Fig. 1. OAM-CCL. (a) Concept of OAM-CCL and (b) intensity profiles and phase fronts for different OAM beams.

Fig. 2. Schematic diagram of the communication system based on OAM-CCL. SL, laser; VOA, variable optical attenuator; FM, fiber optic mirror; PC, polarization controller; DL, delay line; Col., collimator; Pol., linear polarizer; SLM, spatial light modulator; EDFA, erbium-doped fiber amplifier; AM, amplitude modulator; BS, free-space beam splitter; M, mirror; PD, photodetector; DIF, current differential; and OSC, oscilloscope.

Fig. 3. System performance. (a) Timing waveforms of ${\mathrm{SL}}_{\mathrm{D}}$, ${\mathrm{SL}}_{\mathrm{T}}$, and ${\mathrm{SL}}_{\mathrm{R}}$; (b) power spectrum before and after encryption; (c) BER versus masking coefficient at OSNR = 20 dB; and (d) BER versus masking coefficient at different OSNRs.

Fig. 4. System capacity and degrees of freedom. (a) System capacity versus mode number for different masking coefficients at OSNR = 20 dB. (b) The relationship between receiver size and transmission distance for different beam waists, $R_R=10\mathrm{cm}$ and $R_T=10\mathrm{cm}$.

Fig. 5. System performance with misalignment. (a) Synchronization coefficient under simultaneous displacement and angular errors. (b) BER of OAM +3 mode under simultaneous displacement and angular errors, with $\rho =0.187$, ${\omega }_0=2\mathrm{cm}$, and mode space = 2.

Fig. 6. Illustration of the indoor communication simulation for the OAM-CCL-based communication system.