Gain effects in modes of holey fiber

This study utilized the laser simulation code Lyra®, which is essentially an upgrade version of the BPM part of the code LAS-CAD®, in order to calculate the transversal modes of a loaded holey fiber. Lyra calculates the 3D distribution of the signal, as well as the super-Gaussian pump distribution in the fiber, using the FFT-BPM method with a non-paraxial split operator. It also calculates the eigenmodes and the corresponding eigenfunctions of the structure, by sampling the field distribution during the propagation, making a least squares analysis to the corresponding power spectrum. The gain model in Lyra is based on a simple two-level approximation to the rate equations with somewhat artificial pump cross-section to allow efficient absorption of the pump radiation (λ_pump = 0.9 μm, σ_pump = 0.1 cm^-1). The main effect of the gain is to enhance the imaginary refractive index of the core, while somewhat reducing the real index close to the fiber axis. It is found that although higher modes are more easily excited in the unloaded holey fiber, the dominant mode is still very similar in both cases: it has almost the same propagation constant, β = 60766 cm^-1, with a characteristic non-circular mode shape. However, the inner part of the mode, where more than 80% of the power resides, is more Gaussian like, with a circular shape for both the loaded and the unloaded fibers. Some more results of nonlinear effects (like Kerr effect) are also presented in the conference.