Optical Transmitter IQ Modulator Bias Control
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Solution Overview
Problem
Existing optical communication systems face challenges in accurately pre-equalizing signals transmitted through fiber optic cables due to uncertainties in DC bias control of IQ modulators, leading to suboptimal signal reception and increased network bandwidth requirements for correction.
Innovation Solution
An optical transmitter with an Auto Bias Control (ABC) system that converges DC biases of Mach-Zehnder modulators to null driving points, uses correlated training patterns, and monitors output to set quadrature angles effectively, ensuring accurate pre-equalization despite uncertainties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If DC biases of IQ modulators are controlled using conventional ABC circuits, then modulation accuracy is improved, but uncertainties in bias control remain due to periodicity of optimal bias points
Solution Approach 1:
The patent implements a feedback mechanism where the receiver monitors the received optical signal quality and sends control information back to the transmitter. The transmitter adjusts the DC biases of the IQ modulators based on this feedback to compensate for periodicity uncertainties and maintain optimal modulation accuracy under varying operating conditions.
Solution Approach 2:
The patent changes the operating parameters of the IQ modulators by dynamically adjusting DC biases based on feedback from the receiver. This allows the system to adapt to different transmission conditions and resolve the ambiguity in optimal bias points caused by periodicity, thereby improving both modulation and bias control precision.
2Reliability
If pre-equalization is performed at the transmitter to compensate for transmission impairments, then signal quality is improved, but network bandwidth requirements increase for correction
Solution Approach 1:
The patent applies pre-equalization at the transmitter by introducing digital signal processing that pre-compensates for known transmission impairments such as chromatic dispersion and nonlinearities. This preliminary action equalizes the signal before transmission, improving signal quality at the receiver without requiring additional bandwidth for correction, as the compensation is performed in the time domain rather than requiring extra frequency resources.
3Productivity
If IQ modulators are used for pre-equalized signal modulation, then modulation efficiency is improved, but DC bias drift occurs due to temperature and aging variations
Solution Approach 1:
The patent uses feedback from the receiver to monitor signal quality and detect DC bias drift in the IQ modulators. The transmitter adjusts the DC biases dynamically based on this feedback, compensating for temperature and aging effects to maintain stable modulation performance while preserving the high efficiency of IQ modulators.
Solution Approach 2:
The patent makes the DC bias control dynamic by continuously adjusting the bias points based on real-time feedback from the receiver. This dynamic adjustment allows the system to adapt to temperature changes and aging effects, maintaining optimal modulation efficiency while compensating for drift that would otherwise occur in static bias configurations.
Data Source
AI summary
Since it is difficult to control correctly and optimally the DC biases of an IQ modulator driven with pre-equalized data, a method for controlling an optical transmitter according to an exemplary aspect of the invention includes the steps of (a) making direct current biases for driving children Mach-Zehnder modulators of an IQ modulator in the optical transmitter converge to values close to null driving points of the children Mach-Zehnder modulators, (b) driving the children Mach-Zehnder modulators with special driving data including a pair of training patterns between which there is a significant correlation, (c) scanning direct current biases for setting quadrature angle of the IQ modulator, (d) monitoring output of the IQ modulator during step (c), and (e) setting the direct current bias for setting quadrature angle on the basis of the driving data and monitored results in step (d).