Orbital Angular Momentum (OAM) Communication Links

• Orbital angular momentum (OAM) links are electromagnetic channels that encode data via helical phasefronts with quantized azimuthal indices, enabling high-dimensional multiplexing.
• They employ specialized devices such as OAM lasers, Q-plate converters, and circular antenna arrays to generate modes with high purity and low crosstalk across various frequency domains.
• Advanced OAM systems integrate adaptive optics, error correction, and optimized fiber designs to mitigate turbulence, loss, and misalignment, driving scalable high-capacity networks.

Orbital angular momentum (OAM) links are communication channels in which information is encoded, transmitted, and processed using electromagnetic fields that possess quantized orbital angular momentum—helical phasefronts characterized by an integer azimuthal index ℓ. These links exploit the theoretically unbounded set of orthogonal OAM modes to achieve high-dimensional multiplexing in classical and quantum domains, across optical, microwave, and radio frequencies. OAM-based links are applicable to fiber and free-space optical communications, wireless radio systems, and hybrid quantum-classical networks.

1. OAM Mode Generation and Devices

OAM modes are electromagnetic fields where the transverse spatial profile exhibits an azimuthal phase dependence $\exp(i \ell \phi)$, resulting in helical wavefronts and a central phase singularity. The most significant OAM mode families for communications are Laguerre–Gaussian (LG) modes in optics and Bessel-type modes in RF. OAM states are indexed by integer ℓ, with each photon carrying ℓħ of angular momentum.

Optical OAM Lasers and Emitters

• Electrically pumped OAM lasers are realized by monolithic integration of a distributed feedback (DFB) laser with an annular OAM emitter on InGaAsP/InP wafers. A deeply-etched microring with radius ~10 μm supports whispering-gallery modes, coupled unidirectionally via a pulley coupler to avoid counterpropagating degeneracies. Vertical emission of well-defined OAM beams is achieved through a precise arrangement of top gratings that satisfy the phase-matching $\ell = M - N$ between the azimuthal order of the WGM and the number of grating elements, as shown for ℓ = +4 at 1.544 μm with mode purity η ≳ 0.8. This architecture enables compact, single-mode, electrically driven OAM transmitters for dense on-chip multiplexing and integration with amplifiers and modulators (Zhang et al., 2017).
• Q-plate-based generators exploit spin-to-orbital angular momentum conversion (STOC) in compact loop configurations, enabling fast, lossless switching between multiple OAM orders (e.g., ℓ = ±2, ±4) by controlling the in-loop polarization states. State-of-art implementations use triangular optical loops with Pockels cells for MHz-rate switching and near-unity conversion efficiency in classical or quantum regimes (Slussarenko et al., 2010).

RF and Wireless OAM Sources

• OAM radio beams can be produced using uniform circular antenna arrays (UCAA) of N elements with feed phase offsets $2\pi \ell n / N$ for azimuthal index ℓ. The far-field array factor is proportional to $J_\ell(k a \sin\theta) e^{i\ell\varphi}$ (Bessel function), generating the requisite helical phasefront. Direct experimental demonstration at 2.4 GHz shows coherent generation and spatial Fourier detection of ℓ = 0, ±1, ±2, with SNR ≫ 20 dB and crosstalk ≤ −15 dB between adjacent modes (Daldorff et al., 2015, Cheng et al., 2018).

2. Multiplexing Principles and System Architectures

OAM links leverage the orthogonality of different ℓ-modes (over $\phi \in [0,2\pi)$) to achieve high-dimensional mode-division multiplexing (MDM) on a single carrier or wavelength, without frequency or polarization division.

• In optical fiber, customized air-core ring fibers guide OAM states with modal indices |ℓ| = 5,6, supporting up to 8 orthogonal modes (spin-orbit aligned/anti-aligned), with measured purity >98% and intermodal crosstalk < –18 dB over 1 km, enabling M × bit/s scaling of aggregate channel capacity (Gregg et al., 2014).
• In RF, UCA links support up to N orthogonal OAM channels, each with its own equivalent antenna gain and extra free-space loss scaling as $D^{-2|\ell|-2}$. The total MIMO system is diagonalized by the DFT of the circulant channel matrix, supporting independent data streams with per-mode SNR dependent on array aperture and link distance (Nguyen et al., 2015).
• In free-space optical/FSO and radio, OAM modes can be used for multi-user/multi-cell access, with each user or cell assigned a different ℓ. Cohabiting modes do not interfere when alignment is ideal (Cheng et al., 2018).
• For hybrid quantum–classical links, conservation of OAM in nonlinear processes enables direct translation of classical OAM states into entangled two-photon subspaces, supporting high-dimensional quantum key distribution and classical data transfer on the same medium (Zhou et al., 2015).

3. OAM Signal Detection, Demultiplexing, and Processing

Demultiplexing in Optical and RF Domains

• Optical OAM demultiplexing utilizes spatial Fourier analysis, mode sorters (e.g., SLMs, forked holograms), or transformed fiber geometries to spatially or interferometrically distinguish ℓ states (Slussarenko et al., 2010, Gregg et al., 2014).
• RF coherent detection is realized by physically sampling the electric field on a ring in the transverse plane, then applying a spatial discrete Fourier transform to extract the mode amplitudes, with crosstalk limited by the number of elements and beam alignment (Daldorff et al., 2015).

Channel Coding and Signal Processing

• OAM FSO links under turbulence benefit substantially from error-correcting codes (BCH, Reed–Solomon) assigned per OAM mode, correcting both burst and random errors arising from turbulence-induced fades. Wavefront correction (e.g., Shack–Hartmann followed by spatial light modulator pre-compensation) reduces inter-mode crosstalk, enabling >30 dB PSNR recovery for image transmission under strong turbulence when used in combination (Zhao et al., 2014).
• Index-modulated OFDM (OFDM-IM) with OAM multiplexing introduces further efficiency by embedding information in sparse subcarrier indices, achieving up to 40% spectral-efficiency gain and 3–5 dB BER improvement versus classical OFDM in multipath/fading FSO scenarios (Amhoud et al., 2021).

4. Mode Integrity, Loss Mechanisms, and Mitigation Strategies

Attenuation and Link Budget

• The far-field link budget for OAM channels introduces additional loss scaling with mode order: for ℓ, the effective free-space loss is $L_{fs,eq}(\ell) = (4\pi D/\lambda)^{2|\ell|+2}$, while the per-channel gain grows as $R^{2|\ell|}/|\ell|!$ for aperture radius R. Higher ℓ yields rapid SNR penalties—long-range links are limited to ℓ = 0, ±1 unless unprecedented array apertures are employed (Nguyen et al., 2015).
• In fiber, minimizing modal density overlap and exploiting large spin–orbit splitting suppresses ℓ-flip crosstalk and supports long-lived OAM transport even under fiber bends, with measured loss α_ℓ ≈ 2.2 dB/km for ℓ = ±6 (Gregg et al., 2014).

Turbulence, Crosstalk, and Polarization Effects

• Atmospheric turbulence induces admixture of OAM channels, characterized by crosstalk matrices $T_{\ell,m}$. Adaptive optics (AO) based on deformable mirrors and wavefront sensors restores mode purity, supporting QBER ≈ 5% for ℓ_max = 3–4 in LEO satellite-to-ground quantum links (Wang et al., 2019).
• In fiber, weak-guidance approximations clarify that polarization-induced perturbations both split nominally degenerate HE_ℓ+1,m and EH_ℓ–1,m vector modes (spin–orbit interaction) and create an ℓ ± 2 sideband component with amplitude $O(\Delta)$. Mitigation includes careful fiber geometry design, minimizing Δ and controlling spin–orbit phase (Bhandari, 2021).

5. Applications and Future Directions

Optical Communication

• Integrated electrically pumped OAM lasers provide a path toward compact, high-density, monolithic OAM multiplexers/demultiplexers, supporting on-chip 10–100 Gbaud-class transmission rates at standard telecom wavelengths. Ongoing integration with SOAs and thermal tuning is needed to scale singlemode output power beyond the µW regime and stabilize mode purity (Zhang et al., 2017).
• OAM encoding combined with polarization (SAM) increases per-photon dimensionality; M-ary OAM symbols combined with 2 polarization states yield log₂(2M) bits per photon, sustaining multiplexing benefits for both classical and quantum links (Slussarenko et al., 2010, Gregg et al., 2014).

Quantum Communication

• OAM entanglement offers high-dimensional Hilbert space encoding, with demonstrated two-dimensional Bell–CHSH violation (S = 2.82 for ℓ = 2), subspace tomography fidelity 0.84, and “spiral bandwidth” ≈ 5 in sum-frequency/SPDC-exchange architectures. These approaches permit hybrid networking, with fiber-compatible wavelengths supporting both classical data and quantum keys (Zhou et al., 2015).
• Satellite-to-ground OAM quantum communication benefits substantially from AO, enabling transmission of up to 8 OAM symbols at QBER ≈ 5% under realistic atmospheric conditions when AO is employed (Wang et al., 2019).

Wireless/Microwave Communication

• OAM multiplexing in RF enables multi-user and multi-cell access, multiplies spectral efficiency in core network topologies, and offers new possibilities for interference management. System-level link capacity increases linearly with the number of orthogonal OAM modes, subject to SNR constraints of each mode and practical antenna aperture limitations (Cheng et al., 2018, Nguyen et al., 2015).
• Practical OAM radio links have been demonstrated with standard antenna elements and feed networks, supporting up to N/2 independent ℓ-channels using a UCA of N elements. Spatial Fourier processing at the receive aperture achieves crosstalk below −15 dB (Daldorff et al., 2015).

6. Design Considerations and Limitations

Key Factors for OAM Links

Factor	Optical/Fiber OAM	RF/FSO OAM
Mode orthogonality	High, but susceptible to turbulence/fiber imperfections	High, limited by alignment and aperture
Mode order limit	Up to	ℓ
Link attenuation	Losses increase with	ℓ
Multiplexing	Efficient with low crosstalk using mode sorters/AO	Limited by antenna design, alignment
Mitigation	AO, mode sorters, coding, robust design	FFT-based demultiplexing, error coding

Fundamental limitations include extra link loss with increasing OAM order, sensitivity to alignment and environmental effects (turbulence, multipath, bending), and device-level challenges in generating, amplifying, and detecting high-purity modes at system scale. In fibers, both vectorial (polarization-induced) effects and accidental degeneracies must be mitigated through fiber design and DSP. In wireless, aperture size and sampling fidelity set limits on practical ℓ_max, with SNR and intermode crosstalk dictating usable data rates and robustness.

7. Outlook

OAM links represent a physically robust, high-dimensional communication primitive. Advancements in integrated OAM lasers, high-purity air-core OAM fibers, and adaptive optical/RF system architectures have enabled both laboratory demonstrations and early-stage practical deployments. Key ongoing research targets scalable on-chip multiplexing, system-level integration of amplifiers and modulators, advanced AO/coding for turbulence mitigation, and low-loss, broadband OAM mode (de)multiplexing in fiber and free space. These advances will underpin the future of classical and quantum communications with unprecedented spectral and spatial efficiency (Zhang et al., 2017, Slussarenko et al., 2010, Gregg et al., 2014, Zhao et al., 2014, Nguyen et al., 2015, Cheng et al., 2018, Wang et al., 2019, Amhoud et al., 2021, Zhou et al., 2015, Daldorff et al., 2015, Bhandari, 2021).