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A 225-µJ polarization maintaining ytterbium-doped large-mode-a rea multiclad fiber was design ed and fabricated with an effective mode area of 450 µm 2 and a photodarkening maximum excess loss of ~1 dB/m at 1064 nm. The fiber index profile is based on a depressed-clad to obtain a diffraction-limited output. Optimization for low photodarkening and high conversion efficiency while maintaining a good control on the cores refractive index profile has been achieved by adjusting the ytterbium/phosphorus/aluminum concentrations in the fiber core. Concentration ratios of phosphorus/aluminum from 0.12 to 1.25 were experimentally investigated in terms of photodarkening rate and excess loss. Within this range, the photodarkening excess loss was ob served to decrease by a factor of 8. The large-mode-area fiber was used in a 10-ns pulse amplifier at 1064 nm with a repetition rate of 100 kHz and 0.5-nm bandwidth. The diffraction-limited output has a measured M 2 value of 1.04 when the fiber is coiled to a diameter of 12 cm. The fiber amplifier slope efficiency is 70% with a polarization extinction ratio greater than 23 dB. It is shown how the phosphorus/aluminum ratio reduces photodarkening, and how a depressed-clad design improves higher-order mode filtering for reliable, efficient, and compact ytterbium-doped fiber amplifiers. Keywords: Fiber amplifier, large-mode-area, depressed-clad, photodarkening, triple-clad |
