Real-Time Bias Control Technique of Multistage Mach–Zehnder Modulators With Ultra-High Extinction Ratio Based on Dither Signal

Mach-Zehnder modulators (MZMs) are the most commonly used external modulators for optical communication links. The bias point of MZMs may drift due to environmental changes in practical applications, leading to system performance degradation. The latest decoy-state twin-field quantum key distribution (TF-QKD) system requires multiple modulators to generate multi-level light intensities and maintain an ultra-high extinction ratio between the signal-state light and vacuum-state light. This is essential to ensure the QKD security and extend the transmission distance. To meet this tight requirement, this paper presents an efficient bias control technique of multistage MZMs based on the dither signal method. Considering the complex signal processing algorithms involved in the dither signal method, this work designs a high-throughput hardware architecture based on field programmable gate array (FPGA) platform to realize real-time feedback control. It is comprised of three modules: the dither generation module, the fast Fourier transform (FFT) module, and the bias control module. Both static and dynamic extinction ratio tests were conducted to verify the performance of the system. Especially, this paper proposes an innovative and dynamic measurement method based on the highly sensitive single-photon detector to support the direct extinction ratio test of high-speed signals. The experimental results show that the system can stably maintain an ultra-high extinction ratio of 66.92 dB, demonstrating itself suitable for the majority of optical communication systems with very tight extinction ratio requirements. The feedback control time is about only 0.5 seconds per stage, which is fast enough to compensate for the usual environmental drift. In the temperature stability experiments, when the temperature varies between 26–64 °C, the proposed system achieves a 74% reduction of the extinction ratio fluctuation compared to the result without using the feedback system.