Distinct Orbital Angular Momentum Research Articles

Performance investigations on integrated MMF/FSO transmission enabled by OAM beams

This paper introduces a new hybrid system for high-bandwidth and high-transmission-capacity communication. The system integrates a multi-mode fiber (MMF) cable with free space optics (FSO) using orbital angular momentum (OAM) multiplexing. The proposed system uses a single wavelength to simultaneously transmit 40 Gb/s data on four distinct OAM beams ((LG0,0, LG0,17, LG0,40, and LG0,75)) with a symmetric circular shape. The performance of the system was evaluated with a fixed MMF length of 100 m and varying FSO ranges, according to extreme climate changes such as rain and fog, and atmospheric turbulences. The results indicate that the suggested system is capable of transmit up to 1250 m (100 m MMF length + 1150 m FSO range) under clear weather (CW) and weak turbulence (WT), with a minimum transmission distance of 440 m (100 m MMF length + 340 m FSO range) under heavy fog (HF). The performance of the system was also demonstrated based on actual meteorological data for two different cities, Alexandria, Egypt and Chandigarh, India, with different visibility ranges and geographical locations. Since Alexandria has a longer visibility range than Chandigarh, our proposed model could spread over a larger area than it could be in Chandigarh. The obtained ranges were within an acceptable bit error rate (BER) of less than 10−9. The system’s maximum range was 1200 m for Alexandria and 1100 m for Chandigarh. The proposed model can be suggested for use in next generation (NG) passive optical networks (PON) for supporting symmetric 40 Gb/s data.

Revealing the two-dimensional electronic structure and anisotropic superconductivity in a natural van der Waals superlattice (PbSe) 1.14NbSe2

Van der Waals superlattices are important for tailoring the electronic structures and properties of layered materials. Here we report the superconducting properties and electronic structure of a natural van der Waals superlattice (${\mathrm{PbSe})}_{1.14}{\mathrm{NbSe}}_{2}$. Anisotropic superconductivity with a transition temperature ${T}_{c}$ = 5.6 $\ifmmode\pm\else\textpm\fi{}$ 0.1 K, which is higher than monolayer ${\mathrm{NbSe}}_{2}$, is revealed by transport measurements on high-quality samples. Angle-resolved photoemission spectroscopy (ARPES) measurements reveal the two-dimensional electronic structure and a charge transfer of 0.43 electrons per ${\mathrm{NbSe}}_{2}$ unit cell from the blocking PbSe layer. In addition, polarization-dependent ARPES measurements reveal a significant circular dichroism with opposite contrast at K and ${\mathrm{K}}^{\ensuremath{'}}$ valleys, suggesting a significant spin-orbital coupling and distinct orbital angular momentum. Our work suggests natural van der Waals superlattice as an effective pathway for achieving intriguing properties distinct from both the bulk and monolayer samples.

High-order vortex harmonic generation with circular Airy vortex beams

High-order harmonic generation (HHG) driven by an abruptly autofocusing circular Airy vortex beam (CAiVB) is investigated here. The autofocusing property of the CAiVB gives HHG a low initial intensity. The slow divergence after focusing provides a long and sufficiently intense region, which is favorable for HHG. Here, we analyze the phase-matching mechanism in detail by plotting the spatial diagram of the coherence length, while simulating HHG driven by the CAiVB carrying distinct orbital angular momentum in a gas medium, as well as the harmonic distributions in the near and far fields.

Performance analysis of 40 Gb/s free space optics transmission based on orbital angular momentum multiplexed beams

This paper explores the data carrying capacity enhancement in free space optics (FSO) transmission systems by employing orbital angular momentum (OAM) multiplexing technique. Specifically, simulative investigation of OAM-FSO terrestrial transmission is carried out under varying weather conditions. A net data capacity of 40 Gb/s over free space transmission medium is demonstrated using four distinct OAM beams (LG0,0, LG0,13, LG0,40, and LG0,80). The performance is compared for alternate mark inversion (AMI), return-to-zero (RZ), and non-return-to-zero (NRZ) encoding schemes. The results showed that the best performance occurred when using NRZ modulation and a favorable 40 Gb/s transmission along a range varying from 64 to 800 m depending on the external weather conditions is reported.

Robust Demultiplexing of Distinct Orbital Angular Momentum Infrared Vortex Beams Into Different Spatial Geometry Over a Broad Spectral Range

Multi-channel structured light with orbital angular momentum (OAM) can be applied in different applications. For example, OAM modulation and OAM multiplexing in fiber optics communications, high-dimensional quantum cryptography-based OAM states for transmitting secure information across free-space, and independent data streams through OAM beams multiplexed free-space optical links. Using a simple and efficient system consisting of a spiral phase element (SPE) and a multi-channel vortex filter (MVF), we have converted input Gaussian beams into multi-channel OAM-based vortex beams for infrared wavelengths. An SPE has been designed, which generates optical vortices with wavelength-dependent topological charge (including fractional values). The resulting complex fields are optically relayed on a binary MVF designed by modulo- $2\pi $ phase addition of multiple fork gratings with topological charges 1, 2, and 3 and azimuthal orientations. In this way, the MVF generates beams with different OAM states for different carrier waves with different angles and maps them at desired locations in the detector plane. In this study, both $3\times 3$ , as well as a hexagonal configuration, were used. Furthermore, the experimentally obtained OAM spectrum qualitatively agrees with the results of numerical simulations, thus verifying our approach. The presented approach opens a new pathway for developing an efficient multi-channel OAM beams generator designed for a specific wavelength and illuminated by an input beam of different wavelengths over a broad spectral range.

Topological Space‐Time Photonic Transitions in Angular‐Momentum‐Biased Metasurfaces

AbstractIn this paper, topological space‐time photonic transition of light in angular‐momentum‐biased metasurfaces is established, which yields a superposition of orbital‐angular‐momentum (OAM)‐carrying beams at distinct frequency harmonics upon scattering whose topological charges and frequency shifts are correlated. A reflective dielectric metasurface is considered that consists of silicon nanodisk heterostructures integrated with indium‐tin‐oxide and gate dielectric layers placed on a back mirror forming a dual‐gated field effect modulator. The metasurface is divided into several azimuthal sections wherein the nanodisk heterostructures are interconnected via biasing lines. Addressing each azimuthal section with radio‐frequency biasing signals separately allows for flexible implementation of different spatiotemporal modulation profiles. Active tuning of OAM states with high mode‐purity is demonstrated, which yields minimal cross‐talk between OAM channels in a mode‐multiplexed communication system. Moreover, the role of spatiotemporal modulation profile on the spectral and spatial diversity of OAM states is explored. It is shown that angular‐momentum‐biased metasurfaces allow opportunities for hybridized mode‐division and wavelength‐division multiple access through multiplexing and multicasting across distinct OAM states and wavelengths. The nonreciprocity of topological space‐time photonic transitions across the temporal frequency domain and Hilbert space of OAM states is also investigated giving rise to distinct twisted light channels in up‐ and down‐links.

Orbital angular momentum bistability in a microlaser.

Light's orbital angular momentum (OAM) is an unbounded degree of freedom emerging in helical beams that appears very advantageous technologically. Using chiral microlasers, i.e., integrated devices that allow generating an emission carrying a net OAM, we demonstrate a regime of bistability involving two modes presenting distinct OAM (ℓ=0 and ℓ=2). Furthermore, thanks to an engineered spin-orbit coupling of light in these devices, these modes also exhibit distinct polarization patterns, i.e., circular and azimuthal polarizations. Using a dynamical model of rate equations, we show that this bistability arises from polarization-dependent saturation of the gain medium. Such a bistable regime appears very promising for implementing ultrafast optical switches based on the OAM of light. As well, it paves the way for the exploration of dynamical processes involving phase and polarization vortices.

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

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 novel optical phenomena ranging from enhanced sensitivity to chiral light transport. As of now, however, in order to stabilize a system at the vicinity of an exceptional point, its related parameters must be carefully tuned and/or continuously controlled. To overcome this limitation, here we introduce a new family of broadband exceptional points based on unidirectional coupling, implemented by incorporating an Sshaped waveguide in a microring cavity. In active settings, the resulting unidirectionality exhibits unprecedented resilience to perturbations, thus providing a robust and tunable approach for directly generating beams with distinct orbital angular momentum (OAM). This work could open up new possibilities for manipulating OAM degrees of freedom in applications pertaining to telecommunications and quantum computing, while at the same time may expand the notions of non-Hermiticity in the orbital angular momentum space.

Dust-acoustic waves with finite OAM in a self-gravitating dusty plasma with superthermal energetic tails of electrons and ions

The kinetic instability of twisted dust-acoustic (DA) wave with distinct orbital angular momentum states is studied in an unmagnetized self-gravitating superthermal dusty plasma. For this purpose, the linearized Vlasov–Poisson equations are solved together by expressing the perturbed distribution function, electrostatic and gravitational potentials in terms of Laguerre–Gauss functions or solutions. A generalized plasma dielectric constant is derived and the existence conditions for the damping/growth rates are identified. The impact of streaming speed, dust self-gravitation, superthermality and twist parameters are numerically examined on the growth rate instability. It has been found that parametric variation significantly modifies the threshold conditions and associated growth rate instability of twisted DA waves. The relevance of the present results to interstellar dust clouds is illustrated, where massive dust grains follow a Maxwellian distribution in addition to superthermal electrons and ions.

Twisted vortex Gaussian Schell-model beams.

We introduce a new class of partially coherent vortex beams in which the angular momentum of the beam is provided from two different sources: the underlying vortex of the random beam and the "twist" given to the ensemble of beams. The statistical and propagation properties of such beams are investigated, and their orbital angular momentum properties are analyzed. The combination of distinct orbital angular momentum sources allows unusual behaviors that were previously unobserved.

Kinetic study of twisted electron plasma waves in q-nonextensive plasmas

Relying on the kinetic description, the electron-plasma (EP) waves carrying distinct orbital angular momentum (OAM) states are investigated in q-nonextensive plasma consisting of inertial electrons with static background of positive ions. Deriving an expression for the dielectric response function, the dispersion relation and Landau damping frequencies are analyzed both analytically and numerically. It is found that weakly damped regions of the EP waves strongly depend on the non-extensivity (q) and twist (η) parameters. The presence of finite OAM results in the longer range of the values in terms of normalized wavenumber for which the EP waves are weakly damped. However, maximum range of weakly damped wavenumbers is obtained when the longitudinal and azimuthal wavenumbers are equal, viz. twist parameter becomes unity. For very large twist parameter, the results of the twisted wave model coincide with the planer wave model.

Towards nanoscale multiplexing with parity-time-symmetric plasmonic coaxial waveguides

We theoretically investigate a nanoscale mode-division multiplexing scheme based on parity-time (PT) symmetric coaxial plasmonic waveguides. Coaxial waveguides support paired degenerate modes corresponding to distinct orbital angular momentum states. PT symmetric inclusions of gain and loss break the degeneracy of the paired modes and create new hybrid modes without orbital angular momentum. This process can be made thresholdless by matching the mode order with the number of gain and loss sections within the coaxial ring. Using both a Hamiltonian formulation and degenerate perturbation theory, we show how the wavevectors and fields evolve with increased loss/gain and derive sufficient conditions for thresholdless transitions. As a multiplexing filter, this PT symmetric coaxial waveguide could help double density rates in on-chip nanophotonic networks.

Splitting of levels in a cylindrical dielectric waveguide

A splitting of modes in a cylindrical graded-index optical fiber is demonstrated by solving the full Maxwell equations using the perturbation analysis. It is shown that the degeneracy of vortex Laguerre-Gauss modes with distinct orbital angular momentum and polarization (spin) but the same total angular momentum is lifted due to the spin-orbit (vector) and tensor forces. Numerical estimations of group delays of modes in optical fiber and frequency splitting in Fabry-Perot and ring resonators are presented.

Degenerate perturbation theory describing the mixing of orbital angular momentum modes in Fabry-Perot cavity resonators

We present an analytic perturbation theory which extends the paraxial approximation for a common cylindrically symmetric stable optical resonator and incorporates the differential, polarization-dependent reflectivity of a Bragg mirror. The degeneracy of Laguerre-Gauss modes with distinct orbital angular momentum (OAM) and polarization, but identical transverse order $N$, will become observably lifted at sufficiently small size and high finesse. The resulting paraxial eigenmodes possess two distinct OAM components, the fractional composition subtly depending on mirror structure.