Patent ID: 8094692

Claim:
A modulation method in a modulation device including a modulation circuit that passes a bias current through a modulation target that is a direct modulation laser or an external optical modulator, the modulation target being capacitively coupled to the modulation circuit, the modulation circuit driving the modulation target with a differential electric signal including a normal phase and a reverse phase; and a control circuit that sends a control signal to the modulation circuit to control the modulation circuit based on a signal providing an instruction to permit or prohibit transmission, wherein, when fluctuations in average potentials at a normal phase input terminal and a reverse phase input terminal of the modulation target are generated, the control circuit causes the modulation circuit to control at least one of average potentials at the normal phase input terminal and the reverse phase input terminal and the normal phase output terminal and the reverse phase output terminal of the modulation circuit such that transient states of the average potentials generated before and after average potential fluctuation become identical to each other, and the transient state of the average potential at the normal phase input terminal and the transient state of the average potential at the reverse phase input terminal cancel each other as in-phase components of the differential electric signal.