Apparatus and method for controlling an optical modulator

There is provided a control apparatus for an optical modulator, the optical modulator comprising: a first modulation section (10) configured to modulate light with a data signal (DATA) to generate a modulated signal light; and a second modulation section (20) connected to the first modulation section (10) in series. The second modulation section (20) includes a first electrode (20A) and a second electrode (20B) respectively provided drive signals (CLK1, CLK2) to drive the second modulation section (20). The control apparatus comprises: a drive section (11, 21, 23, 25, 25A, 25B) configured to provide the data signal (DATA) to the first modulation section (10), and to provide the drive signals (CLK1, CLK2) to the first and second electrodes (20A, 20B) of the second modulation section (20), respectively; an output monitoring section (40) configured to photoelectrically convert a signal light outputted from said optical modulator to detect an electric spectrum, and to detect the intensity of a specific frequency component from the detected electric spectrum, the intensity being changed most largely corresponding to a relative phase difference between the data signal (DATA) and one of the drive signals (CLK1, CLK2); and a control section (50) configured to detect the relative phase difference based on the detected intensity, and to adjust the relative phase difference between the data signal (DATA) and one of the drive signals (CLK1, CLK2) so as to maximize the detected intensity..