Spectrally efficient free-space optical communications employing orbital angular momentum multiplexing

Similar to other physical dimensions of light such as amplitude, phase, frequency, time and polarization, orbital angular momentum (OAM), which refers to the spatial structure of light (a spiral wavefront), is an additional degree of freedom to modulate or multiplex information in optical communication systems [1, 2]. Theoretically, OAM and linear-polarization (LP) modes in few mode fibers (FMFs) are both spatial orthogonal basis that can be chosen for carrying data streams simultaneously, to greatly increase the capacity and spectrum efficiency of communication systems. Here we review recent progress in free-space optical (FSO) systems employing OAM multiplexing, including: (1) using 16-QAM signals over polarization division multiplexed (PDM) 8 OAM modes in two groups of concentric rings to achieve a spectral efficiency of 95.7 bit/s/Hz [3]; (2) using OFDM/OQAM 64-QAM signals over 22 OAM modes with PDM to achieve a spectral efficiency of 230 bit/s/Hz [4]; (3) using Nyquist 32-QAM signals over 52 orbital angular momentum (OAM) modes with PDM to achieve an ultra-high spectral efficiency of 435 bit/s/Hz [5]; (4) using 30 Gbaud PAM-4 signals over 12 OAM modes with PDM to achieve a single wavelength terabit intensity modulation direct detection (IM-DD) system and the spectrum efficiency is 29.8 bit/s/Hz [6]. The above mentioned systems utilize spatial light modulators (SLMs) to convert Gaussian beams into OAM beams, making the whole systems bulky and unscalable. Therefore, one valuable goal is to realize compact and efficient integrated devices for generating, multiplexing and demultiplexing OAM modes. We will review some promising work in this area: (1) using angular gratings and a microring resonator to realize a silicon optical vortex emitter [7]; (2) fabricating micro-scale spiral phase plates within the aperture of a vertical-cavity surface-emitting laser (VCSEL) [8]; (3) using Dammann optical vortex gratings to achieve the parallel detection of massive individual OAM modes [9].