Large-capacity Communication Research Articles

A low crosstalk seven-core five-mode fiber for high density transmission and large capacity communication

We propose a seven-core five-mode fiber with air hole-assisted and high refractive index ring structure, and it can be fabricated by the stack-and-draw technique. The air hole auxiliary structure can significantly restrain the energy coupling between cores, and the high refractive index ring structure can meet the requirements of low intra-core crosstalk (XT) by improving the effective refractive index difference (Δn eff ) between adjacent modes. Through analyzing the influence of the number and arrangement of air holes on inter-core XT and effective mode field area (A eff ), it can be achieved the results that the inter-core XT is less than −44 dB/100 km and all the A eff values of LP modes are greater than 100 μm2 in the seven-core five-mode fiber when the wavelength is 1550 nm and the bending radius (R) is 80 mm. At the same time, the Δn eff between LP21 and LP02 mode can be controlled to 1.3 × 10−3 by adjusting the parameters of the high refractive index ring. In addition, the relative core multiplexing factor (RCMF) of the seven-core five-mode fiber is as high as 88.34. The simulation results show that the seven-core five-mode fiber has high spatial density and is suitable for the communication system with multi-channel and long-distance transmission.

Orbital angular momentum mode femtosecond fiber laser with over 100 MHz repetition rate

Orbital angular momentum (OAM) lasers have potential applications in large capacity communication systems, laser processing, particle manipulation and quantum optics. OAM mode femtosecond fiber laser has become the research focus due to the advantages of simple structure, low cost and high peak power. At present, OAM mode femtosecond fiber lasers have made some breakthroughs in key parameters such as repetition frequency, pulse width, spectrum width, but it is difficult to achieve good overall performance. Besides, the repetition rate is tens of MHz at present. In this paper, a large-bandwidth mode coupler is made based on the mode phase matching principle. In coupler, the first order mode coupler with 3 dB polarization dependent loss is made by the technology of strong fused biconical taper, and the second order mode coupler with 0.3 dB polarization dependent loss is made by the technology of weak fused biconical taper. By combining the nonlinear polarization rotation mode-locking mechanism, OAM mode femtosecond fiber laser with over 100 MHz repetition rate is built. The achievement of the key parameters is attributed to the selection of dispersion shifted fibers that can accurately adjust intracavity dispersion. Comparing with traditional dispersion compensation fibers (DCF), the group velocity dispersion is reduced by an order of magnitude, so it can better adjust intracavity dispersion to achieve the indexes of large spectral bandwidth and narrow pulse width. In addition, the diameter of the fiber is 8 μm, which is the same as that of a single mode fiber. Comparing with DCF, the fusion loss can be ignored, so only a shorter gain Erbium-doped fiber is required, which ensures a shorter overall cavity length and achieves high repetition frequency. The experimental results show that the first order OAM mode fiber laser has 113.6 MHz repetition rate, 98 fs half-height full pulse width, and 101 nm 10 dB bandwidth. Second-order OAM mode fiber laser has 114.9 MHz repetition rate, 60 fs half-height full pulse width, and 100 nm 10 dB bandwidth. Compared with the reported schemes, our scheme has good performance in key parameters such as repetition rate, pulse width and spectral width. We believe that the OAM mode fiber laser with excellent performance is expected to be widely used in OAM communication, particle manipulation and other research fields.

Integration of Sensing and Communication in a W-Band Fiber-Wireless Link Enabled by Electromagnetic Polarization Multiplexing

The coexistence needs of sensing and communication in millimeter-wave (mmW) bands have urgently driven the seamless integration of sensing and communication in the upcoming mmW era. However, the time-frequency competition between the two functions makes it difficult to accommodate both high sensing resolution and large communication capacity. In this paper, we have designed a W-band fiber-wireless link with the integrated sensing and communication functions enabled by electromagnetic polarization multiplexing. The ultra-wideband fiber-wireless link in W band is enabled by the asymmetrical single-sideband modulation along with the optical heterodyne up-conversion. The electromagnetic polarization multiplexing allocates the sensing and communication functions on two orthogonal electromagnetic polarizations, respectively. Thus, all time-frequency resources of the fiber-wireless link can simultaneously serve these two functions without any resource competition, contributing to an ultra-high spatial resolution and an ultra-large data capacity at the same time. Our experimental results show the spatial resolution of up to 15 mm and data rate as high as 92 Gbit/s were simultaneously realized in W band after delivering over a 10.8-m wireless distance. The overall improvement of both the sensing and communication performance, to the best of our knowledge, led to a record capacity-resolution quotient of 61.333 Gbit/s/cm. In addition, we have qualitatively investigated the integrated sensing and communication fiber-wireless link, in terms of the carrier frequency, system bandwidth, multi-mmW access, and electromagnetic polarization crosstalk.

Transmission Technology for Space-division Multiplexing Supporting Next-generation High-speed, Large-capacity Communications beyond Existing Limits

Transmission Technology for Space-division Multiplexing Supporting Next-generation High-speed, Large-capacity Communications beyond Existing Limits

Multidimensional optical tweezers synthetized by rigid-body emulated structured light

Structured light with more extended degrees of freedom (DoFs) and in higher dimensions is increasingly gaining traction and leading to breakthroughs such as super-resolution imaging, larger-capacity communication, and ultraprecise optical trapping or tweezers. More DoFs for manipulating an object can access more maneuvers and radically increase maneuvering precision, which is of significance in biology and related microscopic detection. However, manipulating particles beyond three-dimensional (3D) spatial manipulation by using current all-optical tweezers technology remains difficult. To overcome this limitation, we theoretically and experimentally present six-dimensional (6D) structured optical tweezers based on tailoring structured light emulating rigid-body mechanics. Our method facilitates the evaluation of the methodology of rigid-body mechanics to synthesize six independent DoFs in a structured optical trapping system, akin to six-axis rigid-body manipulation, including surge, sway, heave, roll, pitch, and yaw. In contrast to previous 3D optical tweezers, our 6D structured optical tweezers significantly improved the flexibility of the path design of complex trajectories, thereby laying the foundation for next-generation functional optical manipulation, assembly, and micromechanics.

Alignment-free twisted-split-ring metasurface on single substrate with 2π phase range for linearly polarized sub-terahertz wave.

To achieve high-speed, large-capacity communication, next-generation mobile communication systems will require manipulation of the propagation of sub-terahertz waves in the propagation channel. In this paper, we propose the use of a novel split-ring resonator (SRR) structure as a metasurface unit cell for manipulating the linearly polarized incident and transmission waves used in mobile communication systems. In this SRR structure, the gap is twisted by 90° to efficiently use cross-polarized scattered waves. By changing the twist direction and gap size of the unit cell, 2π phase designability can be achieved, which enables linear polarization conversion efficiencies of -2 dB with a backside polarizer and -0.2 dB with two polarizers. In addition, a complementary pattern of the unit cell was fabricated, and a measured conversion efficiency of more than -1 dB at the peak with only the backside polarizer on a single substrate was verified. In the proposed structure, the 2π phase designability and efficiency gain are obtained independently by the unit cell and polarizer, respectively, thus enabling alignment-free characteristics, which are highly advantageous from an industrial viewpoint. Metasurface lenses with binary phase profiles of 0 and π were fabricated using the proposed structure with a backside polarizer on a single substrate. The lenses' focusing, deflection, and collimation operations were experimentally verified with a lens gain of 20.8 dB, which agreed well with our calculated results. Our metasurface lens has the great advantages of easy fabrication and implementation, and it has the potential to enable dynamic control by combining it with active devices because of the simple design methodology, which entails only changing the twist direction and the gap's capacitance component.

Performance Analysis of a Drone-Assisted FSO Communication System over Málaga Turbulence under AoA Fluctuations

Future wireless communications have been envisaged to benefit from integrating drones and free space optical (FSO) communications, which would provide links with line-of-sight propagation and large communication capacity. The theoretical performance analysis for a drone-assisted downlink FSO system is investigated. Furthermore, this paper utilizes the Málaga distribution to characterize the effect of atmospheric turbulence on the optical signal for the drone–terrestrial user link, taking into account atmospheric attenuation, pointing errors, and angle-of-arrival fluctuations. The probability density function and cumulative distribution function are then expressed in closed-form using the heterodyne detection and indirect modulation/direct detection techniques, respectively. Thereafter, the analytical expressions including the average bit error rate (BER) and the ergodic capacity are given. Particularly, the asymptotic behavior of the average BER of the considered system is presented using heterodyne detection at high optical power. The Monte Carlo simulation results certify the theoretical analytical results. Correspondingly, the field-of-view of the receiver is analyzed for optimal communication performance.

Demonstration of 12.5 Mslot/s 32-PPM Underwater Wireless Optical Communication System with 0.34 Photons/Bit Receiver Sensitivity

High-capacity, long-distance underwater wireless optical communication (UWOC) technology is an important component in building fast, flexible underwater sensing networks. Underwater communication with light as a carrier has a large communication capacity, but channel loss induced by light attenuation and scattering largely limits the underwater wireless optical communication distance. To improve the communication distance, a low-power 450 nm blue continuous wave (CW) laser diode (LD)-based UWOC system was proposed and experimentally demonstrated. A communication link was designed and constructed with a BER of 3.6 × 10−3 in a total link loss of 80.72 dB in c = 0.51 m−1 water with a scintillation index (S.I.) equal to 0.02 by combining with 32-pulse-position modulation (32-PPM) at a bandwidth of 12.5 MHz and single photon counting reception techniques. The allowable underwater communication distance in Jerlov II (c = 0.528 m−1) water was estimated to be 35.64 m. The attenuation lengths were 18.82, which were equal at link distances of 855.36 m in Jerlov I (c = 0.022 m−1) water. A receiving sensitivity of 0.34 photons/bit was achieved. To our knowledge, this is the lowest receiving sensitivity ever reported under 0.1 dB of signal-to-noise ratio (SNR) in the field of UWOC.

Simulation and design of C + L broadband communication single mode fiber

A novelty CL fiber which can expand L-band communication is proposed to meet the urgent demand of optical fiber transmission bandwidth for large capacity communication system. The influence of L-band attenuation on the OSNR (optical signal-to-noise ratio) of the system is analyzed and calculated; through the simulation of DWDM (dense wavelength division multiplexing) system, the requirements for FAWD (fiber attenuation warping degree) of CL fiber in 100Gbit/s rate DWDM system are obtained; and the influence of waveguide structure design on FAWD is tested. The results show that, the OSNR of L-band for will be 3 dB–3.5 dB worse than traditional C-band on 80 km span system; the FAWD@1625 nm (△α1625) and the FAWDD (fiber attenuation warping degree difference) of CL fiber are recommended to be controlled below 0.015 dB/km and 0.005 dB/km respectively to meet the link requirements in relevant standards. Additionally, it is feasible to improve the attenuation flatness of L-band by optimizing the waveguide structure design which is presented in the paper, that will provide a reference for the large-scale acquisition of novelty CL fibers in the future.

All-dielectric terahertz metasurface for linearly-polarized multichannel transmission and superposition states of spherical and vortex waves

Compared to pure vortex waves, the superposition state of spherical waves and vortex waves has enough degrees of freedom to upgrade applications in particle manipulation, information encryption, and large-capacity communications. Here, we propose a new scheme to achieve superposition states and multichannel transmission of vortex and spherical waves. Two transmissive all-silicon metasurfaces that enable mutual interference between linearly polarized (LP) waves in the terahertz region are demonstrated. Type A can achieve interference between x and y polarized waves, while type B can achieve interference between x (or y) and x (or y) polarized waves. The multichannel transmission and superposition states of topological charges of +3, +2, and +4 are designed and demonstrated from theoretical, simulative, and experimental perspectives at 1.1 THz. In addition, the objective fact that the focused superposition state must be observed close to the focal plane is also revealed. The measured results are in good agreement with the theoretical and simulative results. This work provides an idea for the design of ultrathin terahertz devices and could be applied in the fields of information encryption and high-frequency communications.

Master–slave carrier phase recovery using optical phase conjugation for frequency comb-based long-haul coherent communication systems

Taking optical frequency combs as WDM laser sources for coherent optical communication systems has great potential in low-cost, low-complexity, and large-capacity communications. Master–slave carrier phase recovery (MS-CPR) utilizes the phase coherence of received signals to reduce the computational complexity (CC) in frequency comb-based communication systems. In long-haul systems, chromatic dispersion (CD) weakens the phase coherence and makes MS-CPR ineffective. Besides, nonlinear effects greatly degrade the quality of received signals. A master–slave carrier phase recovery method using optical phase conjugation (OPC) is proposed for frequency comb-based long-haul coherent optical communication systems. By placing the OPC module in the middle of the link, not only the weakening of phase coherence caused by CD is solved, but also nonlinear effects are mitigated. The simulation results show that the proposed method has better performance than conventional MS-CPR and independent carrier phase recovery (ICPR). In the 1000 km long system with 16 quadrature amplitude modulation (QAM) signals, the BER of proposed method is 9.3% of conventional MS-CPR and even 40% lower than ICPR, and the BER of all channels is below the threshold for 7% overhead hard-decision forward error correction (7% HD-FEC).

Applications and prospects of artificial intelligence in covert satellite communication: a review

Satellite communication has the characteristics of wide coverage and large communication capacity, and is not easily affected by land disasters. It is quite suitable as a supplement to terrestrial communication networks and has been widely used in education, navigation, emergency relief, military, etc. However, due to the openness of the channel of satellite communication systems, satellite communication signals are easily eavesdropped on by eavesdroppers. This greatly threatens the privacy and security of countries and individuals. Covert satellite communication can effectively improve the covertness of satellite communication systems and greatly reduce the probability of detection by eavesdroppers. So, it has attracted more and more attention. In addition, with the development of artificial intelligence (AI), AI has been applied in many technics of covert satellite communication, which has achieved higher reliability and stronger concealment in covert satellite communication systems. The research status of key technics in covert satellite communication is discussed in this study, and the applications of AI in covert satellite communication are shown. Finally, future research directions of covert satellite communication are looked forward to. In the future, covert satellite communication technology will be an indispensable part of satellite communication systems.

Orbital angular momentum mode femtosecond fiber laser with over 100 MHz repetition rate

Orbital Angular Momentum (OAM) lasers have potential demand in many applications such as large capacity communication systems, laser processing, particle manipulation and quantum optics. OAM mode femtosecond fiber laser has become the research focus with the advantages of simple structure, low cost and high peak power. The current OAM mode femtosecond fiber lasers have made breakthroughs in the repetition frequency, pulse width, spectrum width and other key parameters, but it is difficult to achieve good overall performance. Besides, the repetition rate is currently in tens of MHz. In this paper, a large-bandwidth mode coupler is made based on the mode phase matching principle. Among them, the first order mode coupler with 3dB polarization dependent loss is made by the technology of strong fused biconical taper, and the second order mode coupler with 0.3dB polarization dependent loss is made by the technology of weak fused biconical taper. Combined with the nonlinear polarization rotation mode-locking mechanism, OAM mode femtosecond fiber lasers with over 100 MHZ repetition rate are built. The achievement of the key parameters is attributed to the selection of dispersion shifted fibers that can accurately adjust intracavity dispersion. Compared to traditional dispersion compensation fibers (DCF), the group velocity dispersion is reduced by an order of magnitude, so it can better adjust intracavity dispersion to achieve the indicators of large spectral bandwidth and narrow pulse width. In addition, the diameter of the fiber is 8μm, which is the same as that of a single mode fiber. Compared to DCF, the fusion loss can be ignored, so only a shorter gain Erbium-doped fiber is required that ensure a shorter overall cavity length and achieve high repetition frequency. The experimental results show that the first order OAM mode fiber laser has 113.6 MHz repetition rate, 98 fs half-height full pulse width, and 101nm 10-dB bandwidth. Second-order OAM mode fiber laser has 114.9 MHz repetition rate, 60 fs half-height full pulse width, and 100nm 10-dB bandwidth. Compared with the reported schemes, our scheme has better performance in key parameters such as repetition rate, pulse width and spectral width. We believe that the OAM mode fiber laser with good over performance is expected to be more widely used in OAM communication, particle manipulation and other research fields.

Satellite vibration effects on communication quality of OISN system

Abstract Over space optical communications are considered as the critical technology for high-bandwidth, high-speed, and large-capacity communications. Indeed, the laser wavelength’s narrow beam divergence requires a precise beam pointing at both ends of the optical link. The precise beam pointing makes the laser beam pointing to or from a moving object is one of the most challenging processes for optical space communications. In this work, the effect of the pointing error due to satellite platform vibration over the performance of the laser communication link of the optical inter satellite network (OISN) system in terms of the quality factor is investigated. Indeed, an optical communication system has been built using the OptiSystem program to simulate the link between satellites in space for the OISN system. In addition, the proposed system shows by simulation the optimal parameters’ values required for the design of the optical communication link between satellites of the OISN system. Moreover, the effect of pointing error due to the platform vibration on the performance of the OISN system is investigated for different scenarios of the pointing error (i.e., no pointing error; one side of the link with pointing error, and two sides of the link with pointing error). The simulation shows that, first, the optimal parameters that can be used for the optical communication link between satellites of the OISN system in terms of the laser wavelength; laser power; optical modulation scheme; optical telescope aperture diameter; and telescope optical efficiency. In addition, the simulation shows that existing pointing error due to vibration at one side of the optical link leads to degradation of the performance of the OISN system in terms of the quality factor for different laser beam power; distances between satellites; telescope diameters; and telescope efficiencies. Moreover, existing pointing errors at the two sides of the optical link lead to rapid degradation of the considered OISN system performance even with the increase of the laser power or telescope diameter, which tend to compensate for its effect initially and then quit.

Dual-Mode 2 × 2 Thermo-Optic Switch Based on Polymer Waveguide Mach-Zehnder Interferometer

A dual-mode <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2\times 2$ </tex-math></inline-formula> thermo-optic switch based on Mach-Zehnder interferometer with symmetric directional couplers is proposed and experimentally demonstrated by utilizing polymer waveguides. The present switch consists of two <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2\times 2\,\,3$ </tex-math></inline-formula> -dB directional couplers, which are designed to be mode-insensitive by choosing the core separation and coupling length optimally. Meanwhile, a mode-insensitive phase shifter is employed to alter the phase of the two different modes simultaneously. With the switching power of ~9.0 mW, the measured extinction ratios for the E11 mode and E21 mode can reach to 17.5 dB and 16.4 dB, respectively, at 1550 nm, and the switching time is ~1.30 ms. The performance of the switch is also independent to the polarization state of light. Our proposed dual-mode switch with the advantages of low power-consumption can accommodate MDM system and would be applied in future large capacity communication systems.

Design of an ultra-broadband and low-loss 3 dB power splitter from the 1.25 to 2.04 µm wave band.

As a key component of the beam splitting and combining in photonic integrated circuits, an optical power splitter requires the characteristics of compactness, broadband, and low loss. Here, we propose a 1×2 ultra-broadband 3dB power splitter with a 5µm long beam splitting area, based on the structure of a subwavelength grating. The 0.45dB bandwidth of the TE mode achieves 790nm from 1.25 to 2.04µm. This power splitter has a large fabrication tolerance, exceeding more than 30nm (±15n m) and obtains the desired power ratio. It is suitable for high-speed and large-capacity communication, wavelength division multiplexing systems, and other multi-wave interconnection applications.

LK-AKA: A lightweight location key-based authentication and key agreement protocol for S2S communication

As a supplement to terrestrial networks, satellite communication networks have a lot of advantages, such as wide coverage, large communication capacity, reliable communication quality, and seamless global connectivity, which are highly competitive with traditional communication technologies. However, there are still many security issues and efficiency problems in satellite networks due to the characteristics of satellite communications, such as highly exposed satellite links, dynamical topology, and the limited computational power and storage resources of satellites. Therefore, it is a crucial issue to design a lightweight and secure authentication mechanism to ensure the security of satellite-to-satellite communication (S2S). This paper proposes a lightweight authentication and key agreement scheme for S2S communication based on the symmetric cryptographic system and the satellite location key (LK) constructed by satellite orbital parameters and location information. The formal verification tool Scyther is employed to evaluate the proposed scheme’s security. The security analysis results show that it has good performance in providing security properties and resistance to typical attacks, such as replay attacks, impersonation attacks, and Man-In-The-Middle (MITM) attacks. In addition, the performance analysis results demonstrate that our scheme has desired efficiency in terms of signaling overhead, computational overhead, and bandwidth overhead.

Generation of Higher-Order Hermite–Gaussian Modes via Cascaded Phase-Only Spatial Light Modulators

The spatial distribution of higher-order Hermite–Gaussian (HG) modes is more complicated than the fundamental mode, and the characteristics of different modes and their orthogonal characteristics have essential applications in the fields of measurement, imaging, and large-capacity communications. The main issue in future applications is how to efficiently generate higher-order HG modes. The spatial light modulator has high spatial resolution and excellent maneuverability, so it is a simple and effective way to generate higher-order HG modes. Here, the efficient generation of higher-order HG modes via cascaded spatial light modulators was experimentally investigated. For the HG90 mode, the conversion efficiency is 61%. The method with high conversion efficiency has promising application potentials in biophotonics, laser physics, and quantum information.

Design of PCF Supporting 86 OAM Modes with High Mode Quality and Low Nonlinear Coefficient

A unique photonic crystal fiber with square and circular air holes (SC-PCF) is designed in this research. Three layers of circular air holes and two levels of square air holes make up the fiber cladding. The finite element approach is used to simulate the fiber construction, and numerical calculations are used to examine the transmission properties in the S+C+L band. The results reveal that the SC-PCF can sustain 86 Orbital Angular Momentum (OAM) modes in the wavelength range of 1400 nm to 1700 nm (300 nm), with an effective refractive index difference (ERID) of 5.88 × 10−3 between them, thus avoiding mode coupling. The mode purity of all modes is greater than 96% at 1550 nm, and the lowest dispersion and dispersion change are 4.939 ps/nm/km and 0.956 ps/nm/km, respectively. The confinement loss (CL) of all modes is lower than 10−9 dB/m, and the nonlinear coefficient (NC) is lower than 1.5 W−1·km−1 in the whole band. The proposed SC-PCF has important value in long-distance and large-capacity communication systems.

Approach of recognition and precision location for the beacon in satellite optical communications

Satellite optical communications have high communication speed and good confidentiality, which can establish high-speed and large-capacity communication links between satellites and the ground or other satellites. The requirements for the PAT (pointing, acquisition and tracking) system of satellite optical communication terminals are very high. In satellite optical communications, the biggest influence on the beacon’s recognition and tracking is the background light on CCD (Charge Coupled Device). Therefore, denoising work is required during the recognition or tracking process. This paper uses corner detection method to extract the beacon frame by frame. Mark the spot, and use the optical flow method to calculate the optical flow displacement based on the two consecutive frames. In addition, calculation accuracy of the beacon’s centroid is compared with that of the tradition method. The feasibility is verified by simulation experiment, the tracking process is completed, and the noise can be precisely eliminated.