A semiconductor-based optical modulator comprising: an input optical waveguide responsive to an incoming continuous wave optical signal; an input optical waveguide splitter, coupled to the input optical waveguide and configured to split the incoming continuous wave optical signal into a pair of separate signals; a pair of optical waveguide arms (16, 18), each waveguide arm coupled to a separate output from the input optical waveguide splitter, and each optical waveguide arm (16, 18) including a semiconductor-based modulating device comprising a first region having dopants of a first conductivity type and a second region having dopants of a second conductivity type, the modulating device being configured to receive an electrical modulating data signal applied along each optical waveguide arm to create a modulated optical signal; an output optical waveguide combiner, coupled to the outputs of the pair of optical waveguide arms (16, 18) to combine the pair of separately modulated optical signals and create an optically modulated output signal thereafter coupled into an output optical waveguide; and characterised by a phase modulation control section disposed along the optical modulator to impart a predetermined phase to the optically modulated output signal as a function of the length of the phase modulation control section, wherein the phase modulation control section comprises a first segment disposed along a first one of the pair of waveguide arms and a second segment disposed along a second one of the pair of waveguide arms, the first and second segments being configured to be driven by the same input signal so as to impart the predetermined phase onto the optically modulated output signal; and a DC bias section disposed along the optical modulator to impart a DC bias to the predetermined phase imparted by the modulator onto the optically modulated output signal about a predetermined operating point. an input optical waveguide responsive to an incoming continuous wave optical signal; an input optical waveguide splitter, coupled to the input optical waveguide and configured to split the incoming continuous wave optical signal into a pair of separate signals; a pair of optical waveguide arms (16, 18), each waveguide arm coupled to a separate output from the input optical waveguide splitter, and each optical waveguide arm (16, 18) including a semiconductor-based modulating device comprising a first region having dopants of a first conductivity type and a second region having dopants of a second conductivity type, the modulating device being configured to receive an electrical modulating data signal applied along each optical waveguide arm to create a modulated optical signal; an output optical waveguide combiner, coupled to the outputs of the pair of optical waveguide arms (16, 18) to combine the pair of separately modulated optical signals and create an optically modulated output signal thereafter coupled into an output optical waveguide; and characterised by a phase modulation control section disposed along the optical modulator to impart a predetermined phase to the optically modulated output signal as a function of the length of the phase modulation control section, wherein the phase modulation control section comprises a first segment disposed along a first one of the pair of waveguide arms and a second segment disposed along a second one of the pair of waveguide arms, the first and second segments being configured to be driven by the same input signal so as to impart the predetermined phase onto the optically modulated output signal; and a DC bias section disposed along the optical modulator to impart a DC bias to the predetermined phase imparted by the modulator onto the optically modulated output signal about a predetermined operating point.