Orbital Angular Momentum Mode Groups Research Articles

Dynamic perturbation mitigation via polarization difference neural network for high-fidelity ring core fiber image transmission

Ring core fibers (RCFs) offer unique advantages in fiber image transmission, as their weakly-coupled orbital angular momentum mode groups result in high resolution images. However, severe image distortion is still exhibited during fiber transmission when subjected to strong disturbances. Here, we present a novel approach with a differential neural network, namely the polarization speckle differential imaging (PSDI) method, to significantly enhance both the robustness and image resolution of RCF-based imaging systems. When the fibers are disturbed, the PSDI method establishes the perturbation mapping relationship between two polarization speckles and utilizes a differential method to eliminate the perturbation effect in the speckles. This approach mitigates environmental disturbances, resulting in an enhancement of the imaging system's robustness in dynamic environments. By addressing the limitations of conventional techniques, this research represents a significant advancement in the field of fiber imaging technology, with potential applications ranging from endoscopy to high-resolution imaging in complex and dynamic settings.

Multi-step-index fiber with a large number of weakly coupled OAM mode groups for IM/DD systems in data centers: design, fabrication, and characterization

Mode division multiplexing (MDM) technique based on weakly coupled few-mode fibers (FMF) is promising to enhance the capacity of short-reach transmission. We design and fabricate a multi-step-index FMF (MSIF), which supports weakly coupled first-order radial orbital angular momentum mode group (OAMl,1 MG) for MDM transmission. We use three layers of core to regulate the minimum effective refractive index difference (min|Δn eff |) between OAMl,1 MG and the adjacent MGs. In experiments, we demonstrate that the fabricated MSIF can support up to OAM6,1 with the interferometric method, and the loss measured by an optical time-domain reflectometer (OTDR) can achieve <0.5 dB/km for the OAMl,1 with an order from |l| = 0 to |l| = 6. The inter-mode-group cross talk (XT) is tested by the power measurement, and the system-level XT after 20 km fiber transmission in the worst case is about −11.1 dB.

MIMO-free OAM-MDM transmission with a ring-core fiber recirculation loop

We experimentally demonstrate the record long-haul orbital angular momentum (OAM) mode-division multiplexed (MDM) transmission over 300-km assisted by a ring-core fiber (RCF) recirculating loop. The recirculation loop consisting of 50-km RCF is controlled mainly by two acousto-optic switches with opposite states, and the signals are cyclically transmitted in the single-span 50-km RCF to achieve long-haul transmission. The RCF features largely separated OAM mode groups, where the effective refractive index difference between adjacent high-order OAM mode groups is larger than 1.0 × 10-3. Due to the low-crosstalk feature of RCF incorporated into fiber optic links, multiple-input multiple-output (MIMO) equalization technology is not used in the OAM MDM system. We transmit 20-Gbit/s quadrature phase-shift keying signals over two high-order OAM modes, and the transmission performance is characterized. The measured average optical signal-to-noise ratio penalties at a bit-error rate of 1.5 × 10-2 are about 2 dB between single OAM mode transmission and OAM modes multiplexing transmission. The obtained experimental results indicate favorable OAM-based MDM transmission performance in a 300-km recirculating loop link.

Design of New Photonic Crystal Fiber for Orbital Angular Momentum Modes Transmission

In order to achieve the flexibility and miniaturization of orbital angular momentum (OAM) fiber coupler, it is necessary to continuously design and optimize the base fiber to cooperate with the efficient transmission of multiplexed beam. It is more advantageous to use the photonic crystal fiber (PCF) instead of ring fiber in the coupler design. We have proposed a new fiber based on circular PCF structure to support high performance OAM modes transmission. The optimal parameters are selected to realize the stable transmission of 4 OAM mode groups with high mode quality. In the wavelength range of 1.2 μm∼2.0 μm, the confinement loss of low order mode is less than 10−8 dB/m, and the mode purity is more than 81.7%. Because of its flexible structure and low loss characteristics, the proposed OAM fiber can be widely used in the design of optical fiber devices in modular division multiplexing (MDM) communication systems.

Mode-group selective photonic lanterns for multiplexing multi-order orbital angular momentum modes.

The lack of research on photonic lanterns multiplexing multi-order orbital angular momentum (OAM) modes hinders the development of OAM space division multiplexing systems. In this paper, an annular multicore photonic lantern (AMCPL) for multiplexing several OAM mode groups is proposed and demonstrated. Comprehensive simulations are carried out to investigate the effect of the multicore arrangements on the crosstalk (XT) between different OAM mode groups. Further optimization provides an inverted multicore arrangement of the OAM AMCPL with balanced XT between high-order OAM mode groups with topological charges |l| = 2 to 5 for the first time, of which the highest XT between target mode groups does not exceed -27.20 dB at wavelengths from 1300 nm to 1600 nm, and mode conversion efficiencies of all target mode groups exceed 99.5%. Furthermore, a quantum interpretation is given to reveal the characteristics of the evolution of the supermodes along the taper of the OAM AMCPL, which has not been reported.

Turbulence-resistant high-capacity free-space optical communications using OAM mode group multiplexing.

Twisted light carrying orbital angular momentum (OAM), which features a helical phase front, has shown its potential applications in diverse areas, especially in free-space optical (FSO) communications. Multiple orthogonal OAM beams can be utilized to enable high-capacity FSO communication systems. However, for practical OAM-based FSO communication links, atmospheric turbulence will cause serious power fluctuations and inter-model crosstalk between the multiplexed OAM channels, impairing link performance. In this paper, we propose and experimentally demonstrate a novel OAM mode-group multiplexing (OAM-MGM) scheme with transmitter mode diversity to increase system reliability under turbulence. Without adding extra system complexity, an FSO system transmitting two OAM groups with a total of 144 Gbit/s discrete multi-tone (DMT) signal is demonstrated under turbulence strength D/r0 of 1, 2, and 4. In our experiments, the proposed OAM-MGM scheme helps to achieve bit-error-rate (BER) mostly less than 3.8 × 10-3 under turbulence strength D/r0 of 1 and 2 with a total transmitted power of 10 dBm. Compared with the conventional OAM mode multiplexed system, the system interruption probability decreases from 28% to 4% under moderate turbulence strength D/r0 of 2.

Structured Beamforming Based on Orbital Angular Momentum Mode-Group

The structured beams, which can manipulate arbitrary intensity and phase of electromagnetic rays, are significant in communication and sensing. The vortex electromagnetic wave carrying orbital angular momentum (OAM) exhibits the phase distribution of the helical wavefront, which can be viewed as a complete set of eigenmodes for electromagnetic waves. It provides a flexible beamforming method to achieve the azimuthal pattern diversity by the spatial superposition of multiple specific OAM modes. We have proposed a novel form of OAM wave with a two-dimensional structure called plane spiral OAM (PS-OAM), which is conducive to overlapping different modes. Based on this, the PS-OAM mode-group can be employed to modify the spatial beam structure and form structured electromagnetic beams. This kind of beam not only retains the inherent vorticity and orthogonality of the OAM waves but also tackles the difficulties in applications stemming from the singularity-caused dark zone and beam divergence of conventional OAM waves. In this paper, the construction and manipulation of the structured beams are investigated, and the properties are analyzed. The potential applications in multiple-input multiple-output (MIMO) communication, orthogonal multiplexing communication, and spatial field digital modulation (SFDM) communication are envisioned.

A Hybrid Cladding Ring-Core Photonic Crystal Fibers for OAM Transmission with Weak Spin–Orbit Coupling and Strong Bending Resistance

A hybrid cladding ring-core photonic crystal fiber (PCF) for transmitting orbital angular momentum (OAM) modes is proposed, which breaks the circular symmetry of the fiber structure to suppress the spin–orbit coupling and promotes bending resistance. Through the optimization of fiber structure parameters, the designed fiber can support 22 OAM modes (6 OAM mode groups) over a 200-nm wide bandwidth (covering the whole C + L band) with large effective refractive index separation between adjacent modes (>10−4) and mode groups (>3.6 × 10−3), low confinement losses (<3.5 × 10−9 dB/m), and high mode purity (>98.3%). Meanwhile, the phase of the OAM modes varies periodically and uniformly with an increase in the azimuth angle, and the polarization of OAM modes maintain nearly circular polarization in the designed fiber, which also demonstrates that the fiber has weak spin–orbit coupling. Moreover, the confinement losses of all vector modes are less than 10−7 dB/m when the bending radius is larger than 0.8 mm, indicating strong bending resistance. Furthermore, the fiber also exhibits large differential group delay, relatively low and flat dispersion, and low nonlinear coefficients (<2.0 W−1/km). Therefore, the novel fiber structure has great potential in the application of mode division multiplexing (MDM) based on OAM modes.

Plane Spiral OAM Mode-Group Orthogonal Multiplexing Communication Using Partial Arc Sampling Receiving Scheme

A variety of novel orbital angular momentum (OAM)-based communication or sensing systems have attracted much attention over the past decade, the superiority is brought about by nothing other than its orthogonality or vorticity. Plane spiral OAM (PSOAM) mode-group (MG) technique as a reconfigurable beamforming method can be used for building a multiple-in-multiple-out (MIMO) system to achieve the reduction in subchannel correlation, which benefits from the spiral phase distribution within the mainlobe. However, the demultiplexing process still depends on the MIMO algorithm, in which the receiver complexity is same as the conventional MIMO systems. In this article, a PSOAM MG orthogonal multiplexing communication link at the X-band using partial arc sampling receiving (PASR) scheme has been demonstrated experimentally. The MG channels have a good isolation of about 15 dB, and the existing performance can ensure the reliable 16-QAM wireless transmission. Besides, a real-time dual-channel video transmission experiment has been carried out to verify the channel isolation caused by MG’s orthogonality intuitively. More importantly, the demultiplexing procedure using the PASR scheme can be implemented by simple analog phase shifting operation with a lower receiver complexity compared with the conventional MIMO systems.

Weakly guiding graded-index ring-core fiber supporting 16-channel long distance mode division multiplexing systems based on OAM modes with low MIMO-DSP complexity.

We propose a weakly guiding graded-index ring-core fiber (RCF) with trench-assisted structure. The simulation analysis indicates that such a special fiber design is able to support 8 orbital angular momentum (OAM) mode groups (MGs) with low inter-group crosstalk (< -25 dB/km) and low intra-group differential mode delay (DMD) (< 125 ps/km) for higher order OAM MGs with topological charge |l| = 4, 5, 6, 7. The designed RCF also shows favorable tolerance characteristics to ellipticity and bending. Moreover, stable and distinguished broadband performance of proposed RCF is verified over the whole C band ranging from 1530 nm to 1565 nm. This kind of fiber design could be employed in small-scale multiple-input multiple-output digital signal processing (MIMO-DSP) intra-group modes multiplexing transmission combined with MIMO-free inter-group mode multiplexing transmission. The simulated results of the designed RCF show its great potential of the 16-channel long-distance mode division multiplexing (MDM) transmission with low MIMO-DSP complexity.

1-Pbps orbital angular momentum fibre-optic transmission

Space-division multiplexing (SDM), as a main candidate for future ultra-high capacity fibre-optic communications, needs to address limitations to its scalability imposed by computation-intensive multi-input multi-output (MIMO) digital signal processing (DSP) required to eliminate the crosstalk caused by optical coupling between multiplexed spatial channels. By exploiting the unique propagation characteristics of orbital angular momentum (OAM) modes in ring core fibres (RCFs), a system that combines SDM and C + L band dense wavelength-division multiplexing (DWDM) in a 34 km 7-core RCF is demonstrated to transport a total of 24960 channels with a raw (net) capacity of 1.223 (1.02) Peta-bit s−1 (Pbps) and a spectral efficiency of 156.8 (130.7) bit s−1 Hz−1. Remarkably for such a high channel count, the system only uses fixed-size 4 × 4 MIMO DSP modules with no more than 25 time-domain taps. Such ultra-low MIMO complexity is enabled by the simultaneous weak coupling among fibre cores and amongst non-degenerate OAM mode groups within each core that have a fixed number of 4 modes. These results take the capacity of OAM-based fibre-optic communications links over the 1 Pbps milestone for the first time. They also simultaneously represent the lowest MIMO complexity and the 2nd smallest fibre cladding diameter amongst reported few-mode multicore-fibre (FM-MCF) SDM systems of >1 Pbps capacity. We believe these results represent a major step forward in SDM transmission, as they manifest the significant potentials for further up-scaling the capacity per optical fibre whilst keeping MIMO processing to an ultra-low complexity level and in a modularly expandable fashion.

Effects of the Intramode-Group Coupling Induced by the Elliptical Deformation of Orbital Angular Momentum Fibers

Based on the coupling mode theory and numerical simulation, intramode-group coupling in orbital angular momentum (OAM) fibers with elliptical deformation is analyzed in depth. The elliptical deformation not only introduces a time delay difference between even and odd modes of the same high-order vector mode but also destroys the structure of the high-order vector modes; the structural damage depends on the strength of the elliptical deformation. When the elliptical deformation is relatively small, the structural damage of high-order vector modes can be ignored and coupling can be thought as occurring only between two OAM modes composed of the same high-order vector mode. While the elliptical deformation becomes larger, the structural damage cannot be ignored, coupling appears among four OAM modes in the same OAM mode group, and the coupling between two OAM modes composed of the same high-order vector mode is more serious. The results will pave the way for the establishment of accurate propagation models and the optimization of the OAM fiber design.

Experimental Study of Plane Spiral OAM Mode-Group Based MIMO Communications

Spatial division multiplexing using conventional orbital angular momentum (OAM) has become a well-known physical layer transmission method over the past decade. The mode-group (MG) superposed by specific single-mode plane spiral OAM (PSOAM) waves has been proved to be a flexible beamforming method to achieve the azimuthal pattern diversity, which inherits the spiral phase distribution of conventional OAM wave. Thus, it possesses both beam directionality and vorticity. In this article, it is the first time to show and verify a novel PSOAM MG based multiple-in–multiple-out (MG-MIMO) communication link experimentally in a line-of-sight (LoS) scenario. A compact multimode PSOAM antenna is demonstrated experimentally to generate multiple-independent controllable PSOAM waves, which can be used for constructing MGs. After several proof-of-principle tests, it has been verified that the beam directionality gain of MG can improve the receiving signal-to-noise ratio (SNR) level in an actual system. Meanwhile, the vorticity can provide another degree of freedom (DoF) to reduce the spatial correlation of MIMO system. Furthermore, a tentative long-distance transmission experiment operated at 10.2 GHz has been performed successfully at a distance of 50 m with a single-way spectrum efficiency of 3.7 bits/s/Hz/stream. The proposed MG-MIMO may have potential in the long-distance LoS backhaul scenario.

Orbital angular momentum communications based on standard multi-mode fiber (invited paper)

Orbital angular momentum (OAM) modes, having unique properties of a helical phase structure and doughnut intensity profile, have been widely studied in fiber-optic communications, in terms of OAM modulation and OAM multiplexing. In general, different types of specialty fibers with a ring-shape structure are preferred for more stable OAM transmission, which, however, may face greater manufacturing challenge and larger fiber loss compared to standard multi-mode fibers (MMFs). Therefore, the widely deployed and commercially available standard MMFs that can support hundreds of OAM modes have recently attracted great attention. In this paper, we review recent research progress in OAM communications based on standard MMFs. First, the basic concept of OAM and different types of specially designed OAM fibers are briefly introduced. Then, the OAM mode properties in MMFs and recent works, including OAM mode modulation, multiple-input multiple-output (MIMO)-free OAM mode group multiplexing, small-scale partial MIMO assisted OAM mode multiplexing, and OAM-based heterogeneous fiber-optic networks, are presented. The OAM communications using other widely deployed standard single-mode fibers are also briefly introduced as supplementary. Finally, key challenges and perspectives of OAM communications based on standard MMF are discussed and summarized.

Deep Learning: Deep Learning Based Recognition of Different Mode Bases in Ring‐Core Fiber (Laser Photonics Rev. 14(11)/2020)

In article number 2000249, Jian Wang and co-workers show the intelligent recognition of various spatial mode bases by convolutional neural network (CNN)-based deep learning. Four mode bases in a ring-core fiber, i.e. linearly polarized mode group, linearly polarized orbital angular momentum (OAM) mode group, circularly polarized OAM mode group, and vector mode group, are successfully recognized with high recognition rate. This may open a door for deep learning enabled robust and intelligent optical communications exploiting diverse spatial modes for sustainable capacity scaling.

Direct Generation of OAM Mode-Group and Its Application in LoS-MIMO System

A partial slotted curved waveguide leaky-wave antenna which can directly generate orbital angular momentum (OAM) mode-groups (MG) with high equivalent OAM order l <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</sub> = ±40 at 60 GHz is proposed in this letter. This partial arc slotted antenna is designed based on the circular traveling-wave antenna which can generate a single mode conventional OAM wave, so it can be termed as a partial arc transmitting (PAT) scheme compared with the full 2π aperture slotting of the circular traveling-wave antenna. The full-wave simulation results show that the generated OAM MGs present a high gain beam with a helical phase distribution. Thus it can be used in a MG based line-of-sight (LoS) multiple-in-multiple-out system (MG-MIMO), whose performance is numerically analyzed. These MGs can improve the signal-to-noise ratio (SNR) and decrease the spatial correlation of sub-channels, thereby the MG-MIMO has a better performance in terms of capacity or BER than conventional MIMO.

Mode-division multiplexed transmission of wavelength-division multiplexing signals over a 100-km single-span orbital angular momentum fiber

We experimentally demonstrate mode-division multiplexed (MDM) transmission using eight orbital angular momentum (OAM) modes over a single span of 100-km low-attenuation and low-crosstalk ring-core fiber (RCF). Each OAM mode channel carries 10 wavelength-division multiplexing (WDM) signal channels in the C band, with each WDM channel in turn transmitting 16-GBaud quadrature phase-shift keying signal. An aggregate capacity of 2.56 Tbit/s and an overall spectral efficiency of 10.24 bit/(s · Hz) are realized. The capacity-distance product of 256 (Tbit/s) · km is the largest reported so far for OAM fiber communications systems to the best of our knowledge. Exploiting the low crosstalk between the OAM mode groups in the RCF, the scheme only requires the use of modular 4 × 4 multiple-input multiple-output processing, and it can therefore be scaled up in the number of MDM channels without increasing the complexity of signal processing.

Low-loss orbital angular momentum ring-core fiber: design, fabrication and characterization

We design, fabricate, and characterize two types of low-loss weakly guiding ring-core fibers (RCFs) supporting orbital angular momentum (OAM) modes. The ring core and cladding have a relative refractive index difference of 1%. The RCFs feature largely separated OAM mode groups (MGs), where the first MG only contains two fundamental modes and the other MGs each have four OAM modes. The effective refractive index difference (Δ neff ) between all MGs in the type-1 RCF is larger than 1.5 × 10−3. For the type-2 RCF, except the first two MGs, the Δ neff between the other MGs is also larger than 1.0 × 10−3. These enable relatively low-level inter-group crosstalk and facilitate multiple-input multiple-output (MIMO) simplified OAM multiplexing transmission systems, where small-scale MIMO assisted intra-group OAM multiplexing and MIMO-free inter-group OAM multiplexing can be considered to take full use of all OAM modes in RCFs. The propagation loss of all supported OAM modes in the fabricated RCFs is measured to be less than 0.25 dB/km. We study the modal effective area and anti-bending performance of the guided OAM modes in RCFs. The effective refractive index, group velocity dispersion, differential group delay, crosstalk, and stability of RCFs are all characterized showing favorable performance. Moreover, using the fabricated two types of low-loss RCFs and adopting 8-ary quadrature amplitude modulation (8-QAM) signals, we also demonstrate data-carrying two OAM mode group multiplexing transmission over 25.32 km type-1 RCF and 50 km type-2 RCF free of MIMO. The demonstrations show a great potential to use low-loss RCFs in long-distance fiber-optic OAM multiplexing communications.

OAM mode multiplexing in weakly guiding ring-core fiber with simplified MIMO-DSP.

We present a low-loss weakly guiding ring-core fiber for orbital angular momentum (OAM) mode group multiplexing (MGM) transmission. This special fiber design supports 50 radially fundamental modes divided into 13 mode groups with only 0.7% relative refractive index difference between the fiber ring core and cladding. Except the first two groups with 10-5 mode spacing, the other mode groups are separated with each other with effective refractive index difference (Δneff) larger than 10-4, indicating relatively low-level inter-group crosstalk. One can directly use different OAM mode groups for MGM communications without multiple-input multiple-output digital signal processing (MIMO-DSP) technique. Besides, one can employ different OAM modes among the same mode group to carry different data information assisted by small-scale MIMO technique. The target fiber exhibits small and flat dispersion within (14.3, 39.7) ps/nm/km which is comparable to that in the standard single-mode fiber (SMF), and extremely large mode area within (787.9, 841.2) µm2 over the whole C + L band. MIMO equalization complexities for modified small-scale MIMO-DSP assisted intra-group modes multiplexing combined with MIMO-free inter-group modes multiplexing method in both time and frequency domain are much simpler compared to traditional 50×50 MIMO equalization.

18 km low-crosstalk OAM + WDM transmission with 224 individual channels enabled by a ring-core fiber with large high-order mode group separation.

The space domain is regarded as the only known physical dimension of lightwaves left to be exploited for optical communications. Very recently, much research effort has been devoted to using orbital angular momentum (OAM) spatial modes to increase the transmission capacity in fiber-optic communications. However, long-distance low-crosstalk high-order OAM multiplexing transmission in fiber is quite challenging. Here we design and fabricate a graded-index ring-core fiber to effectively suppress radially high-order modes and greatly separate high-order OAM mode groups. By exploiting high-order OAM mode group multiplexing, together with wavelength-division multiplexing (WDM), i.e., 12.5Gbaud 8-array quadrature amplitude modulation (8-QAM) signals over OAM+4 and OAM+5 modes on 112 WDM channels (224 individual channels), we experimentally demonstrate 8.4Tbit/s data transmission in an 18km OAM fiber with low crosstalk. Multiple-input multiple-output digital signal processing is not required in the experiment because of the large high-order mode group separation of the OAM fiber. The demonstrations may open a door to find more fiber-optic communication and interconnect applications exploiting high-order OAM modes.