Optical Transmitter Dither Control for QAM Signal Quality
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Solution Overview
Problem
In optical communication systems using QAM modulation, low frequency dither signals used for Auto Bias Control (ABC) cause signal degradation due to varying amplitude and phase of the emitted symbols, especially when the driving voltage is near the high slope area of the Mach-Zehnder Modulator's transmission curve, leading to reduced signal quality and precision in ABC control.
Innovation Solution
An optical transmitter device and method that combines two or more binary electrical signals with a dither signal of frequency lower than the bit rate, adjusts their amplitude, and modulates the optical signal using these signals, where the phase of the dither signal depends on the binary signal value and changes for each bit, to improve signal quality by reducing the amplitude variation caused by low frequency dither signals.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If low frequency dither signals are used for Auto Bias Control in QAM modulation, then ABC control can be implemented to compensate for DC bias drift, but signal degradation occurs due to varying amplitude and phase of emitted symbols especially when driving voltage is near the high slope area of the Mach-Zehnder Modulator's transmission curve
Solution Approach 1:
The patent applies local quality by making the dither signal amplitude dependent on the operating point of the modulator. Specifically, the dither amplitude is reduced when the driving voltage is near the high slope area of the transmission curve (where symbol points are located) and increased when the voltage is near the top of the curve (where null differential value occurs). This localized adjustment of dither amplitude compensates for the position-dependent signal degradation, maintaining ABC control precision without causing excessive signal distortion in critical regions.
Solution Approach 2:
The patent implements dynamics by making the dither signal amplitude variable rather than fixed. The system dynamically adjusts the dither amplitude based on the instantaneous driving voltage level and the slope of the transmission curve. This dynamic adaptation allows the ABC control to effectively track DC bias drift while minimizing signal degradation that would occur with a fixed dither amplitude, particularly in the high slope areas where QAM symbols are transmitted.
2Measurement precision
If dither signal amplitude is increased to improve ABC control precision, then measurement precision of bias drift is improved, but signal degradation increases due to larger amplitude variation of emitted symbols
Solution Approach 1:
The patent applies local quality by making the dither signal amplitude dependent on the operating point of the modulator. Specifically, the dither amplitude is reduced when the driving voltage is near the high slope area of the transmission curve (where symbol points are located) and increased when the voltage is near the top of the curve (where null differential value occurs). This localized adjustment of dither amplitude compensates for the position-dependent signal degradation, maintaining ABC control precision without causing excessive signal distortion in critical regions.
Solution Approach 2:
The patent changes the parameter of dither signal amplitude dynamically based on the modulator's operating conditions. By monitoring the driving voltage level and the slope of the transmission curve, the system adjusts the dither amplitude parameter to optimize the trade-off between measurement precision and signal quality. This parameter adaptation ensures sufficient dither amplitude for accurate bias drift detection while preventing excessive amplitude variation that would degrade the transmitted symbols.
3Object-affected harmful factors
If dither signal amplitude is reduced to minimize signal degradation, then amplitude variation of symbols is reduced, but measurement precision of ABC control deteriorates
Solution Approach 1:
The patent applies local quality by making the dither signal amplitude dependent on the operating point of the modulator. Specifically, the dither amplitude is reduced when the driving voltage is near the high slope area of the transmission curve (where symbol points are located) and increased when the voltage is near the top of the curve (where null differential value occurs). This localized adjustment of dither amplitude compensates for the position-dependent signal degradation, maintaining ABC control precision without causing excessive signal distortion in critical regions.
Solution Approach 2:
The patent implements dynamics by making the dither signal amplitude variable rather than fixed. The system dynamically adjusts the dither amplitude based on the instantaneous driving voltage level and the slope of the transmission curve. This dynamic adaptation allows the ABC control to effectively track DC bias drift while minimizing signal degradation that would occur with a fixed dither amplitude, particularly in the high slope areas where QAM symbols are transmitted.
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 enhances the quality of QAM signals by minimizing signal degradation and maintaining precision in ABC control, even when the modulator is driven at areas with different slopes on the transmission curve, thereby improving the overall signal quality and tolerance to noise and distortions.
Implementation Method 1
two independent Mach-Zehnder devices, which can be called children Mach-Zehnder Modulators (MZM), or nested MZM depending on the sources. The children MZM modulate the phase and amplitude of the same optical carrier wave
Data Source
AI summary
Since it is difficult to emit a control a IQ modulator emitting modulated lightwave according to QAM format without signal degradation of said emitted signal due to low frequency dither used for the control of said modulator, a method for controlling an optical transmitter according to an exemplary aspect of the invention includes: generating a multilevel electrical signal by means of combining two or more binary electrical signals, where said multilevel signal is used to drive the modulator of adding a low frequency dither signal on several of the binary electrical signals, wherein the phase of the added dither signal depends on the value of the binary signal to which it is added; tapping a portion of light after the modulator and generating a monitor signal from the tapped light; controlling the modulator according to the monitor signal.