A multiplexer/demultiplexer optical device comprising: a first beam-splitter cube, which is provided with first input/output ports for receiving/supplying optical beams; a second beam-splitter cube, which is provided with second input/output ports and is optically coupled to the first splitter; a first Porro-prism reflector, which is optically coupled to the second splitter; a second Porro-prism reflector, which is optically coupled to the second splitter and is structured for introducing into optical beams that traverse it a phase delay depending upon an orbital angular momentum of the optical beams and upon an orientation of the second reflector; said device being a Michelson interferometer structured for obtaining constructive/destructive interferences such as to multiplex/demultiplex on/from corresponding input/output ports, on the basis of values of orbital angular momentum, an optical beam comprising a plurality of concentric optical beams with cylindrical symmetry that have different values of orbital angular momentum, wherein the first splitter comprises a first input/output port for a composite optical beam to be demultiplexed and for the composite optical beam obtained from multiplexing, an output port for a portion of the composite optical beam and a second input/output port for a first optical beam that forms part of the composite optical beam and has a first orbital angular momentum, and a first port for coupling to the second splitter; and wherein the second splitter comprises a third input/output port for a second optical beam that forms part of the composite optical beam and has a second orbital angular momentum, and a second port for coupling to the first coupling port of the first splitter, a second coupling port optically coupled to the first reflector and a third coupling port optically coupled to the second reflector. a first beam-splitter cube, which is provided with first input/output ports for receiving/supplying optical beams; a second beam-splitter cube, which is provided with second input/output ports and is optically coupled to the first splitter; a first Porro-prism reflector, which is optically coupled to the second splitter; a second Porro-prism reflector, which is optically coupled to the second splitter and is structured for introducing into optical beams that traverse it a phase delay depending upon an orbital angular momentum of the optical beams and upon an orientation of the second reflector; said device being a Michelson interferometer structured for obtaining constructive/destructive interferences such as to multiplex/demultiplex on/from corresponding input/output ports, on the basis of values of orbital angular momentum, an optical beam comprising a plurality of concentric optical beams with cylindrical symmetry that have different values of orbital angular momentum, wherein the first splitter comprises a first input/output port for a composite optical beam to be demultiplexed and for the composite optical beam obtained from multiplexing, an output port for a portion of the composite optical beam and a second input/output port for a first optical beam that forms part of the composite optical beam and has a first orbital angular momentum, and a first port for coupling to the second splitter; and wherein the second splitter comprises a third input/output port for a second optical beam that forms part of the composite optical beam and has a second orbital angular momentum, and a second port for coupling to the first coupling port of the first splitter, a second coupling port optically coupled to the first reflector and a third coupling port optically coupled to the second reflector.