Optical Transmitter Bias Control via Multi-Frequency Dithering
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Solution Overview
Problem
In optical coherent transceivers using Mach-Zehnder electro-optic modulators, phase fluctuations caused by external noise and disturbances degrade the quality of the main signal, especially with higher degree multilevel modulation schemes like 64-QAM, where the influence of noise on phase modulation becomes significant due to narrower symbol spacing.
Innovation Solution
An optical transmitter is designed with an electro-optic modulator, a monitor circuit, and a microprocessor that superimposes dither signals of different frequencies onto the bias voltage, calculating control errors from these frequencies to determine a control value for bias voltage adjustment, thereby reducing the influence of noise and maintaining signal quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If higher degree multilevel modulation (e.g., 64-QAM) is used to expand transmission capacity, then productivity increases, but the system becomes more sensitive to phase fluctuations caused by disturbance or noise
Solution Approach 1:
The patent implements feedback control by monitoring the optical output of the electro-optic modulator and using a microprocessor to adjust the bias voltage based on detected phase deviations. This closed-loop system continuously compensates for phase fluctuations caused by noise and disturbance, maintaining signal quality despite using high-order modulation schemes like 64-QAM that are more sensitive to phase variations.
Solution Approach 2:
The patent applies periodic dither signals at multiple frequencies to the bias voltage of the electro-optic modulator. By superimposing dither signals with different frequencies and analyzing the resulting optical output, the system can distinguish between actual phase deviations and noise-induced fluctuations. This periodic probing enables selective suppression of noise while maintaining accurate bias control for high-order modulation.
2Reliability
If feedback control is performed to compensate for bias drift, then reliability improves, but the system becomes more complex
Solution Approach 1:
The patent introduces a monitor circuit as an intermediary that extracts a portion of the optical output to measure phase deviations. This separate monitoring path allows the control system to detect bias drift and phase fluctuations without interfering with the main signal transmission, enabling feedback control while maintaining system simplicity through functional separation.
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 effectively suppresses the impact of external noise on phase fluctuations, ensuring stable bias control and maintaining the quality of the main signal, even under conditions of higher multilevel modulation.
Implementation Method 1
Mach-Zehnder electro-optic Modulators (MZMs) to perform quadrature phase shift keying (QPSK)
Implementation Method 2
a monitor circuit that monitors the output light of the electro-optic modulator
Data Source
AI summary
An optical transmitter includes an electro-optic modulator, a monitor circuit that monitors output light of the electro-optic modulator, and a processor that controls a bias voltage of the electro-optic modulator using a monitoring result of the monitor circuit, wherein the processor superimposes a first dither signal with a first frequency and a second dither signal with a second frequency different from the first frequency, onto one bias voltage in a time sharing manner, calculates a first control error based on a first component oscillating at the first frequency and a second control error based on a second component oscillating at the second frequency based on the monitoring result, and determines a control value for controlling the bias voltage using the first control error and the second control error.