An optical system comprising: an optical circuit board having: • a top face, • a bottom face (1b;23), opposed to the top face, • at least one optical layer , embedded between the top and bottom faces, and having a first optical interface , • at least one cut-out formed in the top face, and having a bottom reference layer defining an X-Y plane, - an optical coupling device having: • at least one positioning member extending in a said cut-out, and having a bottom face placed on the reference layer of the optical circuit board, • at least one optical path adapted to transmit light between a first optical interface , optically coupled to the first optical interface of the optical board, and a second interface , - a mechanical fixation system adapted to attach the optical coupling device to the optical circuit board, and comprising a fixation pin extending through at least part of the optical circuit board at the positioning member , characterized in that the mechanical fixation system comprises a bearing surface bearing on at least a top face of the optical coupling device or on any bottom face of the optical circuit board ; and wherein the reference layer is arranged at a predetermined height along the z-axis, and wherein the positioning member contacts the reference layer at the predetermined height along the z-axis due to a fixation of the coupling device to the optical circuit board by the bearing surface of the mechanical fixation system . an optical circuit board having: • a top face, • a bottom face (1b;23), opposed to the top face, • at least one optical layer , embedded between the top and bottom faces, and having a first optical interface , • at least one cut-out formed in the top face, and having a bottom reference layer defining an X-Y plane, • a top face, • a bottom face (1b;23), opposed to the top face, • at least one optical layer , embedded between the top and bottom faces, and having a first optical interface , • at least one cut-out formed in the top face, and having a bottom reference layer defining an X-Y plane, - an optical coupling device having: • at least one positioning member extending in a said cut-out, and having a bottom face placed on the reference layer of the optical circuit board, • at least one optical path adapted to transmit light between a first optical interface , optically coupled to the first optical interface of the optical board, and a second interface , • at least one positioning member extending in a said cut-out, and having a bottom face placed on the reference layer of the optical circuit board, • at least one optical path adapted to transmit light between a first optical interface , optically coupled to the first optical interface of the optical board, and a second interface , - a mechanical fixation system adapted to attach the optical coupling device to the optical circuit board, and comprising a fixation pin extending through at least part of the optical circuit board at the positioning member , characterized in that the mechanical fixation system comprises a bearing surface bearing on at least a top face of the optical coupling device or on any bottom face of the optical circuit board ; and wherein the reference layer is arranged at a predetermined height along the z-axis, and wherein the positioning member contacts the reference layer at the predetermined height along the z-axis due to a fixation of the coupling device to the optical circuit board by the bearing surface of the mechanical fixation system . wherein the reference layer is arranged at a predetermined height along the z-axis, and wherein the positioning member contacts the reference layer at the predetermined height along the z-axis due to a fixation of the coupling device to the optical circuit board by the bearing surface of the mechanical fixation system . <EOS>