One example includes a bias-based Mach-Zehnder modulation (MZM) system. The system includes a Mach-Zehnder modulator to receive and split an optical input signal and to provide an intensity-modulated optical output signal based on a high-frequency data signal to modulate a relative phase of the split optical input signal to transmit data and based on a bias voltage to modulate the relative phase of the split optical input signal to tune the Mach-Zehnder modulator. The system also includes a bias feedback controller to compare a detection voltage associated with the intensity-modulated output signal with a reference voltage to measure an extinction ratio associated with an optical power of the intensity-modulated optical output signal and to adjust the bias voltage based on the comparison to substantially maximize the extinction ratio.