Elliptical Core Multimode Fibers for Reduced DSP Complexity in Coherent Optical Transmission

In this paper, we analyze the performance of elliptical core graded index fibers with a cladding trench, focusing on a wide range of ovality values and supporting up to 90-polarization modes. We demonstrate a six-fold reduction in the intra-mode group delay and an 11 dB decrease in intra-mode group crosstalk for a 10 km long fiber supporting 90-polarization modes (45 spatial modes), with a fiber crosstalk strength of -20 dB/km. These fibers maintain the stable inter-mode group properties, same as the fibers optimized for low differential mode group delay. Further suppression of intra-mode group crosstalk strength is achieved in fibers with fewer supported modes and lower coupling strengths. Additionally, we show a 1.89-fold reduction in receiver equalization complexity to achieve the signal-to-noise ratio of 28 dB in 32 GBd 16-QAM data transmission over 10 km by selecting the worst interferers as inputs to the equalizer determined from the channel matrix, compared to circular fibers. We compare the results of using the worst interferers algorithm to the standard mode group division multiplexing scenario of employing the MIMO for a given mode of the size of mode group and shows the improvement in signal-to-noise ratio for a 6 dB at a given complexity using the worst interferers algorithm. We also propose using modes with minimum interferers determined from the channel matrix for the transmission over the same channel, further enhancing throughput whilst reducing receiver complexity. These findings underscore the potential of elliptical core fibers for space-division multiplexed transmission, offering both improved performance and reduced system complexity.