Generation of femtosecond polygonal optical vortices from a mode-locked quasi-frequency-degenerate laser

Recent advances in femtosecond optical vortices (FOVs)—characterized by helical wavefronts and high peak power—have enabled breakthroughs in attosecond vortex generation and 3D chiral microfabrication, driven by improvements in mode order, pulse duration, phase singularity control, and wavelength tunability. A distinct subclass, polygonal optical vortices (POVs), exhibits closed polygonal intensity profiles, introducing new degrees of freedom for structured light. Extending POVs to the femtosecond regime (FPOVs) merges ultrafast dynamics with nonlinear capabilities, facilitating applications such as multi-photon 3D microfabrication in transparent materials and asymmetric optical trapping. However, conventional phase-modulation techniques for FPOV generation suffer from spatial dispersion and low damage thresholds. Mode-locked Hermite-Gaussian oscillators with astigmatic mode converters (AMCs) offer a robust alternative, delivering high-power FPOVs. The critical challenge remains simultaneous longitudinal mode locking and transverse quasi-frequency-degenerate stabilization to ensure FPOV output coherence—a prerequisite for advancing femtosecond optical tweezers and precision 3D laser processing.

 

In a new paper published in Light: Science & Applications, a team of scientists, led by Professor Jinwei Zhang from School of Optical and Electronic Information and Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, China and co-workers have invented a passive mode locked solid-state Yb:KGW oscillator at quasi-frequency-degenerate state to form the square, pentagonal, and hexagonal shapes FPOVs. By utilizing a mode-locked Hermite-Gaussian (HG) oscillator integrated with an astigmatic mode converter (AMC), this research proposes a robust approach to generate femtosecond polygonal optical vortices. The proposed scheme utilizes quasi-frequency-degenerate (QFD) transverse modes under off-axis pumping conditions, facilitating the conversion of QFD-HG modes into polygonal vortices with distinct intensity distributions. The vortex beam generated directly from the laser oscillator exhibits excellent power stability while maintaining consistent intensity profiles and conserved total topological charges during extended operation. Notably, this work presents the first experimental demonstration of femtosecond polygonal optical vortices (FPOVs) featuring square, pentagonal, and hexagonal intensity distributions. These advancements are anticipated to significantly enhance applications in emerging fields such as femtosecond vortex optical trapping and three-dimensional microstructure fabrication.

 

These scientists summarize the method for the generation of FPOVs from a laser oscillator:

“We design a SESAM mode-locked Yb:KGW laser oscillator working in QFD state for generating FPOVs with square, pentagonal, and hexagonal intensity distribution. It can be divided into two stages: (1) Generation of QFD-HG modes and FD-HG modes in the continuous wave (CW) regime; (2) Generation of FPOV pulses with different intensity distribution. For the first stage, the off-axis pumping is used for generating high-order HG modes and the laser state is changed by adjusting the distance L between the SESAM and R1 for generating QFD-HG modes and FD-HG modes. In terms of the second part, using the proper mode matching and cavity design ensures that the QFD-HG mode has sufficient power density at SESAM and obtain QFD-HG pulses with femtosecond duration.”

 

“The generated femtosecond QFD-HG modes have a great average power stability and high signal-to-noise ratio. By an AMC, the generated femtosecond QFD-HG modes were converted to the corresponding FPOVs. And we used a home-built Mach-Zehnder interferometer to characterize phase singularity and topological charge characteristics of different FPOVs. Specifically, the square FPOV contains 6 topological charges at the beam center and 4 at the corners of the beam pattern. For the pentagonal and hexagonal FPOVs, the beam centers host 11 topological charges, while 5 and 6 charges are distributed at the corners, respectively.” They added.

 

“The measured beam profiles and the recovered phase distribution of the square, pentagonal and hexagonal FPOVs pulse at ten-minute intervals in one hour demonstrated high stability in both the phase structure and topological charge. In addition, the measured interference patterns of pulse trains selected by the high-speed Pockels cells consistently exhibited stable total topological charges.” the scientists forecast.

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