Mode-locked femtosecond vortex laser oscillator with tunable phase singularities.

Femtosecond vortex beams have become increasingly important in various fields such as advanced manufacturing, laser-matter interaction and optical communication. Here, we report the direct generation of femtosecond optical vortex beams with tunable phase singularities from a passive mode-locked Yb:KGW laser oscillator, without any additional optical phase elements. Stable, pure LG vortex pulses are delivered with an average power of 485 mW and a pulse duration of 247 fs. Moreover, tuning the angle between the laser and pumping beam results in the generation of complex vortex pulses with an average power of 244 mW and a pulse duration of 408 fs, which own two singularities in the phase structure with opposite chirality. This work demonstrates a simple and effective way to generate high-quality ultrafast optical vortex beams, which can be extended to other oscillators supporting higher order transverse modes as well as different spectral regions.