Fig. 1. The principle scheme of an HFG for generation of the OAM modes from the fundamental mode. (a) The OAM mode with TC of −1 obtained by using a cHFG and (b) the OAM mode with TC of +1 obtained by using a ccHFG.

Fig. 2. The principle scheme for converting the OAM modes back to the fundamental mode. Fundamental mode obtained from (a) the OAM mode with TC of +1 by using a ccHFG and (b) the OAM mode with TC of −1 by using a cHFG.

Fig. 3. The experimental setup for fabrication and monitoring the spectra of the proposed HFGs.

Fig. 4. The experimental setup for measuring the intensity and phase distributions of the OAM beams generated by the HFGs.

Fig. 5. Transmission spectra of the fabricated HFGs. (a) cHFG-1, (b) ccHFG-1, (c) cHFG-2, and (d) ccHFG-2.

Fig. 6. The experimental setup and the corresponding measurement results for validation of the mode-coupling rules obeyed in cHFG and ccHFG. (a) Mode conversions from the mode HE11 to OAM−1 and finally back to HE11 by sequentially using cHFG-1 and ccHFG-2. (b) Mode conversions from the mode HE11 to OAM+1 and finally back to HE11 by sequentially using ccHFG-1 and cHFG-2.

Fig. 7. The schematic diagram for the HFGs-based all-fiber OAM encoding/decoding system. MZM: Mach-Zehnder modulator; AWG: arbitrary waveform generator; PD: photodetector; SMF: single-mode fiber; FMF: few-mode fiber.

Fig. 8. The measurement results for binary random data series. (a) Input data series and (b) the recovered data series.