Method of fabricating a modulator of the propagation losses and of the index of propagation of a guided optical signal, this method comprising: - providing a stack comprising, successively, a base substrate, a buried layer of dielectric material and a semiconductor layer, the thickness at every point of the buried layer being equal to e 2ini to within approximately 5 nm, where e 2ini is a constant equal to the average thickness of the buried layer, then - etching (514, 516) the semiconductor layer so as to structure a first electrode of the modulator within this semiconductor layer, this first electrode having a proximal end, a distal end and an intermediate part extending, in a transverse direction, from the proximal end up to the distal end so as to mechanically and electrically connect these ends (11, 12), then - encapsulating the structured semiconductor layer in a dielectric material so as to obtain a semiconductor layer encapsulated within a dielectric material in which layer the dielectric material continues, in the transverse direction, until it directly touches the proximal end of the first electrode, then - bonding a substrate onto the encapsulated semiconductor layer, then - forming (524, 528) a second electrode of the modulator having a proximal end facing the proximal end of the first electrode, these proximal ends (12, 32) being separated from each other only by a dielectric layer in such a manner as to form a waveguide able to guide the optical signal to be modulated, characterized in that: - following the bonding of the substrate onto the encapsulated semiconductor layer and prior to the formation of the second electrode, the method comprises the removal of the base substrate in such a manner as to expose one face of the buried layer without modifying its thickness by more than 5 nm, this removal of the base substrate comprising an operation of selective etching of at least a residual thin layer of the base substrate directly in contact with the buried layer, using a chemical agent which etches the base substrate at least 500 times faster than the buried layer, and - the formation (524, 528) of the second electrode is implemented directly on this exposed face of the buried layer such that, once the second electrode has been formed, it is the buried layer that directly forms the dielectric layer interposed between the proximal ends (12, 32) of the electrodes. - providing a stack comprising, successively, a base substrate, a buried layer of dielectric material and a semiconductor layer, the thickness at every point of the buried layer being equal to e 2ini to within approximately 5 nm, where e 2ini is a constant equal to the average thickness of the buried layer, then - etching (514, 516) the semiconductor layer so as to structure a first electrode of the modulator within this semiconductor layer, this first electrode having a proximal end, a distal end and an intermediate part extending, in a transverse direction, from the proximal end up to the distal end so as to mechanically and electrically connect these ends (11, 12), then - encapsulating the structured semiconductor layer in a dielectric material so as to obtain a semiconductor layer encapsulated within a dielectric material in which layer the dielectric material continues, in the transverse direction, until it directly touches the proximal end of the first electrode, then - bonding a substrate onto the encapsulated semiconductor layer, then - forming (524, 528) a second electrode of the modulator having a proximal end facing the proximal end of the first electrode, these proximal ends (12, 32) being separated from each other only by a dielectric layer in such a manner as to form a waveguide able to guide the optical signal to be modulated, - following the bonding of the substrate onto the encapsulated semiconductor layer and prior to the formation of the second electrode, the method comprises the removal of the base substrate in such a manner as to expose one face of the buried layer without modifying its thickness by more than 5 nm, this removal of the base substrate comprising an operation of selective etching of at least a residual thin layer of the base substrate directly in contact with the buried layer, using a chemical agent which etches the base substrate at least 500 times faster than the buried layer, and - the formation (524, 528) of the second electrode is implemented directly on this exposed face of the buried layer such that, once the second electrode has been formed, it is the buried layer that directly forms the dielectric layer interposed between the proximal ends (12, 32) of the electrodes.