Abstract
Abstract Orbital angular momentum (OAM), which describes tailoring the spatial physical dimension of light waves into a helical phase structure, has given rise to many applications in optical manipulation, microscopy, imaging, metrology, sensing, quantum science, and optical communications. Light beams carrying OAM feature two distinct characteristics, i.e., inherent orthogonality and unbounded states in principle, which are suitable for capacity scaling of optical communications. In this paper, we give an overview of OAM and beyond in free-space optical communications. The fundamentals of OAM, concept of optical communications using OAM, OAM modulation (OAM modulation based on spatial light modulator, high-speed OAM modulation, spatial array modulation), OAM multiplexing (spectrally efficient, high capacity, long distance), OAM multicasting (adaptive multicasting, N-dimensional multicasting), OAM communications in turbulence (adaptive optics, digital signal processing, auto-alignment system), structured light communications beyond OAM (Bessel beams, Airy beams, vector beams), diverse and robust communications using OAM and beyond (multiple scenes, turbulence-resilient communications, intelligent communications) are comprehensively reviewed. The prospects and challenges of optical communications using OAM and beyond are also discussed at the end. In the future, there will be more opportunities in exploiting extensive advanced applications from OAM beams to more general structured light.
Highlights
Structured light beams are a variety of special light fields that have tailored spatial structure with variant amplitude, phase and polarization distribution [1,2,3], such as helicallyphased beams, Bessel beams, Airy beams, vector beams and spatiotemporal beams
The fundamentals of Orbital angular momentum (OAM), concept of optical communications using OAM, OAM modulation (OAM modulation based on spatial light modulator, high-speed OAM modulation, spatial array modulation), OAM multiplexing, OAM multicasting, OAM communications in turbulence, structured light communications beyond OAM (Bessel beams, Airy beams, vector beams), diverse and robust communications using OAM and beyond are comprehensively reviewed
Since OAM multiplexing is compatible with WDM, we demonstrated a free-space data link with an aggregate transmission capacity of 1.036 Pbit/s and a high spectral efficiency of 112.6 bit/s/Hz by exploiting N-dimensional multiplexing, i.e., 54.139-Gbit/s OFDM-8QAM signals over 368 WDM pol-muxed 26 OAM modes [87]
Summary
Structured light beams are a variety of special light fields that have tailored spatial structure with variant amplitude, phase and polarization distribution [1,2,3], such as helicallyphased beams, Bessel beams, Airy beams, vector beams and spatiotemporal beams. The light beam carrying orbital angular momentum (OAM), a kind of structured light beams, is featured by a helical phase front of exp(ilθ), where l is the topological charge and θ the azimuthal angle [4]. Due to the helical phase structure, an OAM beam features a doughnut intensity profile with a phase singularity at the beam center Driven by their distinctive properties, i.e., helical phase structure and doughnut intensity profile, OAM beams have attracted increasing interest in recent years, giving rise to many developments in astronomy, manipulation, microscopy, imaging, metrology, sensing, nonlinear interactions, quantum science, and optical communications [5,6,7,8,9,10,11,12,13,14]. Prospects and upcoming challenges of OAM communications and beyond are discussed at the end
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