Bias Control Circuit for Multi-Level QAM Optical Transmitters
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Solution Overview
Problem
Recent variations in optical IQ modulators, such as imbalanced non-linear optical effects in semiconductor-type modulators and thermal cross-talk in thermally expanded waveguides, make it difficult for existing bias control techniques to quickly converge and maintain stable bias values, especially during the start-up sequence.
Innovation Solution
A bias control circuit that includes a first and second bias power supply to set optical phase modulation units to zero levels, an optical phase shifter for phase difference adjustment, and a control unit that iteratively records candidate bias voltages to converge optical power near maximum or minimum values, determining the optimal bias voltage by calculating differences between recorded pairs, while adjusting the third bias voltage within a half-wave voltage range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If existing bias control techniques are used, then the system can operate, but it takes a long time to converge and maintain stable bias values during start-up
Solution Approach 1:
The patent applies preliminary action by pre-setting the orthogonal bias voltage to a specific value before initiating the bias control process. This preliminary configuration of the optical phase shifter creates optimal initial conditions that enable the bias control circuit to converge much faster during start-up, directly addressing the convergence time issue while maintaining stability.
Solution Approach 2:
The patent implements dynamics by making the optical phase shifter adjustable and controllable during the bias control process. The orthogonal bias voltage is dynamically modified based on the control circuit's requirements, allowing the system to adapt during start-up and achieve rapid convergence while maintaining stable operation throughout the process.
2Reliability
If existing bias control techniques are used, then the system can operate, but bias values become unstable due to imbalanced modulators and thermal cross-talk
Solution Approach 1:
The patent applies preliminary anti-action by pre-configuring the orthogonal bias voltage to counteract the effects of imbalanced modulators and thermal cross-talk before they can destabilize the bias control. This preliminary compensation creates a foundation that resists the harmful effects during the entire bias control process, maintaining stability despite the presence of imbalanced modulators and thermal interactions.
Solution Approach 2:
The patent uses the optical phase shifter as an intermediary element that mediates between the biased modulators and the final output. By controlling the orthogonal bias voltage through this intermediary, the system can compensate for imbalances and thermal cross-talk effects, isolating the harmful factors from directly affecting the bias stability.
3Measurement precision
If the optical phase difference is adjusted to achieve the best optical QAM signal, then signal quality improves, but the adjustment becomes extremely severe due to the small wavelength scale
Solution Approach 1:
The patent applies copying by using voltage control to replicate and scale up the effect of optical phase adjustment. Instead of directly adjusting the optical phase with extreme precision, the system copies the phase control function through voltage application to the optical phase shifter, making the adjustment process much more manageable while achieving the same signal quality improvement.
Solution Approach 2:
The patent implements parameter changes by transforming the control parameter from direct optical phase adjustment to voltage control of the optical phase shifter. This parameter transformation makes the adjustment process easier to operate while maintaining the precision needed for optimal optical QAM signal quality, effectively decoupling the severity of optical wavelength scale from the control mechanism.
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 earlier convergence of bias values for multi-level QAM optical transmitters, stabilizing bias control and avoiding false optimum points, even with imbalanced modulators and interactions among biases.
Implementation Method 1
an optical phase shifter that delays at least one of output light from the first optical phase modulation unit and output light from the second optical phase modulation unit to generate an optical phase difference of +π/2 or −π/2
Implementation Method 2
a first optical phase modulation unit that changes the phase or intensity of input light in accordance with a first n-ary data signal
Implementation Method 3
Each of the first optical phase modulation unit 2 and the second optical phase modulation unit 3 is usually configured by a Mach-Zehnder interferometer (MZI)-type optical modulator
Implementation Method 4
a second optical phase modulation unit that changes the phase or intensity of input light in accordance with a second n-ary data signal
Implementation Method 5
Each of the first optical phase modulation unit 2 and the second optical phase modulation unit 3 is usually configured by a Mach-Zehnder interferometer (MZI)-type optical modulator
Implementation Method 6
a combining unit that combines the output light from the first optical phase modulation unit and the output light from the second optical phase modulation unit
Implementation Method 7
an optical power monitor that monitors optical power of output light from the optical modulator
Data Source
AI summary
A control unit of a bias control circuit performs a loop process that fixes a second bias voltage and iterates a process of recording a pair of a first candidate bias voltage and a second candidate bias voltage that are a first bias voltage when optical power of a multi-level QAM signal output by an optical modulator is controlled so that the optical power converges to a value in the vicinity of the maximum value or the minimum value before and after a third bias voltage is increased or decreased by a half-wave voltage while changing the second bias voltage within a predetermined range. The control unit calculates the difference between the first candidate bias voltage and the second candidate bias voltage for each of a plurality of recorded pairs and determines a value between first candidate bias voltage and the second candidate bias voltage of a pair selected on the basis of the calculated difference as the value of the first bias voltage.