An optical transmitter having an optical modulator whose light intensity with respect to a driving signal fluctuates at a predetermined periodic cycle (see Patent Literatures 1-3) has been known. The optical transmitters of Patent Literatures 1-3 each control the amplitude of the driving signal such that the output light from the optical modulator comes to be in a state corresponding to an amplitude of the driving signal corresponding to the above periodic cycle or a state corresponding to the half the amplitude of the driving signal corresponding to the above periodic cycle. The optical transmitter of Patent Literatures 1-3 each control the amplitude of the driving signal and a bias voltage to be applied to the optical modulator on the basis of the power of the output light from the optical modulator.
For example, the Quadrature Phase Shift Keying (QPSK) modulating scheme drives an optical modulator with a driving signal having an amplitude twice the amplitude corresponding to driving from the minimum point to the maximum point of a extinction curve. An extinction curve expresses the relationship between the light intensity and the value of the driving signal. The light intensity comes to be the minimum at the minimum point of an extinction curve and comes to be the maximum at the maximum point of the extinction curve. The amplitude corresponding to the driving from the minimum point to the maximum point of an extinction curve is also expressed by Vπ. The amplitude twice the amplitude corresponding to the driving from the minimum point to the maximum point of an extinction curve is also expressed by 2Vπ.
Patent Literature 1: Japanese Laid-open Patent Publication No. 2008-92172
Patent Literature 2: Japanese Laid-open Patent Publication No. 2011-217003
Patent Literature 3: Japanese Laid-open Patent Publication No. 2007-208472