Fig. 1. Experimental setup of the secure chaos-based OAM multiplexing FSO communication system. DL, drive laser; FC, fiber coupler; PC, polarization controller; VOA, variable optical attenuator; M, mirror; ISO, optical isolator; MZM, Mach–Zehnder modulator; EDFA, erbium-doped fiber amplifier; FDL, fiber delay line; Col., collimator; LP, linear polarizer; BS, beam splitter; SLM, spatial light modulator; SL, slave laser; PD, photodetector; DPO, digital phosphor oscilloscope.

Fig. 2. Measured channel crosstalk for the two OAM modes (a) without chaos feedback loop and (b) with chaos feedback loop. Take mode spacing Δl=1 as an example: CH1-Detection means CH1 sends OAM+2, while the inverse phase hologram OAM−2 is loaded on SLM3 for direct demodulation when CH2 is blocked. CH1-Crosstalk means the crosstalk from CH2 to CH1, that is, CH2 sends OAM+1, while OAM−2 is loaded on SLM3 when CH1 is blocked.

Fig. 3. (a) Measured chaotic synchronization channel crosstalk for the two OAM modes; (b) cross-correlation plot for CH1 when Δl=4; (c) cross-correlation plot for CH2 when Δl=4.

Fig. 4. Performance of the chaos-based OAM multiplexing FSO communication system with different mode spacing. (a) Measured BER curves for different mode spacing for CH2 in the case of direct detection. Take mode spacing Δl=1 as an example: direct detection means the BER value when CH1 sends OAM+2 and CH2 sends OAM+1, while the inverse phase hologram OAM−1 is loaded on SLM3 for CH2 direct demodulation. Eye diagram at R=8  Gbps for (b) Δl=1, (c) Δl=2, (d) Δl=3, and (e) Δl=4.

Fig. 5. Experimental temporal waveforms for CH2 of (a) the original NRZ-OOK signal at the output of AWG, (b) the encrypted signal at the output of MZM, and (c) the decrypted signal after offline DSP. Eye diagrams of (d) the original NRZ-OOK signal, (e) the encrypted signal, and (f) the decrypted signal. (g) BER performances for CH1 and CH2 in the case of legal reception, illegal reception, and encryption.

Fig. 6. Measured BER curves for different cases as a function of masking coefficient β.

Fig. 7. (a1)–(a3) Intensity profiles of demodulated beams for different loading patterns (l=−3, l=−2, l=+3) when transmitting l=−3 and +1; (b1)–(b3) temporal waveforms of corresponding original message (gray line) and recovered message (colored line) at R=10  Gbps.

Fig. 8. Measured BER as a function of the percentage of beam block for secure chaos-based OAM multiplexing FSO transmission link. Insets (I) to (III) show the phase holograms partially blocked in SLM3.

Fig. 9. Intensity profiles of (a1)–(a3) generated OAM beams (l=−3, l=+1, and superposition of l=−3 and +1) at Tx (transmitter); (b1)–(b3) received OAM beams (l=−3, l=+1, and superposition of l=−3 and +1) at Rx (receiver) without turbulence; (c1)–(c3) received OAM beams (l=−3, l=+1, and superposition of l=−3 and +1) at Rx with weak turbulence. Normalized channel crosstalk matrix of OAM multiplexing (d) without turbulence; (e) with turbulence.

Fig. 10. Measured chaotic synchronization channel crosstalk matrix (a) without turbulence; (b) with turbulence. (c) BER performance for CH1 and CH2 as a function of bit rate R, in the case of no turbulence and with turbulence.