Spatiotemporal mode decomposition of ultrashort pulses in linear and nonlinear graded-index multimode fibers

We develop a spatiotemporal mode decomposition technique to study the spatial and temporal mode power distribution of ultrashort pulses in long spans of graded-index multimode fiber,for different input laser conditions.We find that the beam mode power content in the dispersive pulse propagation regime can be described by the Bose-Einstein law,as a result of the process of power diffusion from linear and nonlinear mode coupling among nondegenerate mode groups.In the soliton regime,the output mode power distribution approaches the Rayleigh-Jeans law.