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Orbital angular momentum (OAM), inherent to light beams characterized by helical phase fronts, represents an alternative spatial degree of freedom for photons. In recent years, OAM-based communication has gained significant attention as a potential solution to increase the capacity and spectral efficiency of optical fiber communication systems. In optical fibers, OAM modes can be generated and manipulated by utilizing specialized optical components and techniques. One approach involves using specially designed optical fibers, such as ring–core fibers, which have the capability to support multiple OAM modes. In the manufacturing process, the ring fiber is hardly made perfectly, especially the doped-cores. The introduced defect breaks the symmetry of the ring–core, which has a serious impact on OAM mode transmission. Using a material with a slightly higher refractive index than the cladding to wrap the ring–core is effective in reducing impairment of defects. After numerical simulation, the proposed method improves the walk-off length by a factor of 2–40. [ABSTRACT FROM AUTHOR]

For enhancing channel capacity and spectrum efficiency, vortex beam carrying orbital angular momentum (OAM) has been ardently investigated in optical communication. However, the crosstalk among OAM modes induced by mode coupling has seriously hindered the development of vortex beam communication (OBC). A novel concept of mitigating crosstalk within an OAM-mode group for a short-haul optical fiber communication (OFC) is proposed by reversing the transmission matrix. To support more OAM modes propagation, a step-index OAM fiber with ring-shaped profile is also designed and fabricated, which can support 6 vector/OAM modes propagation. A proof-of-concept experiment is performed for verifying the feasibility of the proposed concept, where a quadrature-phase-shift-keying (QPSK) signal is utilized as an excitation signal. Bit-error-ratio (BER) and constellation diagram (CD) are employed to evaluate the performance of the proposed system. The captured intensity profiles and interference diagrams demonstrate that the propagated four OAM beams within the 2rd OAM-mode group are successfully received. The measured BER and CD illustrate that the crosstalk within the mode group can be significantly mitigated. Further, although the data speed or fiber length is increased, the system performance using the proposed concept is still superior to the case without increasing fiber length or data speed when the proposed concept is missed. Therefore, the proposed concept is effective for mitigating the crosstalk and can be then used for a short-haul OAM-based OFC. [ABSTRACT FROM AUTHOR]

• A mitigating crosstalk method is proposed by simultaneously propagating a probe beam with a data beam when the multiplexed vortex beam are propagated over a atmosphere turbulence (AT) free space. • A conjugated crosstalk is achieved by the probe beam during the square-law mixture in a photoelectric detector. • The conjugated crosstalk phase can effectively synthesize the crosstalk of data beam induced by AT. • A proof of concept is built to investigate the crosstalk, bit error rate, constellation plot, stability of system, etc. for verifying the feasibility of the proposed concept in practice. To overcome the upcoming famine of channel capacity driven by explosive increases of new applications, optical orbital angular momentum (OAM) carried by a vortex beam has been considered to be a promising candidate for space-division multiplexing (SDM) in a novel physics domain. However, owing to the inescapable disturbance induced by atmosphere turbulence (AT), the performance of OAM-based free-space optical communication (FSOC) is seriously restrained, which can be ascribed to the crosstalk among different OAM modes induced by the distorted helical phase. For promoting the development of OAM-based FSOC, a synthesizing-crosstalk method is proposed for mitigating the AT by coaxially propagating an OAM-probe beam (OPB), wherein the OAM mode of the extra OPB is identical to the OAM mode of the propagated OAM-data beam (ODB). The corresponding theory analysis of the proposed method demonstrates that the conjugated distortion of the crosstalk can be generated by a hybrid of the OPB and the ODB in a photoelectric receiver/detector (PD/PR). The generated conjugate distortion of the OPB can effectively counteract the crosstalk of ODB induced by the turbulence. To verify the feasibility of the proposed scheme, an experimental setup is built. The measured results demonstrate that the bit error rate (BER) and constellation plot (CP) of the employed two OAM channels can be significantly improved under different atmosphere turbulence (AT) by utilizing the proposed scheme. A continuous 30 min-long-BER-testing results depict that the proposed concept can stably work well. The proposed concept can be also suitable for multiplexing other OAM modes as well as the employed two OAM channels. Besides, with the increase of data speed, the proposed scheme can also effectively mitigate the crosstalk and improving the system performance. Consequently, the proposed concept is valid and feasible both in theory and practice, which can be used for compensating the distorted helical phase and improving the system performance of an OAM-based FSOC in the future. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

• 16 OAM modes (l = ± 1 ∼ ± 8) and four radial modes (p = 0 ∼ 3) are respectively employed for a 64-ary information coding/decoding. • A specially-designed DVG is used to demodulate the received OAM beams into an 4 × 4 intensity-array profile for decoding. • A proof-of-concept experimental are also proposed and established for verifying the feasibility and validity of the proposed concept in practice. • Bit error rates (BER) with respect to a series of parameters (AT strength, propagation distance, and bit rate) are also employed for evaluating the performance of the proposed concept, respectively. A novel coding/decoding concept based on the orbital angular momentum (OAM) mode and radial mode of a vortex beam is proposed for short-haul free-space optical communication. Sixty-four kinds of vortex beams, which are comprised of sixteen OAM modes and four radial modes, are employed and utilized for coding each of 6-bits-length sequences into one of the 64-ary symbols. A specially-designed Dammam vortex grating (DVG) is utilized to generate an intensity-array profile, where the received vortex beam is demodulated into Gaussian beam in the related coordinate (order). In one of the coordinates, the received vortex beam can be converted into a bright spot (Gaussian beam) with a certain number of intensity rings. According to the coordinate and number of intensity rings, the received vortex beam can be identified and then decoded into the initially-propagated information sequence. The corresponding theory analysis/simulations and experiments demonstrate that the proposed concept is valid and feasible. In addition, the measured results also demonstrate that the performance of the proposed concept declines as the transmission rate/ atmosphere turbulence strength/propagation distance increases. By adopting advanced devices and approaches, the performance of the proposed scenario can be significantly improved, and then the proposed concept can be potentially utilized for short-haul free-space optical communication in the future. [ABSTRACT FROM AUTHOR]

A novel concept of coding/decoding data is proposed by multiplexing vortex beams for short-haul-free-space light communication. Eight vortex beams carrying different orbital angular momentum (OAM) modes (-5, 3, -2, 1, 5, -3, 2, -1) are employed to code the initial information sequences into each 256-ary symbols by different multiplexing approaches. A specially-designed Dammam vortex grating is utilized to decode the received vortex beams into the initial information sequences. In order to certifying the validity and feasibility of the proposed concept in practice, a proof-of-concept experiment is also proposed and established. The measured results demonstrate that a 128 × 128 gray-scale image ("Lena") can be perfectly propagated over a 10 m free space by the proposed concept. In order to further evaluate the performance of the proposed concept, bit error rate (BER) against the received optical signal-to-noise ratio (OSNR) is also employed to analyzing the influences of some parameters on the system performances. The measured BER distributions demonstrate that the BER performance decreases as the data rate/propagation distance/atmosphere turbulence strength increases, respectively. • We proposed a novel scenario of coding and decoding data by multiplexing vortex beams in free space is proposed. • Eight OAM modes are employed to code/decode data by (de)multiplexing for representing 256-ary symbols. • A single hologram is designed to generate the multiplexed vortex beams for coding. • A Dammam vortex grating is also designed for demodulating the received vortex beams. • An experiment is established for verifying the validity and feasibility of the proposed concept and analyzing the performance of the proposed system. [ABSTRACT FROM AUTHOR]