Exploring the self-mode locking and vortex structures of nonplanar elliptical modes in selectively end-pumped Nd:YVO4 lasers: manifestation of large fractional orbital angular momentum

Investigating the generation and propagation evolution of orange optical vortices using continuous-wave KGW Raman lasers with astigmatic mode transformations

Various high-order orange beams (HOBs) at 588 nm are produced via off-center pumped Nd:YVO₄/KGW Raman lasers. We experimentally confirm that the HOBs can be fairly sustained at the incident pump power of 2.88 W, where the average output powers are overall from 300 mW to 160 mW with increasing the off-center displacements from 0.14 mm to 0.21 mm. The HOBs are further transformed by using an astigmatic mode converter to generate a variety of structured lights with optical vortices. Moreover, theoretical wave functions are analytically derived to characterize the propagation evolution of the converted HOBs. The experimental patterns for all propagating positions are excellently reconstructed by the derived wave functions, and the evolution of phase structures is numerically calculated to manifest the robust optical vortices.

Astigmatic hybrid SU(2) vector vortex beams: towards versatile structures in longitudinally variant polarized optics

Structured light with more controllable degrees-of-freedom (DoFs) is an exciting topic with versatile applications. In contrast to conventional vector vortex beams (VVBs) with two DoFs of orbital angular momentum (OAM) and polarization, a hybrid ray-wave structure was recently proposed [Optica 7(7), 820-831 (2020)], which simultaneously manifests multiple DoFs such as ray trajectory, coherent state phase, trajectory combination, besides OAM and polarization. Here we further generalize this exotic structure as the astigmatic hybrid VVB by hatching a new DoF of astigmatic degree. Importantly, the transverse topology varies with propagation, e.g. a linearly distributed hybrid trajectory pattern can topologically evolve to a circularly polygonal star shape, where the number of singularity changes from zero to multiple in a single beam. The propagation-dependent evolution can be easily controlled by the astigmatic degree, including as a vector vortex state such that different astigmatic trajectories have different polarizations. We experimentally generate such beams from a simple laser with a special astigmatic conversion by combined spherical and cylindrical lenses, and the results agree well with our theoretical simulation. With our new structured light, the propagation-multiplexing multi-DoF patterns can be controlled in a single beam, which can largely extend related applications such as high-dimensional large-capacity optical communication, laser machining, and particle trapping.

Optical vortices 30 years on: OAM manipulation from topological charge to multiple singularities

Thirty years ago, Coullet et al. proposed that a special optical field exists in laser cavities bearing some analogy with the superfluid vortex. Since then, optical vortices have been widely studied, inspired by the hydrodynamics sharing similar mathematics. Akin to a fluid vortex with a central flow singularity, an optical vortex beam has a phase singularity with a certain topological charge, giving rise to a hollow intensity distribution. Such a beam with helical phase fronts and orbital angular momentum reveals a subtle connection between macroscopic physical optics and microscopic quantum optics. These amazing properties provide a new understanding of a wide range of optical and physical phenomena, including twisting photons, spin-orbital interactions, Bose-Einstein condensates, etc., while the associated technologies for manipulating optical vortices have become increasingly tunable and flexible. Hitherto, owing to these salient properties and optical manipulation technologies, tunable vortex beams have engendered tremendous advanced applications such as optical tweezers, high-order quantum entanglement, and nonlinear optics. This article reviews the recent progress in tunable vortex technologies along with their advanced applications.

Periodic-trajectory-controlled, coherent-state-phase-switched, and wavelength-tunable SU(2) geometric modes in a frequency-degenerate resonator

We report a method to realize the periodic trajectory controlling, coherent-state phase switching, and wavelength tuning of SU(2) geometric modes (GMs) in a frequency-degenerate resonator (FDR). FDR is a resonator in which the ratio of transverse and longitude modes frequency spacing is a simple rational number, which would lead to a laser wave-packet in a SU(2) coherent state related to a periodic ray trajectory. We demonstrate that the periodic trajectory can be switched with coherent-state phase changing from 0 to π (or π to 0) by simply controlling the position of gain medium in the same FDR. For the period-of-four state, the geometries are switched between "W" and "M" shapes. For the period-of-three state, those are switched between "IV" and "VI" shapes. Moreover, because a special crystal Yb:CALGO with extremely broad emission band is used as the gain medium, our SU(2) GMs have the wavelength-tunable property in contrast to the conventional single-frequency GMs. The center wavelength can be tuned with the range of about 10 nm by adjusting the pump power. These effects can further enrich the various applications of structured light manipulation.

Truncated triangular diffraction lattices and orbital-angular-momentum detection of vortex SU(2) geometric modes

We report, for the first time to the best of our knowledge, a truncated diffraction with a triangular aperture of the SU(2) geometric modes and propose a method to detect the complicated orbital angular momentum (OAM) of an SU(2) wave-packet. As a special vortex beam, a nonplanar SU(2) mode carrying special intensity and OAM distributions brings exotic patterns in a truncated diffraction lattice. A meshy structure is unveiled therein by adjusting the illuminated aperture in the vicinity of the partial OAM regions, which can be elaborately used to evaluate the partial topological charge and OAM of an SU(2) wave-packet by counting the dark holes in the mesh. Moreover, through controlling the size and position of the aperture at the center region, the truncated triangular lattice can be close to the classical spot-array lattice for measuring the center OAM. These effects being fully validated by theoretical simulations greatly extend the versatility of topological structures detecting special beams.

Wavelength-tunable Hermite-Gaussian modes and an orbital-angular-momentum-tunable vortex beam in a dual-off-axis pumped Yb:CALGO laser

A dual-off-axis pumping scheme is presented to generate wavelength-tunable high-order Hermite-Gaussian (HG) modes in Yb:CaGdAlO₄ lasers. The mode and wavelength can be actively controlled by the off-axis displacements and pump power. The purities of the output HG modes are quantified by intensity distributions and the measured M² values. The highest order reaches m=15 for stable HG_m,0 mode, and wavelength-tunable width is about 10 nm. Moreover, through externally converting the HG_m,0 modes, the vortex beams carrying orbital angular momentum (OAM) with a large OAM-tunable range from ±1ℏ to ±15ℏ are produced. This work is effective for largely scaling the spectral and OAM tunable ranges of optical vortex beams.