Mach-zehnder optical modulator and optical modulation method

An optical modulation method that is performed by a Mach-Zehnder optical modulator that includes a first section and a second section that are arranged in at least one arm out of two arms in an optical waveguide through which an optical signal passes, and that optical modulates the optical signal into a quaternary or more value optical modulation signal by applying a driving signal to the first section and the second section, the method comprising:

• creating a first driving signal to be applied to the first section, and a second driving signal to be applied to the second section;
• generating a first dither signal to be superimposed on the first driving signal, and a second dither signal to be superimposed on the second driving signal;
• optical modulating the optical signal into the quaternary or more value optical modulation signal by applying the first driving signal on which the first dither signal is superimposed to the first section, and by applying the second driving signal on which the second dither signal is superimposed to the second section;
• and
• setting, when a length of the second section is n times as long as the length of the first section, the first dither signal and the second dither signal to have a same phase, and such that an amplitude of the first dither signal is n times as large as the amplitude of the second dither signal.