The phase of modes in three-core optical fibers under high thermal load is studied by means of the combined thermal and optical models. In particular, the effects of thermal dissipation in the doped cores, as a result of quantum defect heat generation, and the relative material refractive index variation are investigated. The induced index gradient can influence the guided mode propagation in multiple adjacent cores, even if their distance is enough to decouple them optically, showing a remarkable dephasing of the different fiber output beams. This effect brought by the thermal phenomenon inherent in the amplification process has to be carefully accounted for when designing multi-core high-power optical lasing and amplifying devices. The mode dephasing results also provide useful information for future evaluation of thermo-optic coupling effects that may occur along the fiber under high power regime.
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