Encoding/decoding using superpositions of spatial modes for image transfer in km-scale few-mode fiber

Uniform intensity chiral optical field by multifocal synthesis

Chiral optical beams that carry orbital angular momentum (OAM) have a broad range of applications such as optical tweezers, chiral microstructure fabrication, and optical communications. However, some chiral optical beams have inhomogeneous intensity distribution that limits the application in these fields. In this Letter, two different types of chiral optical fields with uniform intensity and arbitrary length were proposed based on the amplitude encoding method and multifocal synthesis. The intensity distribution of the chiral optical fields is determined by the distance between the focal points that can greatly extend the modulation length of the chiral optical field. Moreover, since each focal point contains modulable amplitude and phase, an arbitrary interception of the optical field can be realized by selectively retaining a part of the focal points. By partitioning the chiral optical field and assigning different topological charges, the OAM space-division multiplexing and independent tunability of the topological charges can be realized. In addition, the composite multi-petal vortex array formed by combining two different chiral optical fields can greatly enhance the information capacity of the optical communications and may have potential applications in fields such as particle manipulation.

Robust orbital-angular-momentum-based underwater acoustic communication with dynamic modal decomposition method

Recently, acoustic communication employing orbital angular momentum (OAM) opens another avenue for efficient data transmission in aquatic environments. Current topological charge (TC) detection of OAM beams relies on the orthogonality among different-order OAM beams. However, such strategy requires measurements of the complete azimuthal acoustic pressure, which inevitably reduces the efficiency and increases the bit error rate (BER). To address these challenges, this study proposes a modified dynamic modal decomposition (DMD) method by partially sampling the acoustic field for precise TC detection. Numerical simulations confirm the accuracy of this approach in extracting single or multiple TCs magnitudes within a partially sampled acoustic field. We theoretically compare the performance of the modified DMD approach with conventional orthogonal decoding method. Simulation results indicate that our modified DMD scheme exhibits lower BER under the same noise interference and is more robust to the array misalignment. This research introduces an efficient demodulation solution for acoustic OAM communication, offering potential benefits for simplifying receiver array design and enhancing long-distance underwater data transmission.

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.

Orbital angular momentum spectra of twisted Laguerre-Gaussian Schell-model beams propagating in weak-to-strong Kolmogorov atmospheric turbulence

The presence of atmospheric turbulence in a beam propagation path results in the spread of orbital angular momentum (OAM) modes of laser beams, limiting the performance of free-space optical communications with the utility of vortex beams. The knowledge of the effects of turbulence on the OAM spectrum (also named as spiral spectrum) is thus of utmost importance. However, most of the existing studies considering this effect are limited to the weak turbulence that is modeled as a random complex "screen" in the receiver plane. In this paper, the behavior of the OAM spectra of twisted Laguerre-Gaussian Schell-model (TLGSM) beams propagation through anisotropic Kolmogorov atmospheric turbulence is examined based on the extended Huygens-Fresnel integral which is considered to be applicable in weak-to-strong turbulence. The discrepancies of the OAM spectra between weak and strong turbulence are studied comparatively. The influences of the twist phase and the anisotropy of turbulence on the OAM spectra during propagation are investigated through numerical examples. Our results reveal that the twist phase plays a crucial role in determining the OAM spectra in turbulence, resisting the degeneration of the detection mode weight by appropriately choosing the twist factor, while the effects of the anisotropic factors of turbulence on the OAM spectra seem to be not obvious. Our findings can be applied to the analysis of OAM spectra of laser beams both in weak and strong turbulence.

Experimental demonstration of multiple dimensional coding decoding for image transfer with controllable vortex arrays

Vortex beams carrying orbital angular momentum (OAM), which featuring helical phase front, have been regarded as an alternative spatial degree of freedom for optical mode coding and multiplexing. For most reported OAM-based mode coding schemes, data information is only encoded by different OAM mode states. In this paper, we introduce a novel design technique to construct vortex array phase grating (VAPGs) for the flexible generation of vortex arrays, and employ the proposed VAPGs to realize multi-dimensional space/mode/amplitude coding/decoding. By designing VAPGs with different parameters and loading them on to a single spatial light modulator (SLM), we successfully generate vortex array with different mode states and relative power in the experiments. Moreover, a 10-bit multi-dimensional space/mode/amplitude data coding/decoding scheme for image transfer in free-space link with a zero bit-error-rate is experimentally demonstrated, which confirm the feasibility of our proposed VAPG-based coding/decoding scheme.

Coupling light to higher order transverse modes of a near-concentric optical cavity

Optical cavities in the near-concentric regime have near-degenerate transverse modes; the tight focusing transverse modes in this regime enable strong coupling with atoms. These features provide an interesting platform to explore multi-mode interaction between atoms and light. Here, we use a spatial light modulator (SLM) to shape the phase of an incoming light beam to match several Laguerre-Gaussian (LG) modes of a near-concentric optical cavity. We demonstrate coupling efficiency close to the theoretical prediction for single LG modes and well-defined combinations of them, limited mainly by imperfections in the cavity alignment.

Image transmission with binary coding for free space optical communications in the presence of atmospheric turbulence

Understanding the influence of atmospheric turbulence on optical information transmission is important for free space optical communication. In this paper, the image transmission through a 1 km horizontal turbulent channel has been numerically investigated, and a simulation model including the process of image pixels encoding and decoding is given. The peak signal-to-noise ratio of the received image is evaluated, and the influences of the channel factors and detector noise are discussed in detail. The critical value of noise level and turbulence strength is given. Our results provide a simulation model for image transmission in a turbulent channel along with insight into the impacts of turbulence parameters and detector noise, which are useful for applications in optical communication.

Experimental demonstration of structural robustness of spatially partially coherent fields in turbulence

Structured fields that are spatially completely coherent have been extensively studied in the context of long-distance optical communication, as the structure in the intensity profile of such fields is used for encoding information. This method of doing optical communication works very well in the absence of turbulence. However, in the presence of turbulence, the intensity structures of such fields start to degrade because of the complete spatial coherence of the field, and this structural degradation increases with the increase in turbulence strength. On the other hand, several theoretical studies have now shown that the structured fields that are spatially only partially coherent are less affected by turbulence. However, to the best of our knowledge, no such experimental demonstration has been reported until now. In this Letter, we experimentally demonstrate the structural robustness of partially coherent fields in the presence of turbulence, and we show that for a given turbulence strength, the structural robustness of a partially coherent field increases as the spatial coherence length of the field is decreased.

Spiral spectrum of a Laguerre-Gaussian beam propagating in anisotropic non-Kolmogorov turbulent atmosphere along horizontal path

Close to the ground, it is generally known that atmospheric turbulence exhibits strong anisotropy, which affects the performance of applications such as free-space optical (FSO) communication. In this paper, we establish a theoretical model for calculating the spiral spectrum, also called the orbital angular momentum (OAM) spectrum, of a Laguerre-Gaussian (LG) beam after propagation through anisotropic turbulence along a horizontal link. This model isolates the effects of anisotropy from other parameters of the turbulence. On the basis of this model, the effects of the anisotropy on the probability density of the OAM spectrum and its corresponding modal crosstalk are studied through numerical examples. Our simulation results show that the width of the OAM spectrum will increase or slightly decrease depending on the specific nature of the anisotropy. In addition, it is demonstrated that the inner scale is more likely to cause modal crosstalk than the outer scale. Some strategies to reduce modal crosstalk in anisotropic turbulence are also discussed. Our results may be useful in OAM-based FSO communication at ground level.

Deep learning based atmospheric turbulence compensation for orbital angular momentum beam distortion and communication

Atmospheric transmission distortion is one of the main challenges hampering the practical application of a vortex beam (VB) which carries orbital angular momentum (OAM). In this work, we propose and investigate a deep learning based atmospheric turbulence compensation method for correcting the distorted VB and improving the performance of OAM multiplexing communication. A deep convolutional neural network (CNN) model, which can automatically learn the mapping relationship of the intensity distributions of input and the turbulent phase, is well designed. After trained with loads of studying samples, the CNN model possesses a good generalization ability in quickly and accurately predicting equivalent turbulent phase screen, including the untrained turbulent phase screens. The results show that through correction, the mode purity of the distorted VB improves from 39.52% to 98.34% under the turbulence intensity of Cn2 = 1 × 10^-13. Constructing an OAM multiplexing communication link, the bit-error-rate (BER) of the transmitted signals in each OAM channel is reduced by almost two orders of magnitude under moderate-strong turbulence, and the demodulated constellation diagram also converges well after compensated by the CNN model.

Interconnecting data based on vortex beams by adjusting the ellipticity of a ring-core fiber

In order to exchange data in a space-division multiplexing (SDM) system, a novel vortex-beam-based data interconnection concept, which is achieved by adjusting the ellipticity of a ring-core fiber, is proposed. A new ring-core fiber is also designed and fabricated for exchanging and propagating the data carried by first- or second-order vortex (orbital angular momentum) beams. The proposed scheme is not only analyzed and simulated in principle, but is also verified through experiments. The numerical results demonstrate that the vortex beams can be exchanged by appropriately adjusting the phase difference (with respect to the ellipticity of a ring-core fiber) between the even and odd vector modes. A new experimental platform is designed and established for the sake of investigating the feasibility of the proposed scheme. The experimental results are consistent with the results of the simulation, and demonstrate that the data carried by the first- or second-order vortex beams can be successfully switched with acceptable bit error rates (BERs) between the first-order vortex beams (L=1 or -1) or between the second-order vortex beams (L=2 or -2, left or right circular polarization), respectively. The measured BERs and constellation diagrams of 16-QAM are employed to evaluate the data exchange performance with respect to different cases (i.e., data exchange once or twice, and data exchange with or without crosstalk). The measured BERs and constellation diagrams also demonstrate that the performance degrades with increase in topological charge or crosstalk. The proposed scheme is flexible, simple, and reliable for data exchange in a SDM system.

LED-based visible light communication for color image and audio transmission utilizing orbital angular momentum superposition modes

Twisted light has recently gained enormous interest in communication systems ranging from fiber-optic to radio frequency regimes. Thus far, the light-emitting diode (LED) has not yet been exploited for orbital angular momentum (OAM) encoding to transmit data, which, however, could open up an opportunity towards a new model of secure indoor communication. Here, by multiplexing and demultiplexing red, green and blue (RGB) twisted beams derived from a white light emitting diode, we build a new visible light communication system with RGB colors serving as independent channels and with OAM superposition modes encoding the information. At the sender, by means of theta-modulation, we use a computer-controlled spatial light modulator to generate two-dimensional holographic gratings to encode a large alphabet with 16 different OAM superposition modes in each RGB channel. At the receiver, based on supervised machine learning, we develop a pattern recognition method to identify the characteristic mode patterns recorded by CCD cameras, and therefore, decoding the information. We succeed in demonstrating the transmission of color images and a piece of audio over a 6-meter indoor link with the fidelity over 96%.

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.

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.

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.

Direct fiber vector eigenmode multiplexing transmission seeded by integrated optical vortex emitters

Spatial modes have received substantial attention over the last decades and are used in optical communication applications. In fiber-optic communications, the employed linearly polarized modes and phase vortex modes carrying orbital angular momentum can be synthesized by fiber vector eigenmodes. To improve the transmission capacity and miniaturize the communication system, straightforward fiber vector eigenmode multiplexing and generation of fiber-eigenmode-like polarization vortices (vector vortex modes) using photonic integrated devices are of substantial interest. Here, we propose and demonstrate direct fiber vector eigenmode multiplexing transmission seeded by integrated optical vortex emitters. By exploiting vector vortex modes (radially and azimuthally polarized beams) generated from silicon microring resonators etched with angular gratings, we report data-carrying fiber vector eigenmode multiplexing transmission through a 2-km large-core fiber, showing low-level mode crosstalk and favorable link performance. These demonstrations may open up added capacity scaling opportunities by directly accessing multiple vector eigenmodes in the fiber and provide compact solutions to replace bulky diffractive optical elements for generating various optical vector beams.

Theoretical analyses on orbital angular momentum modes in conventional graded-index multimode fibre

Orbital angular momentum (OAM) modes are another mode basis to represent spatial modes. There have been increasing interests in exploiting OAM modes in specialty fibres. In this paper, we present a comprehensive characterisation of OAM modes in conventional graded-index multimode fibre (MMF). 1) We synthesise the circularly polarized OAM modes by properly combining two fold degenerate cylindrical vector modes (eigenmodes) and analyse the total angular momentum, i.e. spin angular momentum and orbital angular momentum. 2) We divide all the OAM modes of the conventional graded-index MMF into 10 OAM mode groups with effective refractive index differences between different mode groups above 10⁻⁴ enabling low-level inter-group crosstalk. 3) We study the chromatic dispersion, differential group delay, effective mode area, and nonlinearity for each OAM mode group over a wide wavelength ranging from 1520 to 1630 nm covering the whole C band and L band. 4) We discuss the performance tolerance to fibre ellipticity and bending. 5) We further address the robustness of performance against fibre perturbations including the core size, index contrast and the imperfect index profile of the practically fabricated MMFs. The obtained results may provide theoretical basis for further space-division multiplexing applications employing OAM modes in conventional graded-index MMF.

Generation of flower high-order Poincaré sphere laser beams from a spatial light modulator

We propose and experimentally demonstrate a new complex laser beam with inhomogeneous polarization distributions mapping onto high-order Poincaré spheres (HOPSs). The complex laser mode is achieved by superposition of Laguerre-Gaussian modes and manifests exotic flower-like localization on intensity and phase profiles. A simple optical system is used to generate a polarization-variant distribution on the complex laser mode by superposition of orthogonal circular polarizations with opposite topological charges. Numerical analyses of the polarization distribution are consistent with the experimental results. The novel flower HOPS beams can act as a new light source for photonic applications.

Demonstration of reconfigurable joint orbital angular momentum mode and space switching

We propose and demonstrate space-selective switch functions employing orbital angular momentum (OAM) modes in the space domain for switching network. One is the switching among different OAM modes having different spatial phase structures, called OAM mode switching. The other is the switching among different space locations, called space switching. The switching operation mechanism relies on linear optics. Reconfigurable 4 × 4 OAM mode switching, space switching, and joint OAM mode and space switching fabric using a single spatial light modulator (SLM) are all demonstrated in the experiment. In addition, the presented OAM-incorporated space-selective switch might be further extended to N × N joint OAM mode and space switching with fast response, scalability, cascading ability and compability to facilitate robust switching applications.