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.

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

Time, polarization, and wavelength multiplexing schemes have been used to satisfy the growing need of transmission capacity. Using space as a new dimension for communication systems has been recently suggested as a versatile technique to address future bandwidth issues. We review the potentials of harnessing the space as an additional degree of freedom for communication applications including free space optics, optical fiber installation, underwater wireless optical links, on-chip interconnects, data center indoor connections, radio frequency, and acoustic communications. We focus on the orbital angular momentum (OAM) modes and equally identify the challenges related to each of the applications of spatial modes and the particular OAM modes in communication. We further discuss the perspectives of this emerging technology. Finally, we provide the open research directions and discuss the practical deployment of OAM communication links for different applications.

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Communicating Using Spatial Mode Multiplexing: Potentials, Challenges, and Perspectives

We experimentally demonstrated a high-order optical vortex pulsed laser based on a mode selective all-fiber fused coupler composed of a single-mode fiber (SMF) and a few-mode fiber (FMF). The fused SMF-FMF coupler inserted in the cavity acts as a mode converter from LP $_{01}$ mode to LP $_{11}$ (LP $_{21}$ ) mode with a broadband width. Linearly polarized OAM modes can be obtained by combining different vector modes HE $_{2,m}^{\rm {even}}$ (HE $_{2,m}^{\rm {odd}}$ ) and TE $_{0,m}$ (TM $_{0,m}$ ) when l = 1 or combining HE $_{l+1,m}^{\rm {even}}$ (HE $_{l+1,m}^{\rm {odd}}$ ) and EH $_{l-1,m}^{\rm {odd}}$ (EH $_{l-1,m}^{\rm {even}}$ ) when $l$ > 1 with a $\pi /2$ phase shift. To the best of our knowledge, this is the first report on the generation of high-order pulse vortex beams in a mode-locked fiber laser. The measured time duration of OVB pulses is 273 fs, 140 fs of OAM $_{\pm 1}$ and OAM $_{\pm 2}$ .

Generation of Femtosecond Optical Vortex Beams in All-Fiber Mode-Locked Fiber Laser Using Mode Selective Coupler

Metasurfaces have found broad applicability in free-space optics, while its potential to tailor guided waves remains barely explored. By synergizing the Jones matrix model with generalized Snell’s law under the phase-matching condition, we propose a universal design strategy for versatile on-chip mode-selective coupling with polarization sensitivity, multiple working wavelengths, and high efficiency concurrently. The coupling direction, operation frequency, and excited mode type can be designed at will for arbitrary incident polarizations, outperforming previous technology that only works for specific polarizations and lacks versatile mode controllability. Here, using silicon-nanoantenna-patterned silicon-nitride photonic waveguides, we numerically demonstrate a set of chip-scale optical couplers around 1.55 μm, including mode-selective directional couplers with high coupling efficiency over 57% and directivity about 23 dB. Polarization and wavelength demultiplexer scenarios are also proposed with 67% maximum efficiency and an extinction ratio of 20 dB. Moreover, a chip-integrated twisted light generator, coupling free-space linear polarization into an optical vortex carrying 1ℏ orbital angular momentum (OAM), is also reported to validate the mode-control flexibility. This comprehensive method may motivate compact wavelength/polarization (de)multiplexers, multifunctional mode converters, on-chip OAM generators for photonic integrated circuits, and high-speed optical telecommunications.

Versatile on-chip light coupling and (de)multiplexing from arbitrary polarizations to controlled waveguide modes using an integrated dielectric metasurface

Time, polarization, and wavelength multiplexing schemes have been used to satisfy the growing need of transmission capacity. Using space as a new dimension for communication systems has been recently suggested as a versatile technique to address future bandwidth issues. We review the potentials of harnessing the space as an additional degree of freedom for communication applications including free space optics, optical fiber installation, underwater wireless optical links, on-chip interconnects, data center indoor connections, radio frequency and acoustic communications. We focus on the orbital angular momentum (OAM) modes and equally identify the challenges related to each of the applications of spatial modes and the particular OAM modes in communication. We further discuss the perspectives of this emerging technology. Finally, we provide the open research directions and we discuss the practical deployment of OAM communication links for different applications.

Communicating Using Spatial Mode Multiplexing: Potentials, Challenges and Perspectives

Helical fiber Bragg grating (H-FBG) induced by UV-side exposure exhibits wavelength-dependent coupling between different modes. In this paper, we propose an effective method to generate high-order optical vortices (OVs) using H-FBG written in a multimode fiber. We simulate the conversion of OVs with different topological charges: 0 → ± 1, ±1 → 0, 0 → ± 2, 0 → ± 3 with conversion efficiency as high as 97%. We prove that higher order topological charge can be obtained simply by changing the parameters of the fiber to increase the number of modes in the fiber. We also analyze the conversion of H-FBG under various conditions of the induced asymmetric index profile and the length of H-FBG.

Generation and Conversion of Higher Order Optical Vortices in Optical Fiber With Helical Fiber Bragg Gratings

We demonstrate a novel all-fiber laser generating cylindrical vector beams with high slope efficiency and low threshold by introducing a long-period fiber grating into the laser cavity. Highly efficient mode conversion is realized by an LPFG at ∼1548  nm. Mode selection and spectrum filtering are achieved in combination with a two-mode fiber Bragg grating (TM-FBG). The fiber laser operates at a single wavelength of 1547.95 nm with a 30 dB linewidth of less than 0.18 nm and a side-mode suppression ratio (SMSR) of more than 56 dB. The lasing threshold and slope efficiency of the laser are 24.5 mW and 35.41%, respectively. The output power is 72 mW with an absorbed pump power of 225 mW. The variation of slope efficiency with respect to the reflectivity of the TM-FBG is investigated. Through adjusting the intra-cavity polarization controller, high-purity radially and azimuthally polarized beams are both obtained.

High efficiency all-fiber cylindrical vector beam laser using a long-period fiber grating.

We demonstrate a new method to detect the vortex beams carrying orbital angular momentum by a sectorial screen. When the sectorial screen is illuminated by optical vortex beams, the far-field diffraction pattern can be used to visually determine the modulus and sign of topological charges. We also prove that center alignment is not strictly required. The experimental results agree well with the simulated results.

Detecting the topological charge of optical vortex beams using a sectorial screen.

The invention discloses a random distributed feedback fiber laser generating cylindrical vector beams. Because of the existence of random distributed feedback in the laser, the output cylindrical vector beam has the characteristic of low coherence. The laser mainly includes a highly reflective mirror, a mode output module, an amplifier module and a random distributed feedback module. According tothe invention, random distributed feedback is provided by a fiber resonant ring, and a cylindrical vector beam is output by a mode selection coupler. The random distributed feedback fiber laser of theinvention has the advantages of simple structure, high integration degree and low cost, and is suitable for special fields requiring low-coherence cylindrical vector laser sources.

Random Distributed Feedback Fiber Laser Generating Cylindrical Vector Beams

In this work, an all-fiber-based mode converter for generating orbital angular momentum (OAM) beams is proposed and numerically investigated Its structure is constructed by cascading a mode selective coupler (MSC) and an inner elliptical cladding fiber (IECF) OAM modes refer to a combination of two orthogonal LPlm modes with a phase difference of ±π/2 By adjusting the parameters and controlling the splicing angle of MSC and IECF appropriately, higher-order OAM modes with topological charges of l = ±1, ±2, ±3 can be obtained with the injection of the fundamental mode LP01, resulting in a mode-conversion efficiency of almost 100% This achievement may pave the way towards the realization of a compact, all-fiber, and high-efficiency device for increasing the transmission capacity and spectral efficiency in optical communication systems with OAM mode multiplexing

Ruishan Chen

Papers

Generation and Conversion of Higher Order Optical Vortices in Optical Fiber With Helical Fiber Bragg Gratings

High efficiency all-fiber cylindrical vector beam laser using a long-period fiber grating.

Detecting the topological charge of optical vortex beams using a sectorial screen.

Random Distributed Feedback Fiber Laser Generating Cylindrical Vector Beams

Fiber-based mode converter for generating optical vortex beams