Integrated Optical Modulator Operating Point Stability

Lithium niobate based Mach-Zehnder modulators are widely used in digital opticalcommunication systems, microwave photonic links and various photonics switchingsystems, but the operating bias point of the modulator is known to drift over time due toenvironment perturbations and aging effects. Therefore high precision bias controlbecomes very critical for application where either amplitude or phase information fromM-Z modulators determines the system performance. Conventional lithium niobate M-Zmodulator bias control techniques is input/output optical power ratio based feedbackcontrol. Because the input/output optical power ratio from the modulator is monitored,it can only serve as the feedback signal for the particular operating bias point.Unfortunately, the feedback ratio signal stongly depends on the modulator's input powerlevel fluctuation, the control precision is not too high. Another RF dither tone harmonicsnulling based feedback control can control the opertating bias point more precisionly,but it also depends on the input optical power level. The ratio of the first order harmonicsignal and second order harmonic signal sever as the feedback signal in this paper, thistechnique can improve the control precision of the system to a great extent. The mainwork is provided as follows:1,The drift of the modulator's bias point is analyzed, and the simulated result isused to analyze the effect to the gain, noise and SFDR of the Rof link in theory, wherethe bias phase is different.2,A novel bias control technique is demonstrated for locking a LN MZM at anyarbitrary biasing point, which use the ratio of first order harmonic signal and secondorder harmonic signal as the feedback signal.The system framework is designed.3,The hardware system of the bias control system is proposed, the circuitdiagram of every module are designed detailedly in this paper. The PID controller'sproportion coefficient, differential coefficient and integrating factor are obtained bybuilding the model of the system.4,The harmonic signal measuring experimental investigations are implemented.The result of experiment demonstrated that the output signal of the modulator issuitable for the control system's measuring and the ratio of the two signals isindependent of the optical input power, which is consistent with theoreticalanalysis.This control system can response when the bias phase drift less than 5 degreesfrom the linear operating point.