Abstract
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.
Highlights
Electromagnetic (EM) waves carry linear and angular momentum [1]
The total angular momentum (TAM) of paraxial EM wave beams can be described as the sum of spin angular momentum (SAM) and orbital angular momentum (OAM), where the SAM is associated with the polarization state [2, 3]
The present paper shows that gyrotron oscillators and gyro-amplifiers are natural sources of high-power mm-wave beams with very pure higher-order OAM, which can be used for multiplexing in long-range wireless communication
Summary
Electromagnetic (EM) waves carry linear and angular momentum [1]. The linear momentum is associated with the Poynting vector S⃗S, which represents the directional energy flux (the energy transfer per unit area per unit time, measured in W/m2). In addition to conventional multiplexing methods to achieve higher data rates, OAM can be an effective means to increase the channel capacity and spectral efficiency in wireless communication technology by a factor equal to the number of available OAM states (OAM diversity) [5]. In this article eight multiplexed 28 GHz OAM beams (l = ± 1 and ± 3 on each of two orthogonal polarizations), generated by SPPs, each carrying a 4x1 Gbit/s (4 bits per symbol) quadrature amplitude modulation (16-QAM) signal, thereby achieving a capacity of 32 Gbit/s, were transmitted over a distance of 2.5 m with a spectral efficiency of ≈ 16 bit/(s∙Hz). The present paper shows that gyrotron oscillators and gyro-amplifiers are natural sources of high-power mm-wave beams with very pure higher-order OAM, which can be used for multiplexing in long-range wireless communication.
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.