High-power picosecond laser with Yb-doped tapered low birefringent double-clad fiber

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%.