Process for fabricating a mixed layer containing a first waveguide the core of which is made of silicon and a second waveguide the core of which is made of silicon nitride, this process comprising the following steps: 1) providing a substrate on which are stacked, in order, a buried dielectric layer and a silicon layer, this substrate lying essentially in a plane called the "plane of the substrate" and the various layers lying parallel to the plane of the substrate, then 2) structuring the silicon layer to form the silicon core of the first waveguide, then 3) encapsulating the formed silicon core in dielectric then planarizing the dielectric, this encapsulating step followed by this planarizing step forming an encapsulated-silicon layer containing the silicon core, characterized in that the process then comprises the following steps: 4) producing a trench, in the encapsulated-silicon layer, in the location where the silicon-nitride core of the second waveguide must be produced, then 5) depositing a silicon-nitride layer on the encapsulated-silicon layer, this silicon-nitride layer completely covering the encapsulated-silicon layer and the thickness of the deposited silicon-nitride layer being sufficient to completely fill the trench, then 6) removing (160; 202) the silicon nitride situated outside of the trench to uncover an upper face of the encapsulated-silicon layer, with which the trench filled with silicon nitride is flush and thus to obtain the silicon-nitride core of the second waveguide, then 7) depositing (170; 216) a dielectric layer that covers the uncovered upper face in order to finalise the encapsulation of the silicon-nitride core and thus to obtain the mixed layer containing both the silicon and silicon-nitride cores encapsulated in dielectric. 1) providing a substrate on which are stacked, in order, a buried dielectric layer and a silicon layer, this substrate lying essentially in a plane called the "plane of the substrate" and the various layers lying parallel to the plane of the substrate, then 2) structuring the silicon layer to form the silicon core of the first waveguide, then 3) encapsulating the formed silicon core in dielectric then planarizing the dielectric, this encapsulating step followed by this planarizing step forming an encapsulated-silicon layer containing the silicon core, 4) producing a trench, in the encapsulated-silicon layer, in the location where the silicon-nitride core of the second waveguide must be produced, then 5) depositing a silicon-nitride layer on the encapsulated-silicon layer, this silicon-nitride layer completely covering the encapsulated-silicon layer and the thickness of the deposited silicon-nitride layer being sufficient to completely fill the trench, then 6) removing (160; 202) the silicon nitride situated outside of the trench to uncover an upper face of the encapsulated-silicon layer, with which the trench filled with silicon nitride is flush and thus to obtain the silicon-nitride core of the second waveguide, then 7) depositing (170; 216) a dielectric layer that covers the uncovered upper face in order to finalise the encapsulation of the silicon-nitride core and thus to obtain the mixed layer containing both the silicon and silicon-nitride cores encapsulated in dielectric. <EOS>