Fig. 1. Concept and principle. Upper row: concept of an interbuilding optical interconnects employing OAM multiplexing/multicasting and shows its security. Lower row: layout of a 260-m security OAM multiplexing/multicasting FSO interconnect link between WNLO-E building and WNLO-H building. WNLO, Wuhan National Laboratory for Optoelectronics.

Fig. 2. Experimental setup for 260-m security OAM multiplexing/multicasting link. SLM-1, spatial light modulation; Pol., polarizer; Col., collimator; BS, beam splitter; PC, polarization controller; VOA, variable optical attenuator; OC, optical coupler; EDFA, erbium-doped fiber amplifier; M, mirror; NDF, neutral density filter; Tx, transmitter; Rx, receiver. The practical picture for site #1, site #2, and site #3 are presented in Fig. S2 in the Supplementary Material.

Fig. 3. Generated OAM beams for (a1) l=−3; (a2) l=+3; (a3) superposition of l=±3 and interferograms for (a4) l=−3; and (a5)l=+3 at Tx. Received OAM beams for (b1) l=−3; (b2) l=+3; (b3) superposition of l=±3 and interferograms for (b4)l=−3; and (b5) l=+3 at Rx. (c1)–(c4) Demodulated beams for different loading patterns (l=−3,l=−1,l=+1, and l=+3) when transmitting l=−3. Tx, transmitter and Rx, receiver.

Fig. 4. Statistic results of fluctuations after 260-m OAM multiplexing transmission (recorded in 200 s at the interval of 1 s). (a1), (a2) Center displacement of the received demodulation beam (l=+3,l=−3); (b1), (b2) space light-power fluctuation after the beam reduction (l=+3 and l=−3); (c1), (c2) received power of signal channel after SMF; and (d1), (d2) received power of cross talk channel after SMF.

Fig. 5. BER performance for multiplexing. (a1) Measured BER performance for 260-m 10-Gbaud 16-QAM OAM multiplexing FSO interconnect link. (a2), (a3) BER fluctuation with l=+3 and l=−3 on SLM-3, respectively, at the OSNR of ∼18dB. (b1) Measured BER performance for 260-m security OAM multiplexing FSO interconnect link. (b2), (b3) Average received power distribution according to various angular block parts. (c1) Simulated OAM order spectra of OAM beams with l=+3 wiretapped by the eavesdropper with angular block. Insets are intensity profiles wiretapped by the eavesdropper. (c2) Simulated OAM order spectra of multiplexed OAM beams with l=+3 and l=−3 wiretapped by the eavesdropper with angular block. (c3) Simulated normalized power of OAM beam with l=+3 and cross talk between OAM beam with l=+3 and l=−3 wiretapped by the eavesdropper with angular block. (d1) Simulated OAM order spectra of OAM beams with l=+3 wiretapped by the eavesdropper in a general scenario. Insets are intensity profiles wiretapped by the eavesdropper. re, eavesdropper’s aperture radius and rb, OAM beam radius of l=+3 transmitted over 260 m. (d2) Simulated OAM order spectra of multiplexed OAM beams with l=+3 andl=−3 wiretapped by the eavesdropper in a general scenario. (d3) Simulated normalized power of OAM beam with l=+3 and cross talk between OAM beam with l=+3 and l=−3 wiretapped by the eavesdropper in a general scenario.

Fig. 6. BER performance for multicasting. (a1), (a2) Multicasting power of pattern-designed and experiment before and after offline feedback process respectively. (b) Measured BER performance for 260-m power-controllable 1-to-9 multicasting interconnects link. (c1) Simulated OAM order spectra of multicasting OAM beams wiretapped by the eavesdropper with angular block. Insets are intensity profiles wiretapped by the eavesdropper. (c2) Simulated average power of multicasting channels wiretapped by the eavesdropper with angular block. (d1) Simulated OAM order spectra of multicasting OAM beams wiretapped by the eavesdropper in a general scenario. Insets are intensity profiles wiretapped by the eavesdropper. re: eavesdropper’s aperture radius. rb: OAM beam radius of l=+4 transmitted over 260 m. (d2) Simulated average power of multicasting channels wiretapped by the eavesdropper in a general scenario.