Study on auto bias control of a silicon optical modulator in a four-level pulse amplitude modulation format

Recently, the high-speed silicon optical modulator has played a greater and greater role in optic fiber communication. Due to the silicon material nonlinear electro-optic effect, the quadrature (Quad) bias point of the modulator is difficult to be locked precisely. In this paper, a novel automatic bias control (ABC) method for four-level pulse amplitude modulation format silicon modulator is presented. First, a sinusoidal dither signal is applied to the modulator DC bias, and the normalized output optical signal function, which is modulated by the dither signal, is obtained. Then, a pair of orthogonal reference functions is created, whose numerical size is equal to the absolute value of sinusoidal and cosine functions so that the frequency is the same as the dither signal. Through the cross-correlation integral operation between the normalized and reference functions above, we find that the zero point of the above integral operation is the best Quad bias point through numerical simulation and experiment. The phase accuracy error of ABC is lower than 2 deg, and it satisfies the business specifications of a high-speed silicon optical modulator.