Abstract
A simplified model based on the finite-element method is proposed to study the thermo-optic behavior in high-power fiber amplifiers. It is shown that thermally induced refractive index change results in a thermal lens effect and ultimately leads to multimode behavior even in an intrinsically single-mode fiber. Besides, experimental investigations were carried out to characterize the thermo-optic behavior of an intrinsically single-mode large mode area 20/400 ytterbium-doped fiber with core numerical aperture (NA) ~ 0.04 in an all-fiber amplifier. Output power up to 1.68 kW was achieved with a deterioration of beam quality from M2 < 1.1 to 1.7. It is explained that due to the uneven distribution of axial heat load, an intrinsically single-mode fiber gradually turns into a combination of a multi-mode fiber and a single-mode fiber with the growing heat load and then the leakage core modes trigger transverse mode instability. Moreover, a near diffraction-limited output power of 1.35 kW from an all-fiberized low-NA fiber amplifier is obtained.
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