Series Electro-Absorption Modulators for Bandwidth Extinction Trade-off
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Solution Overview
Problem
Electro-absorption optical modulators face a trade-off between modulation bandwidth and extinction ratio, with broader bandwidth achievable by reducing modulator length but at the cost of decreased extinction ratio, and varying photocurrent affects electrical characteristics, leading to impedance mismatch and increased noise.
Innovation Solution
A series configuration of electro-absorption optical modulators with shorter lengths closer to the light source and adjusted electrode arrangements to equalize optical absorptions and electrical characteristics, using a common drive circuit to synchronize voltage application and reduce noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the modulator length is reduced to broaden modulation bandwidth, then the modulation bandwidth is improved, but the extinction ratio decreases
Solution Approach 1:
The patent divides a single long modulator into multiple shorter modulators connected in series. Each modulator has an optimized length that balances bandwidth and extinction ratio, while the series connection achieves the required total extinction ratio through cumulative effect. This segmentation allows each unit to operate at optimal bandwidth without sacrificing overall extinction ratio.
2Reliability
If multiple EA modulators are arranged in series to improve extinction ratio, then the extinction ratio is improved, but the electrical characteristics vary due to different photocurrents
Solution Approach 1:
The patent introduces local compensating elements (resistors and capacitors) at specific positions in the drive circuits of individual modulators. These elements are tailored to each modulator's photocurrent characteristics, compensating for variations in electrical parameters. This local adjustment equalizes the electrical characteristics across all modulators without requiring complete redesign of the entire drive system.
3Reliability
If the modulator length is increased to improve extinction ratio, then the extinction ratio is improved, but the capacitance increases reducing modulation bandwidth
Solution Approach 1:
Instead of using one long modulator with high capacitance, the patent segments the total modulation function across multiple shorter modulators. Each modulator has lower individual capacitance enabling higher bandwidth, while the series configuration achieves the cumulative extinction ratio of a longer modulator. The total capacitance seen by the drive circuit is reduced due to the series connection effect.
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 allows for simultaneous achievement of target characteristics for modulation bandwidth and extinction ratio with a simple circuit design, reducing noise and manufacturing costs by equalizing electrical characteristics across modulators.
Implementation Method 1
an electro-absorption optical modulator (EA modulator) using an electro-absorption effect of a semiconductor. The electro-absorption effect is a phenomenon that the difference in energy levels between the conduction band and the valence band is varied by applying an electric field to a semiconductor having a quantum well structure, and thus, the absorption of the light propagating through the quantum well structure varies.
Implementation Method 2
the EA modulator varies the absorption of the light propagating through the optical waveguide layer in accordance with the voltage applied to the PIN junction to thereby realize the intensity modulation of the light.
Data Source
AI summary
Preferable simultaneous achievement of the target characteristics with respect to both of the modulation bandwidth and the extinction ratio in the optical intensity modulation using the electro-absorption optical modulator is realized with a simple circuit configuration. The modulator integrated semiconductor laser element includes a plurality of EA modulators disposed in series in an optical signal path, and each adapted to absorb light in accordance with an applied voltage. The modulator driver for supplying the EA modulator with the applied voltage is provided for each of the EA modulators. The plurality of modulator drivers generates the applied voltage common to the plurality of EA modulators in accordance with a control signal. The modulator lengths of the plurality of EA modulators are set so that the closer to the light source the EA modulator is, the shorter the modulator length is.