An optical modulator for producing a modulated optical output having a pre-determined frequency chirp comprises: optical splitting means for receiving and splitting an optical input signal to be modulated into two optical signals to pass along two waveguide arms (36, 38) made of electro-optic material; and optical combining means for receiving and combining the two optical signals into said modulated optical output. At least one electrode pair (40/44, 42/44) is associated with each waveguide arm (36, 38), and is electrically connected in series such as to modulate the phase of said optical signals in antiphase in response to a single electrical signal (Vmod) applied thereto. The modulator is characterised by a capacitive element (60) connected to the electrode pair (42) of one arm (38) such as to modify the division of the single electrical signal (Vmod) such that the magnitude of the electrical signal across the electrode pair (42/44) of one arm (38) is different to that across the electrode pair (40/44) of the other arm (36) thereby imparting the pre-determined frequency chirp in the modulated optical output. |