Probabilistically shaped optical transmission method based on cascaded ROADM

Aiming at the probabilistically shaped dual-polarization 64-ary quadrature amplitude modulation in coherent optical transmission systems, the impact of bandwidth narrowing caused by cascaded reconfigurable optical add-drop multiplexer (ROADM) on bit-wise achievable information rate (BW AIR) was experimentally studied. The three probabilistically shaped distributed signals based on Maxwell-Boltzmann distribution were adopted by transmitter. The experimental device included the generation of probabilistically shaped symbol sequence, the variable bandwidth optical filters that simulated ROADM cascade narrowing the overall frequency response bandwidth, and the simplified detection as well as the BW AIR calculation. The results show that the impact of the bandwidth narrowing is similar for different constellation entropies, and for the given optical signal-to-noise ratio (OSNR), the constellation entropy which produces the maximum BW AIR depends on the extent of bandwidth narrowing caused by cascaded ROADM. Finally, the BW AIR can be maximized by choosing the appropriate probabilistically shaped constellation distribution.