A method for frequency-converting a source optical signal in order to produce a useful optical signal, by mixing a plurality of waves as a result of a non-linear interaction between said waves, comprising producing, within at least one waveguide (1, 2), at least one photon of the useful signal from at least one photon of at least one optical pumping radiation, in accordance with a phase-matching relationship which connects respective propagation constants of the source signal, of said at least one pumping radiation and of the useful signal that are effective in said at least one waveguide, each waveguide (1, 2) having at least one individual normal mode that is effective for each of the source signal, of each pumping radiation and of the useful signal propagating in said waveguide separately from each other waveguide, each waveguide (1, 2) also having at least one individual parameter that modifies a dispersion relationship which is effective for the source signal, each pumping radiation and the useful signal that propagate in said waveguide according to the individual normal mode of said waveguide, said at least one waveguide comprising at least two waveguides (1, 2), which are arranged so that the individual normal mode of each of the waveguides is coupled with an individual normal mode of at least one another of the waveguides, for each of the source signal, of each pumping radiation and of the useful signal, the method comprising simultaneously injecting the source signal and each pumping radiation into at least one of the waveguides (1, 2), and extracting the useful signal from at least one of the waveguides, each of the source signal, of each pumping radiation and of the useful signal that propagates at a location in the waveguides (1, 2) according to the individual normal mode of one of said waveguides having an intensity distribution between several of the waveguides that varies along the waveguides, since at least some of the individual normal modes of said waveguides are coupled, at least one coupling parameter changing an intensity of a coupling that exists between the individual normal modes of at least two of the waveguides (1, 2), wherein respective values of the individual parameters of the waveguides (1, 2) and of said at least one coupling parameter are selected so that the phase-matching relationship is met for at least one set of respective propagation constants of the source signal, of each pumping radiation and of the useful signal, each relating to a collective normal mode of all of the waveguides, that is effective for the propagation of said source signal, of said at least one pumping radiation or of said useful signal, each collective normal mode being such that the source signal, each pumping radiation or the useful signal that propagates in all of the waveguides according to said collective normal mode has an intensity distribution between said waveguides that is constant along the waveguides, the method being characterized in that the source signal has a comb-shaped spectrum within a spectral range that contains at least one portion of said source signal. each waveguide (1, 2) having at least one individual normal mode that is effective for each of the source signal, of each pumping radiation and of the useful signal propagating in said waveguide separately from each other waveguide, each waveguide (1, 2) also having at least one individual parameter that modifies a dispersion relationship which is effective for the source signal, each pumping radiation and the useful signal that propagate in said waveguide according to the individual normal mode of said waveguide, said at least one waveguide comprising at least two waveguides (1, 2), which are arranged so that the individual normal mode of each of the waveguides is coupled with an individual normal mode of at least one another of the waveguides, for each of the source signal, of each pumping radiation and of the useful signal, the method comprising simultaneously injecting the source signal and each pumping radiation into at least one of the waveguides (1, 2), and extracting the useful signal from at least one of the waveguides, each of the source signal, of each pumping radiation and of the useful signal that propagates at a location in the waveguides (1, 2) according to the individual normal mode of one of said waveguides having an intensity distribution between several of the waveguides that varies along the waveguides, since at least some of the individual normal modes of said waveguides are coupled, at least one coupling parameter changing an intensity of a coupling that exists between the individual normal modes of at least two of the waveguides (1, 2), wherein respective values of the individual parameters of the waveguides (1, 2) and of said at least one coupling parameter are selected so that the phase-matching relationship is met for at least one set of respective propagation constants of the source signal, of each pumping radiation and of the useful signal, each relating to a collective normal mode of all of the waveguides, that is effective for the propagation of said source signal, of said at least one pumping radiation or of said useful signal, each collective normal mode being such that the source signal, each pumping radiation or the useful signal that propagates in all of the waveguides according to said collective normal mode has an intensity distribution between said waveguides that is constant along the waveguides, the method being characterized in that the source signal has a comb-shaped spectrum within a spectral range that contains at least one portion of said source signal.