Optical Angular Momentum Research Articles

Performance analysis of modulating retro-reflector link based on orbital angular momentum coding in underwater channels

The modulating retro-reflector (MRR) communication is developed to solve the problem of insufficient terminal power in underwater optical communication (UOC) where size, weight and power are severely restricted. Here, we propose a new MRR link scheme using the optical orbital angular momentum (OAM) for such scenarios to realize high-order coding. Furthermore, we simulate the OAM MRR link in the oceanic turbulence channel and use the convolutional neural network (CNN) to classify OAM patterns. The simulation results show that the OAM MRR link can realize error-free image transmission under weak turbulence strength of Cn2=10−15K2m−2/3 within 15 m distance using 8/16-ary OAM superposed modes, and with the increasing of the distance and the turbulence strength, the performance is decreasing, where the 16-ary format has a higher coding efficiency but lower performance than the 8-ary format. An experiment of the OAM MRR link using CNN to classify 16-ary OAM superposed modes is also demonstrated, where two space light modulators (SLMs) are used to simulate the short-distance oceanic turbulence channel and modulate the OAM beam, respectively. Our experiment result shows the recognition rate of 16-ary OAM set with different types of channel conditions. Our work may provide a novel way for high-order coding MRR links in underwater channels and hopefully increase the data rate of the MRR link in the future.

Controlling Photon Transverse Orbital Angular Momentum in High Harmonic Generation

High harmonic generation (HHG) with longitudinal optical orbital angular momentum has attracted much attention over the past decade. Here, we present the first study on the HHG with transverse orbital angular momentum driven by the spatiotemporal optical vortex (STOV) pulses. We show that the produced spatial-resolved harmonic spectra reveal unique structures, such as the spatially spectral tilt and the fine interference patterns. We show these spatiospectral structures originate from both the macroscopic and microscopic effect of spatiotemporal optical singularity in HHG. Employing two-color counterspin and countervorticity STOV pulses, we further discuss a robust method to control the spatiotemporal topological charge and spectral structure of high-order harmonics. The conservation rule of photon transverse orbital angular momentum in HHG process is also discussed when mixing with photon spin angular momenta.

Nanostructured silica spin–orbit optics for modal vortex beam shaping

Abstract Modality is a generic concept of wave-optics at the basis of optical information and communications. One of the challenges of photonics technologies based on optical orbital angular momentum consists in the production of a modal content for both the azimuthal and radial degrees of freedom. This basically requires shaping the complex amplitude of an incident light beam, which is usually made up from adaptive spatial light modulators or bespoke devices. Here, we report on the experimental attempt of a recent theoretical proposal [Opt. Lett. 42, 1966 (2017)] toward the production of various optical vortex modes of the Laguerre–Gaussian type relying on the spin–orbit interaction of light. This is done in the visible domain from optical elements made out of silica glass. The idea consists in exploiting the combined effects of azimuthally-varying geometric phase with that of radially-varying propagation features. The proposed approach can be readily extended to any wavelength as well as to other families of optical modes, although some dynamic phase problems remain to be solved to make it a turnkey technology.

Nanophotonic manipulation of optical angular momentum for high-dimensional information optics

Light has played a crucial role in the age of information technology and has facilitated the soaring development of information optics. The ever-increasing demand for high-capacity optical devices has prompted the use of physically orthogonal dimensions of light for optical multiplexing. Recent advances in nanotechnology, mainly stemming from functionalized nanomaterials and powerful nanofabrication tools, have propelled the fusion of optical multiplexing and nanophotonics (the study of light at nanoscale and of its interactions with nanostructures) by enabling ultrahigh-capacity information technology. This review aims to introduce the emerging concept of angular momentum (AM)-involved information optics and its implementation in nanophotonic devices. First, previous researches on the manipulation of spin angular momentum (SAM) and orbital angular momentum (OAM) by nanostructures will be reviewed. We then summarize the SAM multiplexing technology on the platform of metasurfaces. Particularly, we elaborately summarize our recent progress in the area of information optics, including OAM holography and on-chip AM multiplexing technology. Finally, a perspective in the combination of this emerging field with optical artificial intelligence (AI) will be given.

Controlling quantum correlations in optical-angle–orbital-angular-momentum variables

The entanglement of optical angular position and orbital angular momentum has become one of the important degrees of freedom for quantum information science. Quantum control of the two conjugate variables may provide a gate to control the production of the entanglement. Here, we theoretically and experimentally show that by controlling the transverse spatial coherence of the pump beam, one can modulate the strength of the quantum correlations between the angular position and orbital angular momentum in down-converted photon pairs. By determining the relation of the Einstein, Podolsky, and Rosen correlations with the transverse spatial coherence of the pump, the boundary of the entanglement can be determined. Our work shows that the transverse spatial coherence of the pump can act as a switch to control the production of quantum entanglement both for discrete and continuous conjugate variables and may provide another platform for the quantum control research field.

Generation, Transmission and Application of Orbital Angular Momentum in Optical Fiber: A Review

Optical orbital angular momentum (OAM) has become a hot research topic because of its unique properties due to its spiral distribution of phases. The production and transmission of OAM has also become a necessary condition for effective use of OAM. As an optical waveguide with good propagation properties, optical fibers are used in optical systems supporting OAM. This paper introduces the OAM generation and transmission system based on fiber, summarizes the current photonic crystal fiber, ring core fiber, fiber grating and other all-fiber systems that can support OAM modes, and explains some experimental principles. Finally, an outlook on OAM generation or transmission devices for all-fiber systems is presented, providing a useful reference for future related research.

Generation of higher-order orbital angular momentum modes and (de)multiplexing based on double-trench SOI waveguide

An ultracompact device, designed with two air-etched trenches embedded in two opposite corners of a silicon bus waveguide, for generation and (de)multiplexing optical orbital angular momentum (OAM) modes is proposed. The double-trench waveguide can generate OAM±1 and OAM±2 modes when the phase difference of two rotated orthogonal LP-like modes is equal to π/2 (or 3π/2). The operating principle and mode purity of the generated OAM modes are theoretically and systematically investigated. The simulation results show that the average purities of OAM±1 modes exceed 90%, and that of OAM±2 modes is greater than 80% over a wide wavelength range of 1.4–1.625 µm. Furthermore, an on-chip five-OAM-mode (de)multiplexer (OAM0, OAM±1, OAM±2) with a compact footprint of <200×4×4 µm3 is designed, whose main mode crosstalk is less than −20 dB. The presented OAM mode generator and (de)multiplexer are promising for applications like optical communications and quantum information processing.

On-chip orbital angular momentum detection using a catenary grating metasurface

Vortex lights with optical orbital angular momentum have shown great promise in the areas of optical communication, optical manipulation and quantum optics. However, traditional methods for detecting the topological charge of vortex beams, such as interference and diffraction, are still challenging in miniaturization of the detection system and perfect matching of wave vectors. Here, a detection approach is proposed for measuring the topological charge of Laguerre–Gaussian (LG) vortex beams based on a catenary grating metasurface. According to the wave vector matching principle, the LG vortex beam can be coupled into surface plasmon polariton (SPP) waves propagating in different directions by using the well-designed catenary grating structure. The positive and negative topological charges can be distinguished by different arrangement of the catenary gratings. Additionally, the propagation angle of the launched SPP waves increases with the value of the topological charge. We believe that the proposed device would have broad application prospects in high compact photonic integrated circuits.

Spectral self-imaging of optical orbital angular momentum modes

The Talbot self-imaging effect is mostly present in the forms of space or time, or in the frequency domain by the Fourier duality. Here, we disclose a new spectral Talbot effect arising in optical orbital angular momentum (OAM) modes. The effect occurs in the context of petal-like beams, which are typically constructed from a number of in-phase equidistant OAM modes with at least one void mode in between. When illuminating such beams on phase masks that are azimuthally modulated with Talbot phases, the initial OAM modes are self-imaged to create new OAM modes, meanwhile preserving the initial OAM spectral profile. Such a phenomenon is theoretically predicted, and a close analogy is drawn with the spectral Talbot effect of frequency combs. The prediction is also experimentally confirmed by observing versatile spectral self-imaging on various optical petal-like beams.

Machine Learning Framework Combining Radial Phase Grating and Channel Information-Assisted Underwater Wireless Optical OAM Communications

In this letter, an enhanced machine learning (ML) framework combining radial phase grating (RPG) and channel information has been proposed for underwater wireless optical orbital angular momentum (OAM) communications. The optical irradiance profile and diffraction pattern are simulated by the combination of split-step angular-spectrum propagation and Monte Carlo methods, then they were embedded by the convolution and fully connected layers. The transfer learning-based ML architectures are trained by the image features and the embedding channel information with fine-tuning. The impacts of oceanic turbulence, seawater types, and transmission distance are considered. Results show that the transfer learning-based OAM detectors could achieve the order-of-magnitude gain in the bit error rate. Besides, the proposed ML framework could combine with a variety of state-of-the-art methods and thus improve the system performance. This work indicates transfer learning-based ML frameworks have great potential in ML-based OAM communication systems.

Polaritonic Vortices with a Half-Integer Charge.

Topological spin textures are field arrangements that cannot be continuously deformed to a fully polarized state. In particular, merons are topological textures characterized by half-integer topological charge ±1/2 and vortex-like swirling patterns at large distances. Merons have been studied previously in the context of cosmology, fluid dynamics, condensed matter physics and plasmonics. Here, we visualized optical spin angular momentum of phonon polaritons that resembles nanoscale meron spin textures. Phonon polaritons, hybrids of infrared photons and phonons in hexagonal boron nitride, were excited by circularly polarized light incident on a ring-shaped antenna and imaged using infrared near-field techniques. The polariton field reveals a half-integer topological charge determined by the handedness of the incident beam. Our phonon polaritonic platform opens up new pathways to create, control, and visualize topological textures.

Partially coherent perfect vortex beam generated by an axicon phase

Vortex beams are structured light fields with a helical phase of the form exp (ilϕ) that carries an optical angular momentum (OAM) of lℏ per photon. Such beams typically have a ring-shaped intensity with a radius that varies with l. Perfect vortex (PV) beams are designed to have a radius that is approximately uniform over a certain OAM range. Here, we report how spatial coherence can be used to maintain a fixed ring shape over a larger propagation distance and for a greater OAM range than is possible for fully coherent vortex beams. Our work is relevant for the application of PV beams in areas such as trapping, tweezing, and optical communications.

Optical diffractive deep neural network-based orbital angular momentum mode add-drop multiplexer.

Vortex beams have application potential in multiplexing communication because of their orthogonal orbital angular momentum (OAM) modes. OAM add-drop multiplexing remains a challenge owing to the lack of mode selective coupling and separation technologies. We proposed an OAM add-drop multiplexer (OADM) using an optical diffractive deep neural network (ODNN). By exploiting the effective data-fitting capability of deep neural networks and the complex light-field manipulation ability of multilayer diffraction screens, we constructed a five-layer ODNN to manipulate the spatial location of vortex beams, which can selectively couple and separate OAM modes. Both the diffraction efficiency and mode purity exceeded 95% in simulations and four OAM channels carrying 16-quadrature-amplitude-modulation signals were successfully downloaded and uploaded with optical signal-to-noise ratio penalties of ∼1 dB at a bit error rate of 3.8 × 10-3. This method can break through the constraints of conventional OADM, such as single function and poor flexibility, which may create new opportunities for OAM multiplexing and all-optical interconnection.

Multidimensional phase singularities in nanophotonics.

Rapid progress in miniaturizing vortex devices is driven by their integration with optical sensing, micromanipulation, and optical communications in both classical and quantum realms. Many such efforts are usually associated with on-chip micro- or nanoscale structures in real space and possess a static orbital angular momentum. Recently, a new branch of singular optics has emerged that seeks phase singularities in multiple dimensions, realizing vortex beams with compact nanodevices. Here, we review the topological phase singularities in real space, momentum space, and the spatiotemporal domain for generating vortex beams; discuss recent developments in theoretical and experimental research for generation, detection, and transmission of vortex beams; and provide an outlook for future opportunities in this area, ranging from fundamental research to practical applications.

Synthetic helical dichroism for six-dimensional optical orbital angular momentum multiplexing

Synthetic helical dichroism for six-dimensional optical orbital angular momentum multiplexing

Application of optical orbital angular momentum to rotation measurements

In recent years, the orbital angular momentum (OAM) of light with a central phase singularity composed of a specific topological charge has attracted increasing attentions. In addition to the wide range of applications of OAM in optical manipulations and optical communications, the special angular spatial structure of OAM offers irreplaceable advantages for use in rotation measurement. Because of the relationship between OAM and angular position, the azimuthal information of optical fields can be acquired via detection of OAM. Several techniques based on this concept have been proposed to sense objects remotely using their angular motion. This article introduces the applications of OAM in object orientation recognition, rotation sensing, small angle measurement and rotating object imaging.

Azimuthally and radially polarized orbital angular momentum modes in valley topological photonic crystal fiber

Abstract Artificially tailoring the polarization and phase of light offers new applications in optical communication, optical tweezers, and laser processing. Valley topological physics provides a novel paradigm for controlling electromagnetic waves and encoding information. The proposed fiber has the inner and outer claddings possessing opposite valley topological phases but the same refractive indices, which breaks through the polarization constraints of the traditional fiber. Robust valley edge states exist at the domain walls between the inner and outer claddings because of bulk edge correspondence. The valley topological fiber modes exhibit the unprecedented radial and azimuthal polarization with high-order azimuthal index. Those topological modes are robust against the disorder of the fiber structure. These results enable guide and manipulate the optical polarization and angular momentum in fiber with high fidelity. The proposed fiber has the potential to become a powerful optical spanner for the application of bio-photonics.

Polarization and Orbital Angular Momentum of Light in Biomedical Applications: feature issue introduction.

In the last decade, consistent and successful innovations have been achieved in the field of lasers and optics, collectively known as 'photonics', founding new applications in biomedicine, including clinical biopsy. Non-invasive photonics-based diagnostic modalities are rapidly expanding, and with their exponential improvement, there is a great potential to develop practical instrumentation for automatic detection and identification of different types and/or sub-types of diseases at a very early stage. While using conventional light for the studies of different properties of objects in materials science, astrophysics and biomedicine already has a long history, the interaction of polarized light and optical angular momentum with turbid tissue-like scattering media has not yet been ultimately explored. Since recently this research area became a hot topic. This feature issue is a first attempt to summarize the recognitions achieved in this emerging research field of polarized light and optical angular momentum for practical biomedical applications during the last years.

Plasmonic spin-Hall effect of propagating surface plasmon polaritons in Ni80Fe20 microstructures

Photoexcitation and shaping of a propagating surface plasmon polariton (SPP) on silver and gold microstructures are well established and lead to the discovery of the plasmonic spin-Hall effect recently. Whereas silver is often the material of choice due to its exceptional low plasma frequency and weak damping, similar observations have not been reported for ferromagnetic metals. In this work, we report on propagating SPPs on Ni80Fe20 microstructures imaged by photoemission electron microscopy (PEEM) in combination with a tunable femtosecond laser system at MHz repetition rate. Circular dichroic (CD) images in threshold PEEM show clear edge-induced SPPs with sub-micrometer wavelength and propagation length of about 3.5 μm. Analysis of the interference patterns as well as the coupling of the optical spin angular momentum to the observed fringe fields reveal propagation characteristics exclusive to evanescent waves and the presence of the plasmonic spin-Hall effect. Our work provides direct evidence that many materials with a high plasma frequency allow for excitation and observation of propagating SPPs at the dielectric/metal interface via CD PEEM imaging, enabling magnetoplasmonic investigation of common ferromagnets on nanometer length and femtosecond time scales.

Ultra-dense perfect optical orbital angular momentum multiplexed holography.

Optical orbital angular momentum (OAM) has been recently implemented in holography technologies as an independent degree of freedom for boosting information capacity. However, the holography capacity and fidelity suffer from the limited space-bandwidth product (SBP) and the channel crosstalk, albeit the OAM mode set exploited as multiplexing channels is theoretically unbounded. Here, we propose the ultra-dense perfect OAM holography, in which the OAM modes are discriminated both radially and angularly. As such, the perfect OAM mode set constructs the two-dimensional spatial division multiplexed holography (conventional OAM holography is 1D). The extending degree of freedom enhances the holography capacity and fidelity. We have demonstrated an ultra-fine fractional OAM holography with the topological charge resolution of 0.01. A 20-digit OAM-encoded holography encryption has also been exhibited. It harnesses only five angular OAM topological charges ranging from -16 to +16. The SBP efficiency is about 20 times larger than the conventional phase-only OAM holography. This work paves the way to compact, high-security and high-capacity holography.