A very large mode-area 2 wt% erbium-doped double-clad phosphate fiber with a core diameter of 60 μm and core numerical aperture of 0.03 was fabricated and used for the last stage amplifier of an all-fiber pulsed laser master-oscillator power amplifier (MOPA) at 1550 nm. The Er3+double-clad phosphate fiber has a cladding-core ratio of 2.17 and a V number of 3.6 at 1550 nm. 3 mJ pulses with 21.5 ns duration were obtained with Gaussian-like spatial beam profile with only a 75 cm long phosphate gain fiber.
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Olson et al. (2020) studied this question.
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