A four arm linearized Mach Zehnder modulator comprising first and second primary electro-optic waveguides; first and second secondary electro-optic waveguides, each secondary electro-optic waveguide being arranged adjacent to a primary electro-optic waveguide; an RF electrode having a stub extending therefrom, the stub having a first foot portion extending partially along the first primary electro-optic waveguide and a second foot portion extending at least partially along the first secondary electro-optic waveguide; an RF ground electrode having a stub extending therefrom, the stub having a first foot portion extending partially along the second primary electro-optic waveguide and a second foot portion extending at least partially along the second secondary electro-optic waveguide; an optical source for providing an optical signal to the waveguides; the RF electrodes being arranged such that when a voltage is applied between the RF electrode and RF ground electrode the magnitudes of the phase shifts experienced by the signals in the primary electro-optic waveguides are the same and the magnitudes of the phase shifts experienced by the signals in the secondary electro-optic waveguides are the same but different to those in the primary electro-optic waveguides.