Optical Transmitter IQ Modulator DC Bias Control
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Solution Overview
Problem
IQ modulators used in QAM formats for optical transmitters face challenges in maintaining stable and reliable modulated lightwave signals due to DC bias drifts, particularly at start-up or reset, which existing Auto Bias Control (ABC) circuits with dither frequencies cannot fully address, leading to local extrema issues and incorrect bias settings.
Innovation Solution
An optical transmitter method involving an ABC circuit that keeps the optical amplitude constant, converges operating point values to predetermined settings, and modulates light with multiple amplitudes and phase levels around these converged values to stabilize the signal, avoiding local minima and ensuring correct bias settings for QAM formats.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If IQ modulator is used for QAM format to increase spectral efficiency, then link capacity is improved, but DC bias drift occurs leading to incorrect bias settings and signal degradation
Solution Approach 1:
The patent applies preliminary action by setting the optical amplitude to a constant value before convergence to predetermined operating point values. This preliminary step ensures that the modulator is properly initialized before QAM modulation begins, preventing DC bias drift and incorrect bias settings that would otherwise degrade signal reliability.
Solution Approach 2:
The patent utilizes parameter changes by dynamically adjusting the optical amplitude during the convergence process. The amplitude is changed from a constant initial value to converged operating point values, allowing the system to adapt to temperature variations and aging effects while maintaining reliable signal transmission for high-capacity QAM formats.
2Stability of the object's composition
If Auto Bias Control circuit with dither frequency is used to control DC bias, then bias stability is improved, but local extrema issues occur at start-up causing incorrect bias settings
Solution Approach 1:
The patent applies preliminary action by establishing a constant optical amplitude condition before initiating the convergence process to predetermined operating point values. This preliminary setup prevents local extrema issues at start-up by ensuring the modulator begins from a known stable state, allowing the ABC circuit to accurately set biases without encountering incorrect local minima.
3Stability of the object's composition
If constant optical amplitude is maintained during convergence, then operating point stability is improved, but modulation flexibility is reduced
Solution Approach 1:
The patent applies preliminary action by maintaining constant optical amplitude only during the convergence phase to predetermined operating point values. Once convergence is achieved, the system transitions to normal QAM modulation operations, thereby maintaining operating point stability during initialization while preserving modulation flexibility during actual data transmission.
Solution Approach 2:
The patent utilizes periodic action by applying constant amplitude conditions periodically during convergence phases (such as at start-up or after reset), while allowing full modulation flexibility during normal operation phases. This periodic application of constant amplitude ensures stability when needed without permanently restricting modulation capabilities.
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 the optical transmitter to emit stable and reliable modulated lightwave signals by effectively controlling DC biases and quadrature angles, ensuring correct modulation even at start-up and during operation, thus overcoming the limitations of existing ABC circuits with QAM formats.
Implementation Method 1
The children MZM modulate the phase and amplitude of the same optical carrier wave
Implementation Method 2
The phase in one of their outputs is relatively delayed by 90 degrees before being recombined
Data Source
Figure 1
Figure 2A~2B
Figure 2C~2D
AI summary
Since it is difficult to emit a stable and reliable modulated lightwave signal by means of IQ modulators used for QAM format, a method for controlling an optical transmitter according to an exemplary aspect of the invention includes the steps of (a) keeping an optical amplitude of a continuous wave light output from the optical transmitter constant, (b) making operating point values in optical modulation converge to predetermined values during step (a), and (c) modulating the continuous wave light with multiple amplitudes and phase levels around the operating point values converged in step (b).