Silicon MZM Bias Control with Single-Detector 3-Level Dither
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Solution Overview
Problem
Existing automatic bias control (ABC) methods for silicon Mach-Zehnder modulators (MZMs) in optical communications require two monitor photodetectors, leading to additional optical insertion loss and driver chip complexity, while methods for LiNbO3 MZMs face inherent second-order harmonic offsets due to nonlinear phase shifts with heater power.
Innovation Solution
A system using a single monitor photodetector and an uneven spacing 3-level square wave dither for silicon MZMs, which generates heater power with no inherent second-order harmonic, allowing accurate bias control by detecting the second-order harmonic in photocurrent to lock and track the quadrature point.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If two monitor photodetectors are used for automatic bias control of silicon MZM, then bias control accuracy is improved, but optical insertion loss increases and device complexity increases
Solution Approach 1:
The patent extracts and eliminates the need for the second monitor photodetector and its associated optical tap from the system. By using only one monitor photodetector with an uneven spacing 3-level square wave dither sequence, the system achieves bias control without the additional optical insertion loss and device complexity introduced by the second mPD and tap structure.
Solution Approach 2:
The patent changes the dither sequence from conventional sinusoidal or two-level square wave to an uneven spacing 3-level square wave. This parameter change in the dither signal characteristics enables the system to generate the necessary second-order harmonic information for bias control while using only a single monitor photodetector, thereby resolving the contradiction between measurement precision and device complexity.
2Reliability
If sinusoidal dither waveform is used for LiNbO3 MZM bias control, then bias control works well due to linear phase-shift relationship, but inherent second-order harmonic offset occurs in silicon MZM due to nonlinear heater power relationship
Solution Approach 1:
The patent changes the dither waveform parameters from sinusoidal to an uneven spacing 3-level square wave. This parameter change is specifically designed to work with the nonlinear heater power relationship in silicon MZMs, eliminating the inherent second-order harmonic offset that plagues sinusoidal dither approaches while maintaining reliable bias control performance.
Solution Approach 2:
The patent converts the harmful nonlinear effect of heater power in silicon MZMs into a beneficial feature. By using an uneven spacing 3-level square wave dither, the system exploits the nonlinear relationship between heater power and phase shift to eliminate the second-order harmonic offset, turning what was previously a source of error into a mechanism that enables precise bias control.
3Measurement precision
If two-level square waveform dither is used to eliminate inherent second-order harmonic, then offset is reduced, but second-order harmonic is not generated in photocurrent making detection ineffective
Solution Approach 1:
The patent modifies the dither waveform parameters to create an uneven spacing 3-level square wave instead of a conventional two-level square wave. This parameter modification enables the system to eliminate the harmful inherent second-order harmonic offset while simultaneously generating the necessary second-order harmonic information in the photocurrent for effective detection and feedback control.
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 approach simplifies the driver chip design, reduces complexity and cost, and achieves stable, reliable bias control by locking the MZM to the quadrature point, enhancing performance in short-reach optical communications.
Implementation Method 1
light in the second arm receives heat for bias control and a phase section for data modulation
Implementation Method 2
silicon MZM which uses heater for phase tuning, the phase shift of MZM has a linear relationship with electric power applied to the heater
Implementation Method 3
providing a complementary output light to a monitor photodetector (mPD)
Implementation Method 4
splitting the laser light into a first arm and a second arm, wherein light in the first arm experiences a phase section for data modulation and light in the second arm receives heat for bias control and a phase section for data modulation; providing a first output light of the MZM
Data Source
AI summary
Systems and methods are disclosed to provide automatic bias control for intensity modulated silicon modulator by providing a laser light to a Mach-Zehnder modulator (MZM) and splitting the laser light into a first arm and a second arm, wherein light in the first arm experiences a phase section for data modulation and light in the second arm receives heat for bias control and a phase section for data modulation; providing a first output light of the MZM; providing a complementary output light a monitor photodetector (mPD); and applying an uneven spacing 3-level square wave as dither for automatic bias control of MZM which operates at quad point and phase tuning with a heater.