Fibre amplifier based on an ytterbium-doped active tapered fibre for the generation of megawatt peak power ultrashort optical pulses

We report a new ytterbium-doped active tapered fibre used in the output amplifier stage of a fibre laser system for the generation of megawatt peak power ultrashort pulses in the microjoule energy range. The tapered fibre is single-mode at its input end (core and cladding diameters of 10 and 80 $\mu\mathrm m$) and multimode at its output end (diameters of 45 and 430 $\mu\mathrm m$), but ultrashort pulses are amplified in a quasi-single-mode regime. Using a hybrid Er/Yb fibre system comprising an erbium master oscillator and amplifier at a wavelength near 1.5 $\mu\mathrm m$, a nonlinear wavelength converter to the 1 $\mu\mathrm m$ range and a three-stage ytterbium-doped fibre amplifier, we obtained pulses of 1 $\mu\mathrm J$ energy and 7 ps duration, which were then compressed by a grating-pair dispersion compressor with 60 % efficiency to a 130 fs duration, approaching the transform-limited pulse duration. The present experimental data agree well with numerical simulation results for pulse amplification in the threestage amplifier.