapping with the substrate, and a conductor pattern with an extension portion that overlaps with the capacitor, reducing parasitic capacitance by eliminating the dielectric substrate between the overhang area and the conductor block, thereby maintaining high characteristic impedance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a bypass capacitor is surface-mounted on a substrate with a large interconnection pattern area, then the capacitor can be easily mounted, but the characteristic impedance decreases and waveform quality deteriorates
Solution Approach 1:
The lower electrode of the bypass capacitor is designed to extend in the vertical dimension beyond the substrate surface, creating an overhang area that overlaps with the conductor block. This three-dimensional configuration allows the electrode to achieve both large effective area for capacitance and close proximity to the conductor block for high impedance, without requiring a large planar interconnection pattern on the substrate.
Solution Approach 2:
The dielectric substrate serves as an intermediary that is strategically positioned only in specific areas. By making the substrate smaller than the lower electrode area, the design allows the overhang area to directly overlap with the conductor block without dielectric interference, while still using the substrate for mounting the capacitor and providing electrical connection through conductor patterns.
2Quantity of substance
If the lower electrode of the bypass capacitor has a large area for high capacitance, then the capacitor performance improves, but parasitic capacitance increases and characteristic impedance decreases
Solution Approach 1:
The lower electrode transitions from a two-dimensional planar configuration to a three-dimensional structure that extends beyond the substrate surface. This vertical extension creates an overhang area that can overlap with the conductor block, effectively increasing capacitance through closer proximity and larger effective area, while the controlled dielectric substrate placement prevents excessive parasitic capacitance.
Solution Approach 2:
The dielectric substrate is selectively positioned only where needed for mounting and electrical connection, rather than covering the entire lower electrode area. This creates different local environments: areas with dielectric substrate for stable mounting and connection, and areas without dielectric (overhang regions) for high-capacitance overlap with the conductor block, optimizing both capacitance and impedance characteristics.
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 suppresses the decrease in characteristic impedance, enhancing waveform quality and reducing power consumption while maintaining high-speed operation in optical modules.
Implementation Method 1
a bypass capacitor being connected in parallel to the semiconductor laser
Implementation Method 2
the dielectric substrate having a conductor pattern on the upper surface, the cathode electrode and the lower electrode being bonded to the conductor pattern
Data Source
AI summary
An optical module includes: an optical semiconductor device in which a semiconductor laser and an optical modulator are integrated; a bypass capacitor including a lower electrode and an upper electrode, the bypass capacitor being connected in parallel to the semiconductor laser; a dielectric substrate having an upper surface and a lower surface, the optical semiconductor device and the bypass capacitor being surface-mounted on the upper surface, the dielectric substrate having a conductor pattern on the upper surface, the cathode electrode and the lower electrode being bonded to the conductor pattern; and a conductor block supporting the lower surface of the dielectric substrate. The lower electrode of the bypass capacitor having an overlap area overlapping with the upper surface of the dielectric substrate, the lower electrode of the bypass capacitor having an overhang area overhanging from the upper surface of the dielectric substrate.