Devices and methods for processing a radiation beam with coherence are disclosed. In one arrangement, an optical system receives a radiation beam with coherence. The radiation beam comprises components distributed over one or more radiation beam spatial modes. A waveguide supports a plurality of waveguide spatial modes. The optical system directs a plurality of the components of the radiation beam belonging to a common radiation beam spatial mode and having different frequencies onto the waveguide in such a way that each of the plurality of components couples to a different set of the waveguide spatial modes, each set comprising one or more of the waveguide spatial modes.