DQPSK Modulator Phase Delay Offset Control via Pilot Signal
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Solution Overview
Problem
Current methods for controlling the phase delay offset point of lithium niobate modulators in DQPSK modulation systems suffer from low accuracy and high implementation costs, due to the sensitivity of lithium niobate modulators to temperature and stress, and the complexity of existing control circuit designs.
Innovation Solution
A method and apparatus that acquire backlight detection current signals from the modulator in different states, determine harmonic amplitude values, and compare these values to a target value to accurately control the phase delay offset point, improving accuracy and flexibility.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the first control method (difference-frequency signal filtering) is used to control the phase delay offset point, then the modulator can be locked to a common offset point, but the accuracy of controlling the offset point position is low because the output amplitude of difference-frequency signals is very small and difficult to detect accurately
Solution Approach 1:
The patent applies periodic action by introducing a pilot signal with a specific frequency (e.g., 100MHz) that is modulated onto the optical carrier. This periodic signal undergoes phase modulation along with the data signal, and after photodetection, produces a difference-frequency signal at a known frequency. By filtering and detecting this periodic difference-frequency signal, the system can accurately determine when the modulator is properly locked to the offset point, solving the detection accuracy problem of the first method.
Solution Approach 2:
The patent uses the pilot signal as an intermediary to indirectly measure the offset point position. Instead of directly detecting the small difference-frequency signals produced by the modulator operation, the system introduces an external reference signal that interacts with the modulator's phase modulation process. The pilot signal's known frequency and phase characteristics serve as a mediator, allowing accurate detection of the offset point through the resulting difference-frequency signal after photodetection.
2Measurement precision
If the second control method (direct sampling for RF harmonic signal detection) is used to control the offset point, then the offset point position can be determined by detecting RF harmonic signals, but the circuit design becomes relatively complex and the implementation cost increases
Solution Approach 1:
The patent simplifies the complex direct sampling method by using periodic action. Instead of continuously sampling and analyzing the entire broadband signal for RF harmonics, the system introduces a periodic pilot signal at a specific frequency. This creates a periodic difference-frequency signal that can be easily filtered and detected using simple band-pass filters and envelope detectors, dramatically reducing circuit complexity while maintaining accurate offset point detection.
Solution Approach 2:
The patent extracts only the necessary information for offset point detection by using a pilot signal at a specific frequency. Instead of analyzing the entire broadband signal spectrum for RF harmonics (which requires complex circuitry), the system extracts the difference-frequency component at the known pilot frequency through simple filtering. This extraction approach isolates the critical detection signal from the complex background, simplifying the control circuit while maintaining detection accuracy.
3Manufacturing precision
If peripheral control circuits are added to the lithium niobate modulator to achieve accurate phase control, then the phase control accuracy improves, but the device complexity and implementation cost increase
Solution Approach 1:
The patent enables the modulator to self-adjust and self-lock to the correct offset point position through the feedback mechanism provided by the pilot signal. The system automatically detects the offset point position through the difference-frequency signal and adjusts the modulator's operating point accordingly, eliminating the need for complex external control circuits. The modulator essentially serves itself by using the pilot signal as an internal reference for automatic locking.
Solution Approach 2:
The patent implements a feedback control mechanism where the pilot signal's difference-frequency component after photodetection provides information about the modulator's offset point position. This feedback information is used to automatically adjust and lock the modulator to the correct operating point. The feedback loop continuously monitors the offset point position through the pilot signal and makes real-time adjustments, achieving accurate phase control without requiring complex open-loop control circuits.
Data Source
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AI summary
The present invention provides a method and an apparatus for controlling a phase delay offset point of a modulator. The method comprises: acquiring backlight detection current signals output from a modulator in different working states, and determining harmonic amplitude values of the backlight detection current signals corresponding to the different working states; determining a detection value of a phase delay offset point corresponding to the modulator according to the determined harmonic amplitude values; comparing the detection value with a set target value of the phase delay offset point, and controlling a position of the phase delay offset point corresponding to the modulator according to the comparison result. The accuracy of controlling the position of the phase delay offset point of the modulator and the performance of the Differential Quadrature Phase Shift Keying (DQPSK) modulation system are improved through the technical solution.