A vortex beam is a special light field possessing a spatial structure, including phase vortex and polarization vortex, and possessing a phase singularity and a polarization singularity correspondingly. A phase vortex laser carrying orbital angular momentum (OAM) has a helical phase front of exp(ilφ), where l is the topological charge value and φ refers to the azimuthal angle. The topological charge represents the twisting rate of the helical phase which is an unlimited value in principle. In addition, phase vortices with different topological charges are orthogonal to each other. A polarization vortex laser is a light beam with spatially variant polarization. Due to the phase singularity and polarization singularity, the intensity of a vortex laser beam at the center is canceled leading to a ring-shape intensity profile. Vortex beams have been widely used in astronomy, optical manipulation, microscopy, imaging, sensing, quantum science, and optical communications owing to their distinct advantages, such as inherent orthogonality and unbounded states in principle.

With the arrival ofa big data era, the dramatic increase of global internet traffic has attracted increasing research efforts for sustainable expansion of capacity. Beyond various advanced modulation formats and multiplexing techniques using the physical dimensions of photons including frequency, amplitude, phase, and time, space-division multiplexing (SDM) is recognized as an alternative technique to increase transmission capacity by exploring the spatial structure of photons. Optical vortices can be regard as a mode set to multiplex data information owing to the inherent orthogonality and unbounded states.

As one of the four major inventions in the 20th century, the laser is the basic part of an optical communication system and plays a very important role. Up to now, on one hand, a vortex beam is generated mainly through the conversion occurring outside the laser cavity, and the various applications based on vortex beam, such as diffractive optical elements, transform optics, spiral phase plate, fiber based devices, photonic integrated devices, metasurfaces, have obvious limitations in some aspects including imperfect spiral wave front phases. On the other hand, a vortex beam can be directly generated from a vortex laser, which can avoid some disadvantages of the conversions occurring outside the laser cavity, such as low conversion efficiency, poor beam quality after conversion, power limitation, and additional converter devices. Therefore, vortex lasers deserve more extensive and sufficient researches.

The ways to output a vortex beam directly from the laser cavity can be mainly divided into three types. 1) Inserting some elements, such as a spiral phase plate, lens, and diaphragm, into the cavity to generate a vortex beam from the laser cavity. 2) Using a special cavity mirror to select the oscillation mode in laser cavity. 3) Using a ring-shaped pumping to generate a vortex beam. By converting the pump beam into a ring-shaped one similar to the shape of the intensity profile of a vortex beam, the oscillation mode in the laser cavity can match the pump beam to the generated vortex beam.

The vortex laser based on discrete components is mainly composed of discrete single optical elements in free space. This laser is simple to construct, stable, and has large number of output modes. In 2005, Kozawa et al. demonstrated a polarization vortex laser by inserting a Brewster prism into the laser cavity based on Nd∶YAG [Fig. 3(a)]. A laser which can generate all states on the higher-order Poincaré sphere was demonstrated in 2016. By exploiting the geometric phase control inside the laser cavity to map polarization to OAM, the OAM degeneracy of a standard laser cavity may be broken to produce pure OAM beams, and the generalized vector vortex beams may be created with high purity at the source. A fiber laser is one with a doped fiber as gain medium which has better stability and better beam quality comparing with the semiconductor lasers. A fiber laser is more compatible to an optical fiber communication system, which can effectively reduce the system complexity and cost. A fiber laser based on a linear resonator and a ring resonator outputs different modes, which are demonstrated in Fig. 11. In recent years, the photonic integration technique has developed rapidly, and the miniaturization of optical devices is also a development trend. In addition, integrated devices are flexible, tunable, and reconfigurable comparing with the traditional discrete components. Therefore, a vortex laser based on integrated devices is of great research value. Vertical-cavity surface-emitting vortex lasers based on lead bromide perovskite and InGaAsP/InP platform are demonstrated (Fig. 15).

Here, we provide an overview of the recent progress of vortex optical lasers. We comprehensively review different types of vortex lasers, including vortex lasers based on discrete components, vortex lasers based on fiber, and vortex lasers based on integrated devices. Meanwhile, the future development trend of vortex lasers is analyzed and the prospect is discussed. Vortex lasers are expected to further promote the wide application of vortex beam in many fields.