Spatial channel multiplexing emulation with a 21-channel multiplane plane light converter

Spatial division multiplexing is a means of transmitting information on independent spatial modes and is being investigated as a means of increasing capacity in optical communications in both optical fibres and free space. Multiplane light converters (MPLC) are a means of deconstructing a wavefront into constituent modes that focus at specific spatial locations, and the reverse - that specific inputs result in controlled modal output. We have used a pair of MPLCs with 21 Hermite Gaussian modes to represent a free space optical connection with multiple single-input multiple-output (SIMO) operation. The effects of atmospheric turbulence have been implemented using a micromirror array and represent atmospheres that vary from weak (C_n²=10^-16 m^-2/3) to strong (C_n²=10^-13 m^-2/3) turbulence. This allows the resulting crosstalk between modes in the receiver to be characterised and modal transmission choice selected to minimise the crosstalk effects. Spatial division multiplexing is shown to improve the resilience against the degrading effects of turbulence, increases detection efficiency and offers an alternative method of compensating for turbulence effects using post detection digital signal processing rather than physical wavefront manipulation using adaptive optics.