Orbital Angular Momentum Demultiplexing Research Articles

Multiplexed manipulation of orbital angular momentum and wavelength in metasurfaces based on arbitrary complex-amplitude control

Due to its unbounded and orthogonal modes, the orbital angular momentum (OAM) is regarded as a key optical degree of freedom (DoF) for future information processing with ultra-high capacity and speed. Although the manipulation of OAM based on metasurfaces has brought about great achievements in various fields, such manipulation currently remains at single-DoF level, which means the multiplexed manipulation of OAM with other optical DoFs is still lacking, greatly hampering the application of OAM beams and advancement of metasurfaces. In order to overcome this challenge, we propose the idea of multiplexed coherent pixel (MCP) for metasurfaces. This approach enables the manipulation of arbitrary complex-amplitude under incident lights of both plane and OAM waves, on the basis of which we have realized the multiplexed DoF control of OAM and wavelength. As a result, the MCP method expands the types of incident lights which can be simultaneously responded by metasurfaces, enriches the information processing capability of metasurfaces, and creates applications of information encryption and OAM demultiplexer. Our findings not only provide means for the design of high-security and high-capacity metasurfaces, but also raise the control and application level of OAM, offering great potential for multifunctional nanophotonic devices in the future.

Structured light enhanced machine learning for fiber bend sensing.

The intricate optical distortions that occur when light interacts with complex media, such as few- or multi-mode optical fiber, often appear random in origin and are a fundamental source of error for communication and sensing systems. We propose the use of orbital angular momentum (OAM) feature extraction to mitigate phase-noise and allow for the use of intermodal-coupling as an effective tool for fiber sensing. OAM feature extraction is achieved by passive all-optical OAM demultiplexing, and we demonstrate fiber bend tracking with 94.1% accuracy. Conversely, an accuracy of only 14% was achieved for determining the same bend positions when using a convolutional-neural-network trained with intensity measurements of the output of the fiber. Further, OAM feature extraction used 120 times less information for training compared to intensity image based measurements. This work indicates that structured light enhanced machine learning could be used in a wide range of future sensing technologies.

Using Standard 2×2 MIMO to Increase Capacity of Spatial Multiplexing with OAM Modes

Modal multiplexing can greatly increase fiber capacity, but often at the cost of high digital processing with multiple-input multiple-output (MIMO) algorithms. The use of orbital angular momentum (OAM) for multiplexing can support MIMO-free reception, but it always operates in an interferencelimited regime. Many previous MIMO-free demonstrations for OAM used a type of optical MIMO that is impractical for deployed systems. We compare the performance of MIMO-free and standard electronic 2 × 2 MIMO in a demonstration with no recourse to optical MIMO. We show that the receiver complexity of these two reception strategies is very similar in hardware and signal processing. We explore commercial coherent receivers with 2 × 2 MIMO, hence this solution for OAM demultiplexing can be deployed effectively with current technology. The 2 × 2 MIMO reception enhances the total bit rate and is more compatible with wavelength division multiplexing (WDM) as it equalizes performance across the C-band. At 1.3 km transmission, the net bit rate spans 426–450 Gb/s across wavelengths with 2 × 2 MIMO, while variability is high for MIMO-free reception, spanning 200–418 Gb/s.

Deep Learning of Coherent Laser Arrays in Angular Domain for Orbital Angular Momentum Beams Customization

Artificial intelligence methods for the phase control of laser arrays have been extensively developed in the recent years. To tailor high-power and fast switchable orbital angular momentum (OAM) beams by the coherent combination of laser arrays, complex phase control difficulties arise, and the introduction of deep-learning (DL) methods to learn the mapping from the intensity information to the relative phases of array elements offers a promising solution. In this work, we theoretically propose a DL-based phase control scheme that can customize OAM beams with desirable topological charges from laser array systems, which relies on the acquisition of the optical field information in the view of angular domain. When the system in closed loops, the optical field and phase errors information of laser array are extracted and decoupled in the angular domain perspective by using an OAM demultiplexer. The processed optical field of the coherently combined beam serves the DL-based phase control servo to realize the efficient phase stabilization, thus ensuring the controllable customization of OAM beams. This work could provide a useful reference on the intelligent control of laser array systems for structured light beams customization.

Learning-enabled recovering scattered data from twisted light transmitted through a long standard multimode fiber

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.

A Novel Demultiplexing Scheme for Vortex Beams in Radio Communication Systems

The orbital angular momentum (OAM) based communication is a promising technology in the wireless communication system for its improving spectrum efficiency. However, for the OAM demultiplexing, it requires a very large aperture size in the receiving end due to the OAM beam divergence and the amplitude null in the beam center, which restricts the full development of OAM based communication. In this paper, we propose a novel partial arc sampling receiving (PASR) scheme based on the augmented dynamic mode decomposition (ADMD) to solve this issue. We explore the use of the phase delay coordinates in the conventional DMD to sort the arbitrary OAM modes with arc sampling receiving. Simulation results demonstrate the accuracy and robustness of the proposed approach in sorting both integral and fractional OAM modes with the PASR scheme. Our work offers a practical approach for demultiplexing OAM beams in radio communication systems with fewer receiving devices, especially for the long-distance transmission case.

10 OAM × 16 Wavelengths Two-Layer Switch Based on an Integrated Mode Multiplexer for 19.2 Tb/s Data Traffic

A two-layer switch exploiting orbital angular momentum (OAM) and wavelength of the light as switching domains is presented, aiming to increase the scalability with respect to the single-layer switches. The switch is able to accept 160 optical Gaussian data inputs on a 16-channel wavelength division multiplexing (WDM) grid and direct each input signals to different output ports exploiting 10 OAMs. The optical switch is based on an integrated OAM multiplexer followed by a compact OAM demultiplexer consisting of two refractive elements. Its experimental characterization confirmed a total enabled throughput of 19.2 Tb/s, thanks to the 30 GHz bandwidth available for each port. The switching time can be lower than 1 μs. The OAM switch power consumption, solely due to the thermal tuning of the OAM emitters, since the OAM demux is passive, is 1.35 mW/Gb/s. In the proposed switching architecture the number of active components, i.e., the power consumption, scales linear with the number of ports. This is favorable in comparison with single-layer switches that cascade e.g., 2 × 2 elementary blocks to obtain large port counts, which scale with the square of the number of ports. The switch accepts input and output signals with Gaussian phase profile that propagate through optical fibers and waveguides, thus making it compatible with standard telecom devices. The suitability of the switch to support real data-traffic is proved by successfully testing it with 10G Ethernet and fiber channel over Ethernet (FCoE) data and video traffic. A possible application scenario is represented by a data-center network where the switch can be used to create a low-power consumption network parallel to the network based on standard electronic routers, to manage large traffic flows.

Dielectric metasurface based polarization and orbital angular momentum demultiplexer

The development of high-speed optical communication calls for compatible multiplexing dimensions to enlarge transmission capacity and process optical information. Although vortex beam multiplexing has been previously demonstrated by exploiting the orthogonality of orbital angular momentum (OAM) modes, the multidimensional demultiplexer remains elusive due to the limited phase and polarization manipulation capability. We introduce dielectric propagation phase metasurface based demultiplexers and investigate their application in simultaneously demultiplexing OAM and orthogonal linear polarization (OLP) channels. Utilizing the size-dependent anisotropy of dielectric rectangular nano-pillars, we reveal that the multimode fork grating phase masks with different off-axis phase items can be integrated on the same substrate with polarization selectivity, which allows simultaneously demultiplexing OAM and OLP channels. The demultiplexed position and the number of channels depend on its off-axis phase items, and a 16-channel (8-OAM modes and 2-polarization states) demultiplexer is demonstrated. The vortex beams with different OAM modes and OLP states are successfully demultiplexed with the maximum crosstalk is below −9.2 dB. Changing the number of superimposed complex amplitudes and the off-axis phase items in multimode fork grating phase masks, we further present two 12-channel demultiplexers with different channel distributions. Our results provide an approach that breaks the polarization-insensitive in manipulating OAM modes, introducing demultiplexers that apply to OAM mode and OLP states simultaneously.

Wavelength and polarization multiplexed optical vortex demultiplexer

Optical vortex beams with orbital angular momentum (OAM) have recently received considerable attention in high-speed optical communication due to their spatial orthogonality. For realizing undamaged demultiplexing, an all-dielectric multiplexed metasurface for demultiplexing wavelength-division multiplexing and polarization-division multiplexing OAM light was proposed. This OAM demultiplexer works in the near-infrared spectral region and the transmitted vortex beams carry original topological charges and phase distribution. It is worth mentioning that this demultiplexer is vastly different from previous demultiplexers, in that it not only eliminates the cumbersome process of converting vortex beams into Gaussian beams but also maintains the original property of the transmitted vortex beams. The presented demultiplexer has flexible and compact features, which are suitable for OAM communication systems. Also, the designed wavelength- and polarization-multiplexed metasurface expands the scope of metasurface compactness multifunctional photonics, which has applications in information security and anti-counterfeiting.

Simultaneous wavelength and orbital angular momentum demultiplexing using tunable MEMS-based Fabry-Perot filter.

In this paper, we experimentally demonstrate simultaneous wavelength and orbital angular momentum (OAM) multiplexing/demultiplexing of 10 Gbit/s data streams using a new on-chip micro-component-tunable MEMS-based Fabry-Perot filter integrated with a spiral phase plate. In the experiment, two wavelengths, each of them carrying two channels with zero and nonzero OAMs, form four independent information channels. In case of spacing between wavelength channels of 0.8 nm and intensity modulation, power penalties relative to the transmission of one channel do not exceed 1.45, 0.79 and 0.46 dB at the hard-decision forward-error correction (HD-FEC) bit-error-rate (BER) limit 3.8 × 10-3 when multiplexing a Gaussian beam and OAM beams of azimuthal orders 1, 2 and 3 respectively. In case of phase modulation, power penalties do not exceed 1.77, 0.54 and 0.79 dB respectively. At the 0.4 nm wavelength grid, maximum power penalties at the HD-FEC BER threshold relative to the 0.8 nm wavelength spacing read 0.83, 0.84 and 1.15 dB when multiplexing a Gaussian beam and OAM beams of 1st, 2nd and 3rd orders respectively. The novelty and impact of the proposed filter design is in providing practical, integrable, cheap, and reliable transformation of OAM states simultaneously with the selection of a particular wavelength in wavelength division multiplexing (WDM). The proposed on-chip device can be useful in future high-capacity optical communications with spatial- and wavelength-division multiplexing, especially for short-range communication links and optical interconnects.

Optical Orbital Angular Momentum Demultiplexing and Channel Equalization by Using Equalizing Dammann Vortex Grating

A novel equalizing Dammann vortex grating (EDVG) is proposed as orbital angular momentum (OAM) multiplexer to realize OAM signal demultiplexing and channel equalization. The EDVG is designed by suppressing odd diffraction orders and adjusting the grating structure. The light intensity of diffraction is subsequently distributed evenly in the diffraction orders, and the total diffraction efficiency can be improved from 53.22% to 82%. By using the EDVG, OAM demultiplexing and channel equalization can be realized. Numerical simulation shows that the bit error rate (BER) of each OAM channel can decrease to 10-4 when the bit SNR is 22 dB, and the intensity is distributed over the necessary order of diffraction evenly.

Manipulation of orbital angular momentum beams based on space diffraction compensation

We put forward a technique to manipulate the size of orbital angular momentum (OAM) beams based on space diffraction compensation. Paraxial Fresnel diffraction which carries a negative spatial quadratic phase distribution can be regarded as a negative diffractive effect. To compensate the negative diffraction, we employ a 4f Fourier lens system containing a phase mask to generate an inverse quadratic phase. The size of OAM beams can be easily controlled by designing the phase mask profile without changing the OAM. The applications of space diffraction compensation in OAM demultiplexing, ring fiber coupling for OAM beams and optical manipulation of micro particles are also discussed.

Free-space optical transmission with orbital angular momentum division multiplexing

The experimental demonstration of a free-space optical transmission system with orbital angular momentum (OAM) division multiplexing is reported. Two beams have been multiplexed with different values of the azimuthal index l, and hence carrying different OAM, at a wavelength of 1550 nm. The first beam (l = 0) is nearly Gaussian and it features an OAM equal to zero. The second beam (l = 1) is an optical vortex produced by means of a suitable computer-generated hologram, which shows an intensity pattern with a doughnut shape and possesses an OAM of –h per photon. A binary intensity modulation at 1.25 Gbit/s has been applied to each OAM channel and an interferometer with Dove prisms has been exploited for OAM demultiplexing.