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.

/pdf/optical-vortices-30-years-on-oam-manipulation-from-2yknluuuga.pdf

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

Structured Light from Lasers

Optical vortex is a promising candidate for capacity scaling in next-generation optical communications. The generation of multi-vortex beams is of great importance for vortex-based optical communications. Traditional approaches for generating multi-vortex beams are passive, unscalable and cumbersome. Here, we propose and demonstrate a multi-vortex laser, an active approach for creating multi-vortex beams directly at the source. By printing a specially-designed concentric-rings pattern on the cavity mirror, multi-vortex beams are generated directly from the laser. Spatially, the generated multi-vortex beams are decomposable and coaxial. Temporally, the multi-vortex beams can be simultaneously self-mode-locked, and each vortex component carries pulses with GHz-level repetition rate. Utilizing these distinct spatial-temporal characteristics, we demonstrate that the multi-vortex laser can be spatially and temporally encoded for data transmission, showing the potential of the developed multi-vortex laser in optical communications. The demonstrations may open up new perspectives for diverse applications enabled by the multi-vortex laser.

/pdf/multi-vortex-laser-enabling-spatial-and-temporal-encoding-5ad6l7427t.pdf

Multi-vortex laser enabling spatial and temporal encoding

Non-Hermitian exceptional points (EPs) represent a special type of degeneracy where not only the eigenvalues coalesce, but also the eigenstates tend to collapse on each other. Recent studies have shown that, in the presence of an EP, light–matter interactions are profoundly modified, leading to a host of unexpected optical phenomena ranging from enhanced sensitivity to chiral light transport. Here we introduce a family of unidirectional resonators based on a novel type of broadband exceptional points. In active settings, the resulting unidirectionality exhibits resilience to perturbations, thus, providing a robust and tunable approach for directly generating beams with distinct orbital angular momenta (OAM). This work could open up new possibilities for manipulating OAM degrees of freedom in applications pertaining to telecommunications and quantum information sciences, while at the same time may expand the notions of non-Hermiticity in the orbital angular momentum space.

https://findresearcher.sdu.dk:8443/ws/files/154419820/Direct_Generation_of_Tunable_Orbital_Angular_Momentum_Beams_in_Microring_Lasers_with_Broadband_Exceptional_Points.pdf

Direct Generation of Tunable Orbital Angular Momentum Beams in Microring Lasers with Broadband Exceptional Points

Structured ultrashort-pulse laser beams, and in particular eigenmodes of the paraxial Helmholtz equation, are currently extensively studied for novel potential applications in various fields, e.g., laser plasma acceleration, attosecond science, and fine micromachining. To extend these prospects further, in the present work we push forward the advancement of such femtosecond structured laser sources into the 2-μm spectral region. Ultrashort-pulse Hermite– and Laguerre–Gaussian laser modes both with a pulse duration around 100 fs are successfully produced from a compact solid-state laser in combination with a simple single-cylindrical-lens converter. The negligible beam astigmatism, the broad optical spectra, and the almost chirp-free pulses emphasize the high reliability of this laser source. This work, as a proof of principle study, paves the way toward few-cycle pulse generation of optical vortices at 2 μm. The presented light source can enable new research in the fields of interaction with organic materials, next generation optical detection, and optical vortex infrared supercontinuum.

/pdf/structured-laser-beams-toward-2-mm-femtosecond-laser-1vbs6ucllj.pdf

Structured laser beams: toward 2-μm femtosecond laser vortices

Wavelength- and OAM- tunable laser with large tunable range is the key source for the application in large capacity optical communications. In this paper, we demonstrate a wavelength- and OAM-tunable vortex laser in a 1.2 W single mode fiber coupled LD pumped Yb:phosphate laser. A z-type cavity has been used to precisely control the laser mode diameter. A thin film polarizer (TFP) is inserted to finely control the intra-cavity loss and tune the wavelength. Corresponding laser fundamental mode to pump beam ratio has been optimized to decrease the pump threshold for high order HG mode running. A pair of cylindrical lenses has been used to convert the HG mode to vortex output. The vortex beam with OAM-tunable range from 1ħ to 14 ħ with wavelength tuning range of ~36.2 nm for LG0,1 vortex beam, and ~14.5 nm for LG0,14 vortex beam at pump power of only 1.2 W have been realized, which is the largest tuning range of both OAM and wavelength at ~1 W pump power range to the best of our knowledge.

Generation of wavelength- and OAM-tunable vortex beam at low threshold

A single mode fiber coupled laser diode was employed to generate a low threshold femtosecond vortex by using an off-axis pumping technique and mode converter with a pair of cylindrical lenses. In this way, the pump threshold of a laser system can be decreased and the slope efficiency can be increased. Two lateral interferometric techniques for optical vortex detection were improved using the circular common-path (CCP) geometry configuration. In the CCP interferometer, the object optical vortex was divided into two beams that transmit oppositely along the interferometer and interfere with themselves in the beam splitter. The two beams traveled along the same path through the same optical elements. These new designs omit delicate adjustment of two interference arms, and can easily manipulate the optical path difference between the two beams. The results show that the experimental results are in good agreement with the numerical simulation. These CCP interferometers are insensitive to the environment and are suitable for the measurement of topological charge of a laser source with a short coherence length, including white lasers.

https://iopscience.iop.org/article/10.1088/1612-202X/aaff51/pdf

Ultrafast vortices generation at low pump power and shearing interferometer-based vortex topological detection

Double-end polarized pumping scheme combined with off-axis pumping technique has been first introduced to generate vortex beams in a z-type cavity. By employing double-end pumping, two different transverse modes can be excited simultaneously. The phase delay between these two modes can be finely tuned by manipulating the cavity structure. Direct emission of a chirality controllable Laguerre Gaussian LG01 vortex beam with slope efficiency of more than 40% has been realized by a double-end polarized pumped Yb:KYW laser. Other modes, such as dual-LG01 mode, cross-shaped mode, and LG10 mode, have also been demonstrated from our laser setup.

Direct generation of vortex beams from a double-end polarized pumped Yb:KYW laser.

In this letter, we report a systematic approach to directly generate chirality controllable Laguerre–Gaussian (LG01) mode in solid-state lasers. This approach is based on two concepts. The first is the off-axis pumping technique, which has been exploited to achieve Hermite–Gaussian modes. The second is intracavity astigmatism manipulation, which has been introduced to the laser cavity by tilting curved mirrors to generate LG modes. The control method of chirality has been discussed in detail. The feasibility of this approach is shown in both theoretical model and experimental analysis of the generation process.

Ping Li

Papers

Generation of wavelength- and OAM-tunable vortex beam at low threshold

Ultrafast vortices generation at low pump power and shearing interferometer-based vortex topological detection

Direct generation of vortex beams from a double-end polarized pumped Yb:KYW laser.

Direct excitation of chirality controllable LG 01 vortex beam in solid-state lasers by intracavity astigmatism manipulation