Abstract: An electro-optical device includes an amplifier having an input receiving a data signal, a crossing adjustment input, and an output. The device includes a light source and a Mach-Zehnder modulator having a first input coupled to the output of the amplifier, an optical input coupled to the laser, a bias input, and an optical output. A bias circuit is coupled to the bias input of the Mach-Zehnder modulator and preferably includes a pilot signal generator connected to the crossing adjustment input of the amplifier, and a feedback circuit coupled between the optical output and bias input of the Mach-Zehnder modulator and coupled to the pilot signal generator. The bias circuit applies the pilot signal to the crossing adjust input of the amplifier which forces the modulator to the correct bias point for duobinary transmission. Modulation on +1 and ?1 levels is eliminated, and only the 0 level is modulated. The feedback loop forces modulated portion of the waveform to null bias.