Adaptive power-controllable orbital angular momentum (OAM) multicasting

On-axis structured beams generation via moiré and Mie resonant metallo-dielectric moiré gratings

Structured beams carrying orbital angular momentum have been generated conventionally using spiral phase plates, fork gratings, and metasurfaces. Spiral phase plates are non-planar, fork gratings do not produce structured beams on the axis, and metasurfaces need subwavelength unit cell level design. In this work, we show a method to generate on-axis structured beams, at the zeroth order of a diffraction grating with experimentally relevant efficiency using moiré patterned binary gratings that are compatible with planar fabrication, do not need subwavelength unit cell level design, and can be used with a spatial light modulator. By logically superposing two binary forked gratings, we create a moiré pattern that enables on-axis structured beam generation at the zeroth order of the diffraction grating. We demonstrate, using experiments and simulations, the generation of on-axis zeroth order structured beams using spatial light modulator based reconfigurable moiré gratings and Mie resonant metallo-dielectric standalone moiré gratings, showcasing the versatility of this approach in different configurations. Simulations and experiments demonstrate that the on-axis structured beam is generated by the moiré pattern within the gratings, and its shape is determined by the topological charges of the overlapping binary forked gratings. Additionally, we demonstrate color-selective on-axis structured beam generation at the zeroth order of the grating, where the color-selectivity of the on-axis structured beam depends on the grating period and arises due to Mie resonance in standalone nanofabricated metallo-dielectric moiré gratings. The on-axis structured beam generation at the zeroth order of the grating using the proposed method may have several applications, including sensing and optical trapping.

Hybrid strategy in compact tailoring of multiple degrees-of-freedom toward high-dimensional photonics

Tailoring multiple degrees-of-freedom (DoFs) to achieve high-dimensional laser field is crucial for advancing optical technologies. While recent advancements have demonstrated the ability to manipulate a limited number of DoFs, most existing methods rely on bulky optical components or intricate systems that employ time-consuming iterative methods and, most critically, the on-demand tailoring of multiple DoFs simultaneously through a compact, single element-remains underexplored. In this study, we propose an intelligent hybrid strategy that enables the simultaneous and customizable manipulation of six DoFs: wave vector, initial phase, spatial mode, amplitude, orbital angular momentum (OAM) and spin angular momentum (SAM). Our approach advances in phase-only property, which facilitates tailoring strategy experimentally demonstrated on a compact metasurface. A fabricated sample is tailored to realize arbitrary manipulation across six DoFs, constructing a 288-dimensional space. Notably, since the OAM eigenstates constitute an infinite dimensional Hilbert space, this proposal can be further extended to even higher dimensions. Proof-of-principle experiments confirm the effectiveness in manipulation capability and dimensionality. We envision that this powerful tailoring ability offers immense potential for multifunctional photonic devices across both classical and quantum scenarios and such compactness extending the dimensional capabilities for integration on-chip requirements.

Customizing Multicolored Orbital Angular Momentum Combs

Current orbital angular momentum (OAM) combs generating technology is hindered by bulky optical systems, limited control, and lack of multicolored information, impeding system integration and practical applications. We present a metasurface approach to realizing multicolored OAM comb engineering along the light propagation direction. The OAM combs are measured based on the intensity of bright spots in the generated intensity patterns that correspond to the weights of the OAM modes. Three OAM combs with different colors are generated at different observation planes. The positioning of transition points along the azimuthal direction is the key to shaping the OAM distribution of the generated beams. OAM combs with customized mode spacings and broad OAM spectra are obtained. Our approach provides a compact platform to realize OAM combs with multidimensional information in the domains of the OAM spectra, frequency, and space, which can significantly enhance the information capacity for potential applications in optical communications.

Research Progress on Router Devices for the OAM Optical Communication

Vortex beams carrying orbital angular momentum (OAM) provide a new degree of freedom for light waves in addition to the traditional degrees of freedom, such as intensity, phase, frequency, time, and polarization. Due to the theoretically unlimited orthogonal states, the physical dimension of OAM is capable of addressing the problem of low information capacity. With the advancement of the OAM optical communication technology, OAM router devices (OAM-RDs) have played a key role in significantly improving the flexibility and practicability of communication systems. In this review, major breakthroughs in the OAM-RDs are summarized, and the latest technological standing is examined. Additionally, a detailed account of the recent works published on techniques related to the OAM-RDs has been categorized into five areas: channel multicasting, channel switching, channel filtering, channel hopping, and channel adding/extracting. Meanwhile, the principles, research methods, advantages, and disadvantages are discussed and summarized in depth while analyzing the future development trends and prospects of the OAM-RDs.

Orbital angular momentum and beyond in free-space optical communications

Orbital angular momentum (OAM), which describes tailoring the spatial physical dimension of light waves into a helical phase structure, has given rise to many applications in optical manipulation, microscopy, imaging, metrology, sensing, quantum science, and optical communications. Light beams carrying OAM feature two distinct characteristics, i.e., inherent orthogonality and unbounded states in principle, which are suitable for capacity scaling of optical communications. In this paper, we give an overview of OAM and beyond in free-space optical communications. The fundamentals of OAM, concept of optical communications using OAM, OAM modulation (OAM modulation based on spatial light modulator, high-speed OAM modulation, spatial array modulation), OAM multiplexing (spectrally efficient, high capacity, long distance), OAM multicasting (adaptive multicasting, N-dimensional multicasting), OAM communications in turbulence (adaptive optics, digital signal processing, auto-alignment system), structured light communications beyond OAM (Bessel beams, Airy beams, vector beams), diverse and robust communications using OAM and beyond (multiple scenes, turbulence-resilient communications, intelligent communications) are comprehensively reviewed. The prospects and challenges of optical communications using OAM and beyond are also discussed at the end. In the future, there will be more opportunities in exploiting extensive advanced applications from OAM beams to more general structured light.

Modal properties of a beam carrying OAM generated by a circular array of multiple ring-resonator emitters

We investigate the modal properties of a beam carrying orbital angular momentum (OAM) generated by a circular array (ring) of multiple micro-ring emitters (rings) analytically and via simulation. In such a "ring-of-rings" structure, N emitters generate N optical vortex beams with the same OAM-order l₀ at the same wavelength. The output beam is a coherent combination of the N vortex beams located at different azimuthal positions, having the same radial displacement. We derive an analytical expression for the output optical field and calculate the OAM-order power spectrum of the generated beam. The analytical expression and simulation results show that (1) the OAM spectrum of the output beam composes equidistant OAM spectral components, symmetrically surrounding l₀ with a spacing equal to N; (2) the envelope of the OAM spectrum broadens with an increased radius of the circular array or the value of l₀; and (3) the OAM components of the generated beam could be tuned either by changing the value of l₀, corresponding to different spectrum envelopes, or by adding different linear phase delays to the micro-ring emitters, which does not affect the envelope of the OAM spectrum.

Polarization controllable generation of flat superimposed OAM states based on metasurface

In this paper, two kind of metasurface-based flat orbital angular momentum (OAM) superposition-state generators are proposed, which can generate OAM superposition states possessing tens of OAM modes being evenly spaced by topological charge number 1. The power spectra of the generated OAM superposition states are flat. The power variation of the OAM modes of the OAM superposition states from one generator is less than 3 dB, and the power variation of the OAM modes of the OAM superposition states from the other one is less than 0.3 dB. By controlling the left-handed and right-handed circular polarization states of the incident light, the OAM spectra of the OAM superposition states generated in the two polarization cases are separated from each other, therefore, the proposed generators are light polarization controllable. In addition, the two generators can operate efficiently on a wide wavelength region ranging from 635nm to 730nm. Our work may have some potential applications, such as used for OAM multicasting, OAM based optical manipulation, or manufacturing integrated OAM-superposition-state generators and OAM modulation devices.

Proposed phase plate for superimposed orbital angular momentum state generation

In this letter, a quadratic phase plate (QPP) whose thickness increases quadratically with azimuthal angle φ is proposed. When a beam with initial topological charge m (m could be an arbitrary integer) is passed through the plate, quadratic phase modulation can be expected to expand the initial single orbital angular momentum (OAM) mode to a superimposed OAM state. The obtained multi-OAM state exhibits a comb-like OAM spectrum, which shows a flat region. The power variation of the OAM modes within the flat region is less than 3 dB, while the power for the OAM modes with charge numbers out of the flat region decays rapidly. Moreover, the number of OAM modes within the flat region can reach dozens. This may have potential applications in the field of communication, quantum information and optical manipulation, etc.

Orbital angular momentum (OAM) conversion and multicasting using N-core supermode fiber

We propose and numerically demonstrate a conversion and multicasting scheme of orbital angular momentum (OAM) states by using N-core supermode fiber (NCSF), where the topological charges of converted OAM states mainly depend on the injected OAM state and the number of fiber cores. The conversion efficiency (CE) of the converted OAM states could be optimized by properly designing the fiber structure. Take N = 6 as an example, ~37% CE could be achieved at telecom bands. Moreover, even for a fabricated NCSF, the CE could be dynamically changed by stretching the fiber or by adjusting the refractive index of the fiber cores through external control of the environmental conditions. Meanwhile, OAM multicasting could also be realized in the designed NCSF. The crosstalk between the multicasted OAM channels and their neighboring ones are assessed to be less than -30 dB. The proposed fiber-based OAM conversion and multicasting system is compatible with the existing optical fiber communication systems, showing potential applications in the future.

Orbital angular momentum complex spectrum analyzer for vortex light based on the rotational Doppler effect

The ability to measure the orbital angular momentum (OAM) distribution of vortex light is essential for OAM applications. Although there have been many studies on the measurement of OAM modes, it is difficult to quantitatively and instantaneously measure the power distribution among different OAM modes, let alone measure the phase distribution among them. In this work, we propose an OAM complex spectrum analyzer that enables simultaneous measurements of the power and phase distributions of OAM modes by employing the rotational Doppler effect. The original OAM mode distribution is mapped to an electrical spectrum of beat signals using a photodetector. The power and phase distributions of superimposed OAM beams are successfully retrieved by analyzing the electrical spectrum. We also extend the measurement technique to other spatial modes, such as linear polarization modes. These results represent a new landmark in spatial mode analysis and show great potential for applications in OAM-based systems and optical communication systems with mode-division multiplexing.

Compensation of a distorted N-fold orbital angular momentum multicasting link using adaptive optics

By using an adaptive feedback correction technique, we experimentally demonstrate turbulence compensation for free-space four-fold and eight-fold 16-ary quadrature amplitude modulation (16-QAM) carrying orbital angular momentum (OAM) multicasting links. The performance of multicasted OAM beams through emulated atmospheric turbulence and adaptive optics assisted compensation loop is investigated. The experimental results show that the scheme can efficiently compensate for the atmospheric turbulence induced distortions, i.e., reducing power fluctuation of multicasted OAM channels, suppressing inter-channel crosstalk, and improving the bit-error rate (BER) performance.

Polarization-insensitive PAM-4-carrying free-space orbital angular momentum (OAM) communications

We present a simple configuration incorporating single polarization-sensitive phase-only liquid crystal spatial light modulator (SLM) to facilitate polarization-insensitive free-space optical communications employing orbital angular momentum (OAM) modes. We experimentally demonstrate several polarization-insensitive optical communication subsystems by propagating a single OAM mode, multicasting 4 and 10 OAM modes, and multiplexing 8 OAM modes, respectively. Free-space polarization-insensitive optical communication links using OAM modes that carry four-level pulse-amplitude modulation (PAM-4) signal are demonstrated in the experiment. The observed optical signal-to-noise ratio (OSNR) penalties are less than 1 dB in both polarization-insensitive N-fold OAM modes multicasting and multiple OAM modes multiplexing at a bit-error rate (BER) of 2e-3 (enhanced forward-error correction (EFEC) threshold).

Demonstration of obstruction-free data-carrying N-fold Bessel modes multicasting from a single Gaussian mode

By designing and optimizing complex phase pattern combining with axicon phase distribution, we report data multicasting from a single Gaussian mode to multiple Bessel modes using a single phase-only spatial light modulator. Under the obstructed path conditions, obstruction-free data-carrying N-fold Bessel modes multicasting is demonstrated in the experiment. We also experimentally study N-fold multicasting of a 20 Gbit/s quadrature phase-shift keying signal from a single Gaussian mode to multiple Bessel modes and measure the link performance. All the multicasted Bessel modes show relatively low crosstalk from their neighboring modes and achieve a bit-error rate of less than 1e-3.