Generation of High-Purity Millimeter-Wave Orbital Angular Momentum Modes Using Horn Antenna: Theory and Implementation

📝 Abstract

Twisted electromagnetic waves, of which the helical phase front is called orbital angular momentum (OAM), have been recently explored for quantum information, high speed communication and radar detections. In this context, generation of high purity waves carrying OAM is of great significance and challenge from low frequency band to optical area. Here, a novel strategy of mode combination method is proposed to generate twisted waves with arbitrary order of OAM index. The higher order mode of a circular horn antenna is used to generate the twisted waves with quite high purity. The proposed strategy is verified with theoretical analysis, numerical simulation and experiments. A circular horn antenna operating at millimeter wave band is designed, fabricated, and measured. Two twisted waves with OAM index of l=+1 and l=-1 with a mode purity as high as 87% are obtained. Compared with the other OAM antennas, the antenna proposed here owns a high antenna gain (over 12 dBi) and wide operating bandwidth (over 15%). The high mode purity, high antenna gain and wide operating band make the antenna suitable for the twisted-wave applications, not only in the microwave and millimeter wave band, but also in the terahertz band.

💡 Analysis

Twisted electromagnetic waves, of which the helical phase front is called orbital angular momentum (OAM), have been recently explored for quantum information, high speed communication and radar detections. In this context, generation of high purity waves carrying OAM is of great significance and challenge from low frequency band to optical area. Here, a novel strategy of mode combination method is proposed to generate twisted waves with arbitrary order of OAM index. The higher order mode of a circular horn antenna is used to generate the twisted waves with quite high purity. The proposed strategy is verified with theoretical analysis, numerical simulation and experiments. A circular horn antenna operating at millimeter wave band is designed, fabricated, and measured. Two twisted waves with OAM index of l=+1 and l=-1 with a mode purity as high as 87% are obtained. Compared with the other OAM antennas, the antenna proposed here owns a high antenna gain (over 12 dBi) and wide operating bandwidth (over 15%). The high mode purity, high antenna gain and wide operating band make the antenna suitable for the twisted-wave applications, not only in the microwave and millimeter wave band, but also in the terahertz band.

📄 Content

Generation of High-Purity Millimeter-Wave Orbital Angular Momentum Modes Using Horn Antenna: Theory and Implementation

Jian Ren1,2*, Kwok Wa Leung1,2
1 Department of Electronic Engineering, City University of Hong Kong, Kowloon, Hong Kong SAR, China
2 State Key Laboratory of Millimeter Waves, City University of Hong Kong, Kowloon, Hong Kong SAR, China
*correspondence to jianren2-c@my.cityu.edu.hk
Abstract
Twisted electromagnetic waves, of which the helical phase front is called orbital angular momentum (OAM), have been recently explored for quantum information, high speed communication and radar detections. In this context, generation of high purity waves carrying OAM is of great significance and challenge from low frequency band to optical area. Here, a novel strategy of mode combination method is proposed to generate twisted waves with arbitrary order of OAM index. The higher order mode of a circular horn antenna is used to generate the twisted waves with quite high purity. The proposed strategy is verified with theoretical analysis, numerical simulation and experiments. A circular horn antenna operating at millimeter wave band is designed, fabricated, and measured. Two twisted waves with OAM index of l=+1 and l=-1 with a mode purity as high as 87% are obtained. Compared with the other OAM antennas, the antenna proposed here owns a high antenna gain (over 12 dBi) and wide operating bandwidth (over 15%). The high mode purity, high antenna gain and wide operating band make the antenna suitable for the twisted-wave applications, not only in the microwave and millimeter wave band, but also in the terahertz band.

Introduction
Exploiting all aspects of electromagnetic (EM) wave properties has been quite a hot topic in the past several decades, from low frequency radio waves to optical waves. EM wave has brought big changes to our daily lives. In the meanwhile, it also attracts a wide range research on exploring its new characteristics, including, but not limited to wavelength, polarization and amplitude. Among these properties, orbital angular momentum (OAM) [1, 2], featuring a helical phase front plane, is a newborn, compared with the spin angular momentum (SAM) [3]. As been known long before, EM waves could carry both OAM and SAM [3, 4]. However, the first detailed research on the OAM was carried out by
Allen in 1992 [1], who showed that a light with a Laguerre-Gaussian amplitude distribution have an OAM of lħ per photon, where l is the OAM mode index and ħ is the Planck’s constant. Since then, how to analyze, generate, and utilize the EM waves carrying OAM attracts researchers of various academic areas all over the world.
In the quantum area, transferring from SAM to OAM of photons was used to transpose the quantum information [5]. With the scattering of an OAM light from a spinning object, the rotate speed of the rotating bodies can be detected, using the characteristic of OAM combining Doppler effects [6]. This can also be used in astrophysics to detect the running state of the celestial body. In the wireless communication area, OAM waves find the most attractive and potential applications, since they can enhance the data capacity of the communication systems as the natural orthogonality between different OAM modes [7]. This idea was demonstrated in the free space information transfer in 2004 for the first time [8]. Thereafter, terabit data transmission has been realized using this concept in the free space in 2012 [9] and in fiber in 2013 [10]. This breakthrough happens not only in the optical region, but also in the microwave and millimeter wave bands [11, 12].
Although different methods have been proposed to generate OAM, the generation of high purity OAM is still a big challenge. The spiral phase plate (SPP) [13] is most often used to create helical phase structure in optical region. When the beam penetrates into the SPP, a phase difference can be generated along the azimuthal angle due to the variation of the SPP thickness, making the traditional plane-wave wave front a helical one along the direction of the wave propagation. This method has an advantage of high precision, which can generate OAM beam with a mode index as high as 5050 [14]. Recently, metasurfaces [15-17] also have been used to generate OAM beams [18-20]. The working principle is somewhat similar with that of the SPP, as the element phases reflected/transmitted by the metasurface have the same phase variation. Another method to create OAM beam is resonator cavity. Using a modified micro-ring resonator, Cai [21] demonstrated a very compact OAM beam emitter on a single chip. At the microwave and millimeter-wave bands, the first OAM beam generation is demonstrated by Thide in 2007 with circular distributed antenna array [22]. The circularly-placed antenna elements have a phase variation along the azimuthal direction. Antenna Array can generate an OAM beam with mo

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