Influence of core NA on Thermal-Induced Mode Instabilities in High Power Fiber Amplifiers

We report on the influence of core NA on thermal-induced mode instabilities (MI) in high power fiber amplifiers. Influence of core NA and V-parameter on MI has been investigated numerically. It shows that core NA has larger influence on MI for fibers with smaller core-cladding-ratio, and the influence of core NA on threshold is more obvious when the amplifiers are pumped at 915nm. The dependence of threshold on V-parameter revealed that the threshold increases linearly as V-parameter decreases when V-parameter is larger than 3.5, and the threshold shows exponentially increase as V-parameter decreases when V-parameter is less than 3.5. We also discussed the effect of linewidth on MI, which indicates that the influence of linewidth can be neglected for linewidth smaller than 1nm when the fiber core NA is smaller than 0.07 and fiber length is shorter than 20m. Fiber amplifiers with different core NA were experimentally analyzed, which agreed with the theoretical predictions.