Here, we propose and experimentally demonstrate a cylindrical vector beam (CVB) sorter based on a spin-dependent spiral transformation. By exploiting the spin–orbital interaction of the geometric phase, a pair of conjugated spiral transformations are applied to modulate the two orthogonal circularly polarized components of the CVB, which are converted into the same linear phase gradient from opposite azimuthal phase gradients. Since the orthogonal spin components of CVBs with different polarization orders carry different phase gradients, under the convergence of a convex lens, the coaxially transmitted CVBs can be sorted with spatially separated positions, and the increased phase gradient provided by the spiral transformation yields the high resolution. We show that five CVB modes from − 2 to + 2 are successfully sorted with a separation efficiency of 3.65. As a proof-of-concept, we demonstrate a two-channel CVB multiplexing communication with a bit error rate approaching 10 − 6 . In addition to providing an avenue for CVB demultiplexing, our results show potential applications in mode filtering and mode routing in all-optical interconnection.
Vortex beams carrying orbital angular momentum (OAM) modes show superior multiplexing abilities in enhancing communication capacity. However, the signal fading induced by turbulence noise severely degrades the communication performance and even leads to communication interruption. Herein, we propose a diversity gain strategy to mitigate signal fading in OAM multiplexing communication and investigate the gain combination and channel assignment to optimize the diversity efficiency and communication capacity. Endowing signals with distinct channel matrices and superposing them with designed channel weights, we perform the diversity gain with an optimal gain efficiency, and the signal fading is mitigated by equalizing the turbulence noise. For the tradeoff between turbulence noise tolerance and communication capacity, multiplexed channels are algorithm-free assigned for diversity and multiplexing according to bit-error-rate and outage probability. As a proof of concept, we demonstrate a 6-channel multiplexing communication, where 3 OAM modes are assigned for diversity gain and 24 Gbit/s QPSK-OFDM signals are transmitted. After diversity gain, the bit-error-rate decreases from 1.41 × 10−2 to 1.63 × 10−4 at -14 dBm, and the outage probability of 86.7% is almost completely suppressed.
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