Estimation of mode instability threshold based on local thermal load

The behavior of the mode instability (MI) threshold in the double cladding Yb-doped fiber amplifier when the amplifier with different local heat load is studied theoretically and experimentally. A theoretical model is constructed, and the effects of different laser parameters on the local thermal load of the gain fiber are analyzed theoretically, such as pump direction, pump linewidth, thermal conductivity of cooling medium. An experimental structure is described. The effects of different local heat load on the MI threshold when the fiber amplifier have the same total or average heat load is studied. The theoretical and experimental results reveal that the MI threshold can be estimated by the local heat load of the gain fiber.