Dither-free low-bias controller for deeply modulated Mach-Zehnder modulators

Abstract

Stable bias control of lithium niobate Mach-Zehnder modulator (MZM) is crucial for radio-over-fiber (ROF) links, since the bias drift causes deviation from the optimum operating point. In this paper, we propose and experimentally demonstrate a novel dither-free low-bias control technique, especially for deeply modulated, low-biased analog photonic links. In our scheme, a second lightwave passes through the MZM, while the output power is locked. Since the reverse modulation efficiency is very low when the input radio frequency (RF) carrier is as high as GHz, the RF power shows little impact on the optical power even when the bias angle is very low, so that the bias stabilization is achieved. Experiment results show that bias locking at a low-bias position (about $-81^{\circ }$) with phase deviations less than $\pm 2.6^{\circ }$ is achieved even when the input RF voltage is as high as $1.3 \,V_{\pi }$. Our proposal needs no low-frequency dither, which is promising for ROF links.