Power scaling of 2- μ m femtosecond optical vortices in a single-crystal fiber

Abstract

By virtue of their controllable orbital angular momentum (OAM), ultrashort pulse duration, and high average power, 2-μm femtosecond optical vortices hold great promise for a range of emerging applications, including attosecond science, integrated photonic device fabrication, and terahertz vortex imaging and sensing. Nevertheless, their direct amplification remains challenging owing to nonlinear phase accumulation, spatial mode distortion, and topological-charge-dependent spectral dispersion. Here, we demonstrate a high-average-power 2-μm femtosecond vortex laser system consisting of a mode-locked laser source and a multi-stage single-crystal fiber (SCF) amplifier, where the large mode area and efficient thermal management of the SCF enable high-fidelity amplification of femtosecond vortex beams with high efficiency and power-handling capability. An average output power of 40.2 W is achieved for the lowest-order topological charge ( l =1) optical vortex at 2.09 μm, corresponding to an amplification gain of >19   dB . Moreover, power scaling of higher-order ( l =3 and 5) femtosecond optical vortices is also demonstrated with both preserving the spatial properties associated with a clear central singularity and a spiral phase front. This work, providing the highest average power of femtosecond optical vortices in the 2-μm spectral range, proves a straightforward way for direct power scaling of structured laser beams with spatially irregular intensity distribution and ultrafast temporal characteristics.