Optical Modulator Bias Control for Multilevel QAM Signals
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Solution Overview
Problem
Existing techniques fail to appropriately adjust the bias voltage of an optical modulator for modulation schemes where the drive electrical signal can have more than two voltage values, such as 16 QAM or 64 QAM, limiting their effectiveness in maintaining optimal transmission characteristics.
Innovation Solution
A transmitter system that includes an optical modulator, a drive signal generator, a deviation identification signal generator to amplify a dither signal, and a bias control unit to adjust the bias voltage based on the deviation, ensuring the bias voltage is optimized for various modulation schemes by generating a uniform signal value corresponding to the transmission characteristics of the optical modulator.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a bias voltage adjustment technique using a low-frequency signal is applied, then the bias voltage can be controlled for binary modulation schemes (BPSK, QPSK), but it cannot appropriately adjust the bias voltage for multilevel modulation schemes (16 QAM, 64 QAM) where the drive electrical signal has more than two voltage values
Solution Approach 1:
The patent changes the parameter of drive signal amplitude to include multiple voltage levels (more than two) while maintaining the low-frequency dither signal technique. By adjusting the amplitude parameter of the drive signal to support multilevel modulation, the system can appropriately adjust bias voltage for 16 QAM, 64 QAM and other high-order modulation schemes, resolving the limitation of binary modulation only.
Solution Approach 2:
The patent makes the bias voltage adjustment technique universal across different modulation schemes by designing the drive signal to accommodate both binary (BPSK, QPSK) and multilevel (16 QAM, 64 QAM) modulation formats. The same low-frequency dither signal method is applied universally to all modulation types, eliminating the need for separate adjustment techniques for different schemes.
2Reliability
If the drive voltage-optical transmission characteristics are adjusted to an optimum point, then the eye opening of transmitted light waveform is increased and power penalty in receiver is minimized, but it becomes difficult to maintain the optimum point due to individual differences, temperature changes, and variations with time
Solution Approach 1:
The patent applies a low-frequency dither signal to the drive signal and measures the resulting optical output to detect deviations from the optimum operating point. This feedback mechanism continuously monitors transmission characteristics and automatically adjusts the bias voltage to maintain the optimum point, compensating for individual differences, temperature changes, and temporal variations without requiring manual intervention.
Solution Approach 2:
The system performs self-adjustment of the bias voltage by using the low-frequency dither signal to automatically detect and correct deviations from the optimum operating point. The optical modulator system serves itself by continuously optimizing its own bias voltage based on real-time transmission characteristics, eliminating the need for external manual adjustment and ensuring long-term stability.
Data Source
AI summary
A transmitter includes an optical modulator, a drive unit, a deviation identification signal generation unit, and a bias control unit. The deviation identification signal generation unit generates a signal value in accordance with a deviation of a bias voltage of the optical modulator by amplifying a signal value of a dither signal extracted from a modulated signal output from the optical modulator by a factor set for each of a plurality of modulation schemes so as to generate a uniform signal value. The bias control unit generates a bias value at which a center of a variation range, which is a range from a maximum value to a minimum value of the drive signal, corresponds to transmission characteristics of the optical modulator according to one predetermined adjustment method by using the signal value generated by the deviation identification signal generation unit.