A beam of laser light is composed of multiple longitudinal temporal modes and transverse spatial modes. Photons carry spin angular momentum which defines the laser beam’s polarisation, and orbital angular momentum (OAM) which defines the spatial modes in a beam. This project focuses on the orbital angular momentum. The OAM carried by photons in a laser has a theoretically infinite number of orthogonal states, so can be defined by an infinite dimensional Hilbert Space. This discrete and unbounded state-space provides an opportunity to send a large amount of information per photon. OAM is present in beams with a helical (vortex) phase structure. The projection of the vortex phase structure demonstrates an intensity profile forming a Laguerre Gaussian (LG) doughnut shaped mode.

A Multi Plane Light Converter (MPLC), is a system composed of multiple phase plates, designed to perform a unitary spatial transform. In practice this is achieved by reflecting a laser beam between a Spatial Light Modulator (SLM) and a mirror multiple times. The MPLC is useful for spatial separation because it performs Fourier plane wave decomposition allowing it to receive multiple input signals and output a single superposition (multiplexing) or the reverse process (demultiplexing). In this work, the simulation is derived from the theoretical assumptions made when looking at the optics of a thin lens. This approximation works in a SLM because the phase planes are equivalent in behaviour to thin lenses - except they reflect light instead of allowing it to pass through. A lens is named “thin” if the change in x,y coordinates from input to output is negligible, whereas in a thick lens this coordinate change is significant.