Multiplexing heralded single photons in orbital-angular-momentum space

A pivotal issue of the conventional optical fiber communications networks is to meet the explosively increasing requirement in data traffic. In order to meet this ever-increasing demand, there have been a lot of research and industrial development efforts to utilize the photon in various dimensions such as wavelength division multiplexing (WDM), space division multiplexing (SDM), mode division multiplexing (MDM) and so on. Fueled by emerging bandwidth-hungry applications, orbital angular momentum (OAM) modes and their multiplexing have recently gained much attention due to its special doughnut-shaped intensity distribution, as well as its unique helical phase wavefront with the theoretically infinite topological value. The OAM modes with different topological charge values are orthogonal to each other, which has provided a new degree of freedom in MDM. In this paper, we propose and design a Ge-doped air-core ring fiber, which can support numerous OAM modes. By varying the mole fraction of GeO2 and adjusting the structure parameter, including the air-core radius and the GeO2-doped ring width, we study the influence of the different fiber parameters on the total supported OAM mode number. The hollow silica fiber with a 50-μm air core and a 1.5-μm thickness of Gedoped ring is designed in simulation to support fiber eigenmodes up to HE112,1 and EH107,1. This provides 436 OAM modes at 1550 nm while maintaining radially single mode condition. Moreover, it can support more than 400 OAM modes from 1260 nm to 1625 nm, covering O, E, S, C, and L bands.

In this paper the possibility to use orbital angular momentum (OAM) modes on a multi-conductor cable to mitigate crosstalk is presented for the first time. OAM modes are understood here as a general method to excite an electromagnetic system that shows cyclic symmetry. It is demonstrated that the OAM modes, known from antenna communication, are an extension of the known modes in two and three conductor systems. It will be shown analytically that the OAM modes are the eigenmodes of a cable showing cyclic symmetry and can therefore propagate on the cable theoretically free of crosstalk. The OAM modes are then compared to the pairwise differential modes in a simulation with regard to crosstalk. For this comparison a circularly arranged multi-conductor cable as well as a cable where the conductor pairs are closer together are evaluated. For the modeling of the per-unit-length parameters a two-dimensional (2-D) solver based on a boundary element method (BEM) is used.

Orbital angular momentum (OAM) mode division multiplexing and encoding have been widely investigated for achieving higher information capacity and performance in both classical and quantum communication systems [1]. A critical component in such systems is an OAM mode sorter that can efficiently sort different OAM modes exp(ilθ) according to the mode index l. To this end, an elegant scheme is to transform the vortex phase exp(ilθ) of OAM modes to a linear phase exp(ilv / β) of tilted plane waves through geometrical transformations, so that they can be separated in the focal plane of a lens. A log-polar transformation was firstly employed [2] and recently a spiral transformation has been proposed to achieve high-resolution OAM mode sorting by the authors [3].

The orbital angular momentum (OAM) of photons provides a pivotal resource for carrying out high-dimensional quantum Pauli-X operations due to its unique discrete infinite orthogonality, which opens enticing perspectives for high-capacity quantum communication and multi-parallel quantum computation. However, the state-of-the-art Pauli-X gates that require cyclic transformation of OAM modes are mostly limited to four modes, face scalability challenges owing to the system redundancy and fidelity degradation induced by increased processing modes. Here, we propose a general strategy to design scalable quantum Pauli-X gates in high-dimensional OAM space by taking full advantage of the order-correlated spatial divergence property of OAM modes. Leveraging the mechanism of balance mode filtering, partitioned phase is embedded between two structurally symmetric Q-plate elements that enable bidirectional mode shifting to filter out the highest-order OAM mode for spatially selective phase modulation. Derived from radial matching between modes and partitioned phase profiles, this design of parallel mode transformation underpins the extension to higher dimensions by merely adjusting filter parameters. We demonstrate the five-mode cyclic transformation with mode purity and diffraction efficiency higher than 92% and 92.7%, respectively. By incorporating polarization-entangled photon sources, we further realize five-dimensional quantum Pauli-X gates encoded on OAM space and experimentally verify the entanglement of output mode states. Low modulation loss and efficient quantum state tomography (QST) techniques endow the reconstructed density matrices with fidelities exceeding 92.3% ± 0.1%, indicating the well-performed entanglement preservation of output states. These results confirm that our approach achieves scalable quantum Pauli-X gate operations through a simple balanced architecture with high coherence, consolidating a theoretical and experimental foundation for large-scale quantum computation encoded on high-dimensional OAM space.

Multiplexing multiple orbital angular momentum (OAM) modes of light has proven to be an effective way to increase data capacity in fiber-optic communications. However, existing techniques for distributing the OAM modes rely on specially designed fibers or couplers. Direct transmission of multiplexed OAM modes through a long standard multimode fiber remains challenging because the strong mode coupling in fibers disables OAM demultiplexing. Here, we propose a deep-learning-based approach to recover the scattered data from multiplexed OAM channels without measuring any phase information. Over a 1-km-long standard multimode fiber, our method is able to identify different OAM modes with an accuracy of more than 99.9% in the parallel demultiplexing of 24 scattered OAM channels. To demonstrate the transmission quality, color images are encoded in multiplexed twisted light and our method achieves decoding the transmitted data with an error rate of 0.13%. Our work shows that the artificial intelligence algorithm could benefit the use of OAM multiplexing in commercial fiber networks and high-performance optical communication in turbulent environments.

We present a study on the potential and challenges of guiding orbital angular momentum (OAM) modes in hollow-core photonic bandgap fibers (HC-PBGFs). Two 19-cell HC-PBGFs with different structural parameters are comparably investigated. The OAM mode properties in a 37-cell HC-PBGF are also discussed to explore the scalability of OAM states. Characteristics of vector modes and OAM modes are comprehensively analyzed with numerical simulations. The results show HC-PBGF with a larger core could effectively support more OAM modes with lower confinement loss and a larger effective area. In addition, HC-PBGF with a deeper and wider photonic bandgap is advantageous for achieving low crosstalk OAM transmission over a broader band-width. 19-cell HC-PBGFs could support OAM modes with purity beyond 0.9, and the value can be further improved by exploiting the 37-cell HC-PBGF.

In this paper, a nested dual-arm spiral antenna (NDASA) is proposed to generate multiple orbital angular momentum (OAM) modes at the same operating frequency. The NDASA consists of the feeding network and three nested dual-arm spiral structures with different sizes. Meanwhile, a metal reflector is located between the spiral structures and the feeding network to enhance the radiation directivity, which can also reduce the impact of the radio frequency (RF) coaxial cables. Both the experimental and simulated results show OAM waves with topological charges of $l = 1$ , $l = 3$ and $l = 5$ can be independently generated at $f = 3$ GHz, and their maximum gains are 6.94dBi, 6.76dBi and 5.49dBi, respectively. In addition, the OAM modes multiplexing and demultiplexing technology are investigated in simulations and experiments. The isolations between the OAM modes of $l = 1$ , $l = 3$ and $l = 5$ and the other different OAM modes are greater than 13.8dB, 13.2dB and 25.1dB, respectively, which proves the potential ability of the proposed NDASA for multiplexing and demultiplexing multiple OAM modes.

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.

Single photons are the key ingredient for many photonic quantum technologies including quantum key distribution and measurement-based quantum computing. However, it remains difficult to create devices with the appropriate specifications for use in non-laboratory environments. The optical microcavity platform provides an attractive route towards a room temperature single photon source device. Our ultra-small focused ion beam (FIB) milled open-access cavities offer enhancement of the spontaneous emission rate, tunability of the emission spectrum and increased light collection. The embedment of solid-state emitters within these cavities enables us to create a robust room temperature single photon source device, with the potential for high efficiencies and single photon purities. Defects such as the nitrogen-vacancy (NV) centre in diamond have been shown to be stable room-temperature sources of single photons. There are new single emitters emerging in two-dimensional materials such as hexagonal boron nitride (hBN). Here we present developments in room-temperature coupling of single defects to open-access microcavities of a planar-hemispherical geometry with mode volumes down to λ3. We report enhancements in the spectral density of photons into a single cavity mode, combined with improved single photon purities. It will be shown that the NV-cavity system provides a ~3% single photon emission efficiency with purities of up to 94%. The hBN-cavity system provides count rates >1Mcts/s into a single cavity mode with purities up to 96%. With these high single photon purities, such devices would be robust against photon number splitting attacks making them attractive for applications in quantum cryptography.

In this paper, a novel design, photonic crystal fiber (PCF) has been reported to support more orbital angular momentum (OAM) modes for the first time. The cladding region formed by employing the three-layer chain shaped air cavities. The OAM properties and parameters are calculated by optimizing the design of the PCF. The simulation results can support 26 OAM modes with a wider bandwidth (750 nm). The confinement loss of the most OAM modes is below average 10−7 dB/m and the nonlinear coefficient is less than 4 W−1/km. Some OAM modes with comparatively flat dispersion variation (such as 3.8684 ps/km-nm) for HE2,1 are noticed from the design. Moreover, fundamental optical characteristics have been rigorously computed by utilizing the finite element method. Now, it can be anticipated that, these excellent optical characteristics confirm the introduced design as a prominent candidate for the OAM transmission and other relevant areas of optical communications.

In this paper, a regular octagonal photonic crystal fiber (ROPCF) is proposed to support and improve the stable transmission of orbital angular momentum (OAM) modes. The proposed ROPCF is composed of a doped high refractive index ring and four layers of air hole rings, ensuring that the effective refractive index difference between OAM modes of the same order or OAM mode group is greater than 10 − 4 , and can stably transmit 114 OAM modes in the wavelength range of 1.5 ∼ 1.6 μ m . A finite element method (FEM) is used to analyze the designed ROPCF. The measured results show that the ROPCF has high mode quality (MQ) (all modes are greater than 95%), and the confinement losses (CL) of all vector modes are in the range of 10 − 14 to 10 − 11 dB/m. In addition, it also has a large effective mode area between 197.64 to 297.82 μ m 2 . Moreover, the nonlinear coefficients and chromatic dispersions (CD) are less than 0.6 W − 1 / km and 276 ps/nm/km, respectively. And the dispersion change is relatively flat. The 10ps walk-off lengths of all vector modes supported by ROPCF at a wavelength of 1.55 μ m are also analyzed. The obtained data results show that the 10ps walk-off lengths of the most of supported vector modes are within the range of 10 3 to 10 5 m, which is beneficial for long-distance transmission of OAM modes. These significant performances indicate that ROPCF has great application value in improving communication channel capacity (CC) and spectral efficiency (SE).

We propose and demonstrate a hybrid few-mode fiber configuration (HFMFC) that enables ultrahigh-channel-count orbital angular momentum (OAM) mode conversion. The HFMFC consists of periodically twisted graded-index few-mode fiber segments and a step-index few-mode fiber segment. Our proposed HFMFC-based multichannel OAM mode converter (OAM-MC) offers an exceptionally high channel count in a wide bandwidth, with customizable channel spacing down to 50 GHz (∼0.4 nm), achieved through optimization of the structural parameters of the HFMFC. By employing this methodology, we have successfully demonstrated 10, 32, 117, and 233 channel OAM mode conversions covering the entire C + L band, representing the highest performance among all reported fiber-based multichannel OAM-MCs to date, for the first time to the best of our knowledge. The suggested ultrahigh-channel-count OAM-MC exhibits promising potential for applications in various fields such as OAM fiber communication, OAM holography, OAM information processing, and OAM metrology.

We study orbital angular momentum (OAM) mode coupling in ring-core fibers (RCFs) due to elliptical shape deformation. We introduce a coupling model based on numerical mode solver outputs of perturbation. We show improved predictions in calculating coupling strength compared to the classical modeling approach. Our model captures and quantifies the disparate behaviors of coupling in lower and higher order degenerate OAM modes. The ideal orthogonality of modes is undermined by fiber imperfections. Our model predicts the OAM order at which the orthogonality within OAM mode pair is maintained despite elliptical deformation. We use our coupling model to simulate propagation effects and compare the performance of two fibers (thin and thick RCF) designed under the same constraints. Our numerical propagation results show different performance for the two fibers under the same level of elliptical deformation. This model uncovers distinct digital signal processing requirements for these two types of fiber, and predicts their signal-to-noise ratio penalty. For each fiber, we examine the large number of supported modes and find the optimal subset of mode groups, i.e., the groups with the lowest penalty. We show that this optimal subset is different from that predicted during the fiber design optimization.

Orbital angular momentum (OAM) modes have their phase distribution as exp (jlθ), which resembles the plane wave in the Cartesian coordinates. Like the traditional lens, which can focus the plane wave on the focal plane, the angular lens can focus the OAM beam in the angular domain, albeit with a relatively long tail due to the unsatisfied angular focal condition for the non-ring shape beams. In this paper, a hybrid lens in the angular domain and the radial domain is proposed. The radial lens with the specific radially distributed phase guarantees the angular focal condition is met for the beams with an arbitrary beam waist or radial field distribution, which significantly improves the performance for the OAM modes sorting by the angular lens. The discrimination of the different OAM modes can be achieved efficiently based on such a single optical component, i.e., the proposed hybrid radial-angular lens, with the OAM modes inter-mode crosstalk as 3.7% when the topological charge difference is 3.

Each photon in a vortex beam carries an orbital angular momentum (OAM). OAM detection plays a very important role in unlocking the new capability of vortex beams in both fundamental research and technological advancements. As the number of information channels continues to grow, there is an urgent need to improve the detection capability of the OAM sorting system. The unprecedented capability of optical metasurfaces has provided an opportunity to realize OAM detection in a compact platform. Different OAM modes are typically separated into distinct light spots in OAM sorting. However, each spot in an OAM sorting system is tailored to recognize only one predesigned OAM mode. A metasurface approach to detecting more OAM modes with the same number of light spots in the observation plane is demonstrated. The conservation of momentum in the design is realized with a customized multifoci metalens. The efficacy of this method is exemplified through the detection of two OAM modes in an individual light spot, dramatically increasing the detection capability of the OAM sorter. It is anticipated that this design will reduce the demand for spatial resources, creating new opportunities to explore OAM applications across many research fields such as optical communications and quantum science.