A B-integral managing strategy with multi-stage SCF amplifier: towards direct power scaling of femtosecond sources near 2 μm

Abstract Intense femtosecond laser pulses at 2 µm hold tremendous promise for applications ranging from fundamental scientific research particularly in high-order harmonic generation (HHG) and next-generation laser-driven particle accelerator, to industry and life sciences. Compared with the well-designed chirped-pulse amplification (CPA) technique, direct amplification is the most predictable way for the realization of intense femtosecond laser pulses in the advantages of the simplicity and compactness but suffered from the strong nonlinearity in the past tens of years. Here, we propose a B-integral managing strategy for manipulating the nonlinear effects by employing discrete single-crystal fiber (SCF) configuration, enabling direct amplification of 2-µm femtosecond pulses at high repetition rates without additional pulse picking, stretching and compression. The system delivers > 56 W average power at 75.45 MHz with extremely high extraction efficiency (> 55%) and near diffraction-limited beam quality (M2 < 1.2). The dynamic evolution of the optical spectra and temporal properties in power amplifier reveals that detrimental nonlinear effects are largely suppressed due to the low accumulated nonlinear phase shift in the discrete SCF layout. This straightforward, compact and relatively simple for realization approach is expected to open a new route to the amplification of 2-µm ultrashort pulses at MHz and kHz repetition rates towards high average/peak powers, and thus exciting future prospects for the applications in the modern nonlinear photonic regimes.