In this work, we investigated an active ytterbium double-clad tapered spun fiber with low intrinsic birefringence (1.45×10⁻⁸ rad/m) and a mode field diameter of 35 μm. These fiber properties significantly increase the nonlinear threshold in amplifiers, enabling high average and peak output powers. The low birefringence also ensures stable output polarization despite variations in pump power. We demonstrated a MOPA system based on this fiber, operating at 1040 nm, achieving a peak power of 160 kW (50 ps pulses at 20 MHz, 160 W average power, 63% slope efficiency) with high beam quality (M² = 1.15 at 115 W). We explored polarization changes under pump power up to 270 W at 976 nm, finding that polarization drift due to heating (quantum defect) caused minimal azimuth and ellipticity changes. The degradation in DOP was attributed to unpolarized ASE rather than fiber polarization properties. Over two hours, polarization drift was minimal, with azimuth and ellipticity deviations of 0.4° and 0.5°, and a DOP variation of 1.5%.
We demonstrate the dramatic progress in Yb-doped spun tapered double-clad fiber amplifiers delivering up to 550 W of average power with single mode spatial profile and 50 ps pulses at 20 MHz repetition rate. The special geometrical architecture of the fiber enables the direct amplification of short pulses from tens of mW to hundreds of watt levels in a single amplification stage, leading towards the realization of a compact and highly efficient picosecond fiber-based laser system with excellent output spatial and temporal characteristics.
We have demonstrated for the first time, to the best of our knowledge, the successful direct amplification of a cylindrical-vector beams with axially symmetric polarization and doughnut-shaped intensity profile in picosecond MOPA system based on a double-clad ytterbium-doped spun tapered fiber with a ring-shaped active core. The output radially polarized beam with absolute contrast between bright and dark zones carries 10 ps pulses at 1030 nm with a 14.5 W average power level, 91 kW peak power and 0.97 μJ pulse energy.
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