We describe a multimode reconfigurable optical spatial mode multiplexing system having first and second first and second input beams and a beam combiner to combine these into an optical output. At least one of the paths comprises a polarisation-independent reconfigurable phase modulator to impose a controllable phase profile on an input beam in an input beam phase modulating optical path, to controllably convert a spatial mode order of the input beam from a lower to a higher order spatial mode. The system also has a control input to control the phase modulator to configure the phase profile for the mode conversion. The input beams are combined into a multiple spatial mode combined beam output independent of a polarisation of the input beams. The number of spatial modes of the combined beam can be more than a number of spatial modes in either of the first and second input beams separately.