Optical Orbital Angular Momentum Shift-Keying Communication Using Direct Demodulation

Orbital angular momentum (OAM) shift-keying (SK), which manifests as the rapid switching of OAM modes, is crucial but severely hampered for lacking effective demodulation techniques in OAM communication. In this paper, we propose and investigate a novel direct-demodulation method for the OAM-SK communication. After passing through a well-designed beam slit, the optical vortex beam will possess varying output energies associated with the OAM modes. Hence, splitting the optical vortex beam into two sub-beams and one of them passing through the slit, we can directly extract the OAM-SK signals from the energy ratios of the two sub-beams. The results show that the bit-error-rate (BER) of demodulated OAM-SK signals can reach 3.7 x 10(-3) even under the influence of strong atmospheric turbulence with C-n(2)= 3 x 10(-12) and misalignment of 1.8 mm. Encoding digital signals to OAM modes and amplitude of optical vortex beams simultaneously, we construct a novel multi-modulation format composed of 16 QAM-OFDM and OAM-SK signals, which is also demodulated with the BER of 10(-5) at the optical signal-to-noise ratio (OSNR) of 18 dB. These demonstrate that the direct-demodulation method provides a feasible way to demodulate OAM-SK signals and may show potential in complex OAM modulation communications.