In this paper, a method to automatically control the bias voltages of LiNbO<sub>3</sub> Mach–Zehnder modulator (MZM) is proposed and demonstrated experimentally. The automatic bias control (ABC) method is based on low cost coherent detection employing 3 × 3 optical coupler, which enhances the dither detection sensitivity. By minimizing the monitored dither frequency components which represents the residual carrier component in the optical signal, the modulator biases are optimized. A 0–π/2 square-wave phase modulation is introduced to mitigate the detection fluctuation induced by the imbalance of 3 × 3 coupler and improve the monitoring accuracy. Experiments show that the proposed method is universal for different modulation formats. High bias control precision is achieved for inner in-phase/quadrature (I/Q) branches and outer phase bias of optical quadrature modulator. The penalty caused by the proposed ABC is negligible for both single carrier (SC) modulation and optical orthogonal frequency division multiplexing modulation. The tolerance to temperature disturbance is also demonstrated.
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