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IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 14, 2015
Near-Field Experimental Verification of Separation of OAM Channels Elettra Mari, Fabio Spinello, Matteo Oldoni, Roberto A. Ravanelli, Filippo Romanato, and Giuseppe Parisi
Abstract—The experimental proof that near-field radio communication channels based on orbital angular momentum (OAM) are naturally isolated is presented. In near-field zone, we show that two antennas, built for producing a beam with the same value of OAM, have a good throughput. On the other hand, antennas of different types, i.e., built for generating beams with different OAM values, including a standard antenna, exhibit a good modal isolation. Index Terms—Orbital angular momentum, radio communication, twisted parabola.
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LECTROMAGNETIC (EM) waves carry energy and both linear and angular momentum. The angular contribution, namely orbital angular momentum (OAM), was discovered in the 1990s [1]. The possibility of using OAM states for transmission of many independent channels without increasing the bandwidth has attracted increasing interest both in the optical domain [2]–[4] and in RF and microwave [5], [6]. In fact, the infinite set of OAM states forms an orthogonal basis [2], and the orthogonality among beams with different OAM states could allow, under certain experimental conditions, a natural insulation between OAM channels. In this letter, we report the experimental verification of modal insulation between OAM channels, that is the base of a possible radio communication system based on OAM. We performed experimental tests at 17.2 GHz ( cm) in the near-field zone to measure the total modal isolation between channels with different OAM value. The choice of the near-field zone for the experiments ensures that a good fraction of power is received, avoiding the spread of the OAM beam due to propagation [6], [7]. For the generation of OAM beams in radio domain, several techniques have been proposed such as staircase spiral reflector [8], spiral antennas [9], or arrays [10], [11]. In our tests, we used some specifically built prototypes [12], based on a modified parabolic 36-cm antenna. The manufacManuscript received October 13, 2014; revised November 05, 2014; accepted November 07, 2014. Date of publication November 11, 2014; date of current version February 06, 2015. This work was supported by SIAE Microelectronics. E. Mari and G. Parisi are with Twistoff s.r.l., 35129 Padova, Italy (e-mail:
[email protected]). F. Spinello is with Twistoff s.r.l., 35129 Padova, Italy, and also with the Department of Information Engineering, University of Padova, 35131 Padova, Italy. M. Oldoni and R. A. Ravanelli are with SIAE Microelettronica, 20093 Cologno Monzese, Milan, Italy. F. Romanato is with the Department of Physics and Astronomy, University of Padova, 35100 Padova, Italy. Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LAWP.2014.2369536
Fig. 1. Prototype of a parabolic antenna modified for the generation of OAM beams.
tured antenna is shown in Fig. 1. Inspired by the optical phase masks [13], the twisted reflector converts the field generated by the feeder into a helical beam with a value of OAM that depends on the height of the step of the twisted reflector. The modified parabolic reflector designed to generate a vortex with order is described, in a cylindrical coordinate system , with -axis conventionally oriented, by the formula [12] (1) m of the twisted Equation (1) ensures that the focus reflector is independent of the angular coordinate . Both the left-handed ( ) and the right-handed ( ) OAM-mode antennas as well as the untwisted one (a standard parabola generating ) have the same focus. The feeder, whose structure is shown in Fig. 2, is mounted in the same position as for a standard parabolic Cassegrain antenna. The OAM values are defined for twisted parabolas used in transmission mode for which the EM field propagates toward the positive -axis. When a twisted antenna is used in reception side, it performs as an inverse phase reflector because of the change of propagation direction. In other words, a transmitting antenna receives like a [12]. The antennas were produced by sintering, starting from ceramic powder. The bodies were made of photo-polymeric ceramic material. The surfaces were then covered with copper in an electroplating bath, up to a thickness of about 0.1 mm. This treatment was performed only on their internal surface, and the final tolerance was better than . First of all, we have verified the correct behavior of any single twisted antenna by measuring in far-field zone (100 m) intensity
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MARI et al.: NEAR-FIELD EXPERIMENTAL VERIFICATION OF SEPARATION OF OAM CHANNELS
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TABLE I DENOTES THE OAM VALUE OF THE RETURN-LOSS MEASUREMENTS. TRANSMITTED BEAM, RL RETURN LOSS, OPEN SPACE, AND SHIELDED CONDITION
Fig. 2. Layout of the Cassegrain antenna configuration for the twisted ( ) parabolic reflector. Inset shows the technical data of the feeder, used for numerical simulations, which is present in both the used twisted and standard parabolas.
Fig. 3. Normalized intensity (a) and phase, in spherical coordinates, (b) maps radio beam vortex measured in free space, at a distance of the m from the transmitter. Frequency is set to 17.2 GHz, and polarization is kept vertical. The central dark zone of the intensity distribution, corresponding to the phase singularity, is clearly visible. The inner and outer radii of the doughnut are about 6 and 9 m, respectively (color online).
and phase of the beam produced. The twisted transmitting antenna was mounted on a computer-controlled rotator, moving both in elevation and azimuth with a resolution of 0.2 . The signal was received by a standard connected to a vector network analyzer (VNA), that can measure instantaneous phase and amplitude. No post-processing was required. The choice of mapping the field at a distance of 100 m was convenient due to facilities already installed for other OAM experiments we are developing. In Fig. 3, measured intensity and phase of the beam radiated by the antenna are reported. We then performed two series of near-field tests for the verification and evaluation of the separation between OAM channels, which is a consequence of the orthogonality between OAM states, when the beam, in magnitude and phase, is collected. The first test was the measurement of the return loss (RL). As is well known, this parameter is defined as the ratio between the incident power on the antenna from the feeding line and the reflected one ; namely . We placed all the antennas in open space (OP), freely radiating. We performed measurements only at 17.2 GHz because this is the design frequency of the twisted parabolic antennas (recall that these devices are narrowband [12]). We connected each antenna, one at a time, to a port of the VNA and measured the RL; to perform these measurements, the VNA transmitted a constant power of
about 20 dBm. We found that, for both the twisted and the standard antennas, its value is approximately equal to 14 dB, as expected from the specifications of the standard parabola. After that, we repeated the RL measurements with each parabola in contact with a metal plate (brass) that covers the whole aperture (this is the short-circuit or shielded condition, SH). The obtained results are reported in Table I. For the standard ( ) antenna, the measured RL value, in SH condition, drops dramatically to 2.3 dB. This is a clear indication of an expected and almost complete reflection of the radiated power back into the feeder. In this case, theoretically, RL should be equal to 0. The difference of this measured RL value and the theoretical one is due to several experimental factors such as power loss (approximately 1 dB/m) of 2-m flexible cables, use of waveguide cable adapters, or the distance between antennas that are not in contact. On the other hand, the twisted parabola ( ), even if its aperture is shut by the brass plate, still shows a high RL ( dB), similar to the one measured in the OP condition. This different behavior with respect to the standard parabola is linked to the OAM geometrical properties. In fact, the emitted field’s reflection on the brass plate reverses its propagation direction with respect to the fixed -axis; as a consequence, the OAM topological charge changes sign. Thus, what comes back from the plate and impinges on the reflector (designed to emits an beam) is an beam. Recalling that the same antenna in reception mode acts like a antenna, a doughnut field profile (corresponding to ) is formed on the secondary reflector, and a very small fraction of the power reaches the detector. In fact, the beam focused toward the feeder of the receiving antenna acquires the topological charge of the impinging beam plus the topological charge of the antenna in reception mode. For example, when a transmitting antenna operates in pair with an opposite one (i.e., an antenna built as that acts in reception mode as a , according to the change of propagation direction), the received twisted beam is transformed into an wave, which cannot be collected by the feeder because the dark region inside the ring is mostly overlapping the feeder aperture. On the other hand, a transmitting antenna in communication with an identical receiving antenna ( in reception mode, as stated above), produces an untwisted beam ( ) that is focused into the feeder. As proof of concept, we report in Fig. 4 method of moments (MoM)-based numerical simulations of the normalized intensity of the electric field produced by twisted and antennas, in reception mode, illuminated by a beam. In the first case, an untwisted beam is focused into the feeder, whereas with the twisted receiving antenna, a
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Fig. 4. Intensity of the electric field (normalized to 1) produced by a wave received by antenna A and a antenna B, in reception mode. External circles represent a top view of the antennas, and solid black circles represent the dimension of the feeder. A) An untwisted beam ( ) is produced, and it is focused into the feeder. B) A beam is produced showing that the feeder receives only a small fraction of the beam because of the characteristic central dark zone of the OAM beam (color online).
TABLE II INSERTION LOSS (IL) MEASUREMENTS. TX AND RX COLUMNS IDENTIFY THE TRANSMITTING AND RECEIVING ANTENNAS
IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 14, 2015
domain have been proposed [5], [10], opening a discussion [14] about the possibility of overcoming some difficulties like the effect of the channel propagation or the misalignment between transmitting and receiving antennas [15]. In fact, for increased distances, less power will be received by a fixed receiver aperture due to the divergence of the OAM beam. In this letter, we have limited our study in near-field conditions, strengthening the idea of exploiting OAM in near-field radio communication [14], when both the transmitting and receiving antenna arrays are facing each other perfectly on the same axis in free space. A possible application could be in shortrange high-data-rate links [6] in indoor environments by taking advantage of separation between OAM channels that is a physical consequence of the orthogonality of OAM states. ACKNOWLEDGMENT The authors gratefully thank C. G. Someda and F. Tamburini for the interesting discussion during this work. The authors acknowledge the logistic support of SIAE Microelectronics in the designing, building, and testing of the setup. REFERENCES
beam is produced. In the latter case, with OAM beam, characterized by a doughnut-shaped intensity distribution, the central dark zone hits on the feeder, and only a small portion of signal is transmitted. This particular behavior also explains the results of the second series of experimental tests, i.e., the measurements of insertion loss (IL) for combinations of aligned transmitting (Tx) and receiving (Rx) antennas of different type. The antennas faced each other at a short distance (less than 1 cm from their outer border), keeping the (vertical) polarization concordant. The alignment of the antennas ensures the collection of the whole OAM beam. The IL is defined as the ratio between the Tx and the Rx powers measured at two different sides of a radio link. It is defined, as is well known, as: and quantifies the signal power lost between the transmitting and the receiving points. The lower its value, the better the signal transmission. We connected the Tx antenna to the first port of the VNA and the Rx antennas to the second port; then we configured the instrument to measure the IL. The frequency and power parameters are identical to those used for the measurement of RL. Table II shows the experimentally obtained results. As expected, pairs of identical antennas, facing each other, show a good throughput ( and ). On the other hand, we see that pairs of twisted antennas of different type exhibit an excellent modal isolation; we measure an insertion loss of 18.6 dB when a standard antenna receives a twisted beam, and an even better isolation of 26.2 dB when Tx and Rx antennas have opposite OAM values. This is compatible with the expected behavior of OAM beams impinging on surfaces with opposite topological charge. All these experimental results suggest that it is possible to separate OAM channels in near-field conditions. Various ideas of exploiting OAM radio waves orthogonality in the far-field
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