Disclosed is an optical system including a polychromatic optical source emitting multiple transverse modes, an optical link having at least one portion of multimode optical fiber, and an optical device positioned between the optical source and the input of the multimode optical fiber. The optical device can modify the distribution of the energy coupling of the transverse modes emitted by the source in the propagation modes of the multimode optical fiber. The optical system makes it possible to use low-cost transverse multimode optical sources for producing high-bandwidth Ethernet transmission networks having excellent performance.