Large mode area optical fibres refractive index reconstruction from top-hat mode near field

In this work we present a new and efficient algorithm for the exact synthesis of large mode area (LMA) single mode (SM) optical fibers supporting `perfect top-hat´ modal electric field profiles. In order to achieve this flexibility in field shape we use complex refractive index profile waveguides, where we determine the required refractive index profile on both real and imaginary refractive index. We develop a technique for calculating directly and accurately the refractive index profiles of such fibers from knowledge of the top-hat modal electric field. The method we use to solve this inverse problem is via modeling the waveguide transversely as a transmission line. We demonstrate this algorithm with a number of example reconstructions of LMA SM optical fibers with perfect `top-hat´ electric field profile and an effective mode area (A_eff) size that in some cases exceeds the 500 μm² and in one the 1000 μm². Such fiber designs reduce or suppress nonlinear effects, such as four-wave-mixing (FWM), self-phase-modulation (SPM), stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS), in high power applications.