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Orbital Angular Momentum Waves: Generation, Detection and Emerging Applications
Rui Chen, Hong Zhou, Marco Moretti, Xiaodong Wang, Jiandong Li
TL;DR
OAM research addresses how structured waves can expand communication capacity while supporting imaging and particle manipulation. This survey compares optical, radio, and acoustic OAM generation and detection, reviews communication applications and challenges, and discusses imaging and manipulation prospects. It concludes that OAM offers a broad platform whose practical use remains constrained by propagation, alignment, medium, and hardware issues.
Problem
Communication systems seek additional capacity, while OAM's potential for multiplexing, imaging, and particle manipulation requires coordinated understanding across wave types and applications.
Method
The paper comprehensively reviews and compares optical, radio, and acoustic OAM generation and detection methods, applications, challenges, and emerging uses.
Results
The survey identifies common and wave-specific generation and detection approaches and examines OAM communication applications together with particle manipulation and imaging.
Takeaways & Limitations
OAM provides orthogonal modes for multiplexing and has potential in communications, particle manipulation, and target imaging, subject to practical technical challenges.
Abstract
from arXiv · showhide
Orbital angular momentum (OAM) has aroused a widespread interest in many fields, especially in telecommunications due to its potential for unleashing new capacity in the severely congested spectrum of commercial communication systems. Beams carrying OAM have a helical phase front and a field strength with a singularity along the axial center, which can be used for information transmission, imaging and particle manipulation. The number of orthogonal OAM modes in a single beam is theoretically infinite and each mode is an element of a complete orthogonal basis that can be employed for multiplexing different signals, thus greatly improving the spectrum efficiency. In this paper, we comprehensively summarize and compare the methods for generation and detection of optical OAM, radio OAM and acoustic OAM. Then, we represent the applications and technical challenges of OAM in communications, including free-space optical communications, optical fiber communications, radio communications and acoustic communications. To complete our survey, we also discuss the state of art of particle manipulation and target imaging with OAM beams.
I. INTRODUCTION
OAM is a spatial form of electromagnetic angular momentum carried by vortex waves with helical phase fronts and an axial dark core. This survey reviews OAM fundamentals, generation and detection, communication applications, and prospects for particle manipulation and imaging.
- The survey compares optical, radio, and acoustic OAM generation and detection, reviews communication applications and challenges, and discusses particle manipulation and imaging.
- OAM is the angular-momentum component associated with the spatial distribution of electromagnetic waves, distinct from polarization-linked spin angular momentum.
- Vortex waves carry OAM through a helical phase factor exp(iℓθ), with zero field amplitude along the phase singularity axis.
- Laguerre-Gaussian beams use azimuthal index ℓ for topological charge and radial index p for the number of radial intensity nodes.
- Orthogonal OAM modes can provide independent multiplexing channels that complement time, frequency, and polarization to improve communication capacity.
- OAM beams have potential for particle capture or rotation and for imaging resolutions beyond the Rayleigh limit.
III. GENERATION OF OAM WAVES
The paper surveys state-of-the-art techniques for generating OAM in optical, radio, and acoustic waves. A taxonomy organizes the large number of generation methods discussed.
- Generation methods span optical, radio, and acoustic waves, reflecting the broadening of OAM research beyond optical frequencies.
- Figure 2 presents a taxonomy summarizing the generation techniques covered in this section.
A. Optical OAM
Optical OAM can be generated from laser cavities or Gaussian beams transformed by optical converters, with methods differing in flexibility, efficiency, purity, complexity, and integration suitability.
- Optical vortex beams arise either directly from laser cavities or by converting Gaussian beams with cylindrical lenses, SPPs, holograms, metamaterials, or q-plates.
- Cylindrical-lens converters rephase Hermite–Gaussian modes into Laguerre–Gaussian beams, offering high efficiency and purity but requiring precise construction and incident-field alignment.
- SPPs convert Gaussian light into a beam with topological charge ℓ through azimuthally increasing thickness, supporting high efficiency and high-power operation but only a single mode.
- SLMs dynamically impose phase holograms, enabling different topological charges, controlled beam directions, and multiple diffraction orders.
- Metamaterials use subwavelength artificial structures, while q-plates modify optical angular momentum through patterned birefringence and polarization-dependent charge variation.
- Method selection depends on practical trade-offs: SLMs provide the greatest flexibility, whereas SPPs perform well in cost, conversion efficiency, and structural complexity.
- In optical communication systems, SLMs are most widely used, SPPs are second, and metamaterials are suited to small, integrated vortex-beam generators.
B. Radio OAM
Radio OAM generation uses wavelength-scaled structures and antenna arrays, with different methods suited to high- or low-frequency operation and multiplexed vortex production.
- Spiral reflectors create electromagnetic vortices by introducing position-dependent path differences across the reflected wavefront.
- A UCAA uses N uniformly spaced antenna elements with adjacent phase difference Δϕ = 2πℓ/N to generate a vortex mode ℓ.
- Theoretically, a UCAA with N elements produces a distortion-free vortex wave for |ℓ| < N/2 and can support multiplexed radio OAM through amplitude and phase control.
- Metamaterials convert incident radio waves into vortex waves, while q-plates formed from concentric dielectric rings can also generate radio-frequency vortices.
- Radio OAM generators include SPPs, holographic gratings, spiral reflectors, UCAAs, metamaterials, and q-plates.
- SPPs and holographic gratings suit high radio frequencies, whereas spiral reflectors and UCAAs are more suitable at lower frequencies because device size depends on wavelength.
C. Acoustic OAM
Acoustic OAM can be generated with transducer arrays, spiral diffraction gratings, and metamaterials, each offering different trade-offs in frequency range, flexibility, efficiency, and size.
- Method comparison: The acoustic generation landscape includes SPPs, active sources, UCTAs, spiral gratings, and metamaterials, with implementation choices affecting control, efficiency, and integration.The survey compares these approaches alongside optical and radio generation methods.
- Spiral diffraction gratings: Spiral diffraction gratings generate higher OAM modes by increasing the number of arms, with logarithmic spirals supporting a wider frequency range than other spiral types.A grating with m arms produces ℓ=mn at the nth diffraction order, but passive versions have low efficiency because they block incident sound.
- Metamaterials: Acoustic metamaterials manipulate the azimuthal output phase through sub-wavelength resonators and pipes, converting planar waves into vortex sound waves.The illustrated structure uses eight fanlike sections and achieves a 0 to 2π exit-phase range with high transmittance.
- Method comparison: Generation methods occupy different operating regimes: SPPs suit high-frequency fields, UCTAs suit low-frequency fields, and both spiral gratings and metamaterials have subwavelength dimensions.Metamaterials are attracting attention because they can combine small size, low cost, and high efficiency.
D. Summary and Open Challenges
The survey compares OAM generation methods across optical, radio, and acoustic waves, while identifying practical constraints that remain before broader deployment.
- Summary: Optical generation is relatively mature, while radio and acoustic generation adapt methods such as SPPs and metamaterials alongside UCAAs and UCTAs.The survey explicitly compares generation methods across all three wave types.
- Summary: SLMs are relatively optimal for optical generation, SPPs and UCAAs perform well for radio generation, and UCTAs offer flexible acoustic generation in air and water.UCTAs are limited by their inherent transducer size for many miniaturized applications.
- Open challenges: Stable underwater acoustic vortex generation remains an important research direction because current small metamaterials generate OAM only in air.Long-distance transmission also requires convergent OAM beams, which remain challenging across optical, radio, and acoustic systems.
IV. DETECTION OF OAM WAVES
OAM detection methods exploit phase structure, interference, holography, and coordinate transformations, with trade-offs among efficiency, flexibility, complexity, and mode coverage.
- Radio and acoustic detection: Radio OAM detection includes inverse SPPs, interference phase detection, phase-gradient estimation, circular arrays, and partial-aperture sampling.Acoustic OAM detection is described as being in its early stages.
- Optical detection: Dove prism interferometers separate even and odd modes and can be cascaded to sort additional modes with theoretical efficiency close to 100%.Detecting N modes requires N−1 interferometers, making the architecture complex for practical applications.
- Optical detection: Optical OAM modes can be detected by inverse SPPs that remove a mode-specific helical phase or by holographic gratings that convert selected modes to Gaussian light.Dammann holographic gratings support parallel detection of multiple states but require high precision and have conversion efficiency no higher than 1/N.
- Optical detection: Interference detection produces spiral or forked patterns, but the resulting pattern complexity can make the intended optical OAM mode difficult to identify correctly.Interference, inverse SPPs, mode sorters, and holographic gratings are compared as common optical detection methods.
- Optical detection: OAM mode sorters use log-polar coordinate transformations to map helical ring beams into tilted rectangular beams that focus at mode-dependent positions.This spatial separation enables composite mode sorting, although adjacent modes are not effectively separated.
B. Radio OAM
Radio OAM detection uses phase, interference, and array-based measurements, with alignment and computational complexity shaping practical method selection.
- Detection methods: Inverse SPPs and vortex-plane-wave interference provide radio-frequency analogues of optical OAM detection, while other optical methods are not directly applicable.Radio systems therefore require additional detection approaches tailored to the frequency range.
- Interference detection: A phase interferometer separates ℓ=0 and ℓ=1 beams by measuring the 180° phase difference produced between aligned receive antennas.Two Yagi antennas are connected by a 180° phase-shift cable to reduce background interference.
- Phase-gradient detection: The phase-gradient method estimates ℓ from phase measurements at two points as ℓ= (φ1 −φ2)/β, requiring β < π/|ℓ| and accurate beam-axis alignment.It generally detects single-mode OAM beams and produces large errors when the receiving-circle center is misaligned.
- Array detection: UCAAs require the receiving array to capture the aligned wavefront and can demultiplex several radio OAM modes through spectral-analysis-like processing.PASR reduces the receiving aperture to an arc of 1/P of a circumference sampled by M antennas.
- Method comparison: Interference phase detection and single-point estimation have high practical complexity, whereas phase-gradient detection is simpler but limited to one mode.PASR has computational support but requires further experimental verification.
C. Acoustic OAM
Acoustic OAM communications remain an emerging field, with detection methods established but requiring further research and underwater links facing major unresolved effects.
- Acoustic OAM detection is still at an early stage, with inner-product separation and passive metamaterials among the reported approaches.
- Optical and radio OAM have many detection methods, whereas effective acoustic OAM detection requires further research, investigation, and testing.
- Inverse SPP detection is described as easiest for optical and radio OAM, while acoustic mode demultiplexing remains an open research problem.
- OAM communications use orthogonal modes either to encode information in topological charge or to multiplex data across multimodal beams.
- Acoustic OAM links must account for divergence, transmitter–receiver misalignment, atmospheric turbulence, and underwater creatures.
A. Free-Space Optical OAM Communications
Free-space optical OAM communications demonstrate information transmission and multiplexing, but divergence, misalignment, and turbulence create major link-design challenges requiring optical or signal-processing mitigation.
- Free-space optical OAM-SK maps information onto topological charges and detects modes with vertically stacked forked gratings.
- OAM-DM can transmit up to L bits per beam when multiplexing L modes, compared with log2(L) bits per beam for OAM-SK.
- Three-dimensional multiplexing successively combines OAM, polarization, and wavelength to carry multiple data streams.
- OAM beam divergence increases with mode number, causing power loss, while transmitter–receiver misalignment causes power loss and mode crosstalk.
- Atmospheric turbulence distorts helical wavefronts, producing received-energy fluctuations and crosstalk between OAM channels.
- Mitigation approaches include adaptive optics, DSP algorithms, wavefront sensing, and phase-retrieval methods.
B. Optical OAM Fiber Communications
Optical fiber OAM communications seek stable transmission of multiple modes while avoiding free-space divergence and turbulence, using specialized fibers and mode converters.
- Optical fiber links avoid the beam divergence and atmospheric turbulence affecting free-space optical OAM communications.
- Vortex-fiber experiments transmitted OAM modes through fibers 20 m and 900 m long using microbend-grating conversion to HE21 modes.
- A ring fiber was designed to support up to 10 OAM modes while maintaining radial single-mode conditions.
- A multi-OAM-mode multi-ring fiber uses multiple rings to support multimode OAM transmission.
- All-fiber and microstructured-fiber approaches explore LP-to-OAM conversion and multimode OAM transmission.
C. Radio OAM Communications
Radio OAM communications have demonstrated multiplexed transmission and high capacities, but divergence, alignment, and multipath effects constrain links and motivate steering and signal processing.
- Radio OAM experiments separated two same-frequency signals over 442 meters and later demonstrated a 4 Gbps uncompressed video link at 60 GHz.
- A 28 GHz experiment achieved 32 Gbit/s capacity and about 16 bit/s/Hz spectral efficiency using four OAM modes, two polarization states, and 16-QAM.
- UCAA-based OAM is described as a subset of MIMO, with equivalent channel spatial multiplexing capacity over line-of-sight links.
- OAM receivers can have lower complexity because mode orthogonality can mitigate inter-channel interference.
- Radio OAM faces stronger beam-divergence constraints than free-space optical OAM because radio wavelengths are larger, limiting achievable link distance.
- Beam steering simulations showed almost no capacity loss after correcting non-parallel and off-axis misalignment.
- Multipath reflections produce stronger intra- and inter-channel interference for OAM channels with larger topological charges.
E. Discussion and Lessons Learned
OAM systems offer diverse communication, manipulation, and imaging capabilities, but practical deployment remains constrained by propagation effects, alignment, component limitations, and experimental validation gaps.
- Communication systems: OAM communication systems use SLMs, phase holograms, SPPs, UCAAs, and UCTAs across optical, radio, and acoustic frequency bands.Free-space optical links commonly use SLMs for multiplexing and demultiplexing, while radio and acoustic systems benefit from circular arrays.
- Communication systems: Atmospheric turbulence, mode coupling, beam divergence, multipath, and transmitter–receiver misalignment remain major communication-link challenges.Suggested responses include adaptive-optics or DSP mitigation, improved fibers, beam convergence, and misalignment compensation.
- Particle manipulation: OAM beams can manipulate particles through angular-momentum transfer, with optical beams stopping or rotating particles and acoustic vortices exerting torque.For optical beams with ℓ=1, the reported angular momentum is approximately 0.06ℏ or 2.06ℏ per photon depending on spin direction.
- Imaging: Optical and radar imaging applications include diffraction-limit-beyond resolution, spiral phase contrast, target imaging, and azimuth estimation.Radar vortex waves provide spatial diversity and can improve resolution without relative motion or an aperture-limited azimuth-resolution increase.
- Imaging: Super-resolution radar imaging has motivated UCAA-based autoregressive models and power spectral density estimation algorithms.These approaches address the limited azimuth resolution imposed on traditional radar imaging by the Rayleigh limit.
- Imaging: OAM radar imaging faces limited echo energy because different modes illuminate targets unevenly, causing amplitude modulation and degraded imaging capability.The vortex beam’s null center and mode-dependent main-lobe direction make simultaneous illumination with multiple modes difficult.
VII. CONCLUSIONS AND PERSPECTIVES
The survey synthesizes OAM generation, detection, and applications across optical, radio, and acoustic systems, while emphasizing unresolved practical barriers. It concludes that deployment depends on smaller, cheaper, compatible components and better handling of propagation channels.
- Conclusions and perspectives: The survey reviews OAM generation, detection, communications, particle manipulation, and imaging across optical, radio, and acoustic waves.It compares common generation and detection methods and discusses applications and technical challenges.
- Conclusions and perspectives: Practical deployment remains limited by bulky and expensive components, finite-aperture generation of high-order multimode beams, and difficult channel conditions.The paper identifies transmitter, multiplexer, demultiplexer, and receiver development as central to future progress.
- Conclusions and perspectives: Future OAM systems require lower-cost, smaller, and existing-technology-compatible components alongside solutions for propagation-channel problems.The survey remains optimistic that these challenges can gradually be resolved for communications and radar target detection.