The present invention relates to an optical modulator, divided into at least two active segments separated by at least one passive segment. The modulator comprises: an optical waveguide with an optical group index n.sub.o having an optical signal propagating at an optical velocity v.sub.o, and a microwave transmission line with an electrical propagation index n.sub.p, having an electrical signal propagating at an electrical velocity V.sub.e. The electrical propagation index n.sub.p of the unloaded microwave transmission line is lower than the optical group index n.sub.o of the optical waveguide. The loading and length of the microwave transmission line are adjusted for a specific Bloch impedance and electrical velocity v.sub.e. The invention also relates to a method for adapting the impedance of an optical modulator.