Space-time coding and optimal scrambling for mode multiplexed optical fiber systems

Approaching the capacity limits of single-mode fiber based optical transmission systems, new fibers supporting the propagation of up to six orthogonal spatial modes, called few-mode fibers, stand as promising candidates for future high-capacity systems. Extensive research is being carried out to further increase the number of modes to multiplex more data. This technique is known as spatial division multiplexing (SDM). However, the co-existence of modes in the same space leads to inevitable modal crosstalk that may induce a loss of their orthogonality as well as power disparities. This phenomenon is called mode dependent loss (MDL) and mainly arises from optical components such as few-mode amplifiers. Although optical solutions were suggested to reduce MDL by inserting mode scramblers or using fibers with strong modal coupling, MDL was unfortunately not completely removed. In this work, we propose a DSP solution based on Space-Time (ST) coding along with OFDM, originally designed for multi-antenna channels, to mitigate MDL in SDM systems. We show that a combination of ST coding at the transmitter and an optimal distribution of mode scramblers in the optical link can completely absorb the penalties induced by important levels of MDL in 6-mode SDM systems. Later on, we address the complexity and scalability of the ST-coding solution and propose a sub-optimal decoding scheme that keeps the MDL-induced penalty low while considerably reducing the decoding complexity.