Orbital Angular Momentum Wave Generation and Multiplexing: Experiments and Analysis Using Classical and Quantum Optics

A circular-polarized orbital angular momentum (OAM) generator and multiplexer based on silicon on insulator is designed and simulated. This device can generate right hand and left hand circular polarized channels that can be coupled to OAM fiber without losses associated with filtering out unwanted polarization.

A 3D-printed sub-terahertz lens for manipulating quasi-non-diffractive orbital angular momentum (OAM) waves is proposed in this paper. The proposed lens comprises all-dielectric elements that impose desired phase profiles on the incident waves by varying their height. Based on the concept of the Bessel beam launchers and the beam deflectors, quasi-non-diffractive OAM waves with a pre-defined deflection angle can be produced by the proposed lens. For verification, two lenses were 3D printed for producing OAM waves with a non-diffractive depth of 55.58λ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> (λ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> is the free-space wavelength at 140 GHz). Measured by a terahertz imaging camera, the desired quasi-non-diffractive OAM waves were observed behind the proposed lenses with expected non-diffractive performance and a large deflection angle of up to 47°.

The orbital angular momentum (OAM) wave has shown great potential for improving radar imaging and underwater communication performance due to its helical wavefront phase and infinite orthogonal modes. However, there are currently no known applications of this technology in underwater imaging. In this paper, we employed acoustic OAM wave for underwater imaging and established transceiver signal models using the uniform circular array. We concurrently achieved two-dimensional imaging of azimuth and elevation angles, which differs from radar imaging. We proposed a matching process for the echo signal in the modal domain, the OAM wave beam image's sidelobe decreased by 7.9 dB in the elevation direction and 6.1 dB in the azimuth direction compared to the plane wave, with the mainlobe decreased by 0.2° in the elevation direction and 0.4° in the azimuth direction. Furthermore, this paper introduced OAM wave high-resolution image reconstruction based on the orthogonal matching pursuit (OMP) algorithm. Finally, we implemented broadband acoustic OAM wave for underwater imaging and introduced an image reconstruction method based on the modal domain OMP algorithm. Simulation results demonstrate that the use of OAM wave in underwater imaging is feasible, and the proposed scheme can achieve high-resolution imaging.

Transmissive programmable metasurface can offer notable benefits for the precise agile manipulation of electromagnetic (EM) waves, which is a promising candidate for radar detection, wireless communications, and spectral imaging. However, most previous studies typically face the challenge of narrow bandwidth and are constrained by relatively simple and single functionality. Here, an ultra‐wideband (UWB) transmissive programmable metasurface is proposed to conduct agile multifunctional EM modulation. The meta‐atom is designed with a butterfly‐shaped configuration and manipulated using two pin diodes, demonstrating high‐efficiency transmission with low insertion loss less than 1 dB and stable phase difference of 180° from 15.77 to 28.59 GHz, ranging from Ku to Ka‐band with a fractional bandwidth up to 57.7%. The transmissive programmable metasurface is further constructed, and its effectiveness for multifunctional EM modulation is verified both numerically and experimentally, which can conduct different functionalities, including near‐field holographic imaging at lower‐frequency Ku‐band and far‐field high‐purity orbital angular momentum (OAM) generation at higher‐frequency Ka‐band. The proposed strategy may enrich the functionalities of programmable metasurfaces, as well as open a new pathway toward UWB multifunctional applications.

In the realm of free-space optical (FSO) communication systems, the utilization of orbital angular momentum (OAM) multiplexing has garnered significant attention. However, OAM beams are subject to atmospheric turbulence (AT) during transmission in free space, resulting in distortions of the phase front, intermodal crosstalk, and mode-dependent loss (MDL). While techniques such as adaptive optics and digital signal processing can mitigate these effects, inconsistencies in OAM mode performance persist due to MDL. Some studies have evaluated an MDL-impaired mode division multiplexing system for non-unitary multiple input multiple output (MIMO) channels based on maximum likelihood detection. Nevertheless, the computational complexity remains considerably higher compared to simpler yet suboptimal advanced MIMO decoders, such as successive interference cancellation (SIC) with zero-forcing/minimum mean square error (ZF/MMSE). On the other hand, an OAM mode and radial index are two distinct indices of Laguerre-Gaussian (LG) modes, with the potential to carry a large amount of information. To the best of our knowledge, no prior studies have explored the impact and mitigation strategies for MDL induced by atmospheric turbulence in an FSO system utilizing both OAM mode and radial index multiplexing with MMSE-SIC receiver. This paper introduces OAM multiplexing holography to generate OAM modes. We perform both analytical and numerical analyses of the orbital angular momentum spectra associated with distorted single-mode and multiplexed LG beams. This is achieved by generating holograms, incorporating random phase screens along the propagation path, and varying the refractive index structure constants. We mathematically develop and simulate an MMSE-SIC receiver for OAM multiplexed FSO for different radial values for the first time and compared its BER to Zero Forcing-SIC and K-best detectors. Additionally, orthogonal space-time block coding is employed at the transmitter to enhance the overall performance of the system.

Orbital angular momentum (OAM) has gained great interest due to its most attractive feature of high dimensionality, and several ground-breaking demonstrations in communication based on OAM multiplexing have been carried out. Accordingly, a rapid data-density growth from OAM multiplexing has posed a great challenge to the signal-processing layer. Meanwhile, in another area, optical signal-processing circuit based on photon-phonon conversion has received considerable attention and made rapid progress. Here, with an aim of finding the intersection between OAM multiplexing and photon-phonon conversion, we report on the observation of reversible OAM photon-phonon conversion. A specific OAM state can be flexibly and controllably interconverted between photonic and phononic domains via Brillouin photon-phonon coupling within the decay time of acoustic signal, in which OAM and spin angular momentum are independently conserved. Our result demonstrates the controllable OAM transfer between photons and phonons, shows the potential of using OAM multiplexing to extend the capacity of photon-phonon conversion based signal-processing scheme, and may trigger the development of OAM-multiplexed photon-phonon circuit.

SummaryUtilization of planar printed uniform circular antenna arrays (UCA) to generate the orbital angular momentum (OAM) carrying waves in the millimeter‐wave (mmWave) frequency band is advantageous from the viewpoints of easy signal modulation and mode reconfiguration, low cost, low profile, and straightforward integration with the existing broadband wireless infrastructure. The OAM mmWave UCA are highly promising as the enablers of very high transmission data rates required by hybrid 5G/6G optical and wireless communication systems by complementing and enhancing other technologies currently in use. Therefore, we here contribute a detailed electromagnetic analysis of important constraints of such antenna arrangements aimed at short‐range multimode OAM wave transmission. We investigate (i) the required antenna array dimensions and optimized UCA arrangements for a particular link range and (ii) the corresponding mode structure of OAM waves in the plane of receiving arrays. Four relatively simple antenna configurations operating in the 60‐GHz band are compared. Theoretical assumptions based on ideal OAM modes are critically assessed and, using state‐of‐the‐art numerical electromagnetic analysis, compared to realistically generated OAM waves. The proposed “cyclic transmission setup” resulted in much lower unwanted field components in the region of receiving arrays. RMS magnitudes of unwanted modes are on average about 64% of the received mode, in comparison with 80% (up to 94%) for sequential transmission. The observed mode impurities and mode mixing effects at the receiver indicate the need to dedicate more attention to the system‐level design, the development of efficient receiving arrays, the MIMO processing, and the stream separation.

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

The generation of rapidly tunable optical vortex (OV) beams is one of the most demanding research areas of the present era as they possess orbital angular momentum (OAM) with additional degrees of freedom that can be exploited to enhance signal‐carrying capacity by using mode division multiplexing and information encoding in optical communication. Particularly, rapidly tunable OAM devices at a fixed wavelength in the telecom band stir extensive interest among researchers for both classical and quantum applications. This article demonstrates the realistic design of a Si‐integrated photonic device for rapidly tunable OAM wave generation at a 1550‐nm wavelength by using an ultra‐low‐loss phase change material (PCM) embedded with a Si‐ring resonator with angular gratings. Different OAM modes are achieved by tuning the effective refractive index using rapid electrical switching of Sb2Se3 film from amorphous to crystalline states and vice versa. The generation of OAM waves relies on a traveling wave modulation of the refractive index of the micro‐ring, which breaks the degeneracy of oppositely oriented whispering gallery modes. The proposed device is capable of producing rapidly tunable OV beams, carrying different OAM modes by using electrically controllable switching of ultra‐low‐loss PCM Sb2Se3.

Optical beams carrying orbital angular momentum (OAM) play an important role in micro-/nanoparticle manipulation and information multiplexing in optical communications. Conventional OAM generation setups require bulky optical elements and are unsuitable for on-chip integration. OAM generators based on metasurfaces can achieve ultracompact designs. However, they generally have limited working bandwidth and require complex designs and multistep time-consuming fabrication processes. In comparison, graphene metalenses based on the diffraction principle have simple designs and can be fabricated by laser nanoprinting in a single step. Here, we demonstrate that a single ultrathin (200 nm) graphene OAM metalens can integrate OAM generation and high-resolution focusing functions in a broad bandwidth, covering the entire visible wavelength region. Broadband graphene OAM metalenses with flexibly controlled topological charges are analytically designed using the detour phase method considering the dispersionless feature of the graphene material and fabricated using ultrafast laser nanoprinting. The experimental results agree well with the theoretical predictions, which demonstrate the accuracy of the design method. The broadband graphene OAM metalenses can find broad applications in miniaturized and integrated photonic devices enabled by OAM beams.

EM waves with finite orbital angular momentum (OAM) have shown potential for a dramatic increment in the RF channel capacities. Generation of OAM waves using microstrip antennas makes the most economical configuration and thus can further boost the research toward realization of RF OAM channels. So far, all such microstrip configurations that generate OAM waves require more than one feed, especially for modes higher than the fundamental. This article gives a new and simple OAM antenna design method for radiating OAM waves of desired topological charge using a direct single point feed connected to a circular microstrip antenna (CMA) for corresponding mode. The presented method manipulates cylindrical harmonics using tiny modifications in the CMA. Subsequently, a generalized equivalent circuit model has been given for the same. Additionally, the presented method has also been qualitatively extended to annular ring microstrip antennas (ARMAs). Simulated and measured results have been presented to corroborate the proposed concept.

In order to extend the applications of vortex waves, we propose a water-immersed rectangular horn antenna array for generating underwater vortex waves carrying the orbital angular momentum (OAM). Firstly, a single dielectric-loaded rectangular horn antenna with the central frequency of 2.6 GHz was designed for generating underwater electromagnetic (EM) waves. Due to the supplementing dielectric-loaded waveguide in this single antenna, the problems with difficult sealing and fixation of the feed probe could be solved effectively. The simulation results show that it has a good impedance characteristics (S11 &lt; −10 dB) and reasonable losses (less than 3.5 dB total for two antennas and a coaxial line) from 2.5 GHz to 2.7 GHz. Experiments on the single antenna were also carried out, which agree well with the simulations. Based on the designed single antenna, the water-immersed rectangular horn antenna array was proposed, and the phase gradient from 0~2π was fed to the horn antennas for generating underwater OAM waves. The simulation results demonstrate high fidelity of the generated OAM waves from the intensity and phase distributions. The purity of the generated OAM modes was also investigated and further verifies the high fidelity of the generated OAM waves. The generated high-quality OAM waves meet the requirements for underwater applications of OAM, such as underwater communication and underwater imaging.

We designed and fabricated a silicon photonic orbital angular momentum (OAM) multiplexer (MUX) chip that enables simultaneous wavelength-division multiplexing (WDM) and OAM mode-division multiplexing (MDM) in optical fiber communication systems. We successfully utilized the fabricated MUX chip to demonstrate simultaneous 3 OAM MDM transmission as well as OAM MDM/WDM transmission with an 800-m OAM ring-core fiber. In both cases, 10 Gbit/s OOK signals were utilized and BERs below the 7% FEC limit were obtained.

We propose a helically twisted hollow-core photonic bandgap fiber (PBGF) for orbital angular momentum (OAM) generation. Results demonstrate that this fiber can effectively separate and transmit higher-order OAM modes while reducing crosstalk. The mode effective refractive index curves as a function of wavelength and the transmission characteristics of OAM are investigated in this letter. We show that this fiber has the potential to be used in OAM generators, light-controlling devices, and all-fiber optical communication applications.

Beams carrying orbital angular momentum possess a significant potential for modern optical technologies ranging from classical and quantum communication to optical manipulation. In this paper, we theoretically design and experimentally demonstrate an ultracompact array of elliptical nanoholes, which could convert the circularly polarized light into the cross-polarized vortex beam. To measure the topological charges of orbital angular momentum in a simple manner, another elliptical nanoholes array is designed to generate reference beam as a reference light. This approach may provide a new way for the generation and detection of orbital angular momentum in a compact device.