Characterizing the variation of propagation constants in multicore fibre
  We demonstrate a numerical technique that can evaluate the core-to-core variations in propagation constant in multicore fibre. Using a Markov Chain Monte Carlo process, we replicate the interference patterns of light that has coupled between the cores during propagation. We describe the algorithm and verify its operation by successfully reconstructing target propagation constants in a fictional fibre. Then we carry out a reconstruction of the propagation constants in a real fibre containing 37 single-mode cores. We find that the range of fractional propagation constant variation across the cores is approximately $\pm2 \times 10^{-5}$. 
Peter J. Mosley, Itandehui Gris-Sanchez, James M. Stone, Robert J. A.   Francis-Jones, Douglas J. Ashton, Tim A. Birks
physics.optics