Closed-Loop Bias Control of Optical Quadrature Modulator

We describe a relatively simple but effective closed-loop bias control of LiNbO
₃
quadrature modulator (QM) for optical quaternary phase-shift-keyed (QPSK) transmission. Simulation and experimental results of the feedback control loop for maintaining near optimal operating points of the QM are described for 12.5-GSym/s optical QPSK. The control loop uses nonhigh-speed off-the-shelf components independent of the symbol rate. Similar performance of the control loop at higher symbol rates using the same components is expected