A semiconductor optical modulator includes optical input and output portions; a plurality of Mach-Zehnder modulators, each including first and second waveguide arms having first and second modulation electrodes, respectively; an optical demultiplexer coupled between the optical input portion and the Mach-Zehnder modulators through an optical waveguide; an optical multiplexer coupled between the optical output portion and the Mach-Zehnder modulators; a plurality of electrical inputs; and a plurality of differential transmission lines electrically connecting the electrical inputs to the Mach-Zehnder modulators. The first modulation electrode and the second modulation electrode of at least one of the Mach-Zehnder modulators, one of the electrical inputs, and one of the differential transmission lines that connects such one electrical input to such one Mach-Zehnder modulator form an electrical circuit having a differential impedance in a range of 80Ω to 95Ω.