Integrated mode converter and multiplexer, comprising: - at least a first monomode input waveguide adapted to receive a first input zero-order signal and a second monomode input waveguide adapted to receive a second input zero-order signal; - a multimode output waveguide adapted to transmit at least a first zero-order output signal and a second first-order output signal; and - a multimode interference coupler configured to equally split optical signals from a first input and a second input into a first output and a second output, and to induce a first phase shift of 90° between the first output and second output, one region in the multimode interference coupler comprising an arrangement of periodically alternated sections of a core material and of a cladding material, with a period smaller than a value of the shortest wavelength in the bandwidth of the mode converter and multiplexer; - a first phase-shifter configured to induce a second phase shift between the first output and second output, wherein the first phase-shifter comprises a first waveguide connected to the first output, and a second waveguide connected to the second output, the first waveguide and the second waveguide having different widths, wherein the first waveguide comprises a first delay with a third width and the second waveguide comprises a second delay with a fourth width, the first delay and the second delay further comprising arrangements of periodically alternated sections of the core material and of the cladding material, with a period of the arrangements of periodically alternated sections being smaller than a value of the shortest wavelength in the bandwidth of the mode converter and multiplexer; and - one symmetrical Y-junction arranged next to the first phase-shifter, configured to combine outputs of the first phase-shifter into the multimode output waveguide. - at least a first monomode input waveguide adapted to receive a first input zero-order signal and a second monomode input waveguide adapted to receive a second input zero-order signal; - a multimode output waveguide adapted to transmit at least a first zero-order output signal and a second first-order output signal; and - a multimode interference coupler configured to equally split optical signals from a first input and a second input into a first output and a second output, and to induce a first phase shift of 90° between the first output and second output, one region in the multimode interference coupler comprising an arrangement of periodically alternated sections of a core material and of a cladding material, with a period smaller than a value of the shortest wavelength in the bandwidth of the mode converter and multiplexer; - a first phase-shifter configured to induce a second phase shift between the first output and second output, wherein the first phase-shifter comprises a first waveguide connected to the first output, and a second waveguide connected to the second output, the first waveguide and the second waveguide having different widths, wherein the first waveguide comprises a first delay with a third width and the second waveguide comprises a second delay with a fourth width, the first delay and the second delay further comprising arrangements of periodically alternated sections of the core material and of the cladding material, with a period of the arrangements of periodically alternated sections being smaller than a value of the shortest wavelength in the bandwidth of the mode converter and multiplexer; and - one symmetrical Y-junction arranged next to the first phase-shifter, configured to combine outputs of the first phase-shifter into the multimode output waveguide.