Optical Module Impedance Matching for Signal Reflection
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Solution Overview
Problem
The existing optical transmission circuits face challenges in downsizing and cost reduction due to impedance mismatch between the drive IC and TOSA, leading to signal reflection and attenuation issues, especially with the transition from 2-level to 4-level pulse amplitude modulation.
Innovation Solution
The optical module employs a balanced line configuration with a drive circuit having a differential output impedance of 100Ω and a TOSA with a differential input impedance of 50Ω, utilizing a resistive element to set the reflection coefficient to 0.10 or less, thereby reducing signal reflection and attenuation while maintaining a satisfactory waveform.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Volume of moving object
If the drive IC and TOSA are connected through a balanced line with impedance mismatch (100Ω to 50Ω), then the TOSA can be downsized and cost reduced, but signal reflection and attenuation occur
Solution Approach 1:
A matching circuit is introduced as an intermediary component between the drive IC and TOSA to bridge the impedance gap. The matching circuit includes a series inductor and shunt capacitor that together form an impedance transformation network, converting the 100Ω differential impedance from the drive IC to match the 50Ω differential impedance of the TOSA, thereby eliminating signal reflection while maintaining the downsized TOSA structure
Solution Approach 2:
The impedance parameters of the transmission line are changed along its length. The balanced line is designed with varying characteristic impedance - starting at 100Ω near the drive IC and transitioning to 50Ω near the TOSA through carefully controlled inductor and capacitor values. This gradual parameter change allows impedance matching without requiring the entire transmission line to be redesigned
2Volume of moving object
If the differential impedance is set to 50Ω for downsizing, then the TOSA package can be reduced, but impedance mismatch causes signal attenuation
Solution Approach 1:
The matching circuit acts as an energy-efficient intermediary that transforms impedance without significant power loss. The inductor and capacitor are designed with high quality factors to minimize resistive losses, ensuring that the majority of signal energy is successfully transferred from the 100Ω drive IC output to the 50Ω TOSA input, reducing both reflection loss and attenuation
Solution Approach 2:
The impedance matching is performed in advance through pre-calculated inductor and capacitor values that are optimized for the specific frequency range and power levels. This preliminary design ensures that the impedance transformation is already optimized before signal transmission begins, preventing energy loss from improper matching conditions
3Reliability
If the differential impedance is set to 100Ω to maintain signal quality, then the TOSA cannot be downsized
Solution Approach 1:
The transmission path is segmented into distinct impedance zones. The first segment (from drive IC to matching circuit) operates at 100Ω differential impedance, while the second segment (from matching circuit to TOSA) operates at 50Ω differential impedance. This segmentation allows each portion to be optimized for its specific function - the first for drive compatibility and the second for compact packaging
Solution Approach 2:
Different impedance characteristics are applied to different locations along the transmission path. The matching circuit introduces localized impedance transformation only where needed - at the interface between drive IC and TOSA - while leaving the rest of the transmission lines at their respective optimal impedances. This local quality change enables downsizing without compromising overall signal integrity
Data Source
AI summary
In an optical module, optical signal output having satisfactory waveform and intensity can be obtained. A differential transmission line includes a first differential transmission line, which has a first characteristic impedance and is connected to a drive IC, a second differential transmission line, which has a second characteristic impedance and is connected to a light output element, the second characteristic impedance being smaller than the first characteristic impedance, and connecting portions configured to connect the first differential transmission line and the second differential transmission line in series with each other. A resistive element is arranged between the connecting portions. The resistive element has a resistance value that is set to a value with which an absolute value of a reflection coefficient for a signal traveling from the second differential transmission line side to the first differential transmission line side is 0.10 or less.