The generation of a flexibly modulated vortex beam with super-high topological charge is always attractive for practical applications. In addition, a high-quality vortex beam with flat-top intensity and a sharp edge is more desired for potential power amplification and optical trapping. Here, a perfect vortex beam (PVB) with topological charge as high as 140 has been realized using super-pixel wavefront shaping. To address the non-uniform intensity distribution of the obtained PVBs, a globally adaptive feedback algorithm (GAFA) based wavefront-shaping is proposed which can efficiently suppress the original intensity fluctuation. Flat-top PVB is then obtained with dramatically reduced beam speckle contrast, while the total mean intensity remains relatively stable. The GAFA based flat-top PVB generation not only alleviates the requirements for ideal incident illumination but also facilitates applications such as inertial confinement fusion and optical trapping.

Recently, the Institute of Micro-scale Optoelectronics at Shenzhen University realized a low-coherence perfect vortex beam (PVB) with a topological charge of up to 140 based on a random fiber laser (RFL) and the wavefront shaping of a digital micromirror device (DMD). The results are published in the article of High Power Laser Science and Engineering, Vol.12, e5. (Rui Ma, Ke Hai Luo, Jing Song He, Wei Li Zhang, Dian Yuan Fan, Anderson S. L. Gomes, Jun Liu. Digital generation of super-Gaussian perfect vortex beams via wavefront shaping with globally adaptive feedback[J]. High Power Laser Science and Engineering, 2024, 12(1): 010000e5).

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Figure 1 depicts the schematic diagram of the flat-top PVB generation and the corresponding characterization. The experimental setup consists of three major parts: the RFL light source, the vortex beam generation using super-pixel wavefront shaping, and the Mach-Zehnder interferometer for vortex phase verification. The RFL light source has advantages such as low coherence, low relative intensity noise, simple structure, and agile lasing wavelength. In addition, compared with traditional liquid crystal spatial light modulators, the DMD based wavefront shaping has advantages such as high refresh rate, polarization independence, high power tolerance, and wide wavelength operation window, which is conducive to real-time beam shaping of polarization-independent and low coherence lasers.

Figure 2 shows the result of the GAFA-based wavefront shaping in realizing a flat-top PVB. With the increase of the feedback iteration number, the original intensity profiles in Figure 2(a) gradually become flat with strong suppression of the intensity in the inner annulus region. When the iteration number reaches 40, the intensity profiles in all four PVB cases exhibit excellent intensity uniformity, as shown in Figure 2(d). With another 10 iterations, the mean intensity of the PVB even increases. In addition, during the GAFA wavefront shaping, all the features of the annulus, that is, the radius, the width, and the sharpness of the edge, remain unaffected, which indicates the robustness of the proposed method.

In conclusion, super high topological charge PVB has been demonstrated by optimizing the DMD based super-pixel wavefront shaping. To address the non-uniform intensity distribution of the experimentally obtained PVBs, a GAFA based wavefront-shaping is proposed and flat-top PVBs with dramatically suppressed beam speckle contrast have been realized. It is speculated that the GAFA based wavefront-shaping could alleviate the requirements for uniform incident illumination in the generation of flat-top vortex beams and facilitate applications that need flat-top beam profiles such as inertial confinement fusion, laser processing, and optical trapping.