Optical Phase Modulator Bias Control for DQPSK Stability
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Solution Overview
Problem
Conventional optical transmitting apparatuses lack effective control over phase shifting and DC drift, particularly for phase shift keying techniques like DQPSK, leading to instability and accuracy issues due to temperature changes and aging, which affects the quality of optical signals.
Innovation Solution
The optical transmitting apparatus includes a phase modulator with a driving signal generation unit, a phase shift unit, a data modulation unit, a monitor unit, and a control unit to control the phase difference between split optical signals, ensuring proper phase modulation and stabilization by monitoring maximum power, minimum power, or phase of low-frequency signals, and adjusting the phase shift accordingly.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional NRZ modulation with ABC circuit is used, then bias control compensating for operating point deviation is achieved, but phase shifting control for phase shift keying is insufficient
Solution Approach 1:
The optical modulator is designed to perform multiple functions: it serves as both an intensity modulator for NRZ modulation with ABC circuit bias control, and as a phase modulator for phase shift keying operations. This multi-functionality allows the same device to handle both conventional intensity modulation and advanced phase modulation techniques, resolving the contradiction between reliability of bias control and adaptability for phase shift keying.
2Adaptability or versatility
If phase shift unit is added for phase shift keying, then phase shifting capability is improved, but control over phase shift amount and DC drift is insufficient
Solution Approach 1:
A feedback control mechanism is implemented where the optical output is monitored and the phase shift amount is adjusted based on the detected signal characteristics. The system monitors the optical signal after phase modulation and uses this information to control the phase shift unit, ensuring accurate phase shifting and compensating for DC drift through continuous feedback adjustment.
3Device complexity
If simple bias control is used, then device complexity is low, but temperature drift and aging effects cause signal degradation
Solution Approach 1:
The system implements self-service through automatic monitoring and adjustment mechanisms. The optical modulator continuously monitors its own output signal and automatically adjusts its operating point to compensate for temperature drift and aging effects. This self-service capability maintains signal quality stability without requiring complex external control circuits, resolving the contradiction between device simplicity and reliability.
Data Source
AI summary
A phase shift unit provides a prescribed phase difference (π/2, for example) between a pair of optical signals transmitted via a pair of arms constituting a data modulation unit. A low-frequency signal f0 is superimposed on one of the optical signals. A signal of which phase is shifted by π/2 from the low-frequency signal f0 is superimposed on the other optical signal. A pair of the optical signals is coupled, and a part of which is converted into an electrical signal by a photodiode. 2f0 component contained in the electrical signal is extracted. Bias voltage provided to the phase shift unit is controlled by feedback control so that the 2f0 component becomes the minimum.