On the thermal origin of mode instabilities in high power fiber lasers

Detailed simulations on the temperature profile inside of a large mode area fiber in high power operation with mode interference are presented. These simulations show that the pump power is not homogeneously absorbed along the fiber, which in turn gives rise to an oscillating temperature profile along the fiber. This longitudinal temperature profile creates an index grating with the right period to transfer energy between the interfering modes. Two cases are analyzed: mode beating between the fundamental mode and a radially anti-symmetric mode and mode beating between the fundamental mode and a radially symmetric mode.