A photonic assembly is described. The assembly comprises a substrate. An optical modulator (100) in or on the substrate has an output port coupled to an output waveguide (106) mounted in or on the substrate. A spiller waveguide (107, 108) is mounted in or on the substrate. The spiller waveguide (107, 108) has an input end (109, 110) physically separated from but proximate to the output waveguide (106) so as to collect light spilt from the output port or output waveguide (106). The modulator (106) may be a MZI modulator.