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 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 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 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 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.