Yb-doped tapered double-clad fibers with polarization maintenance for half-kW power amplifiers

Amplifying short pulses directly within a single fiber laser system has proven to be a challenging task, primarily due to thermally induced transverse mode instabilities and detrimental nonlinear effects. Another demanding aspect is preserving the linear polarization state at high power levels, which is even more pronounced for ultra-large-mode area fibers. This study demonstrates significant advancement in the direct amplification of narrow linewidth short pulses from tens of mW to several hundreds of Watts in a single-stage amplification, maintaining a high degree of linear polarization at the maximum output power. Through a comprehensive experimental investigation, two distinct types of Ytterbium-doped tapered double-clad fibers (T-DCFs), namely, PANDA (PT-DCF), with high built-in birefringence, and spun (sT-DCF), with ultra-low built-in birefringence, are examined. The unique geometrical architecture of the amplifiers is exploited for the realization of a compact and highly efficient picosecond fiber-based laser system, achieving more than 75% slope efficiency. In a single amplification stage, 50 ps pulses at a repetition rate of 20 MHz and an average power of 65 mW are amplified up to 457 W and 573 W of average power using PT-DCF and sT-DCF amplifiers, respectively. Both amplifiers exhibit near diffraction limited beam quality, M² < 1.4 at the highest power level. At the maximum power levels, the system maintains a high degree of linear polarisation, achieving ∼ 90% and ∼ 94% for the sT-DCF and PT-DCF, respectively. These ultra-large mode area fiber amplifiers are verified as versatile solutions for direct amplification of short pulses up to half-kW level with excellent spectral, spatial, and polarization characteristics.