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Vortex beams (VBs) carrying orbital angular momentum (OAM) have shown promising potential in enhancing communication capacity through the possession of multiple multiplexing dimensions involving the OAM mode, polarization, and wavelength. Although many research works on multidimensional multiplexing have been conducted, the (de)multiplexer compatible with these dimensions remains elusive. Following the expanded concept of the Pancharatnam–Berry (PB) phase, we designed a polarization-dependent phase-modulation metasurface to phase-modulate the two orthogonal linearly polarized components of light, and two Dammann vortex gratings with orthogonal polarization responses were loaded to simultaneously (de)multiplex OAM mode and polarization channels. As a proof of concept, we constructed a 16-channel multidimensional multiplexing communication system (including two OAM modes, two polarization states, and four wavelengths), and 400 Gbit/s quadrature-phase shift-keying (QPSK) signals were transmitted. The results demonstrate that the OAM mode and polarization channels are successfully (de)multiplexed, and the bit-error-rates (BERs) are below 1.67 × 10−6 at the received power of −15 dBm.

Vortex beams (VBs), characterized by helical phase front and orbital angular momentum (OAM), have shown perspective potential in improving communication capacity density for providing an additional multiplexing dimension. Here, we propose a diffractive deep neural network (D2NN) method for OAM mode multiplexing and demultiplexing. By designing the D2NN model and simulating light propagation through multiple diffractive screens, the phase and amplitude values can be automatically adjusted to manipulate the wavefront of light beams. Training the D2NN model with mode coupler and separator functions, we convert VBs into target light fields with the diffraction efficiency exceeds 97%, and the mode purities are over 97%. Constructing an OAM multiplexing link, we successfully multiplex and demultiplex two OAM channels that carry 16-QAM signals in simulation, and the demodulated bit-error-rates are below 1×10-4. It is anticipated that the D2NN can perform flexible modulation of multiple OAM modes, which may open a new avenue for high-capacity OAM communication and all-optical information processing, etc.

The objective of this study was to comparative evaluate the protective efficacy of live attenuated and inactivated porcine reproductive and respiratory syndrome vaccines. Fifty-five healthy piglets free of PRRSV antigen and antibody were randomly assigned into four groups. Piglets of group 1 (n = 20) were inoculated with PRRS live vaccine TJM-F92 strain (Live vaccine TJM-F92). Piglets of group 2 (n = 20) were inoculated with commercial PRRS inactivated vaccine (Inactivated vaccine). Piglets of group 3 (n = 10) were inoculated with PBS (Negative control). Group 4 (n = 5) remained as a not-immunized and not-challenged control (Mock group). And half of piglets from groups 1 and 2 were randomly selected and challenged with HP-PRRSV TJ strain and PRRSV NADC30-like strain respectively at 49 days post immunization (dpi). Our data showed the level of antibodies arose from live vaccine was significantly higher and faster than that from inactivated vaccine after immunization. Neither high fever nor signs of clinical disease were observed in piglets immunized with live vaccine, whereas piglets in inactivated vaccine group exhibited serious clinical symptoms, pathological lesions, and viremia load after HP-PRRSV TJ strain challenge. No obvious clinical signs, gross pathology lesions and viremia load were found in live vaccine group, while severe clinical signs, pathology change of lungs and high viremia load were recorded in inactivated vaccine group after NADC30-like strain challenge. The results indicated that PRRS live vaccine (TJM-F92 strain) could effectively prevent the infection of HP-PRRSV TJ strain and virulent NADC30-like strain. Immunization with PRRS inactivated vaccine could not effectively arouse immune system and provide strong immune cross-protection. Therefore, it is necessary to select suitable, widely application and effective PRRS vaccine for PRRS control according to the situation of pig farms.

Here we propose a polarization-dependent gradient phase modulation strategy and fabricate a local polarization-matched metasurface to add/drop polarization multiplexed cylindrical vector beams (CVBs). The two orthogonal linear polarization states in CVB multiplexing will represent as radial- and azimuthal-polarized CVBs, which means that we must introduce independent wave vectors to them for adding/dropping the polarization channels. By designing the rotation angle and geometric sizes of a meta-atom, a local polarization-matched propagation phase plasmonic metasurface is constructed, and the polarization-dependent gradient phases were loaded to perform this operation. As a proof of concept, the polarization multiplexed CVBs, carrying 150-Gbit/s quadrature phase shift keying signals, are successfully added and dropped, and the bit error rates approach 1 × 10−6. In addition to representing a route for adding/dropping polarization multiplexed CVBs, other functional phase modulation of arbitrary orthogonal linear polarization bases is expected, which might find potential applications in polarization encryption imaging, spatial polarization shaping, etc.

Optical vortex beams (VBs) carrying orbital angular momentum (OAM) have shown significant multiplexing ability in radio-over-fiber communication by virtue of their intrinsic mode orthogonality. However, the large bandwidth required by optical microwave and the narrow bandwidth of mode converters severely hinder its practical applications. By exploiting the Pancharatnam-Berry phase of metasurface, we experimentally demonstrate a mode conversion solution for OAM multiplexing based radio-over-fiber communication. Based on the dispersion-free Pancharatnam-Berry phase design, these metasurfaces show broadband working wavelengths (from 1540 nm to 1600 nm) and polarization-insensitive characteristic. These make them compatible with polarization division multiplexing (PDM) and wavelength division multiplexing (WDM). As a proof-on-concept, a three-dimensional multiplexing based radio-over-fiber communication is demonstrated with 2 OAM modes, 2 polarization states and 4 wavelengths, and these 16 channels are demultiplexed with crosstalk less than −13.29 dB. The communication sensitivity decreases about 2.5 dB after introducing PDM and WDM because of the increased crosstalk. The 96 Gbit/s QPSK-OFDM signals carried by 16 channels are demodulated with bit-error-rate lower than 3.26 × 10−5 at 4.5 dBm.

The characteristics of positive DC corona discharge in SF 6 /N 2 gas mixtures under different gas-mixture ratios are investigated with a needle-to-plane electrode configuration. With increasing applied voltage, a discharge-stage transition process including the Townsend stage, streamer-pulse stage, brush-pulse stage and breakdown stage is observed. The variation in current-pulse characteristics, including pulse-time parameters, mean pulse amplitude and pulse-repetition rate, is investigated under different gas-mixture ratios. The pulse-time parameters are not affected by gas-mixture ratio. The pulse-repetition rate increases with the increase of SF 6 concentration and mean current, ranging from hundreds of kilohertz to thousands of kilohertz. The streamer-pulse amplitude increases with increasing SF 6 concentration. With increasing mean current, the small-amplitude brush pulses become dominant, thus the mean pulse amplitude decreases. In order to further explore the influence mechanism of gas-mixture ratio on positive corona discharge, a fluid model of positive corona discharge is developed and relevant simulations show the formation process of current pulse, and verify the validity of the pulse characteristics changing with gas-mixture ratio.

Structured light beams (SLBs), mainly including vortex beams (VBs) and cylindrical vector beams (CVBs), have received significant attention in optical communications due to orthogonal orbital angular momentum (OAM) or vector modes. However, the mode conversion between OAM and vector modes and the working bandwidth are always challenges restricting the practical applications. Here, we demonstrate broadband SLB multiplexing in optical communication with dielectric Pancharatnam-Berry phase-based metasurfaces (PBMs). By virtue of the dispersion-free characteristic and spin-orbital interaction, the designed metasurfaces have a broad working wavelength ranging from C- to L-bands and can flexibly convert VBs and CVBs. With these properties, SLBs are successfully multiplexed with conventional physical dimensions (including wavelengths and polarization states) to construct complex multiplexing formats. In a back-to-back free-space transmission experiment, 50 Gbit/s quadrature-phase shift-keying (QPSK) signals are used for multiplexing with four OAM modes, seven wavelengths, and two polarization states. When the transmission rate reaches 2.73 Tbit/s, the bit-error rate (BER) is below 10-6 at an optical signal-to-noise ratio (OSNR) of 17 dB. By changing the polarization states of the incident light, CVB multiplexing is realized, and the BER is below 10-6 at an OSNR of 19 dB. Owing to the silica substrate, PBM is capable of photonic integration, and it is anticipated that this PBM may provide a new avenue for high-order multiplexing and all-optical signal processing of SLBs.

Atmospheric turbulence in free-space will distort the helical phase-front of vortex beams (VBs) and cause mode diffusion, seriously hindering the practical application of optical orbital angular momentum (OAM) communications. Here, we propose and experimentally investigate a convolutional neural network (CNN)-based atmospheric turbulence compensation method for OAM multiplexing communication. Taking advantage of signal processing, we design a CNN model that can automatically extract the characteristic parameters from the distorted intensity distribution of VBs. After supervisory training, the CNN model possesses a strong generalization ability and can efficiently predict the equivalent turbulence phase screen. Under the influence of the turbulence with $\boldsymbol{C}_n^{\bf{2}} = {\bf{5}} \times {\bf{1}}{{\bf{0}}^{ - {\bf{13}}}}{\textbf{m}^{ - {\bf{2}}/{\bf{3}}}}$ and $\Delta {\bf{z}}{\kern 1pt} = {\kern 1pt} {\bf{{50}}}\,{\mathbf{m}}$ , the mode purity of the distorted VB improves from 26.91% to 93.12% through the compensation. By constructing an OAM multiplexing communication link with the bit-rate of 100 Gbit/s and employing the CNN model to equalize the OAM channels, the bit-error-rates are decreased by three orders of magnitude, and the measured crosstalk is reduced from −23.15 dB to −29.46 dB. Moreover, the constellations converge obviously at the signal-to-noise ratio of 20 dB, and the error-vector-magnitude decreases from 0.3337 to 0.1622. These results indicate that the CNN model can well compensate the atmospheric turbulence induced distortion in VBs, and may open new avenues for improving the performance of OAM communications.

Structured light beams (SLBs) with spatially nonuniform phase or polarization distributions have attracted enormous interest in optical multiplexing communications due to the mutual orthogonality o...

Detecting orbital angular momentum (OAM) modes with high-speed and -accuracy is always a challenge in the practical applications of vortex beams (VBs). Here, we design a multilayer feed-forward neural network (FNN) model to rapidly identify OAM modes. After trained with loads of learning samples influenced by the transmission distance, beam waist, and atmospheric turbulence, the model shows a good generalization ability in quickly identifying the OAM mode of VBs ranging from −25 to +25. The classification accuracy reaches 99.55% under the influence of moderate turbulence and achieves 68.53% even under strong turbulence. Furthermore, we construct a communication link to deliver a 100 × 100 pixels Lena gray image by encoding OAM modes. After free-space transmission, the OAM-SK signals are demodulated by the FNN model, and the Lena gray image is well reconstituted. With an i5-6500 CPU, the identification process takes only 7.5 × 10−5 s per OAM mode. These numerical simulation results demonstrate that the proposed FNN model provides a feasible way to rapidly identify the OAM modes of VBs and may show great potential in the optical OAM communication, etc.

Rapidly detecting orbital angular momentum (OAM) modes is crucial for optical OAM shift-keying (OAM-SK) communication. Here, we propose and investigate a novel coherent detection method for demodulating optical OAM-SK signals. A reference light beam with desired OAM modes and weights is well-designed and employed to interfere with the light beam which carries OAM-SK signals. Then the OAM-SK signals representing as the rapid switching of OAM modes are converted to intensity changes for demodulation. In the numerical simulation, a pseudo-random sequence is mapped and encoded to different OAM modes (+1, +2, +3, +4) to produce OAM-SK signals, and the demodulated bit-error-rate (BER) of 9 . 7656 × 1 0 − 4 is obtained at the signal-noise ratio (SNR) of 12-dB. By modulating the OAM modes and phase of the optical carrier synchronously, a multi-dimensional modulation format (16-ary OAM-Phase modulation) is constructed, and the demodulated BER is less than 4 . 8828 × 1 0 − 4 at the SNR of 14-dB. These results demonstrate that the coherent detection method provides an effective way to demodulate OAM-SK signals, and may shows great potential in high-speed optical OAM communications.

Vortex beams have application potential in multiplexing communication because of their orthogonal orbital angular momentum (OAM) modes. OAM add–drop multiplexing remains a challenge owing to the lack of mode selective coupling and separation technologies. We proposed an OAM add–drop multiplexer (OADM) using an optical diffractive deep neural network (ODNN). By exploiting the effective data-fitting capability of deep neural networks and the complex light-field manipulation ability of multilayer diffraction screens, we constructed a five-layer ODNN to manipulate the spatial location of vortex beams, which can selectively couple and separate OAM modes. Both the diffraction efficiency and mode purity exceeded 95% in simulations and four OAM channels carrying 16-quadrature-amplitude-modulation signals were successfully downloaded and uploaded with optical signal-to-noise ratio penalties of ∼1 dB at a bit error rate of 3.8 × 10−3. This method can break through the constraints of conventional OADM, such as single function and poor flexibility, which may create new opportunities for OAM multiplexing and all-optical interconnection.

Here, we propose and experimentally demonstrate a cylindrical vector beam (CVB) sorter based on a spin-dependent spiral transformation. By exploiting the spin–orbital interaction of the geometric phase, a pair of conjugated spiral transformations are applied to modulate the two orthogonal circularly polarized components of the CVB, which are converted into the same linear phase gradient from opposite azimuthal phase gradients. Since the orthogonal spin components of CVBs with different polarization orders carry different phase gradients, under the convergence of a convex lens, the coaxially transmitted CVBs can be sorted with spatially separated positions, and the increased phase gradient provided by the spiral transformation yields the high resolution. We show that five CVB modes from − 2 to + 2 are successfully sorted with a separation efficiency of 3.65. As a proof-of-concept, we demonstrate a two-channel CVB multiplexing communication with a bit error rate approaching 10 − 6 . In addition to providing an avenue for CVB demultiplexing, our results show potential applications in mode filtering and mode routing in all-optical interconnection.

The emergence of cylindrical vector beam (CVB) multiplexing has opened new avenues for high-capacity optical communication. Although several configurations have been developed to couple/separate CVBs, the CVB multiplexer/demultiplexer remains elusive due to lack of effective off-axis polarization control technologies. Here we report a straightforward approach to realize off-axis polarization control for CVB multiplexing/demultiplexing based on a metal–dielectric–metal metasurface. We show that the left- and right-handed circularly polarized (LHCP/RHCP) components of CVBs are independently modulated via spin-to-orbit interactions by the properly designed metasurface, and then simultaneously multiplexed and demultiplexed due to the reversibility of light path and the conservation of vector mode. We also show that the proposed multiplexers/demultiplexers are broadband (from 1310 to 1625 nm) and compatible with wavelength-division-multiplexing. As a proof of concept, we successfully demonstrate a four-channel CVB multiplexing communication, combining wavelength-division-multiplexing and polarization-division-multiplexing with a transmission rate of 1.56 Tbit/s and a bit-error-rate of 10−6 at the receive power of −21.6 dBm. This study paves the way for CVB multiplexing/demultiplexing and may benefit high-capacity CVB communication.

This paper presents and compares the results of two simulation methods for negative corona current pulses in SF 6 at 0.3 Mpa. A needle-plane electrode configuration is constructed in a 2D axisymmetric model using COMSOL Multiphysics®. The first model is constructed based on fluid dynamics and plasma chemical reactions. The second is a simplified fluid model which reduces the whole particles and reactions into three continuity equations of electrons, positive ions and negative ions. The results show that in the first simulation, the current pulse bears a close resemblance to that in experiment, with a magnitude of 1.31 mA and a pulse-width of 48.7 ns. The second simulation can also successfully simulate the current waveform, but there exists obvious deviation from experimental current pulses in numerical values. Based on the spatiotemporal distribution of mean electron energy and charged particles, the mechanism of pulse formation is analyzed.

Due to the spatial non-uniform distribution of polarization, cylindrical vector beam (CVB) shows many superior properties and has drawn many research interests. In this letter, we propose and investigate a cross-polarized modulation method to generate the stable CVBs with arbitrary state on the high-order Poincare sphere (HOPS). It demonstrates that the CVB can be decomposed into a left-handed and a right-handed circularly polarized vortex beam. Exploiting the polarization sensitivity of spatial light modulators (SLMs), the spatial spiral phases are modulated to the horizontal and vertical parts of the linearly polarized beam to produce the CVB. By manipulating the phase difference between the two orthogonal-polarized components, the CVBs with arbitrary rotation at the equator of HOPS are experimentally obtained. These results indicate that the cross-polarized modulation method may provide a simple and flexible way to generate the CVBs with arbitrary polarization states and high stability.

Vector light beams (VLBs), manifesting as the non-uniform spatial distribution of polarization states, have shown broad applications in increasing communication capacity. Here, we propose and experimentally demonstrate a spatial phase cross-polarized method for achieving a mode coded modulation of VLBs. By employing the polarization sensitivity of the spatial light modulators (SLMs), the left and the right-handed circularly polarized parts of light beams are modulated with opposite spatial spiral phase to produce VLBs by the coherent combination. Manipulating the spatial phase distribution of the left and the right-handed circularly polarized light beams, the VLBs with 16 modes ([m; l]=[2, 3, 4, 5, 3, 4, 5, 6, 4, 5, 6, 7, 5, 6, 7, 8; 0, −1, −2, −3, 1, 0, −1, −2, 2, 1, 0, −1, 3, 2, 1, 0]) are obtained and used to code digital signals. In the experiment, a 60 × 60 pixels flower gray image with 14 400 quaternary numbers are then coded to the 16 modes and decoded successfully after 1 m transmission in free space. And the flower gray image is also recovered accordingly.

Orbital angular momentum (OAM) shift-keying (SK), which manifests as the rapid switching of OAM modes, is crucial but severely hampered for lacking effective demodulation techniques in OAM communication. In this paper, we propose and investigate a novel direct-demodulation method for the OAM-SK communication. After passing through a well-designed beam slit, the optical vortex beam will possess varying output energies associated with the OAM modes. Hence, splitting the optical vortex beam into two sub-beams and one of them passing through the slit, we can directly extract the OAM-SK signals from the energy ratios of the two sub-beams. The results show that the bit-error-rate (BER) of demodulated OAM-SK signals can reach 3.7 ×10-3 even under the influence of strong atmospheric turbulence with Cn2 = 3 ×10-12 and misalignment of 1.8 mm. Encoding digital signals to OAM modes and amplitude of optical vortex beams simultaneously, we construct a novel multi-modulation format composed of 16 QAM-OFDM and OAM-SK signals, which is also demodulated with the BER of 10-5 at the optical signal-to-noise ratio (OSNR) of 18 dB. These demonstrate that the direct-demodulation method provides a feasible way to demodulate OAM-SK signals and may show potential in complex OAM modulation communications.

We have experimentally demonstrated an effectively full-duplex radio-over-fiber (ROF) system, which only used a single light source at central station (CS). Frequency quadrupling modulation scheme was employed to generating 100-GHz optical millimeter wave (mm-wave) for downlink transmission while the central optical carrier was reused at base station (BS) for uplink connection. Using this proposed system design, 2.5-Gb/s OOK data was successfully transmitted over 10-km dispersion shifted fiber (DSF) and 3-m wireless link for downstream channel with about 3.5-dB power penalty, and 2.5-Gb/s OOK data was transmitted over 50-km standard single-mode fiber (SMF) for upstream channel with power penalty less than 0.5-dB.This system shows good performance in long distance transmission and provides an efficient solution for high frequency mm-wave communication.

Vortex beams (VBs), possessing a helical phase front and carrying orbital angular momentum (OAM), have attracted considerable attention in optical communications for their mode orthogonality. A platform for achieving all-optical signal processing of VBs, however, remains elusive due to the limited light-field-manipulation capability. We introduce diffractive deep neural networks (${\mathrm{D}}^{2}$NNs) and their applications to process VBs. Exploiting the multiple-light-field-modulation ability of multilayer diffraction structures and the strong data-processing capability of deep neural networks, we reveal that ${\mathrm{D}}^{2}$NNs can manipulate multiple VBs by configuring the phase and amplitude distribution of diffractive screens. The diffraction efficiency and converted-mode purity are greater than 96%. After being trained, ${\mathrm{D}}^{2}$NNs with functions of hybrid-OAM-mode generation, identification, and conversion are obtained, and three typical types of all-optical signal-processing communication, (OAM-shift keying (OAM-SK), OAM multiplexing and demultiplexing, and OAM-mode switching) are successfully achieved. Our simulation results provide an approach that breaks the limitations of poor functionality and complex design in processing VBs, introducing the ${\mathrm{D}}^{2}$NN as a universal light-field-modulation platform.

The development of high-speed optical communication calls for compatible multiplexing dimensions to enlarge transmission capacity and process optical information. Although vortex beam multiplexing has been previously demonstrated by exploiting the orthogonality of orbital angular momentum (OAM) modes, the multidimensional demultiplexer remains elusive due to the limited phase and polarization manipulation capability. We introduce dielectric propagation phase metasurface based demultiplexers and investigate their application in simultaneously demultiplexing OAM and orthogonal linear polarization (OLP) channels. Utilizing the size-dependent anisotropy of dielectric rectangular nano-pillars, we reveal that the multimode fork grating phase masks with different off-axis phase items can be integrated on the same substrate with polarization selectivity, which allows simultaneously demultiplexing OAM and OLP channels. The demultiplexed position and the number of channels depend on its off-axis phase items, and a 16-channel (8-OAM modes and 2-polarization states) demultiplexer is demonstrated. The vortex beams with different OAM modes and OLP states are successfully demultiplexed with the maximum crosstalk is below −9.2 dB. Changing the number of superimposed complex amplitudes and the off-axis phase items in multimode fork grating phase masks, we further present two 12-channel demultiplexers with different channel distributions. Our results provide an approach that breaks the polarization-insensitive in manipulating OAM modes, introducing demultiplexers that apply to OAM mode and OLP states simultaneously.

Insulator is a very typical weak point of inner insulation in gas-insulated power equipment, which is commonly made of alumina-filled epoxy resin. When a corona discharge is caused by tip defect near the insulator, it may lead to the corona develop into a surface discharge, resulting in insulation degradation. In this paper, we investigate the effect of epoxy resin spacers on corona characteristics in SF 6 /N 2 (5:95) mixtures under positive DC voltages. A three-electrode arrangement corona discharge experimental platform was built. For a constant electrode gap spacing, the influence of tip defect location on corona characteristics is analyzed by using the same size epoxy resin spacers. Characteristics of corona were studied including the corona onset voltage, breakdown voltage, current pulse, and changes in corona pattern. The experimental results have shown that the corona onset voltage and breakdown voltage increase first and then saturate with increasing distance between the needle electrode and the dielectric surface. The corona pulse characteristics are not affected by dielectric solid. If the needle electrode is located at the surface of the epoxy resin spacer, the corona will be attracted to the dielectric surface, forming so called surface discharge. If the needle electrode is set away from the surface, corona discharge will also be attracted by the surface and form a “towards dielectric” component. But the surface effect decreases with increasing distance, in our case, the dielectric effect can be neglected if the distance is larger than 12 mm.