Fig. 1. Schematic of an all fiber OAM communication system based on the integrated (de)multiplexer. Through the multiplexing grating, the incident Gaussian beams are converted to optical vortex beams carrying OAM states. After propagation in the few-mode fiber, the coaxial OAM beams are demultiplexed to Gaussian beams by an identical grating at the output facet of the fiber.

Fig. 2. Simulated mode purity of (a) +2 and (b) −2 order OAM beams converted from incident Gaussian beams at various coupling angles. (c) Simulated coupling efficiency for the +2 (black) and −2 (red) OAM modes with various numerical apertures.

Fig. 3. FDTD simulation of the excitation and evolution of OAM modes with an integrated multiplexer on the fiber facet. (a) Intensity and (b) phase excitation and evolution of the OAM mode of order +2. Corresponding (c) intensity and (d) phase of the −2-order OAM mode.

Fig. 4. Experimental setup of the OAM multiplexing fiber communication system. (a) Experimental setup comprises a transmitter, OAM (de)multiplexer, and receiver. The inserts show the measured intensity of the incident Gaussian beam, the generated OAM mode, and the two demultiplexed Gaussian beams, respectively. PC: polarization controller; SMF: single-mode fiber; EDFA: erbium-doped fiber amplifier; AWG: arbitrary-waveform generator; MZM: Mach–Zehnder electro-optical modulator; VOA: variable optical attenuator; BPF: band-pass filter; PD: photodiode; OSC: oscilloscope. (b) and (c) are the measured intensity and interferograms, respectively, for the generated +2-order OAM mode. (d) and (e) are the corresponding measured results for the −2-order OAM mode.

Fig. 5. Experimental results of OAM multiplexing fiber communication using integrated vortex grating. Demultiplexing results of (a) +2 mode, (b) −2 mode, (c) +2 and −2 modes. (d) Measured bit error rate (BER) of the OAM communication system.

Fig. 6. Image showing the fabrication of a vortex grating on the facet of a few-mode fiber using two-photon lithography. (a) Fiber holder used to adjust and to fix the position of the fiber tip with respect to the 3D writing system. (b) Schematic of the geometrical arrangement of the few-mode fiber, photoresist, glass slide, optical matching oil, and objective lens during 3D writing using two-photon lithography.

Fig. 7. Coupling efficiency for the +2 OAM mode as a function of numerical aperture obtained from FDTD simulations (red) and from measurements (cyan).