Automatic bias control suitable for a microwave photonic vector modulator

摘要

The microwave photonic vector modulator (MPVM) sometimes shows an unmodulated carrier peak, which appears in the modulated signal. This peak results from an incorrect bias value or a Mach-Zehnder modulator (MZM) bias drift over time, which in turn leads to an imbalance in the I/Q signals. This paper describes a mathematical model of an MPVM output by showing the unmodulated carrier peak behavior. It also discusses a constellation imbalance resulting from this non-modulated peak. To control this non-modulated carrier peak and restore the constellation balance, an automatic bias control (ABC) is proposed and simulated through a co-simulation technique. Unlike previous methods, this ABC is based on the analysis of the I/Q signals to generate voltage steps for the photonic modulator bias. The I/Q signals were recovered through a direct-conversion receiver, and an algorithm was developed to analyze the I/Q amplitude and phase imbalance. Consequently, the photonic modulator bias is continuously monitored by analyzing the resulting modulated microwave signal. In this way, its operation is transparent under MZM bias variations or extinction ratio (ER) imperfection since only the I/Q and microwave signals are considered in the algorithm implementation. ABC is validated through a co-simulation technique using low and high-order modulation formats such as QPSK, 64QAM, and 256QAM.