A travelling wave electro-optic modulator comprising a substrate; first and second parallel spaced apart electrode strips (9, 10) arranged on the substrate; first and second optical waveguides (5, 6) arranged on the substrate, the optical waveguides (5, 6) being positioned between the first and second electrode strips (9, 10) and extending parallel thereto; the first electrode strip comprising at least one portion extending proximate to the first optical waveguide; the second electrode strip comprising at least one portion extending proximate to the second optical waveguide; a semiconductive backplane layer arranged within the substrate and extending between the waveguides (5, 6); and, a matched termination connected to the first and second electrode strips (9, 10), the matched termination comprising (a) a serpentine electrically conductive strip arranged on the substrate and connecting the first and second electrode strips (9, 10) together; and, (b) a semiconductive backplane matching element, the backplane matching element comprising a plurality of semiconductive backplane plates connected together by at least one semiconductive backplane arm, the plates and at least one backplane arm being arranged within the substrate, the plates being arranged proximate to the electrode strips (9, 10) such that each electrode strip (9, 10) is capacitively coupled to at least one backplane plate; the serpentine electrically conductive strip being arranged such that at least a portion of its length is proximate to at least one backplane arm such that the two are electrically coupled together. a substrate; first and second parallel spaced apart electrode strips (9, 10) arranged on the substrate; first and second optical waveguides (5, 6) arranged on the substrate, the optical waveguides (5, 6) being positioned between the first and second electrode strips (9, 10) and extending parallel thereto; the first electrode strip comprising at least one portion extending proximate to the first optical waveguide; the second electrode strip comprising at least one portion extending proximate to the second optical waveguide; a semiconductive backplane layer arranged within the substrate and extending between the waveguides (5, 6); and, a matched termination connected to the first and second electrode strips (9, 10), the matched termination comprising (a) a serpentine electrically conductive strip arranged on the substrate and connecting the first and second electrode strips (9, 10) together; and, (b) a semiconductive backplane matching element, the backplane matching element comprising a plurality of semiconductive backplane plates connected together by at least one semiconductive backplane arm, the plates and at least one backplane arm being arranged within the substrate, the plates being arranged proximate to the electrode strips (9, 10) such that each electrode strip (9, 10) is capacitively coupled to at least one backplane plate; (a) a serpentine electrically conductive strip arranged on the substrate and connecting the first and second electrode strips (9, 10) together; and, (b) a semiconductive backplane matching element, the backplane matching element comprising a plurality of semiconductive backplane plates connected together by at least one semiconductive backplane arm, the plates and at least one backplane arm being arranged within the substrate, the plates being arranged proximate to the electrode strips (9, 10) such that each electrode strip (9, 10) is capacitively coupled to at least one backplane plate; the serpentine electrically conductive strip being arranged such that at least a portion of its length is proximate to at least one backplane arm such that the two are electrically coupled together.