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High-order femtosecond vortices converted from a tunable high-order Hermite–Gaussian Yb:CALGO laser

Femtosecond optical vortices open a variety of applications, ranging from femtosecond micro-processing to vortex strong-field physics. Thus far, the generation of high-order femtosecond vortices remains a challenge. The 1st to 4th order tunable femtosecond vortices generated by a z-type cavity semiconductor saturable absorber mirror mode-locked Yb:CALGO laser are reported in this study. The anti-reflection coated Yb:CALGO crystal is used as the gain material and is positioned perpendicular to the laser beam to avoid an angle dispersion, which results in multi-transverse-mode running. The astigmatism caused by the tilted curved mirrors is compensated by elevating the folding mirrors adjacent to the curved mirrors. The 1st to 5th order tunable femtosecond Hermite–Gaussian (HGn0) modes are realized by employing the off-axis pumping technique. The mode-locked pulse width is in the range of 220–580 fs for various HG modes. The mode-locked HG modes are converted into femtosecond vortices using a cylindrical-lens mode converter. The work provides a reliable way to generate high-order femtosecond vortex beams.