Figure 2

Experimental set-up of the OAM-based FSO system. A laser source with 80 wavelengths carrying QPSK/16-QAM signals and two polarizations is divided into 10 branches. Each branch is amplified by an EDFA and coupled into one of the OAM channels in the coaxial OV beams via a 1×10 DOVG. After propagation in free space for 1 m, the OAM channels are then diffracted into the corresponding angles and converted into Gaussian beams by an identical DOVG. The focused Gaussian beams pass through pinholes and are coupled into single mode fibers. The wavelength, polarization and QPSK/16-QAM signals are demultiplexed and all the encoded signals are detected in parallel. DOVG, Dammann optical vortex grating; EDFA erbium-doped fiber amplifier; FSO, free-space optical; OAM, orbital angular momentum; OV, optical vortex; 16-QAM, 16-quadrature amplitude modulation; QPSK, quadrature phase-shift keying ;WDM, wavelength division multiplexing; OC, optical coupler.