Structured ray-wave vector vortex beams in multiple degrees of freedom from a laser

In this work, we introduce a rotatable diffractive deep neural network (R-D2NN) architecture that enables the generation of multi-mode vector vortex beams (VVBs) through controlled rotation of its diffractive layers. This reconfigurable design allows a single physical device to transform an input Gaussian beam into diverse VVB outputs without the need for retraining. The system simultaneously produces orthogonally polarized optical fields carrying distinct orbital angular momentum (OAM) modes, which are coherently superposed to form target VVBs. The vectorial nature and topological charges of the generated beams are verified via Stokes parameter measurements. Numerical simulations demonstrate the generation of up to 16-mode VVBs with a mode purity exceeding 99% using five diffractive layers. Furthermore, we experimentally realize a proof-of-concept two-layer system using a single spatial light modulator (SLM) with two specific architectures, achieving an average mode purity of 85%. The proposed architecture provides a training-free, single-element solution for high-fidelity dynamic VVBs generation, promising for applications in advanced optical communications and sensing.

The vector vortex (VV) beam, originally introduced to exhibit a form of single-particle quantum entanglement between different degrees of freedom, has specific applications for quantum-information protocols. In this paper, by combining measurement-device-independent quantum key distribution (MDIQKD) with a spontaneous parametric-downconversion source (SPDCS), we present a modified MDIQKD scheme with pairs of VV beams, which shows a structure of hybrid entangled entanglement corresponding to intrasystem entanglement and intersystem entanglement. The former entanglement, which is entangled between polarization and orbit angular momentum within each VV beam, is adopted to overcome the polarization misalignment associated with random rotations in quantum key distribution. The latter entanglement, which is entangled between the two VV beams, is used to perform the MDIQKD protocol with SPDCS to inherit the merit of the heralded process. The numerical simulations show that our modified scheme has apparent advances both in transmission distance and key-generation rate compared to the original MDIQKD. Furthermore, our modified protocol only needs to insert $q$ plates in a practical experiment.

A pivotal issue of the conventional optical fiber communications networks is to meet the explosively increasing requirement in data traffic. In order to meet this ever-increasing demand, there have been a lot of research and industrial development efforts to utilize the photon in various dimensions such as wavelength division multiplexing (WDM), space division multiplexing (SDM), mode division multiplexing (MDM) and so on. Fueled by emerging bandwidth-hungry applications, orbital angular momentum (OAM) modes and their multiplexing have recently gained much attention due to its special doughnut-shaped intensity distribution, as well as its unique helical phase wavefront with the theoretically infinite topological value. The OAM modes with different topological charge values are orthogonal to each other, which has provided a new degree of freedom in MDM. In this paper, we propose and design a Ge-doped air-core ring fiber, which can support numerous OAM modes. By varying the mole fraction of GeO2 and adjusting the structure parameter, including the air-core radius and the GeO2-doped ring width, we study the influence of the different fiber parameters on the total supported OAM mode number. The hollow silica fiber with a 50-μm air core and a 1.5-μm thickness of Gedoped ring is designed in simulation to support fiber eigenmodes up to HE112,1 and EH107,1. This provides 436 OAM modes at 1550 nm while maintaining radially single mode condition. Moreover, it can support more than 400 OAM modes from 1260 nm to 1625 nm, covering O, E, S, C, and L bands.

The flat-top (FT) beam has plenty of applications in the high-power fiber laser, automotive processing industry and aerospace semiconductor. In this work, an all-fiber mode-locked fiber laser with FT beam output based on mode superposition is proposed. According to the principle of mode superposition, the output of FT beam is realized by the orthogonal superposition of orbital angular momentum (OAM) mode and linear polarization (LP) fundamental mode (LP<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub>). This designed fiber laser can output high quality FT beam with 98% flatness and high purity OAM mode with 90.24% mode purity. And the tunable mode output between LP<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub>, LP<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub>(including even and odd modes), OAM and FT beam is experimentally realized. The duration of signal mode-locked pulse, pulse interval and the repetition rate are 59 ns, 175.4 ns and 5.7 MHz, respectively.

The crosstalk of orbital angular momentum (OAM) modes is one of the most important issues for free-space optical communication. We theoretically derive a crosstalk model for Laguerre–Gaussian (LG) vortex beams propagating in turbulent atmosphere. The calculated crosstalk of different OAM modes in different turbulence by analytical expression is consistent with experimental measurements. Both theoretical and experimental results reveal that the crosstalk of high order LG vortex beams is more obvious in atmospheric turbulence. Different from the numerical calculation of the integral expression of the previous studies, this analytical expression can directly and quantitatively characterize the crosstalk and the law of different topological charges in vortex beams. Based on this analytical expression, the crosstalk widths of vortex beams with different topological charges can be calculated accurately, so that the optimal sets of OAM modes can be found more easily under each atmospheric turbulence condition.

Based on angular spectrum diffraction theory, the scattering field of vector vortex beams (VVBs) passing through a rotating random rough surface is derived. The speckle produced by VVBs passing through the rotating ground glass is collected experimentally. The effects of ground glass roughness and rotational speed on the speckle and orbital angular momentum (OAM) of different order VVBs are discussed in detail. Furthermore, the speckle characteristics of the VVB are compared with the scalar vortex beam. The change in the OAM states of VVBs before and after passing through the rotating ground glass was detected by double-slit interference. The results show that the speckle of VVBs is generated after passing through the static ground glass, and the speckle contrast increases with the increment of roughness. Compared with the scalar vortex beam, the speckle contrast of the VVB changes faster with the roughness of ground glass. When the ground glass rotates, the speckle reduction effect appears and the contrast is reduced. When the rotational speed is >15 r / s, the speckle reduction effect is the strongest. The intensity distributions and OAM states of VVBs are not affected by the rotating ground glass. The research results provide a theoretical basis for the application of VVBs in optical communication, optical detection, and other fields.

In this paper the possibility to use orbital angular momentum (OAM) modes on a multi-conductor cable to mitigate crosstalk is presented for the first time. OAM modes are understood here as a general method to excite an electromagnetic system that shows cyclic symmetry. It is demonstrated that the OAM modes, known from antenna communication, are an extension of the known modes in two and three conductor systems. It will be shown analytically that the OAM modes are the eigenmodes of a cable showing cyclic symmetry and can therefore propagate on the cable theoretically free of crosstalk. The OAM modes are then compared to the pairwise differential modes in a simulation with regard to crosstalk. For this comparison a circularly arranged multi-conductor cable as well as a cable where the conductor pairs are closer together are evaluated. For the modeling of the per-unit-length parameters a two-dimensional (2-D) solver based on a boundary element method (BEM) is used.

In this article, a spoof surface plasmon (SSP) ring resonator to generate dynamically reconfigurable vortex beams is proposed. According to the phase constant control of the SSP and the harmonic radiation principle, the tunable cylindrical vector vortex beams (VVBs) with different topological charges can be obtained. The tunable system that operates by electronic control would utilize varactors to change the wavenumber of SPP on the ring resonator and realize the reconfigurable VVBs modes at a fixed frequency. All VVBs have excellent orbital angular momentum (OAM) purity (above 90%). Interestingly, the antenna also simultaneously radiates the scalar vortex beams with tunable OAM modes. Both numerical simulations and experimental characterizations confirm the theoretical predictions. Because the designed antenna has the advantages of simple feeding network, easy bias circuit, as well as miniaturization and integration compatibility, we anticipate that the antenna will enable a wide range of applications for future wireless communication and imaging technologies.

Orbital angular momentum (OAM) mode division multiplexing and encoding have been widely investigated for achieving higher information capacity and performance in both classical and quantum communication systems [1]. A critical component in such systems is an OAM mode sorter that can efficiently sort different OAM modes exp(ilθ) according to the mode index l. To this end, an elegant scheme is to transform the vortex phase exp(ilθ) of OAM modes to a linear phase exp(ilv / β) of tilted plane waves through geometrical transformations, so that they can be separated in the focal plane of a lens. A log-polar transformation was firstly employed [2] and recently a spiral transformation has been proposed to achieve high-resolution OAM mode sorting by the authors [3].

Light beams carrying both spin (polarization) and orbital angular momentum (OAM) can be represented by mapping the total angular momentum onto a higher-order Poincaré sphere [1]. Due to the non-separability of the polarization and OAM degrees-of-freedom, these beams, commonly known as vector vortex beams, find varieties of applications in scientific and technological fields including photon entanglement [2], material processing [3] and microscopy [4]. Such beams are commonly generated using q-plates [5] and liquid crystal based spatial light modulators [6]. However, the limitation of low power handling and restricted wavelength coverage of such techniques can be addressed through the vector vortex beam generation using nonlinear parametric processes. Unfortunately, the dependence of phase-matching of the parametric processes on the polarization of the interacting beams puts the major hindrance to the nonlinear generation of vector vortex beams. Here we report a novel experimental scheme to generate vector vortex beams using single pass second harmonic generation (SHG) of ultrafast higher order vector vortex beams.

Experimental generation and orbital angular momentum properties of focused field for vector vortex beams on Hybrid-order Poincaré sphere

Multiplexing multiple orbital angular momentum (OAM) modes of light has proven to be an effective way to increase data capacity in fiber-optic communications. However, existing techniques for distributing the OAM modes rely on specially designed fibers or couplers. Direct transmission of multiplexed OAM modes through a long standard multimode fiber remains challenging because the strong mode coupling in fibers disables OAM demultiplexing. Here, we propose a deep-learning-based approach to recover the scattered data from multiplexed OAM channels without measuring any phase information. Over a 1-km-long standard multimode fiber, our method is able to identify different OAM modes with an accuracy of more than 99.9% in the parallel demultiplexing of 24 scattered OAM channels. To demonstrate the transmission quality, color images are encoded in multiplexed twisted light and our method achieves decoding the transmitted data with an error rate of 0.13%. Our work shows that the artificial intelligence algorithm could benefit the use of OAM multiplexing in commercial fiber networks and high-performance optical communication in turbulent environments.

We present a study on the potential and challenges of guiding orbital angular momentum (OAM) modes in hollow-core photonic bandgap fibers (HC-PBGFs). Two 19-cell HC-PBGFs with different structural parameters are comparably investigated. The OAM mode properties in a 37-cell HC-PBGF are also discussed to explore the scalability of OAM states. Characteristics of vector modes and OAM modes are comprehensively analyzed with numerical simulations. The results show HC-PBGF with a larger core could effectively support more OAM modes with lower confinement loss and a larger effective area. In addition, HC-PBGF with a deeper and wider photonic bandgap is advantageous for achieving low crosstalk OAM transmission over a broader band-width. 19-cell HC-PBGFs could support OAM modes with purity beyond 0.9, and the value can be further improved by exploiting the 37-cell HC-PBGF.

In this paper, a nested dual-arm spiral antenna (NDASA) is proposed to generate multiple orbital angular momentum (OAM) modes at the same operating frequency. The NDASA consists of the feeding network and three nested dual-arm spiral structures with different sizes. Meanwhile, a metal reflector is located between the spiral structures and the feeding network to enhance the radiation directivity, which can also reduce the impact of the radio frequency (RF) coaxial cables. Both the experimental and simulated results show OAM waves with topological charges of $l = 1$ , $l = 3$ and $l = 5$ can be independently generated at $f = 3$ GHz, and their maximum gains are 6.94dBi, 6.76dBi and 5.49dBi, respectively. In addition, the OAM modes multiplexing and demultiplexing technology are investigated in simulations and experiments. The isolations between the OAM modes of $l = 1$ , $l = 3$ and $l = 5$ and the other different OAM modes are greater than 13.8dB, 13.2dB and 25.1dB, respectively, which proves the potential ability of the proposed NDASA for multiplexing and demultiplexing multiple OAM modes.

Each photon in a vortex beam carries an orbital angular momentum (OAM). OAM detection plays a very important role in unlocking the new capability of vortex beams in both fundamental research and technological advancements. As the number of information channels continues to grow, there is an urgent need to improve the detection capability of the OAM sorting system. The unprecedented capability of optical metasurfaces has provided an opportunity to realize OAM detection in a compact platform. Different OAM modes are typically separated into distinct light spots in OAM sorting. However, each spot in an OAM sorting system is tailored to recognize only one predesigned OAM mode. A metasurface approach to detecting more OAM modes with the same number of light spots in the observation plane is demonstrated. The conservation of momentum in the design is realized with a customized multifoci metalens. The efficacy of this method is exemplified through the detection of two OAM modes in an individual light spot, dramatically increasing the detection capability of the OAM sorter. It is anticipated that this design will reduce the demand for spatial resources, creating new opportunities to explore OAM applications across many research fields such as optical communications and quantum science.