Orbital angular momentum mode groups multiplexing transmission over 2.6-km conventional multi-mode fiber

Optical mode manipulation using deep spatial diffractive neural networks

In this paper, we investigate the theoretical models and potential applications of spatial diffractive neural network (SDNN) structures, with a particular focus on mode manipulation. Our research introduces a novel diffractive transmission simulation method that employs matrix multiplication, alongside a parameter optimization algorithm based on neural network gradient descent. This approach facilitates a comprehensive understanding of the light field manipulation capabilities inherent to SDNNs. We extend our investigation to parameter optimization for SDNNs of various scales. We achieve the demultiplexing of 5, 11 and 100 orthogonal orbital angular momentum (OAM) modes using neural networks with 4, 10 and 50 layers, respectively. Notably, the optimized 100 OAM mode demultiplexer shows an average loss of 0.52 dB, a maximum loss of 0.62 dB, and a maximum crosstalk of -28.24 dB. Further exploring the potential of SDNNs, we optimize a 10-layer structure for mode conversion applications. This optimization enables conversions from Hermite-Gaussian (HG) to Laguerre-Gaussian (LG) modes, as well as from HG to OAM modes, showing the versatility of SDNNs in mode manipulation. We propose an innovative assembly of SDNNs on a glass substrate integrated with photonic devices. A 10-layer diffractive neural network, with a size of 49 mm × 7 mm × 7 mm, effectively demultiplexes 11 orthogonal OAM modes with minimal loss and crosstalk. Similarly, a 20-layer diffractive neural network, with a size of 67 mm × 7 mm × 7 mm, serves as a highly efficient 25-channel OAM to HG mode converter, showing the potential of SDNNs in advanced optical communications.

Generalized Newton's rings with vortex beams

The Newton's rings are interference patterns with concentric rings, and Newton's rings experiment is one of the most famous classic optics experiments. Here, we show that if we use a vortex beam, we can obtain generalized Newton's rings. Unlike traditional Newton's rings, the generalized ones are no longer concentric rings but spiral arms, and fork-shaped dislocations appear in spiral arms. More interesting, we reveal that both the number of spiral arms and the number of fork-shaped dislocations are equal to the value of topological charge of incident vortex beams. Our theoretical results are demonstrated experimentally. This novel interference pattern can be used for measuring the topological charge of vortex beams.

The Performance of Orbital Angular Momentum Mode (|l| = 1~3) Amplification Based on Ring-Core Erbium-Doped Fibers

We demonstrated that a ring-core erbium-doped fiber amplifier (RC-EDFA) can support orbital angular momentum (OAM) modes with topological charges (|l| = 1~3). The dependence of the characteristics on the length of the RC-EDF was investigated experimentally, including an investigation of gain and 3 dB gain bandwidth over the whole C band (i.e., 1530~1565 nm). The 3 dB gain bandwidth was improved to 21 nm. At a signal wavelength of 1550 nm, the maximum gain of all signal modes was up to 30.1 dB. Differential modal gain was maintained below approximately 1.3 dB.

Terahertz device utilizing a transmissive geometric metasurface

Due to potential applications in next generation high-capacity wireless communication systems, generating and controlling vortex beams carrying orbital angular momentum (OAM) has received considerable attention. In this work, a scheme is proposed to generate two/four splitting vortex beams and focusing vortex beams with different topological charges under left circularly polarized and right circularly polarized terahertz waves under incidence. The meta-unit cell consists of a two-flying-fish-shaped patterned metallic top layer and an identical metallic patterned bottom layer separated by a silica layer. Full-wave simulation results agree well with that of calculation predictions. The proposed terahertz metasurface-based devices are able to carry different OAM modes and can abruptly manipulate during propagation, which indicates that such metasurface-based devices may have promising applications in terahertz wireless communication links in the future.

Selective excitation of optical vortex modes with specific charge numbers in band-tuned topological waveguides

We propose a method for selectively propagating optical vortex modes with specific charge numbers in a photonic integrated circuit (PIC) by using a topological photonic system. Specifically, by performing appropriate band tuning in two photonic structures that comprise a topological waveguide, one specific electromagnetic mode at the Γ point of a band diagram can be excited. Based on theoretical analysis, we successfully propagated optical vortex modes with specific charge numbers over a wide range in the C band in the proposed topological waveguide. The proposed method could be useful in controlling optical vortex signals at the chip level in future orbital angular momentum multiplexing technologies.

Spin-orbit coupling suppressed high-capacity dual-step-index ring-core OAM fiber

We design and fabricate a dual-step-index ring-core fiber (RCF) for orbital angular momentum (OAM) mode transmission. It has been proven that the proposed novel, to the best of our knowledge, dual-step-index ring-core structure can suppress the spin-orbit coupling and cut off the radial higher-order modes when the mode number becomes very large. In experiments, we demonstrate that the fabricated fiber can support OAM_8,1 with the interferometric method, where four higher-order mode groups are weakly-coupled. We also measure the loss of each mode according to the cut-back method and the loss can achieve <0.3 dB/km for the OAM modes with an order from |l| = 1 to |l| = 5. The exploration of this novel optical fiber structure may provide ideas and knowledge for the improvement of the optical fiber communication capacity.

Optical Fiber-Integrated Metasurfaces: An Emerging Platform for Multiple Optical Applications

The advent of metasurface technology has revolutionized the field of optics and photonics in recent years due to its capability of engineering optical wavefronts with well-patterned nanostructures at subwavelength scale. Meanwhile, inspired and benefited from the tremendous success of the “lab-on-fiber” concept, the integration of metasurface with optical fibers has drawn particular interest in the last decade, which establishes a novel technological platform towards the development of “all-in-fiber” metasurface-based devices. Thereby, this review aims to present and summarize the optical fiber-integrated metasurfaces with the current state of the art. The application scenarios of the optical fiber metasurface-based devices are well classified and discussed accordingly, with a brief explanation of physical fundamentals and design methods. The key fabrication methods corresponding to various optical fiber metasurfaces are summarized and compared. Furthermore, the challenges and potential future research directions of optical fiber metasurfaces are addressed to further leverage the flexibility and versatility of meta-fiber-based devices. It is believed that the optical fiber metasurfaces, as a novel all-around technological platform, will be exploited for a large range of applications in telecommunication, sensing, imaging, and biomedicine.

High-fidelity spatial mode transmission through a 1-km-long multimode fiber via vectorial time reversal

The large number of spatial modes supported by standard multimode fibers is a promising platform for boosting the channel capacity of quantum and classical communications by orders of magnitude. However, the practical use of long multimode fibers is severely hampered by modal crosstalk and polarization mixing. To overcome these challenges, we develop and experimentally demonstrate a vectorial time reversal technique, which is accomplished by digitally pre-shaping the wavefront and polarization of the forward-propagating signal beam to be the phase conjugate of an auxiliary, backward-propagating probe beam. Here, we report an average modal fidelity above 80% for 210 Laguerre-Gauss and Hermite-Gauss modes by using vectorial time reversal over an unstabilized 1-km-long fiber. We also propose a practical and scalable spatial-mode-multiplexed quantum communication protocol over long multimode fibers to illustrate potential applications that can be enabled by our technique.

The use of long multimode fibers for multiplexed quantum communication is hindered by modal crosstalk and polarisation mixing. Here, the authors use an auxiliary laser beam sent backwards from Bob to Alice, allowing her to pre-compensate for the spatial distortions and polarisation scrambling.

Deep-learning-based high-resolution recognition of fractional-spatial-mode-encoded data for free-space optical communications

Structured light with spatial degrees of freedom (DoF) is considered a potential solution to address the unprecedented demand for data traffic, but there is a limit to effectively improving the communication capacity by its integer quantization. We propose a data transmission system using fractional mode encoding and deep-learning decoding. Spatial modes of Bessel-Gaussian beams separated by fractional intervals are employed to represent 8-bit symbols. Data encoded by switching phase holograms is efficiently decoded by a deep-learning classifier that only requires the intensity profile of transmitted modes. Our results show that the trained model can simultaneously recognize two independent DoF without any mode sorter and precisely detect small differences between fractional modes. Moreover, the proposed scheme successfully achieves image transmission despite its densely packed mode space. This research will present a new approach to realizing higher data rates for advanced optical communication systems.

OAM-basis transmission matrix in optics: a novel approach to manipulate light propagation through scattering media

Transmission matrix (TM) is an ideal theoretical model describing light propagation through scattering media. Until now, most of the present TMs utilize the eigenstates of spatial position as input and output bases. Thus, they describe the relationship between the spatial distributions of two light fields. Here, we demonstrate that wider relationships between the light fields could be described by a TM. As a significant example, we propose a generalized TM with the eigenstates of OAM as input bases - OAM-basis TM. With the measured OAM-basis TM, we achieved single-spot and multiple-spot focusing, verifying its availability in light propagation manipulation. The distinct eigenchannels property was also discussed. The OAM-basis TM has broadened the definition of TM. Meanwhile, it will open new perspectives for OAM-based communication, as well as the applications of wavefront shaping technology in biomedical photonics and optical communication.

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.

Photonic crystal fibers supporting fully separated eigenmodes

We present two kinds of fully degeneracy-lifted solid-core photonic crystal multi-mode fibers aiming at multiple-input multiple-output-free direct fiber vector eigenmode multiplexing transmission. One of the target fiber structures is able to support 52 fully separated eigenmodes with effective index difference above 1.37×10^-4 and confinement loss below 3.28×10^-5  dB/km over the whole C+Lband (1530-1625 nm). Specifically, it is suggested that >10^-4 effective index separation is sufficient to possess polarization-maintaining properties, while for short-reach stable mode-division multiplexing transmission, the effective index difference >10^-3 is highly desired. In particular, the other target fiber structure can further increase effective index separation above 1.10×10^-3 among 24 eigenmodes with confinement loss below 6.68×10^-4  dB/km over the whole C+L band.

Optical orbital-angular-momentum-multiplexed data transmission under high scattering

Multiplexing multiple orbital angular momentum (OAM) channels enables high-capacity optical communication. However, optical scattering from ambient microparticles in the atmosphere or mode coupling in optical fibers significantly decreases the orthogonality between OAM channels for demultiplexing and eventually increases crosstalk in communication. Here, we propose a novel scattering-matrix-assisted retrieval technique (SMART) to demultiplex OAM channels from highly scattered optical fields and achieve an experimental crosstalk of -13.8 dB in the parallel sorting of 24 OAM channels after passing through a scattering medium. The SMART is implemented in a self-built data transmission system that employs a digital micromirror device to encode OAM channels and realize reference-free calibration simultaneously, thereby enabling a high tolerance to misalignment. We successfully demonstrate high-fidelity transmission of both gray and color images under scattering conditions at an error rate of <0.08%. This technique might open the door to high-performance optical communication in turbulent environments.

Optical vortices 30 years on: OAM manipulation from topological charge to multiple singularities

Thirty years ago, Coullet et al. proposed that a special optical field exists in laser cavities bearing some analogy with the superfluid vortex. Since then, optical vortices have been widely studied, inspired by the hydrodynamics sharing similar mathematics. Akin to a fluid vortex with a central flow singularity, an optical vortex beam has a phase singularity with a certain topological charge, giving rise to a hollow intensity distribution. Such a beam with helical phase fronts and orbital angular momentum reveals a subtle connection between macroscopic physical optics and microscopic quantum optics. These amazing properties provide a new understanding of a wide range of optical and physical phenomena, including twisting photons, spin-orbital interactions, Bose-Einstein condensates, etc., while the associated technologies for manipulating optical vortices have become increasingly tunable and flexible. Hitherto, owing to these salient properties and optical manipulation technologies, tunable vortex beams have engendered tremendous advanced applications such as optical tweezers, high-order quantum entanglement, and nonlinear optics. This article reviews the recent progress in tunable vortex technologies along with their advanced applications.

Reconfigurable and tunable twisted light laser

Twisted light, having a helical spatial phase structure and carrying orbital angular momentum (OAM), has given rise to many developments ranging from optical manipulation to optical communications. The laser excitation of twisted light in a reconfigurable and tunable way is of great interest. Here, we propose and experimentally demonstrate an OAM reconfigurable and wavelength tunable twisted light laser with achievable high-order OAM modes on a hybrid free-space and fiber platform. The excited twisted light laser is enabled by a ring resonator incorporating spatial light modulators (SLMs) and bandpass filter (BPF). By appropriately switching the phase pattern loaded onto SLMs and adjusting the BPF, twisted light laser with reconfigurable OAM and tunable wavelength is implemented. In the experiment, the OAM value is varied from -10 to +10 and the wavelength is adjusted from 1530 to 1565 nm covering the whole C band. The obtained results indicate successful implementation of a reconfigurable and tunable twisted light laser with favorable operation performance. Reconfigurable and tunable twisted light laser may open up new perspectives to more extensive OAM-enabled applications with improved flexibility and robustness.

Fully degeneracy-lifted bow-tie elliptical ring-core multi-mode fiber

We present a bow-tie elliptical ring-core multi-mode fiber (BT-ERC-MMF) that features an elliptical ring-core structure and two symmetrical bow-tie stress-applying parts (SAPs). This special fiber design breaking geometry and stress symmetry fully separates the two- or four-fold degenerate modes in traditional circular symmetry fiber with strong mode coupling during mode-division multiplexing (MDM) transmission. The designed fiber is able to support 53 fully degeneracy-lifted eigenmodes with minimum effective index difference between adjacent modes larger than 1.59 × 10^-4 at 1550 nm, facilitating potential fiber eigenmode multiplexing transmission without using multiple-input multiple-output digital signal processing (MIMO-DSP) technique. The effect of bending on the designed fiber is investigated based on conformal mapping. Broadband performance including effective modal index (neff), effective index difference (Δneff), effective mode area (Aeff), nonlinearity and chromatic dispersion (D) is also comprehensively studied over the whole C band ranging from 1530 to 1565 nm. The designed fiber targets emerging applications in low-crosstalk direct fiber eigenmode-division multiplexing combined with the mature wavelength-division multiplexing (WDM) technique to increase transmission capacity and spectral efficiency.

Generating and synthesizing ultrabroadband twisted light using a compact silicon chip

Compact and broadband manipulation of spatial modes is important in applications exploiting the space domain of light waves. Here, we demonstrate chip-scale generation and synthesization of ultrabroadband orbital angular momentum (OAM) modes (twisted light) on a silicon platform. By introducing a subwavelength holographic fork grating on top of a silicon waveguide, the in-plane guided mode is converted to the free-space OAM mode. Inputs from both sides of the waveguide enable the synthesization of OAM modes. We also characterize wavelength-dependent emission efficiency, offset angle, and purity with favorable performance. The chip-scale ultrabroadband OAM generator and synthesizer may find potential applications in multidimensional optical communications and quantum key distribution.

Feedback-enabled adaptive underwater twisted light transmission link utilizing the reflection at the air-water interface

Line-of-sight link is widely used in common free-space optical (FSO) laser communications between two fixed locations. While in practical underwater wireless optical communications (UWOC), the environment is relatively complicated. In some scenarios there exist irremovable obstacles, which block the line-of-sight optical link. Fortunately, the air-water interface can function as a natural mirror to enable non-line-of-sight optical link using the total internal reflection. Very recently, twisted light beams carrying orbital angular momentum (OAM) have attracted researchers' great attention to improve the transmission capacity in UWOC. Here, we propose and experimentally demonstrate a non-line-of-sight underwater twisted light transmission link utilizing the total internal reflection at the air-water interface. To overcome the beam fluctuation and drift caused by the change of interface states, we develop a proof-of-concept adaptive feedback system to provide a stable output. Moreover, we study the degrading effects of the slight wind effect, the salinity (turbidity) effect, and the vertical thermal gradient-induced turbulence effect. The results show that the water wave caused by the slight wind causes the most beam drift, the thermal gradient causes the most distortions, and the salinity causes the most power loss.

Orbital angular momentum modes emission from a silicon photonic integrated device for km-scale data-carrying fiber transmission

We experimentally demonstrate orbital angular momentum (OAM) modes emission from a high emission efficiency OAM emitter for 20-Gbit/s quadrature phase-shift keying (QPSK) carrying data transmission in few-mode fiber (FMF). The device is capable of emitting vector optical vortices carrying well-defined OAM efficiently with the efficiency of the device >37%. Seven modes propagate through a 2-km two-mode and a 3.6-km three-mode FMF with measured optical signal-to-noise ratio (OSNR) penalties less than 4 dB at a bit-error rate (BER) of 2 × 10^-3. The demonstrations with favorable performance pave the way to incorporate silicon photonic integrated devices as transceivers in an OAM-enabled optical fiber communication link.

Metasurface-assisted orbital angular momentum carrying Bessel-Gaussian Laser: proposal and simulation

Bessel-Gaussian beams have distinct properties of suppressed diffraction divergence and self-reconstruction. In this paper, we propose and simulate metasurface-assisted orbital angular momentum (OAM) carrying Bessel-Gaussian laser. The laser can be regarded as a Fabry-Perot cavity formed by one partially transparent output plane mirror and the other metasurface-based reflector mirror. The gain medium of Nd:YVO₄ enables the lasing wavelength at 1064 nm with a 808 nm laser serving as the pump. The sub-wavelength structure of metasurface facilitates flexible spatial light manipulation. The compact metasurface-based reflector provides combined phase functions of an axicon and a spherical mirror. By appropriately selecting the size of output mirror and inserting mode-selection element in the laser cavity, different orders of OAM-carrying Bessel-Gaussian lasing modes are achievable. The lasing Bessel-Gaussian₀, Bessel-Gaussian₀₁⁺, Bessel-Gaussian₀₂⁺ and Bessel-Gaussian₀₃⁺ modes have high fidelities of ~0.889, ~0.889, ~0.881 and ~0.879, respectively. The metasurface fabrication tolerance and the dependence of threshold power and output lasing power on the length of gain medium, beam radius of pump and transmittance of output mirror are also discussed. The obtained results show successful implementation of metasurface-assisted OAM-carrying Bessel-Gaussian laser with favorable performance. The metasurface-assisted OAM-carrying Bessel-Gaussian laser may find wide OAM-enabled communication and non-communication applications.

Dielectric metasurfaces enabling twisted light generation/detection/(de)multiplexing for data information transfer

We propose, design, fabricate and demonstrate nanophotonic all-dielectric metasurfaces enabling the generation, detection and (de)multiplexing of twisted light having helical phase structure and carrying orbital angular momentum (OAM). The designed metasurfaces are based on dielectric elliptical resonators on standard silicon-on-insulator (SOI) platform. One can achieve full-phase control of 0-2π and flexible amplitude adjustment by properly changing the geometric dimensions (long axis, short axis) and orientation of dielectric elliptical resonator based on the Mie resonance effect. Using the designed and fabricated all-dielectric metasurfaces, we demonstrate the generation and detection of OAM beams with topological charge number from l = -4 to 4. The crosstalk matrix of generated OAM beams is also characterized showing -16 dB crosstalk. We further demonstrate the (de)multiplexing of two OAM beams (OAM₊₁ & OAM₊₄ or OAM₊₂ & OAM₊₃) each carrying a binary image ("A" & "B" or "HUST" & "WNLO"). The obtained results show error-free data information transfer with favorable performance. The presented alternative approach of all-dielectric metasurfaces shows distinct features of easy fabrication process and easy chip-scale integration facilitating ultrathin optical applications. The demonstrations may open a door to find more interesting applications in all-dielectric metasurfaces enabled spatial light manipulation and optical communications and interconnects.

Directly using 8.8-km conventional multi-mode fiber for 6-mode orbital angular momentum multiplexing transmission

Twisted light carrying orbital angular momentum (OAM), which featuring helical phase front, has shown its potential applications in diverse areas, especially in optical communications in free space and specially designed fibers, e.g. a vortex fiber. Instead of specially designed fibers extensively used in the reported OAM-based fiber transmission experiments, here we demonstrate the viability of a conventional graded-index multi-mode fiber (MMF) for OAM multiplexing transmission with less digital signal processing (DSP) complexity. We demonstrate a 120-Gbit/s quadrature phase-shift keying (QPSK) signal transmission in an 8.8-km OM4 MMF by using OAM mode multiplexing with all the modes in the first two mode-groups (OAM01L, OAM01R, OAM+11L, OAM+11R, OAM-11L, OAM-11R) with only 2×2 and 4×4 multiple-input-multiple-output (MIMO) equalization. Moreover, we demonstrate the data-carrying two OAM mode groups multiplexing transmission over the 8.8-km MMF without MIMO equalization. These demonstrations may open up new perspectives to enable the realistic use of OAM-based MMF solution in data centers and super-computers.

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., 12.5 Gbaud 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 8.4 Tbit/s data transmission in an 18 km 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.

Orbital angular momentum mode multiplexed transmission in heterogeneous few-mode and multi-mode fiber network

Mode-division multiplexing (MDM), which employs the spatial modes of light as information carriers, has been widely investigated to increase the transmission capacity. Few-mode fibers (FMFs) and multi-mode fibers (MMFs) have been used for MDM fiber transmission. One of the MDM techniques known as twisted light multiplexing using orbital angular momentum (OAM) modes has recently attracted increasing interest. In this Letter, by splicing two FMFs together with a conventional OM3 MMF, we propose and demonstrate OAM-based MDM in a heterogeneous fiber-optic network, i.e., two OAM mode (OAM₀₁ and OAM_-11) multiplexing transmission in the heterogeneous FMFs and MMF network. We transmit 20-Gbit/s quadrature phase shift keying signals over two OAM modes in different mode groups without multiple-input multiple-output equalization techniques and achieve less than 2.8 dB optical signal-to-noise ratio penalties at a bit-error rate of 2×10^-3. The experimental results show favorable transmission performance of OAM-based MDM in heterogeneous FMFs and MMF network compared to the one in FMF.

Adaptive water-air-water data information transfer using orbital angular momentum

With the increasing demands for underwater monitoring and military applications, underwater wireless optical communication (UWOC) is desired to be an alternative approach to provide higher data rate than acoustic communication. Twisted light carrying orbital angular momentum (OAM) has recently gained increasing interest in diverse areas, especially in free-space and fiber-based optical communications. OAM-based UWOC between underwater and aerial users, a promising technique to enable a variety of applications, which however, has not yet been reported so far. Here we experimentally demonstrate an adaptive water-air-water data information transfer using OAM. According to the feedback information of the received intensity distribution, the reflection element is adjusted for mitigating the misalignment-induced degradation effect due to water level change. The experimental results show favorable performance of the feedback-assisted water-air-water twisted light data information transfer.