Simple generation of orbital angular momentum modes with azimuthally deformed Cassegrain subreflector

Abstract

A parabolic reflector antenna with an azimuthally deformed Cassegrain subreflector is proposed to effectively generate arbitrary orbital angular momentum (OAM) modes. The Cassegrain dual-reflector antenna was fabricated for 18 GHz and measured in the near-field range. The nearfield to far-field transformed radiation phase around a full azimuth shows that the proposed antenna generates fields with the l = 1 OAM mode. Simulated and measured feed reflection coefficients are below −10 dB for 15.1-21.2 GHz, even though the distance between a horn feed and a deformed subreflector is 15.6 mm, thus it is expected that the proposed structure can be used for a broadband low-profile antenna.Introduction: Various researches based on orbital angular momentum (OAM) modes have been extensively studied in optics [1]. Recently, communication systems analogous to the optical one were demonstrated in radio frequency [2][3][4]. To utilise OAM characteristics for radio communications, generating and detecting arbitrary OAM modes is essential. In [2], a helicoidally parabolic reflector, which is similar to [5], is used as the main reflector to transmit electromagnetic waves with a fundamental OAM mode (l = 1). From the standpoint of reflector antenna design, fabricating the main parabolic surface as a helicoid [2, 5] is expensive and inefficient, because ordinary main reflectors should be modified for the generation of non-zero OAM modes. Therefore, we need a simple and efficient method to generate arbitrary OAM modes while keeping the main reflector unchanged. In this Letter, we propose an alternative structure [6] in which a Cassegrain subreflector is azimuthally deformed instead of the main parabolic reflector. A parabolic reflector antenna with an azimuthally deformed Cassegrain subreflector enables us to process small area of a subreflector without modifying a large main reflector. Owing to this merit, we can adopt a variety of ordinary main reflectors for the efficient generation of arbitrary OAM modes. In addition, the proposed antenna has very good reflection characteristics due to the fact that a deformed subreflector radiates higher-order OAM modes which cannot be received by a normal feed horn.

Orbital Angular Momentum (OAM) Antennas via Mode Combining and Canceling in Near-field

“…1b , are reflected using a sub-reflectarray designed for a fixed OAM mode ( ). The effective deformation of a subreflector was proposed to generate the specific OAM mode 19 ( ). The main reflectarray with OAM mode, which was placed very close to the sub-reflectarray, was used to apply the OAM mode combining of l sub and l main .…”

Communicating Using Spatial Mode Multiplexing: Potentials, Challenges, and Perspectives

“…Another similar method uses a flat phase plate with different permittivities along its surface [49]. Twisted parabolic reflectors have been used to generate OAM beams, as well [50]- [52]. Gui and his co-workers fabricated two circular slot antenna systems to generate OAM beams at 2.4 GHz [53].…”

Microwave Orbital Angular Momentum Mode Generation and Multiplexing Using a Waveguide Butler Matrix

“…The OAM mode is characterized by an intensity vortex on the beam axis and a helical phase front. Spiral phase plates (SPPs) [7] and helical reflectors [8] can be used to transform the zero OAM Hermite-Gaussian mode into finite OAM Laguerre-Gaussian modes, and vice-versa. Optical OAM modes can be used as micro-particle rotating tweezers [2] and for quantum key distribution [3], and their use in high-capacity data communications has been attempted [4]- [6].…”

“…The first one is to first generate single OAM modes, and then multiplex them. Spiral phase plates (SPPs) [7] and helical reflectors [8] can be used to transform the zero OAM Hermite-Gaussian mode into finite OAM Laguerre-Gaussian modes, and vice-versa. However, a single SPP or helical reflector can only generate one OAM mode, and the several different OAM modes should be optically multiplexed-with a beam splitter, for example [9]-before transmission or transmitted individually without multiplexing [10].…”