A technology to automatically control the biasing of an optical duobinary transmitter using a single-drive LN-MZ modulator is provided. A low-frequency signal is amplitude modulated onto a voltage signal input into a Mach-Zehnder optical modulator 22. The optical output from the optical modulator 22 is detected by an optical detection subsystem 30. In a bias control subsystem 40, the low-frequency signal component amplitude modulated onto the electrical signal is detected from the optical output, and a DC bias voltage applied to the optical modulator is controlled such that the low-frequency signal component is either minimized or maximized.