An improved fiber-optic communications system comprises a multi-mode waveguide carrying an optical signal, a single-mode waveguide optically coupled to and receiving the optical signal from the multi-mode waveguide and an adiabatic coupler optically coupled between the multi-mode waveguide and the single-mode waveguide. The multi-mode and single-mode waveguides may be optical fibers. The adiabatic coupler may comprise a tapered core surrounded by a cladding. Alternatively, the adiabatic coupler may comprise a core surrounded by a cladding, wherein the refractive index of at least one of the core and the cladding varies over the length of the adiabatic coupler.