Orbital Angular Momentum Detection Research Articles

Encoding independent wavefronts in a single metasurface for high-order optical vortex recognition

The orbital angular momentum (OAM) of vortex beams has great potential in optical communications due to its communication confidentiality and low crosstalk. It is necessary to design a plausible OAM pattern recognition mechanism. Abandoning AI models that require large datasets, a single passive all-dielectric metasurface consisting of TiO2 nanopillars on a SiO2 substrate is used to recognize high-order optical vortexes. In this configuration, the proposed device is capable of simultaneously encoding the wavefront and the transmission paths in different incident OAM beams. Due to the presence of spin angular momentum (SAM), the vortex beam to be identified is spatially separated after passing through the metasurface. As a proof of concept, 14 signal channels are considered in the constructed metasurface, 12 of them can be encoded at will for the detection of any vortex beam with a predefined topological charge. These results make use of metasurfaces to enable OAM pattern recognition in an effective way, which may open avenues for the ultimate miniaturization of optical vortex communication and advanced OAM detection technologies.

Separation and detection of orbital angular momentum states of composite vortex beams in atmospheric turbulence channels

The separation and detection of compound orbital angular momentum (OAM) modes are the basis for achieving high-speed and high-capacity communication, but the atmospheric turbulence causes distortion of the phase fronts of vortex beams, which hinders the recognition of OAM modes. To solve this problem, in this work, we propose and investigate a joint scheme of combining the Gerchberg–Saxton (GS) algorithm and the phase of addition and subtraction model that can achieve high recognition accuracy and a wide range of OAM modes of compound perfect optical vortex beam under long-distance turbulence environment. At the receiving end, the GS algorithm and phase addition and subtraction are used to compensate and modulate the incident vortex light field to obtain the pre-processed light field, and then the OAM is detected by the coordinate transformation method. The simulation results show that: The detection range of the OAM of the three optical path detection model for the vortex beam transmitting 1000 m reaches −37 to +37 in the vacuum environment, and the mode purity of −37 to +37 can reach more than 0.3 when the turbulence intensity is Cn 2 = 2 × 10−15 m−2/3. The detection range of the OAM of the vortex can be further expanded by increasing the optical path. This study provides a new method for the separation and detection of OAM of composite vortex beams.

Ultracompact Metasurface for Simultaneous Detection of Polarization State and Orbital Angular Momentum

AbstractIn the field of optical communications and quantum informatics, polarization and orbital angular momentum (OAM) offer a promising way to expand the dimension of information. However, detecting the polarization state and topological charge of the OAM beam on‐chip way can be challenging. To address this, an ultracompact metasurface is proposed and demonstrated. The metasurface is composed of six polarization‐sensitive metalenses that occupy 60°annular sectors each and are capable of simultaneous polarization state and orbital angular momentum detection. The metasurface focuses the coaxial polarized vortex beam into six locations, and the foci displacement is related to the value of the topological charge. The corresponding polarization state can be obtained through the reconstructed Stokes parameter using the six focus light intensities. For proof‐of‐concept, an ultracompact metasurface operating at a wavelength of 1064 nm is fabricated, which exhibits exceptional polarization and orbital angular momentum recognition ability. The proposed metasurface possesses the merits of ultra‐compact size and simultaneously entire polarization state and orbital angular momentum identification capability, which may encourage the development of on‐chip optical communication systems.

Unveiling detection probability for multi-Gaussian correlated anomalous vortex modes in maritime atmospheric turbulence

In this study, we employ the Rytov approximation to investigate the detection probability of orbital angular momentum (OAM) in multi-Gaussian correlated anomalous vortex (MGCAV) beams under non-Kolmogorov maritime atmospheric turbulence. Our results demonstrate that the OAM detection probability of a MGCAV beam is influenced by various factors, including beam parameters and the characteristics of maritime atmospheric turbulence. Specifically, an increase in propagation distance, beam order, and beam index, or a decrease in inner scale, spatial coherence width, and non-Kolmogorov parameter, leads to a decrease in the OAM detection probability. The phase characteristics of partially coherent vortex modes are affected by both atmospheric turbulence phase and initial random phase, resulting in reduced robustness compared to fully coherent vortex modes. Furthermore, a comparative analysis between Gaussian–Schell correlated anomalous vortex (GSCAV) beams and MGCAV beams reveals the superior resilience of GSCAV beams in mitigating the impact of maritime atmospheric turbulence. Moreover, specific combinations of beam order, topological charge, and beam waist, or the optimal beam width, yield maximum OAM detection probability or minimum scintillation. These findings provide valuable insights applicable to optical communication, particularly in scenarios above sea and ocean levels.

Detection of orbital angular momentum carried high-order radial vortex beams using CNN-OAM mode classifier

Vortex beams provide an innovative multiplexing mechanism for the field of optical communication due to its specific orthogonality. The precise and efficient detection of orbital angular momentum (OAM) modes carried by vortex beams is an essential technology for OAM multiplexing communication systems. An OAM mode classifier based on convolutional neural network (CNN) is proposed for the mode detection of distorted high-order vortex beams (radial index p > 0). The effect of different atmospheric turbulence (AT) intensities, radial indexes, and propagation distances on the detection accuracy of the OAM modes are investigated. The ability of the classifier models for generalization after training on both specific and mixed AT dataset is analyzed. The results show that the higher-order vortex beams have a greater detection accuracy in AT environments. For the high-order vortex beam carrying OAM modes, the CNN-OAM mode classifier has an average detection accuracy of 97.3% and can detect up to 30 OAM modes under any AT intensities ranging from 1 × 10−16 m−2/3 to 1 × 10−14 m−2/3 at a propagation distance of 1000 m. The great accuracy and extensive detection range of the CNN-OAM mode classifier proposed in this paper can offer a novel approach to the recognition and detection of high-order vortex beams.

Sub‐THz 3D printed lens based on diffractive neural network for low‐cost detection of orbital angular momentum states

AbstractAccurate detection of orbital angular momentum (OAM) is one of the critical techniques for OAM‐related communications. However, conventional OAM identification approaches are often complicated and costly. In this letter, a convenient method based on diffraction neural network (DNN) is presented for OAM states detection in the sub‐terahertz (sub‐THz) regime. The optimal phase distribution of the lens for OAM state detection is obtained from DNN training. Then, a prototype based on three‐dimensional printing is fabricated according to the training results. Numerical simulation and experiment are performed at 140 GHz to verify the proposed approach. The results show that the proposed method can accurately detect the OAM state with lower complexity. This convenient detection approach can provide a novel pathway for realizing low‐cost THz OAM receiving systems, promoting the development of future communication technology.

Orbital angular momentum detection device for vortex microwave photons

Orbital angular momentum (OAM), which was first discovered in the optical field, represents a new dimension of electromagnetic waves. However, the detection of OAM microwave photons, i.e., vortex microwave photons, at room temperature is difficult due to their low energy. Here we report a prototype of a vortex microwave photon detection device based on vortex electrons. Our OAM detection device efficiently distinguishes the intrinsic OAM in the microwave band, which is helpful for exploring new physical dimensions. In addition, the detection device can be enhanced with a vortex electron sorting device designed with electron holograms so that OAM microwave photon demultiplexing can be achieved. Finally, the OAM detection device has high practicability; i.e., not only it can be used at room temperature, but also it is much smaller than a particle accelerator system. To illustrate the significance of this method, we demonstrate an on-off keying transmission system based on our OAM detection device.

Vortex X-wave propagation through von Kármán oceanic turbulence with anisotropy.

Vortex X-waves with coupling effects of orbital angular momentum (OAM) and spatiotemporal invariance are introduced into the research of underwater wireless optical communication systems (UWOCSs). We establish the OAM probability density of vortex X-waves and channel capacity of UWOCS using Rytov approximation and correlation function. Furthermore, an in-depth analysis of OAM detection probability and channel capacity is performed on vortex X-waves carrying OAM in von Kármán oceanic turbulence with anisotropy. The results show that an increase in OAM quantum number results in a "hollow X" shape in the received plane, where the energy of vortex X-waves is injected into the lobes, reducing the received probability of the vortex X-waves transmitted to the receiving end. As the Bessel cone angle increases, the energy gradually concentrates toward the energy distribution center, and the vortex X-waves become more localized. Our research may trigger the development of UWOCS for bulk data transfer based on OAM encoding.

Mode Recognition of Orbital Angular Momentum Based on Attention Pyramid Convolutional Neural Network

In an effort to address the problem of the insufficient accuracy of existing orbital angular momentum (OAM) detection systems for vortex optical communication, an OAM mode detection technology based on an attention pyramid convolution neural network (AP-CNN) is proposed. By introducing fine-grained image classification, the low-level detailed features of the similar light intensity distribution of vortex beam superposition and plane wave interferograms are fully utilized. Using ResNet18 as the backbone of AP-CNN, a dual path structure with an attention pyramid is adopted to detect subtle differences in the light intensity in images. Under different turbulence intensities and transmission distances, the detection accuracy and system bit error rate of basic CNN with three convolution layers and two full connection layers, i.e., ResNet18 and ResNet18, with a specified mapping relationship and AP-CNN, are numerically analyzed. Compared to ResNet18, AP-CNN achieves up to a 7% improvement of accuracy and a 3% reduction of incorrect mode identification in the confusion matrix of superimposed vortex modes. The accuracy of single OAM mode detection based on AP-CNN can be effectively improved by 5.5% compared with ResNet18 at a transmission distance of 2 km in strong atmospheric turbulence. The proposed OAM detection scheme may find important applications in optical communications and remote sensing.

Orbital angular momentum optical communications enhanced by artificial intelligence

Angular momentum of light can be divided into spin angular momentum and orbital angular momentum (OAM). Due to the theoretically unlimited orthogonal states, the physical dimension of OAM provides a potential solution to boost the information capacity. The OAM multiplexing and modulation techniques have been implemented to meet the continuous growth of bandwidth requirements, resulting in the concept of OAM optical communication. However, the performances of the traditional optical OAM detection techniques degrade seriously in the practical application of OAM optical communications. Thanks to the powerful data analysis advantages, the cutting-edge machine learning (ML) algorithms have been widely used in the field of image processing, laying the technical foundation for OAM recognition. This paper reviews the recent advances on OAM optical communications that are enhanced by ML methods. More than the traditional OAM detection methods, the OAM demodulation methods based on multiple network architectures, including the support vector machine, self-organizing map, feed-forward neural network, convolutional neural network, and diffractive deep optical neural network (D2NN), have been summarized. We also discuss the development of the spiking neural network and on-chip D2NN, opening a possible way to facilitate the future ultra-low power and ultra-fast OAM demodulation technology.

Sensing optical phase distortion via beatnote detection of a dual probe beam encoded with orbital angular momentum.

Laser based optical applications such as imaging, ranging, and wireless communications are susceptible to environmental distortions. Inferring the strength of these optical distortions is crucial to obtaining information about the environment in which the system is operating. Our technique of inferring environmental distortion strength leverages the spreading of light's orbital angular momentum (OAM) spectrum combined with heterodyne detection. A laser encoded with OAM can be decomposed into a basis set of helical modes that spreads upon interaction with optical distortions. This mode spreading is quantified using the OAM spectrum that can be measured using mode projection or mode sorting techniques. This new technique, to the best of our knowledge, provides benefits compared to the latter two OAM detection methods such as: low-frequency noise rejection, a simpler optical receiver, lower noise floor, and an inherent optical phase component. Central to the method is the heterodyne detection of the zeroth-order OAM coefficient of a superimposed two-beam, two-frequency, probe. The measured heterodyne signal power is seen to be proportional to the coupling power of each beam's OAM spectra. To test the idea, wave-optic simulations and experiments using spatial light modulators are implemented using a simplified optical turbulence model to represent the environment. The experimental implementation agrees well with simulated and theoretical results.

Direct Light Orbital Angular Momentum Detection in Mid-Infrared Based on the Type-II Weyl Semimetal TaIrTe4.

The direct photocurrent detection capability of light orbital angular momentum (OAM) has recently been realized with topological Weyl semimetals, but it is limited to the near-infrared wavelength range. The extension of the direct OAM detection capability to the mid-infrared band, which is a wave band that plays an important role in a vast range of applications, has not yet been realized. This is because the photocurrent responses of most photodetectors are neither sensitive to the phase information nor efficient in the mid-infrared region. In this study, a photodetector based on the type-II Weyl semimetal tantalum iridium telluride (TaIrTe4 ) is designed with peculiar electrode geometries to directly detect the topological charge of the OAM using the orbital photogalvanic effect (OPGE). The results indicate that the helical phase gradient of light can be distinguished by a current winding around the optical beam axis, with a magnitude proportional to its quantized OAM mode number. The topologically enhanced responses in the mid-infrared region of TaIrTe4 further help overcome the low responsivity issues and finally render direct OAM detection capability. This study enables on-chip-integrated OAM detection, and thus OAM-sensitive focal plane arrays in the mid-infrared region.

Detection of the orbital angular momentum state of light using sinusoidally shaped phase grating

We demonstrate a method based on a unique sinusoidally shaped phased grating for efficient and nearly alignment free detection of both signs and modulus of the orbital angular momentum (OAM) of light. OAM detection efficiency is almost the same over the whole grating area. The capability and robustness of this method are demonstrated by detection of optical vortices with OAM topological charge up to 150 using a reflective phase-only liquid crystal on a silicon spatial light modulator.

Study on the propagation characteristics of elliptical Airy vortex beam

The propagation characteristics of elliptical Airy vortex beams (EAVB) with different elliptical parameter and topological charges are demonstrated experimentally and numerically. Besides, the relationship between elliptical parameter and detection probability of the orbital angular momentum(OAM) is analyzed numerically. The results show that elliptical parameter has a significant impact on the OAM spectral distribution and the propagation characteristics of the EAVB. The EAVB possess an elliptical shaped transverse pattern in the main lobe surrounded by a series of rings and has the line of intensity nulls in the focal plane. It also has two focal planes during the propagation, and the distance between the two focal plane becomes shorter with the increase of elliptical parameter. The long axis of the intensity ellipse of the EVAB rotates 90°between the second focal plane and the first focal plane. There is a linear relationship between OAM detection probability and the elliptical parameter. The experimental results are in good agreement with theory, which will be helpful for the application of EAVB in optical trapping and optical communication.

Simultaneous Detection of Spin and Orbital Angular Momentum of Light through Scattering from a Single Silver Nanowire

AbstractIn recent times the spin angular momentum (SAM) and orbital angular momentum (OAM) of light have gained prominence because of their significance in optical communication systems, micromanipulation, and sub‐wavelength position sensing. To this end, simultaneous detection of SAM and OAM of light beam is one of the important topics of research from both application and fundamental spin‐orbit interaction (SOI) point of view. While interferometry and metasurface based approaches have been able to detect the states, the presented approach involves elastic scattering from a monocrystalline silver nanowire for the simultaneous detection of SAM and OAM state of a circularly polarized Laguerre–Gaussian beam. By employing Fourier plane (FP) microscopy, the transmitted scattered light intensity distribution in the FP is analyzed to reconstruct the SAM and OAM state unambiguously. The SAM and OAM induced transverse energy flow as well as the polarization dependent scattering characteristics of the nanowire is investigated to understand the detection mechanism. This method is devoid of complex nanofabrication techniques and to the authors' knowledge, is a first example of single nano‐object based simultaneous SAM and OAM detection. The study will further the understanding of SOI effects and can be useful for on‐chip optical detection and manipulation.

Measuring high-order multiple vortex beams with fork-shaped grating

Orbital angular momentum (OAM) is a degree of freedom independent of polarization and frequency of vortex beams. Therefore, the measurement of OAM has attracted great interest. In this paper, we propose a method to detect OAM beams utilizing far-field diffraction feature fringes caused by fork-shaped grating (FSG), which has characteristics of wide-range, high efficiency, and excellent extensibility for detection of high-order OAM beam. We give a detailed explanation of the concept and feature of FSG detection theoretically, and improve detection reliability of fringes by increasing the number of forks and setting appropriate offset. Experimentally, we achieve detection of OAM+50. Moreover, although only two OAMs were detected at the same time in our experiment, FSG can also be used in the detection of multi-OAM beams due to the excellent extensibility of this method. Our finding opens up a new method for the detection of high-order OAM and multi-OAM beams, which is of great significance to the application of FSG.

Nanophotonic Materials for Twisted-Light Manipulation.

Twisted light, an unbounded set of helical spatial modes carrying orbital angular momentum (OAM), offers not only fundamental new insights into structured light-matter interactions, but also a new degree of freedom to boost optical and quantum information capacity. However, current OAM experiments still rely on bulky, expensive, and slow-response diffractive or refractive optical elements, hindering today's OAM systems to be largely deployed. In the last decade, nanophotonics has transformed the photonic design and unveiled a diverse range of compact and multifunctional nanophotonic devices harnessing the generation and detection of OAM modes. Recent metasurface devices developed for OAM generation in both real and momentum space, presenting design principle and exemplary devices, are summarized. Moreover, recent development of whispering-gallery-mode-based passive and tunable microcavities, capable of extracting degenerate OAM modes for on-chip vortex emission and lasing, is summarized. In addition, the design principle of different plasmonic devices and photodetectors recently developed for on-chip OAM detection is discussed. Current challenges faced by the nanophotonic field for twisted-light manipulation and future advances to meet these challenges are further discussed. It is believed that twisted-light manipulation in nanophotonics will continue to make significant impact on future development of ultracompact, ultrahigh-capacity, and ultrahigh-speed OAM systems-on-a-chip.

Vortex beam: generation and detection of orbital angular momentum [Invited

Recent advances in the research of vortex beams, structured beams carrying orbital angular momentum (OAM), have revolutionized the applications of light beams, such as advanced optical manipulations, high-capacity optical communications, and super-resolution imaging. Undoubtedly, the methods for generation of a vortex beam and detection of its OAM are of vital importance for the applications of vortex beams. In this review, we first introduce the fundamental concepts of vortex beams briefly and then summarize approaches to generating and detecting the vortex beams separately, from bulky diffractive elements to planar elements. Finally, we make a concise conclusion and outline that is yet to be explored.

Spiral Spectrum of a Laguerre-Gaussian Beam Propagating in Anisotropic Turbulent Plasma

The explicit expression of orbital angular momentum (OAM) spiral spectrum of Laguerre-Gaussian (LG) beam propagating in anisotropic plasma turbulence is derived based on Rytov theory. The relationship between the detection probability of OAM and the crosstalk with the anisotropy parameters, topological charge, radial index, wavelength, beam width, receiving aperture, inner and outer scales are discussed in detail. In order to quantify the effect of plasma turbulence, we compared it with atmospheric turbulence based on restriction function. To optimize OAM detection probability, we also compared the effects of circular aperture method (CAM) with focusing mirror method (FMM) on the propagation characteristics of the spiral spectrum. The results show that even in the strong turbulence region of atmospheric turbulence, the radial dimension of LG beam in plasma turbulence is nearly twice that of atmospheric turbulence and the modes crosstalk has almost reached its upper bound. Besides, by comparing the two optimization methods, we found the ability to reduce crosstalk is related to the choice of beam width and receiving aperture. Therefore, we give a method to optimally select between the CAM and the FMM. We found the corresponding relationship and gave an analytical formula. The FMM has a greater ability to reduce crosstalk than the CAM when the beam width ≥4 cm and the aperture ≥3 cm. We quantified the effect of plasma turbulence and provide a guideline for the selection of beam width and receiving aperture under these two optimization methods. The research results may be helpful in the field of optical communication.

Broadband decoupled spin and orbital angular momentum detection via programming dual-twist reactive mesogens.

The introduction of spin and orbital angular momentum mode division multiplexing to existing wavelength division multiplexing will significantly enlarge the capacity of optical networks. Therefore, components compatible with the above techniques are in high demand. Here, a geometric phase combined a Dammann vortex grating, and a polarization grating is designed and encoded to a dual-twist reactive mesogen. It can generate a couple of vortex channel arrays highly efficiently in broadband. Meanwhile, orthogonal spins are spatially separated, facilitating spin identification. A vortex will recover to a Gaussian beam when it is diffracted to an order with opposite topological charge, which enables the detection of orbital angular momentum. It supplies a parallel and efficient way for decoupled spin and orbital angular momentum detection operating at the entire visible range, and the design may be extended to many other compatible optical communication components.