Modelling of thermal effects and gain competition in Yb-doped large mode area photonic crystal fibers

In the last few years Yb-doped double cladding fibers have become the key component for the development of reliable and high-performance lasers. Despite an effective cooling of the fiber medium, a significant heat load is generated when high pump power is involved, which alters the mode propagation characteristics, causing unwanted coupling among the modes and destroying the output beam quality. This work presents a new tool for the analysis of the amplification and modal properties of Yb-doped double-cladding fibers, which comprises a full-vector modal solver, based on the finite-element method, an amplifier model and a thermal one. Simulation results, shown for two large pitch fiber designs, both in co-propagating and counter-propagating pumping schemes, have demonstrated the influence of the generated heat load on the overlap integral and on the power evolution of the guided modes.