  Mode division multiplexing (MDM) is mooted as a technology to address future
bandwidth issues, and has been successfully demonstrated in free space using
spatial modes with orbital angular momentum (OAM). To further increase the data
transmission rate, more degrees of freedom are required to form a densely
packed mode space. Here we move beyond OAM and demonstrate multiplexing and
demultiplexing using both the radial and azimuthal degrees of freedom. We
achieve this with a holographic approach that allows over 100 modes to be
encoded on a single hologram, across a wide wavelength range, in a wavelength
independent manner. Our results offer a new tool that will prove useful in
realising higher bit rates for next generation optical networks.
