Patent ID: 8400702

Claim:
A method for modulating an inputted light with a Mach-Zehnder modulator, the method comprising: supplying a drive signal to the Mach-Zehnder modulator; supplying a bias signal to the Mach-Zehnder modulator; modulating the inputted light with the Mach-Zehnder modulator based on the drive signal and the bias signal; receiving information of a selected modulation format that is designated by an external input and that is selected from a plurality of modulation formats; and controlling the drive signal and to which of the drive signal or the bias signal a predetermined frequency signal is superimposed so as to provide a bias voltage to the Mach-Zehnder modulator in accordance with a frequency component of the predetermined frequency signal in a modulated signal light from the Mach-Zehnder modulator, in accordance with the selected modulation format.