A tunable optical filter comprising an index grating produced in a waveguide defining a longitudinal direction of propagation of an incident optical signal, said grating being defined by a plurality of optical parameters, is characterized in that the optical guide contains semiconductor nanoparticles and in that the index grating is made up of a refractive index variation in the guide that is at least partly induced by a Kerr effect linked to the 3.sup.rd order dielectric susceptibility (.chi..sup.3) of said nanoparticles, at least one of said parameters of said index grating being modified by application of an optical control signal coupled into the guide and propagating longitudinally in said guide.