The electro-optic phase modulator intended to modulate the optical phase of a lightwave incident on the modulator, includes an electro-optic substrate having an entrance face and an exit face, an optical waveguide of refractive index (ng) higher than that (ns) of the substrate, continuously rectilinear from a guide entrance end located on the entrance face to a guide exit end located on the exit face, and which is adapted to guide the incident lightwave partially coupled in the waveguide into a guided lightwave propagating along the optical path of the waveguide between the guide entrance end and exit end, and at least two modulation electrodes arranged parallel to the waveguide, so as, when a modulation voltage (Vm(t)) is applied between these modulation electrodes, to introduce a modulation phase-shift, function of the modulation voltage, in the guided lightwave. The phase modulator further includes elements for the electric polarization of the substrate.