Research progress of all-solid-state intra-cavity vortex beam generation in visible laser (invited)

  Significance   In recent years, visible vortex laser carrying orbital angular momentum (OAM) has been widely used in the fields of astronomy, optical manipulation, microscopic imaging, sensing, quantum science and optical communication. Especially for the underwater communication or the super-resolution imaging, further optimizing the output of vortex beams in the visible range is of great significance in enhancing imaging resolution and communication capacity. This not only holds importance in scientific research but also holds vast potential for wide-ranging applications in real-life scenarios, paving the way for advancements in high-resolution imaging, high-speed communication, and other fields.

  Progress  Visible vortex beams can be generated through both extra-cavity and intracavity conversion methods. This study focuses on the intracavity conversion approach to obtain visible vortex beams. With the development of the vortex lasers operating at 1 μm, nonlinear frequency doubling has become a common technique for generating visible vortex beams. By utilizing techniques such as intracavity thermal lens effect, etched point defects, and design of a hemispherical resonator cavity, combined with frequency doubling method, visible vortex beams can be generated without the need for additional components. Alternatively, an extra-cavity mode converter can be used to generate vortex beams, which is then combined with frequency doubling method to produce visible vortex beams. Compared to nonlinear frequency conversion techniques, direct pumping the visible laser crystals to obtain visible vortex beams in the visible range can improve conversion efficiency. For the LD pumped Pr³⁺ doped all-solid-state laser, visible vortex beams can be generated through intracavity mode conversion techniques such as off-axis pumping, annular light pumping, spherical aberration mode selection. Visible vortex fiber lasers offer advantages of compact structure and high conversion efficiency. They mainly utilize techniques such as fiber core misalignment fusion splicing or specially designed mode selectors to generate visible vortex lasers.

  Conclusions and Prospects   Currently, visible vortex solid-state lasers are mainly achieved by combining near-infrared vortex beams with frequency doubling or by utilizing LD direct pumped Pr³⁺-doped crystals combined with intracavity vortex beam conversion technology. The former approach typically requires the insertion of laser crystals and frequency doubling crystals inside the cavity, leading to a complex system structure and lower optical-to-optical conversion efficiency. In the future, visible vortex solid-state lasers have great potential for development in terms of tunability, multi-wavelength operation, high power, and single longitudinal mode characteristics. Achieving multi-wavelength visible vortex beam output and generating ultra-short pulse vortex beams (such as picosecond and femtosecond vortex beams) are among the directions for further advancement. Furthermore, if visible vortex solid-state lasers can be extended to the realm of spatiotemporal mode locking, it will inject new vitality into the development of vortex beams.