A multimode reconfigurable optical spatial mode multiplexing system (600, 650), the system comprising: a first optical path to carry a first input beam from a first optical input; a second optical path to carry a second input beam from a second optical input; a beam combiner to combine beams from said first and second optical paths and provide a combined beam optical output; wherein at least one of said first and second optical paths is a phase modulating optical path and comprises a reconfigurable phase modulator (608a, 608a', 608b, 608b') to impose a controllable phase profile on an input beam in the phase modulating optical path to controllably convert a spatial mode order of the input beam from a lower order spatial mode to a higher order spatial mode; and further comprising a control input to control said reconfigurable phase modulator to configure said controllable phase profile to alter said conversion of said spatial mode order of said input beam; wherein said first and second input beams are combined into a multiple spatial mode combined beam fibre optical output independent of a polarisation of said first and second input beams; and wherein a number of spatial modes of said combined beam is controllable with said control input to increase a number of spatial modes in said combined beam to more than a number of spatial modes in either of said first and second input beams separately; characterised in that said reconfigurable phase modulator is polarisation-independent and comprises a ferroelectric liquid crystal, FLC, spatial light modulator, SLM, which comprises exactly 4 pixels tiling an active area of the SLM, one in each quadrant of the modulator. a first optical path to carry a first input beam from a first optical input; a second optical path to carry a second input beam from a second optical input; a beam combiner to combine beams from said first and second optical paths and provide a combined beam optical output; wherein at least one of said first and second optical paths is a phase modulating optical path and comprises a reconfigurable phase modulator (608a, 608a', 608b, 608b') to impose a controllable phase profile on an input beam in the phase modulating optical path to controllably convert a spatial mode order of the input beam from a lower order spatial mode to a higher order spatial mode; and further comprising a control input to control said reconfigurable phase modulator to configure said controllable phase profile to alter said conversion of said spatial mode order of said input beam; wherein said first and second input beams are combined into a multiple spatial mode combined beam fibre optical output independent of a polarisation of said first and second input beams; and wherein a number of spatial modes of said combined beam is controllable with said control input to increase a number of spatial modes in said combined beam to more than a number of spatial modes in either of said first and second input beams separately; characterised in that said reconfigurable phase modulator is polarisation-independent and comprises a ferroelectric liquid crystal, FLC, spatial light modulator, SLM, which comprises exactly 4 pixels tiling an active area of the SLM, one in each quadrant of the modulator.