699 research outputs found
Orbital Angular Momentum Waves: Generation, Detection and Emerging Applications
Orbital angular momentum (OAM) has aroused a widespread interest in many
fields, especially in telecommunications due to its potential for unleashing
new capacity in the severely congested spectrum of commercial communication
systems. Beams carrying OAM have a helical phase front and a field strength
with a singularity along the axial center, which can be used for information
transmission, imaging and particle manipulation. The number of orthogonal OAM
modes in a single beam is theoretically infinite and each mode is an element of
a complete orthogonal basis that can be employed for multiplexing different
signals, thus greatly improving the spectrum efficiency. In this paper, we
comprehensively summarize and compare the methods for generation and detection
of optical OAM, radio OAM and acoustic OAM. Then, we represent the applications
and technical challenges of OAM in communications, including free-space optical
communications, optical fiber communications, radio communications and acoustic
communications. To complete our survey, we also discuss the state of art of
particle manipulation and target imaging with OAM beams
Novel Insights into Orbital Angular Momentum Beams: From Fundamentals, Devices to Applications
It is well-known by now that the angular momentum carried by elementary particles can be categorized as spin angular momentum (SAM) and orbital angular momentum (OAM). In the early 1900s, Poynting recognized that a particle, such as a photon, can carry SAM, which has only two possible states, i.e., clockwise and anticlockwise circular polarization states. However, only fairly recently, in 1992, Allen et al. discovered that photons with helical phase fronts can carry OAM, which has infinite orthogonal states. In the past two decades, the OAM-carrying beam, due to its unique features, has gained increasing interest from many different research communities, including physics, chemistry, and engineering. Its twisted phase front and intensity distribution have enabled a variety of applications, such as micromanipulation, laser beam machining, nonlinear matter interactions, imaging, sensing, quantum cryptography and classical communications. This book aims to explore novel insights of OAM beams. It focuses on state-of-the-art advances in fundamental theories, devices and applications, as well as future perspectives of OAM beams
Gyro-devices – natural sources of high-power high-order angular momentum millimeter-wave beams
The Orbital Angular Momentum (OAM) carried by light beams with helical phasefront (vortex beams) has been widely employed in many applications such as optical tweezers, optical drives of micro-machines, atom trapping, and optical communication. OAM provides an additional dimension (diversity) to multiplexing techniques, which can be utilized in addition to conventional multiplexing methods to achieve higher data rates in wireless communication. OAM beams have been thoroughly studied and used in the optical regime but in the mm-wave and THz-wave region, they are still under investigation. In these frequency bands, there are difficulties associated with beam-splitting and beam-combining processes as well as with the use of spiral phase plates and other methods for OAM generation, since the wavelength is much larger compared to those at optical frequencies, leading to higher diffraction losses.
The present paper describes the natural generation of high-power OAM modes by gyro-type vacuum electron devices with cylindrical interaction circuit and axial output of the generated rotating higher-order transverse electric mode TEm,n, where m > 1 and n are the azimuthal and radial mode index, respectively. The ratio between the total angular momentum (TAM) JN and total energy WN of N photons is given by m/ω, where ω is the angular frequency of the operating mode, which in a gyrotron oscillator is close to the TEm,n-mode cutoff frequency in the cavity. Therefore, m/ω = Rc/c, where Rc is the caustic radius and c the velocity of light in vacuum. This means that the OAM is proportional to the caustic radius and at a given frequency the same for all modes with the same azimuthal index m. Right-hand rotation (co-rotation with the electrons) corresponds to a positive value of m and left-hand rotation to negative m. The corresponding OAM mode number (topological charge) is l = m – 1. Circularly polarized TE1n modes only possess a Spin Angular Momentum (SAM: s = ±1). TE0n modes have neither SAM nor OAM.
This is the result of the photonic (quasi-optical) approach to derive the TAM of modes generated in gyrotrons. The same result follows from the electromagnetic (EM) wave approach for the TAM within a given waveguide volume per total energy of the EM wave in the same volume.
Such high-power output beams with very pure higher-order OAM, generated by gyrotron oscillators or amplifiers (broadband) could be used for multiplexing in long-range wireless communications. The corresponding mode and helical wavefront sensitive detectors for selective OAM-mode sorting are available and described in the present paper
Integrated circularly polarized OAM generator and multiplexer for fiber transmission
Unlike linearly polarized modes in fiber, modes
exploiting orbital angular momentum (OAM) are circularly
polarized when propagating in fiber. The use of OAM modes for
spatial multiplexing requires efficient, low cost mode generators
and multiplexers. We propose such a device based on the
standard 220-nm silicon-on-insulator platform, taking multiple
single-mode data-modulated signals, and imprinting these signals
on right- and left-circularly polarized OAM channels on a single,
multiplexed output. The device is designed to easily couple to
an OAM fiber with a ring shaped core. This approach treating
circular polarization within the multiplexer allows us to avoid
the losses associated with filtering out unwanted polarization
to create a single polarization. Designing the device to have
an output matched to the OAM fiber mode profile also avoids
mode size conversion. We describe our design methodology
and optimization techniques using a transfer-matrix model and
the finite-difference time-domain method. A candidate design is
simulated and modal crosstalk is examined, showing that lowcrosstalk
OAM multiplexing can be achieved through direct fiberto-chip
coupling
Integrated phased array for scalable vortex beam multiplexing
Orbital angular momentum (OAM) modes have low model interactions during fiber propagation at data center distances, and thus are suitable for ultra-high capacity systems at low digital signal processing. Generating OAM modes using free-space setups is useful for proof-of-concept experiments, but is not a scalable solution. We use an optical phased array (OPA) with two-dimensional antennas for on-chip circularly polarized OAM beam generation. Our previous work demonstrated an OAM multiplexer for lower-order modes. In this work, we demonstrate an OAM multiplexer that supports a record of 46 (23 per polarization) simultaneous spatial modes up to OAM order 11. We also improve the crosstalk performance of our multiplexer. We incorporate an intensity tuning capability that substantially improves the OAM quality by enabling a uniform power distribution across the antennas. The worst-case crosstalk for the supported OAM5 to OAM11 are found experimentally to be better than -12 dB, with OAM10 achieving -17.2 dB
ORBITAL ANGULAR MOMENTUM ORTHOGONALITY-BASED CROSSTALK REDUCTION: THEORY AND EXPERIMENT
Full duplex communication systems allow a single channel to be used for simultaneous two-way communication, increasing spectral efficiency. However, full duplex communication systems suffer from the issue of self-interference between local transmitter and receiver antennas. Analog subtraction and signal processing methods have previously been used to reduce this problem. This dissertation proposes the use of waves carrying orbital angular momentum (OAM) to mitigate the problem of self-interference by offering a means of additional isolation between local antennas.
Orbital angular momentum has been widely studied both in the photonics and radio domain. The theoretically infinite orthogonal states of an OAM signal make it highly desirable in the field of communication. The application of OAM in a full duplex system, may be the answer to the problem of self-interference. This dissertation shows how the use of OAM waves may create an additional isolation between local antennas in a full duplex system. Motivated by the promise that OAM orthogonality holds, this dissertation explores the crosstalk reduction achieved through OAM. One of the main contributions of this dissertation is to provide insight into the nature of the effect. It motivates OAM orthogonality as a direction of research for use in future full duplex systems.
The effect of OAM on crosstalk must be studied experimentally and theoretically. To this effect, a patch array antenna was designed using the High Frequency Simulation Software (HFSS), to generate OAM beams. The designed antennas are fabricated and characterized. This dissertation discusses the experiments carried out to determine the amount of crosstalk reduction achieved due to the OAM nature of the signal transmitted. The impact of the change in distance between the local transmitter and receiver antennas on crosstalk is also studied. The results obtained are verified through theoretical analysis using simulations in HFSS. This dissertation reports a maximum theoretical crosstalk reduction of 3.6dB, and a crosstalk reduction of 2.6 dB realized experimentally.
Building on these results, a compact, more practical antenna configuration was designed. This nested design yields more than 60dB crosstalk reduction and provides for a more elegant system realization. The dissertation includes the design of a parabolic dish antenna to build a complete system, which is also studied in this dissertation.
The symmetry of the nested antenna configuration allows for analytic theoretical study which is included herein. The study mathematically proves the orthogonality of OAM modes, and the isolation between two antennas with different OAM modes. A similar study is simulated in HFSS using coaxial based loop antennas, and the crosstalk in the nested design is investigated. The design offers a crosstalk isolation of more than 90dB, and further affirms the mathematical analysis.
This dissertation provides a detailed analysis of the isolation offered by OAM orthogonality in local antennas which can be useful in a full duplex system. The work consists of practical, simulated, and mathematical investigation, and considers various antenna configurations and designs. Additionally, it presents and analyses a design for a full duplex system
Map-Assisted Power Allocation and Constellation Design for mmWave WDM with OAM in Short-Range LOS Environment
We consider a system that integrates positioning and single-user millimeter
wave (mmWave) communication, where the communication part adopts wavelength
division multiplexing (WDM) and orbital angular momentum (OAM). This paper
addresses the power allocation and high dimensional constellation design in
short-range line-of-sight (LOS) environment, with stable communication links.
We propose a map-assisted method to reduce transmission delay and online
computing overhead. We explore the possibility of using a few patterns in the
maps, and investigate its performance loss. For power allocation, we first
characterize the performance loss outside the OAM beam regions with only plane
waves, and figure out that the loss is always small. However, in OAM beam
regions, the performance loss has similar characteristics only at some specific
positions. Based on numerical results, we illustrate that a few patterns can be
adopted for all receiver locations in the map. We also investigate high
dimensional constellation design, and prove that a fixed constellation can be
adopted for the positions where the channel matrices are sufficiently close to
be proportional. Similarly, we figure out that the constellation design for all
receiver locations can be represented by a few constellation sets
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