Design and Fabrication of a Weakly-Coupled OAM Ring-Core Fiber Supporting 5-Mode-Group-Multiplexed MIMO-Free Transmission

We propose a center-raised ring-core fiber (RCF), which can support 5-mode-group-multiplexed multiple-input and multiple-output free (MIMO-free) transmission. The center region in the index profile of the ring core is lifted from silica to enlarge the separation of lower-order mode groups (MGs) so that the RCF supports more weakly-coupled MGs. Moreover, the 5-MG RCF was fabricated, and the loss of each mode can maintain about 0.3 dB/km. The minimum effective index separation between the adjacent radial first-order MGs is 1.35
$\times\; 10^{-3}$
. In addition, we also adopted the swept-wavelength interferometry (SWI) technique to evaluate the coupling coefficients between the radial first-order MGs in the RCF, which are all less than −30 dB/km. Furthermore, it is demonstrated that 5 MGs modulated by 32-Gbaud QPSK signals are multiplexed and transmitted in a 10-km RCF MIMO-free transmission system. To the best of our knowledge, it is the largest number of the multiplexed radial first-order MGs of the RCF at present. At the same time, we also verified the wavelength division multiplexing (WDM) performance of the fiber in the C-band. In the future, in order to further increase the transmission capacity of this fiber, 4 × 4 MIMO and dense WDM in C+L bands can be utilized. Therefore, we believe that this fiber can increase the number of weakly-coupled MGs to increase the single-fiber transmission capacity of mode division multiplexing (MDM).