Optical Transmitter Bias Control via Nested MZI Dithering
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Solution Overview
Problem
Conventional auto bias control methods for electro-optic modulators in optical coherent transceivers face challenges in achieving both noise-tolerance and stability in convergence to a target bias voltage, as these requirements are contradictory and difficult to satisfy simultaneously.
Innovation Solution
The proposed solution involves a processor that controls the bias voltages of child and parent Mach-Zehnder interferometers by superimposing different dither signals and extracting phase error information from the output light, doubling the extraction frequency of phase error information for the parent Mach-Zehnder interferometer within a control loop to smooth noise while maintaining stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional auto bias control methods are used to extract phase error information from optical output, then bias voltage can be controlled to optimum point, but noise tolerance deteriorates due to low extraction frequency and instability in convergence
Solution Approach 1:
The patent applies periodic dither signals at different frequencies to the bias voltages of child MZMs and parent MZM, enabling periodic extraction of phase error information at multiple sampling points within one control loop. This periodic action increases the extraction frequency and provides multiple measurements per cycle, improving noise tolerance through statistical processing while maintaining precise bias control.
2Measurement precision
If dither signals are applied to extract phase error information, then bias control accuracy is improved, but system complexity increases due to multiple dither patterns and simultaneous equations
Solution Approach 1:
The patent segments the control process into distinct phases within a single control loop: first applying dither signals to child MZMs for their phase error extraction, then applying dither signals to parent MZM for its phase error extraction. This segmentation avoids the need to solve complex simultaneous equations while achieving accurate control of all MZMs through sequential processing.
3Speed
If multiple dither patterns are used for simultaneous control of child MZMs and parent MZM, then convergence speed is improved, but stability deteriorates due to interference terms between dither patterns
Solution Approach 1:
The patent employs asymmetric dither signal application where different dither frequencies and timing are assigned to child MZMs versus parent MZM. Specifically, dither signals are applied to child MZMs in the first phase and to parent MZM in the second phase within the same control loop, creating an asymmetric control sequence that eliminates interference between dither patterns while maintaining fast convergence.
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 enables stable convergence of bias voltages to optimum points while improving noise tolerance, effectively addressing the limitations of conventional methods by doubling the sampling frequency of phase error information for the parent Mach-Zehnder interferometer.
Implementation Method 1
Optical coherent transceivers that perform quadrature phase-shift keying (QPSK) using Mach-Zehnder electro-optic Modulators (MZMs)
Implementation Method 2
a first child Mach-Zehnder interferometer and a second child Mach-Zehnder interferometer are nested to form a parent Mach-Zehnder interferometer
Data Source
AI summary
In an optical transmitter having an electro-optic modulator with first child MZI and a second child MZI nested to form a parent MZI, and a processor that controls the bias voltages of electro-optic modulator. In the first section of a control loop, the processor simultaneously superimposes different dither signals onto the first bias voltage of the first child MZI and the second bias voltage of the second child MZI, and extracts the first phase error information for the first child MZI and the first-round third phase error for the parent MZI from a first monitoring result. In the second section of the control loop, the processor simultaneously superimposes different dither signals onto the first and second bias voltages, and extracts the second phase error information for the second child MZI and the second-round third phase error for the parent MZI from a second monitoring result.