Design and Implementation of an Automatic Bias Controller for Drift Free Signal Generation Using Mach-Zehnder Electro-Optic Modulator’s Monitor Photodiodes

Abstract

This paper presents the design, implementation, and experimental validation of a low-cost, dither-free automatic bias controller for a Mach-Zehnder modulator (MZM). To overcome the inherent operating-point drift in lithium niobate (LiNbO3)-based modulators caused by thermal fluctuations and aging, we developed a closed-loop feedback system that uses the MZM’s integrated monitor photodiode. The controller locks to the operating point at quadrature bias perfectly for RF-over-Fiber (RFoF/AM) or null bias for DSB-SC and BPSK modulation formats. It has a Raspberry Pi interface and an STM32F411CEU6 microcontroller development board. Experimental results with 10 mW and 50 mW laser powers show that the system can maintain target photocurrents (e.g., 0.6 mA and 3 mA at quadrature) stable even when there is a change due to drift. This method is a cost-effective alternative to expensive commercial controllers for optical communication in the laboratory and in the field.