Contents Introduction
The exponential growth in data traffic is driving an ever-increasing demand for higher data capacity and more efficient spectral usage in transmission links. Different multi-level modulation formats such as m-ary phase-shift keying (m-PSK) and m-ary quadrature amplitude modulation (m-QAM) and various multiplexing techniques such as wavelength-division multiplexing (WDM), time-division multiplexing (TDM), and polarization-division multiplexing (PDM) have been used to improve the transmission capacity and spectral efficiency[1]. Additionally, space-division multiplexing (SDM) is attracting more interest as it explores the only remaining unused space dimension2–4. In fiber optical communications, few-mode fiber (FMF) and multi-core fiber (MCF) are well-known promising candidates enabling efficient SDM[5]–[7]. More recently, orbital angular momentum (OAM), which is also related to the spatial structure of an electromagnetic wave (spiral phase front)[8], has shown its possible use both in free-space and fiber transmission links[9]–[17]. The distinct features are unlimited charge values of OAM and intrinsic orthogonality among different OAM states which facilitate an alternative multiplexing technique, i.e. OAM-division multiplexing (ODM). Lots of research efforts have been devoted to free-space and fiber transmission links using OAM multiplexing: 1) using 20-Gbaud/s 16-QAM signals over two OAM modes at 10 wavelengths, 1.6- Tbit/s transmission capacity through a 1.1-km specially designed vortex fiber was reported[13], [14]; 2) using 20-Gbaud/s 16-QAM signals over pol-muxed eight OAM modes in two groups of concentric rings (32 channels in total), a transmission capacity of 2.56 Tbit/s and a high spectral efficiency of 95.7 bit/s/Hz were gained[15]: 3) using 100-Gbit/s quadrature phase-shift keying (QPSK) signals over pol-muxed 12 OAM modes at 42 wavelengths, an aggregate transmission capacity of 100.8 Tbit/s was obtained[16]; 4) using orthogonal frequency-division multiplexing (OFDM) offset quadrature amplitude modulation (OFDM/OQAM) 64-QAM signals over pol-muxed 22 OAM modes, a transmission capacity of 736.0 Gbit/s and an ultrahigh spectral efficiency of 230 bit/s/Hz were achieved[17].
