A system includes an optical Y-junction coupler to receive a first modulated optical signal on a wide input path of the optical Y-junction coupler and to receive a second modulated optical signal on a narrow input path of the optical Y-junction coupler, wherein the optical Y-junction coupler generates a combined optical signal from signals received on the wide input path and the narrow input path. A multimode waveguide receives the combined optical signal from the optical Y-junction coupler and propagates a spatially multiplexed optical output signal along a transmission path.