Bias voltage control system and bias voltage control method

A bias voltage control system for an optical IQ modulator, the optical IQ modulator including an I-branch Mach-Zehnder Modulator to which an I-branch bias voltage is applied and which performs I-modulation on incident light, a Q-branch Mach-Zehnder Modulator to which a Q-branch bias voltage is applied and which performs Q-modulation on the incident light, and an optical phase shifter to which an orthogonal bias voltage is applied and which is connected in series with the I-branch Mach-Zehnder Modulator or the Q-branch Mach-Zehnder Modulator to perform a phase shift on an output thereof, the bias voltage control system comprising:

• a photo-detector configured to convert at least a part of an optical signal output by the optical IQ modulator to an electrical signal;
• an analog-to-digital converter configured to convert the electrical signal to a digital signal;
• and
• a digital signal processor configured to superimpose a first dither signal on the I-branch bias voltage and superimpose a second dither signal on the Q-branch bias voltage thereby generating a first order harmonic signal and a second order harmonic signal in the digital signal, and control the I-branch bias voltage, the Q-branch bias voltage and the orthogonal bias voltage according to powers of a DC component, the first order harmonic signal and the second order harmonic signal of the digital signal.