Joint Auto-Bias Control for MZM-Based High Extinction Ratio Optical PPM Modulation

Pulse-position-modulation (PPM) with photon-counting detectors has gained significant attention in deep-space optical communications due to its high energy efficiency and massive bandwidth potential. However, the non-ideal characteristics of photon-counting detectors and the sensitivity of the Mach-Zehnder modulator (MZM) to environmental disturbances pose challenges in maintaining a high extinction ratio (ER). This paper introduces a Joint Auto-Bias Control (JABC) Algorithm to address these issues by optimally setting and continuously tracking both the MZM bias voltage and the peak-to-peak voltage of the driving electrical signal. First, we derive the control criteria for both the bias voltage and peak-to-peak voltage in an MZM-based PPM system, establishing a comprehensive framework to achieve the desired ER. Next, an integrated JABC algorithm is proposed. During initialization, the bias and peak-to-peak voltages are jointly optimized for maximal ER. Subsequently, the algorithm employs an early-late gate tracking mechanism to counteract phase drift in real time, ensuring maximum ER output. A theoretical model is also developed to characterize phase bias voltage jitter under environmental disturbances. Finally, extensive experiments validate the efficacy of the proposed approach, demonstrating that the JABC algorithm reliably maintains an optimal ER despite fluctuations in temperature and other system parameters. Finally, extensive experiments validate the efficacy of the proposed approach, demonstrating that the JABC algorithm reliably maintains an optimal ER of approximately 16.5 dB despite temperature and system parameter fluctuations, with stability within $\pm 0.5$ dB over 30 minutes of continuous operation. Moreover, the results highlight a strong dependency of ER on DFB laser power stability, underscoring the importance of robust bias control under deep-space optical communication conditions.