A supercontinuum optical pulse source provides a combined supercontinuum, and can comprise one or more seed pulse sources; first and second optical amplifiers arranged along first and second respective optical paths, wherein the first and second optical amplifiers are configured to amplify one or more optical signals generated by the one or more seed pulse sources; a first microstructured light-guiding member arranged along the first optical path and configured to generate supercontinuum light responsive to an optical signal propagating along the first optical path; a second microstructured light-guiding member arranged along the second optical path and configured to generate supercontinuum light responsive to an optical signal propagating along the second optical path; a supercontinuum-combining member to combine supercontinuum generated in at least the first and second microstructured light-guiding members to form a combined supercontinuum; wherein the supercontinuum-combining member comprises an output fibre, wherein the output fibre comprises a silica-based multimode optical fibre supporting a plurality of spatial modes at one or more wavelengths of the combined supercontinuum; wherein the supercontinuum-combining member has one or more input fibres which support no more than four spatial modes at any wavelength within the combined supercontinuum; and wherein the output fibre of the supercontinuum-combining member supports no more than four spatial modes at any wavelength within the combined supercontinuum.