A bias-based Mach-Zehnder modulation, MZM, system comprising: 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 (V BIAS ) to modulate the relative phase of the split optical input signal to tune the Mach-Zehnder modulator; and a bias feedback controller to compare a detection voltage (DET) associated with the intensity-modulated output signal with a reference voltage (V REF ) to measure an extinction ratio associated with an optical power of the intensity-modulated optical output signal and to adjust the bias voltage (V BIAS ) based on the comparison to substantially maximize the extinction ratio, characterised in that the bias feedback controller comprises a bias controller to set an amplitude of the bias voltage (V BIAS ) in response to the comparison of the peak voltage with the reference voltage (V REF ); and a peak voltage detector to detect a peak voltage (V PK ) associated with a peak amplitude of the detection voltage (DET) wherein the bias controller is further to iteratively increase the reference voltage (V REF ) in response to detecting that the peak voltage (V PK ) is equal to the reference voltage (V REF ) and to iteratively increase the bias voltage (V BIAS ) based on the comparison until the bias controller detects that the peak voltage (V PK ) is equal to the reference voltage (V REF ) during each iterative increase of the reference voltage (V REF ) up to a point where the bias controller detects that the substantially maximum extinction ratio is achieved. 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 (V BIAS ) to modulate the relative phase of the split optical input signal to tune the Mach-Zehnder modulator; and a bias feedback controller to compare a detection voltage (DET) associated with the intensity-modulated output signal with a reference voltage (V REF ) to measure an extinction ratio associated with an optical power of the intensity-modulated optical output signal and to adjust the bias voltage (V BIAS ) based on the comparison to substantially maximize the extinction ratio, characterised in that the bias feedback controller comprises a bias controller to set an amplitude of the bias voltage (V BIAS ) in response to the comparison of the peak voltage with the reference voltage (V REF ); and a peak voltage detector to detect a peak voltage (V PK ) associated with a peak amplitude of the detection voltage (DET) wherein the bias controller is further to iteratively increase the reference voltage (V REF ) in response to detecting that the peak voltage (V PK ) is equal to the reference voltage (V REF ) and to iteratively increase the bias voltage (V BIAS ) based on the comparison until the bias controller detects that the peak voltage (V PK ) is equal to the reference voltage (V REF ) during each iterative increase of the reference voltage (V REF ) up to a point where the bias controller detects that the substantially maximum extinction ratio is achieved. <EOS>