Photonic crystal fibers (PCFs) have been extensively studied to enhance communication capacity and spectral efficiency by supporting orbital angular momentum (OAM) modes. However, the structural fragility resulting from the central air hole in PCFs undermines the fiber’s roundness. We introduce a hybrid PCF that replaces the central air hole with a material with a high refractive index. This innovation not only enhances the fiber’s structural strength but also introduces an additional communication channel. Furthermore, this method increases the amount of OAM modes supported for transmission in the ring. Numerical results demonstrate that significant differences in refractive indices exist between adjacent vector modes, with high mode quality (ranging from 74.6% to 97.6%) and a small effective mode area (ranging from 23.5 to 59.5 μm2). The proposed hybrid PCF can transmit 20 OAM modes in the inner core and 54 OAM modes in the outer ring. The spacing between the two layers of air holes has a minimal impact on the effective mode area and mode quality. However, the effective mode area decreases and the mode quality increases with the enlargement of the diameter of air holes. In addition, a decrease in doping concentration results in lower mode quality and a larger effective mode area. Moreover, the proposed hybrid PCF can be employed in OAM-based mode division multiplexing for high-capacity short-range fiber optic communication.
A concept of noncoaxially propagating discrete data by coding/decoding orbital angular momentum (OAM) of vortex beam through multiple channels is proposed. Four independent channels along different directions (diffraction orders/angles) are established by a series of specially designed holograms. Sixteen vortex beams are used to code each sequence with the length of 4 bits into an OAM mode in each channel. The theoretical analysis (simulation) demonstrates that the proposed scheme is reliable. In addition, an experiment is also designed and performed for practically certifying the feasibility of the proposed scheme. A 16-bits-length sequence, which is coded into four OAM modes in four channels (one channel with one OAM mode), is successfully received/decoded by observing the intensity-profile arrays (OAM-mode spectrum) generated by a specially designed Dammann vortex grating. The measured results show that the proposed concept is also viable in practice. Moreover, the received/recovered images indicate that the performance of the system decreases as the atmosphere turbulence strength increases when a color image and a gray-scale image are simultaneously propagated. Besides, the measured bit error rate (BER) gradually decreases with the increase of bit rate or propagation distance. The acceptable BER under the threshold of forward error correct can be achieved for a short-distance communication network.
Due to the characteristics of spatially inhomogeneous polarization and unlimited spatial coordinates, a concept of hybrid multiplexing and encoding/decoding based on spatial coordinates and mode states of vector beams is proposed for potentially enhancing the channel capacity and spectrum efficiency. For the sake of verifying the potentiality, a new encoding/decoding method by the mode states (i.e., polarization and mode) and spatial coordinates of vector beams is also proposed in this paper. In addition, the hybrid multiplexed vector beams are also utilized as carriers (16 channels) for transmitting data by four spatial coordinates and four mode states. In order to verify the feasibility of the proposed concepts, the relevant experimental setups are elaborated and established in this paper. Meanwhile, a fast mode recognition method based on the lookup table (LUT), image processing, and digital signal processing are employed to decode data when the hybrid vector beams propagate in free space (FS). The experimental results demonstrate that the encoded sequences (i.e., 0, 66, and D9) by the mode states and spatial coordinates can be successfully received and demodulated to the original data without errors after propagating 115 cm when the hybrid multiplexed vector beams are used for encoding/decoding. The total transmission rate of 320 Gbit / s can be acquired by combining the 16 channels (four mode states and four spatial coordinates) when the hybrid multiplexed vector beams are used as carriers. Furthermore, bit error rate, constellation diagrams (different transmission distance, transmitting data rate), and crosstalk among different channels or coordinates are employed to evaluate the propagation performances when the hybrid multiplexed vector beams are used as carriers.
By exploiting the non-Kolmogorov model and Rytov approximation theory, a propagation model of Bessel–Gaussian vortex beams (BGVB) propagating in a subway tunnel is derived. Based on the propagation model, a model of orbital angular momentum (OAM) mode probability distribution is established to evaluate the propagation performance when the beam propagates along both longitudinal and transverse directions in the subway tunnel. By numerical simulations and experimental verifications, the influences of the various parameters of BGVB and turbulence on the OAM mode probability distribution are evaluated, and the results of simulations are consistent with the experimental statistics. The results verify that the middle area of turbulence is more beneficial for the vortex beam propagation than the edge; when the BGVB propagates along the longitudinal direction in the subway tunnel, the effects of turbulence on the OAM mode probability distribution can be decreased by selecting a larger anisotropy parameter, smaller coherence length, larger non-Kolmogorov power spectrum coefficient, smaller topological charge number, deeper subway tunnel, lower train speed, and longer wavelength. When the BGVB propagates along the transverse direction, the influences can be also mitigated by adopting a larger topological charge number, less non-Kolmogorov power spectrum coefficient, smaller refractive structure index, shorter wavelength, and shorter propagation distance.
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