Modulator Bias Control Based on Bi-PID Algorithm for Optical Time and Frequency Transmission

To resist the bias drift of the Mach–Zehnder modulator (MZM), we proposed and demonstrated a dither-free bias control method based on the bidirectional proportion integration differentiation (Bi-PID) algorithm. We theoretically analyzed the characteristics of time and frequency signals and made the bias point stable at the null point and quadrature point through optical power feedback and harmonic component feedback, respectively. 726.6 km transmission experiments were conducted to measure the effectiveness of the proposed modulator bias control (MBC) method. The results demonstrated that the stability of the time-sync system with the proposed MBC was better than that of the traditional method in the long-term measurement, whose time deviation (TDEV) was 2.8×10 ^-12 @10 ⁴ s. The frequency stability characterized by Allan deviation (ADEV), reaching 3.1×10 ^-17 @10 ⁴ s, outperformed traditional methods as well.