Optical Modulator HF Line Inductance for Capacitance Cancellation
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Solution Overview
Problem
The frequency response characteristics and reflection characteristics of high-speed optical modulators are limited by the depletion layer capacitance of the light absorption part, hindering the realization of optical modulators with higher speed and wider bandwidth.
Innovation Solution
An optical semiconductor chip is designed with a high frequency line connecting the electrode pad and the modulation electrode, providing inductance in series with the depletion layer capacitance of the optical waveguide, thereby canceling out the capacitive impedance and improving frequency response and reflection characteristics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the optical modulator uses a light absorption layer with depletion layer capacitance, then the electro absorption effect enables optical modulation, but the depletion layer capacitance limits the frequency response and bandwidth
Solution Approach 1:
An inductor is introduced as an intermediary element connected in series between the electrode pad and the light absorption layer. This inductor acts as a mediator that compensates for the capacitive effect of the depletion layer, thereby improving frequency response characteristics and enabling higher modulation speeds without sacrificing modulation effectiveness
2Ease of operation
If the electrode pad is made large (40-100 μm square) for proper electrical connection, then the electrical connection is improved, but the parasitic capacitance increases limiting high frequency performance
Solution Approach 1:
The inductor serves as an intermediary that compensates for the parasitic capacitance inherent in the electrode pad structure. By introducing this inductive element in series, the system can maintain larger electrode pads for good electrical connection while the inductor counteracts the capacitive effect, preserving high frequency response characteristics
3Speed
If the optical modulator is designed for high speed operation, then the modulation bandwidth increases, but the reflection characteristics deteriorate
Solution Approach 1:
The inductor is designed to provide preliminary anti-action against the capacitive impedance of the depletion layer. By pre-compensating for the capacitive effect through the series inductor, the system maintains good reflection characteristics even at high modulation bandwidths, preventing the deterioration that would otherwise occur
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
The implementation of the high frequency line in the optical semiconductor chip enhances the frequency response characteristics and reflection characteristics, achieving a higher speed and wider bandwidth for the optical modulator.
Implementation Method 1
a high frequency line connecting the electrode pad and the modulation electrode and providing inductance in series with respect to a depletion layer capacitance of the optical waveguide
Implementation Method 2
An electro absorption (EA) optical modulator using an electro absorption effect of a semiconductor is one typical high-speed optical modulator
Data Source
AI summary
An optical semiconductor chip of the present disclosure includes a high frequency line between an electrode pad receiving a modulation signal and a modulation electrode on the optical waveguide having a light absorption layer. The depletion layer capacitance generated in the light absorption layer is canceled by an inductor component of the high frequency line. When a portion directly below the high frequency line is embedded with a low-dielectric-constant material or is made hollow, the parasitic capacitance is further reduced. The high frequency line may have a zigzag shape as well as a linear shape. The electrode pad on the optical semiconductor chip can be connected to other substrates including RF lines for modulation signal input by bumps or wire bonding.