Abstract
Orbital angular momentum (OAM) multiplexing technique has recently emerged and generated widespread interests since the OAM was discovered as a property of electromagnetic wave and acoustic wave. It was widely acknowledged that OAM multiplexing can achieve very high effective degrees of freedom (EDOF) and improve the spectral efficiency in optical, radio and acoustic communications. However, in the field of free-space optical (FSO) communications, it was demonstrated that OAM multiplexing is not the optimal multiplexing technique and the spatial bandwidth product (SBP) limits the EDOF. Is there any EDOF limits of OAM multiplexing in radio and acoustic communications? Could OAM multiplexing be safely scaled to far field? Here, we discover that the azimuthal resolution of OAM mode generator in OAM multiplexing limits its EDOF. Furthermore, we also verify that the OAM multiplexing in radio and acoustic communication fails to enable a long distance transmission and high EDOF simultaneously incurred by the inherently imperfect OAM mode generator.
Highlights
It was recently discovered that orbital angular momentum (OAM) and spin angular momentum (SAM) jointly compose the angular momentum of photons, where the Orbital angular momentum (OAM) is a brand new fundamental angular momentum carried by twisted photons[1,2,3]
We discover that the azimuthal resolution of OAM mode generators and detectors limits the effective degrees of freedom (EDOF) in OAM multiplexing
We discuss the physical resources of uniform circular array (UCA) and the EDOF of OAM multiplexing in radio and acoustic communications
Summary
It was recently discovered that orbital angular momentum (OAM) and spin angular momentum (SAM) jointly compose the angular momentum of photons, where the OAM is a brand new fundamental angular momentum carried by twisted photons[1,2,3]. It was demonstrated that the coaxially transmitted optical OAM beams sharing the same frequency, phase lag, and polarization are capable of transmitting multiple independent data streams in free-space optical (FSO) communications[17]. Based on this result, combining the OAM multiplexing with classical multiplexing was verified to yield a further increase of the channel capacity. Given the inevitable flaw of UCA, the decrease of EDOFs and the inherent severe power attenuation, we conclude that OAM multiplexing in radio and acoustic communications is incapable of realizing high EDOF and long transmission distance simultaneously. In the radio domain and the acoustic domain, spiral phase plate (SPP) and UCA are widely utilized to generate
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