Thermo-optic behaviour of intrinsically single-mode low-numerical aperture (NA) ytterbium-doped fibre

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