SignificanceThe exponential growth in global data traffic, driven by the rapid development of the digital economy and diversified emerging services, presents unprecedented challenges and opportunities for optical transport networks (OTNs) as fundamental communication infrastructure. Traditional fixed-rate, fixed-grid optical transport technologies demonstrate limitations in spectrum utilization efficiency and service adaptation flexibility, rendering them inadequate for future differentiated service provisioning requirements. Flexible-rate grid-less optical transport technology has emerged as a solution, enabling next-generation high-efficiency, intelligent, and elastic optical transport networks through advancements in key technologies, including multi-format signal generation, flexible-rate signal reception, and multi-granularity elastic wavelength switching.ProgressThis paper systematically examines the core innovations of flexible-rate grid-less optical transport technology through an in-depth exploration of three dimensions: the transmitter, receiver, and switching node.At the transmitter side, traditional optical networks utilize fixed modulation formats and coding rates, limiting their adaptability to flexible-rate requirements of diverse services. Flexible coded modulation technology addresses this limitation by enabling on-demand transmission of multi-format optical signals through dynamic adjustment of multi-dimensional parameters, including modulation order (e.g., QPSK, 16QAM, 64QAM), symbol rate, and forward error correction (FEC) coding rate. The core technologies comprise flexible coding and flexible modulation. Flexible coding dynamically adjusts coding parameters and strategies according to transmission conditions and service requirements, ensuring efficient and reliable optical signal transmission. Flexible modulation incorporates probabilistic constellation shaping (PCS) and time-domain hybrid modulation (TDHM). PCS optimizes power efficiency, enabling flexible rate adaptation without bandwidth increase, while TDHM achieves high-efficiency transmission and dynamic adaptation by combining different modulation signals in the time domain. These technologies enable optimal selection of modulation formats and coding schemes under varying transmission distances and channel conditions, establishing the foundation for dynamic and flexible optical signal generation.At the receiver side, traditional optical transport equipment faces limitations due to single-rate reception, restricting effective processing of diverse modulation formats. Modulation format identification (MFI) and adaptive equalization technologies serve as essential components for enhancing receiver processing capability and adaptability. MFI technology precisely analyzes received optical signal characteristics to automatically identify modulation formats (e.g., QPSK, 16QAM, or 64QAM), providing vital information for subsequent signal processing. Adaptive equalization employs advanced digital signal processing (DSP) algorithms to dynamically compensate for signal impairments caused by chromatic dispersion, polarization mode dispersion, or nonlinear effects during transmission. Real-time adjustment of equalization parameters enables the receiver to optimize demodulation schemes based on signal characteristics and channel conditions, ensuring efficient signal recovery across diverse transmission scenarios. Additionally, deep learning-based MFI methods enhance identification accuracy and real-time performance, providing intelligent solutions for flexible-rate optical signal reception.In terms of switching nodes, traditional fixed-grid wavelength-division multiplexing (WDM) systems exhibit limitations in bandwidth allocation and increasing spectrum fragmentation. Grid-less flexible optical switching technology overcomes these constraints through dynamic scheduling and reconfiguration of wavelength-level or sub-wavelength-level optical paths, eliminating conventional 50 GHz fixed channel spacing restrictions and enabling dynamic adjustment of channel bandwidth and center frequency. This technology utilizes advanced devices such as optical cross-connects (OXCs) and wavelength-selective switches (WSSs), combined with intelligent routing algorithms and resource virtualization techniques, substantially improving spectrum utilization and service adaptability. A software-defined networking (SDN)-based centralized control mechanism enhances switching node intelligence and automation, enabling real-time network traffic variation perception and dynamic spectrum resource allocation. Channel bandwidth can be expanded for high-throughput scenarios requiring high-order modulation signals, while narrower channels improve spectral efficiency for low-rate services. This on-demand flexibility minimizes resource wastage and strengthens network support for diversified services.Conclusions and ProspectsThe optical transmission network increasingly aggregates and carries massive differentiated data generated from various application scenarios, driven by rapid emerging business development. Flexible rate grid-less optical transmission technology represents a crucial enabler for all-optical infrastructure. The convergence with cutting-edge technologies such as quantum communications and photonic neural networks may transform optical networks from “elastic adaptation” to “cognitive autonomy”. However, challenges persist in addressing energy efficiency degradation from dynamic modulation and coding strategy switching, real-time optimization of multi-rate adaptive equalization algorithms, and coordinated operation of high-dimensional optical switching nodes. Academia and industry must strengthen collaboration in device fabrication, algorithm architecture, and operational frameworks to advance optical communication networks toward intelligent all-optical networking.

SignificanceFiber-optic communication represents a fundamental technology for access networks, metropolitan area networks, backbone networks, and trans-oceanic communication networks. This technology enables long-distance, high-speed data transmission and serves as the foundation for digitalization and information transformation. The real-time monitoring of fiber-optic communication networks is essential for maintaining reliable and secure data transmission. Fiber sensing technology, when integrated into fiber-optic communication networks and forming fiber-based integrated sensing and communication (F-ISAC) networks, enables continuous monitoring of network status, ensuring optimal operational performance. Additionally, F-ISAC networks possess the capability to monitor environmental parameters surrounding optical cables, including temperature, strain, vibration, and displacement. While fiber sensing technology can be incorporated into fiber-optic networks, establishing a system that optimizes both sensing and transmission performance presents significant challenges. The development of an integrated system combining high performance, integration, and efficiency remains crucial for advancing the intelligence of optical communication networks.ProgressFirst, we introduce the origin of the F-ISAC technology, from technologies to applications. Then the main F-ISAC techniques are introduced according to the difference of sensing technologies, including forward transmission-based and backscatter-based F-ISAC techniques. With respect to the above two major types of F-ISAC technologies, the integration techniques, including space division multiplexing (SDM), wavelength division multiplexing (WDM), frequency division multiplexing (FDM), time-frequency reuse, and communication-sensing sharing, are further introduced. With regard to the above technologies, this paper introduces some representative solutions in each technology category, hoping to help readers have a better understanding of the advantages and characteristics of these solutions.Conclusions and ProspectsF-ISAC technology and applications demonstrate potential to enhance the extensive coverage capabilities of optical communication networks, offering novel approaches for intelligent network operation and maintenance, while contributing to advancements in smart cities and smart transportation. Current challenges and research directions in F-ISAC include 5 aspects. 1) Communication-sensing crosstalk and suppression: addressing the integration of high-performance sensing technology while maintaining communication quality represents a critical challenge. 2) Development of multi-functional optoelectronic devices: designing devices that optimize both sensing and communication capabilities remains essential for F-ISAC implementation. 3) Integrated signals design and processing: developing efficient ISAC signals and appropriate receiving and demodulation algorithms to enhance F-ISAC efficiency and performance constitutes a significant challenge. 4) Sensing and communication collaboration mechanisms: while fiber-optic communication and sensing systems demonstrate inherent trade-offs, opportunities exist for synergistic integration, such as sensing-assisted communication optimization through dynamic channel parameter adjustment and communication-enhanced sensing through improved event detection accuracy. 5) Standardized deployment frameworks: conventional fiber-optic sensing requirements often conflict with standardized telecommunication fiber deployment methods. Additional research must evaluate F-ISAC implementation strategies and their adaptability across various application scenarios.

SignificanceThe evolution of sixth-generation (6G) mobile communication systems necessitates unprecedented demands for ultra-high data rates, ultra-low latency, and ultra-reliable connectivity across terrestrial, aerial, maritime, and extraterrestrial environments. While traditional radio frequency (RF) communication has served as a foundation, it encounters significant limitations including spectrum scarcity, interference, and high energy consumption. These constraints particularly affect emerging applications such as holographic communication, real-time industrial automation, autonomous vehicles, and massive-scale Internet of Things (IoT), where current technologies struggle to meet stringent performance requirements.Visible Light Communication (VLC), operating in the 380?700 nm wavelength range of the electromagnetic spectrum, presents a viable solution. The visible spectrum provides an extensive, unlicensed bandwidth of approximately 400 THz, enabling high-capacity communication without increasing spectral congestion. Furthermore, VLC integration with existing lighting infrastructure through LEDs and laser diodes (LDs) facilitates cost-effective deployment while serving dual purposes of illumination and communication. This approach enhances energy efficiency and supports sustainable communication technologies.VLC demonstrates distinct advantages in specific environments. Its resistance to electromagnetic interference makes it particularly suitable for hospitals, airplanes, and factories, while its line-of-sight characteristics ensure robust physical-layer security—crucial for defense, financial, and industrial applications. Additionally, VLC proves effective for underwater communication where RF signals face severe attenuation, and in dense indoor environments where RF performance deteriorates. The high directionality and limited signal dispersion of visible light enable efficient spatial reuse, improving system capacity in multi-user scenarios.The academic and industrial communities worldwide have identified VLC as a crucial component of 6G development. Various governmental and research initiatives actively promote VLC standardization and commercialization. As data traffic increases exponentially and ubiquitous connectivity becomes essential, VLC emerges as a vital complement to conventional wireless systems. This technology represents not merely an alternative communication method but a fundamental shift toward secure, high-capacity, low-latency networks. Consequently, advancing VLC research constitutes both a technological necessity and a critical element in developing future digital infrastructure.ProgressRecent years have witnessed significant advancements in VLC technology across light sources, receivers, modulation and coding, signal processing, channel modeling, and MIMO systems. VLC systems utilize either LED or Laser Diode (LD) light sources. While LED systems offer cost advantages and lighting compatibility, their bandwidth remains limited. However, innovations in multi-color LEDs and micro-LED arrays have achieved data rates exceeding 25 Gbit/s over short distances (Table 1, Fig. 1). LD-based systems demonstrate superior bandwidth and coherence, with recent implementations reaching over 600 Gbit/s using 50-wavelength WDM systems (Table 2, Fig. 1).Receiver technology developments include large field-of-view detectors, wearable perovskite receivers, and GaN-based photodetectors achieving speeds above 15 Gbit/s (Fig. 2). Signal processing advances have adapted and optimized modulation formats such as carrierless amplitude and phase (CPA), orthogonal frequency division multiplexing (OFDM), and discrete multi-tone modulation for VLC, incorporating sophisticated bit-loading algorithms and hybrid modulation schemes (Figs. 4?6). These developments enable flexible spectrum utilization and robust performance in challenging conditions.Machine learning applications, particularly deep learning, have transformed channel modeling and equalization. Neural networks effectively model nonlinearities and turbulence-induced distortions in VLC channels (Figs. 3 and 9). Implementations of sparse multi-layer perceptrons and hybrid neural networks have optimized computational efficiency while improving post-equalization performance (Figs. 7?9). These approaches demonstrate significant effectiveness in both indoor and underwater VLC applications.MIMO VLC systems combined with WDM have expanded capacity through spatial multiplexing. Contemporary systems utilizing compact multi-wavelength LDs and advanced precoding techniques have achieved 805 Gbit/s with spectral efficiency exceeding 50 bit/(s/Hz) (Fig. 10). Additionally, beamforming implementations using meta-surfaces, liquid crystal spatial modulators, and intelligent reflective surfaces have improved system adaptability, supporting mobile multi-user environments (Fig. 11).Integration efforts between VLC and other communication technologies have increased. A hybrid VLC-PON system demonstrates potential for indoor high-capacity access, showing scalability toward 100 Gbit/s requirements [Fig. 12(a)]. In vehicular applications, MIMO VLC systems incorporating car headlights and structured receivers demonstrate promise for reliable high-speed in-vehicle networking [Fig. 12(b)].Conclusions and ProspectsVLC continues its evolution from experimental prototypes to practical implementations addressing critical limitations in current communication networks. Its vast unlicensed spectrum, lighting infrastructure compatibility, and RF interference immunity establish VLC as an innovative solution for 6G networks, particularly where RF systems prove inadequate. Nevertheless, technical challenges persist, specifically in developing efficient, cost-effective transceiver components for visible wavelengths. Future VLC systems require externally modulated coherent devices, integrated photonic circuits, and AI-driven design methodologies to overcome bandwidth and integration constraints.Moreover, VLC demonstrates considerable potential in emerging applications such as optical interconnects in data centers, where short-range high-speed communication is essential. Laser-based VLC systems, enhanced with WDM and advanced equalization, can deliver Tbit/s-class links with reduced power consumption and system complexity. In space and underwater environments, VLC provides superior directionality and immunity to electromagnetic interference. However, the technology must address challenges related to turbulence and scattering, which can be mitigated through predictive modeling and adaptive optics techniques.The future prospects for VLC technology appear highly promising. Ongoing research should emphasize multi-disciplinary integration, combining advancements in materials science, AI, signal processing, and nano-photonics. The development of chip-scale VLC transceivers, intelligent beam steering, and adaptive modulation strategies will facilitate VLC’s implementation in smart homes, factories, autonomous vehicles, satellite constellations, and underwater exploration systems. In the emerging 6G era, where communication demands pervasive, reliable, and instantaneous connectivity, VLC is positioned to become an integral component of the global communication infrastructure.

SignificanceThe exponential growth of emerging big data services and global internet traffic has driven traditional single-mode optical fiber communication systems toward their capacity limits. Mode division multiplexing (MDM) technology has emerged as a promising solution to expand communication capacity by enabling multi-mode parallel transmission within a single fiber core and utilizing spatial dimensions. However, variations in the intensity and phase distributions of different modes present significant challenges: the precise manipulation of modes and accurate separation and conversion of mode channels. This precision is essential for achieving high-performance communication systems. Consequently, research on mode manipulation and its applications in MDM-based optical fiber communication holds substantial importance for addressing the growing demand for communication capacity and advancing optical fiber communication technology.ProgressMode manipulation and MDM-based optical fiber communication research has demonstrated significant advancements in recent years. Scientists have explored the fundamental principles of mode manipulation, encompassing effective refractive index matching, mode coupling, and optical field wavefront control. Multiple mode multiplexing/demultiplexing technologies have emerged, including planar waveguide-based, fiber-based, and free-space approaches. Planar waveguide-based technologies achieve high integration and miniaturization through waveguide construction on substrates. Shanghai Jiao Tong University demonstrated 4-channel multiplexing/demultiplexing using a silicon-based waveguide with a multimode interferometer structure. Zhejiang University developed a compact 4-mode multiplexer/demultiplexer with low insertion loss and high crosstalk suppression utilizing multimode micro-ring waveguides. Fiber-based technologies, employing fibers as the control medium, provide direct integration with few-mode fibers. Long-period fiber gratings, mode-selective couplers, and photonic lanterns represent key implementations. Shanghai University and Peking University have achieved significant advances in mode conversion and multiplexing/demultiplexing using these methods. Free-space-based technologies facilitate manipulation of optical field amplitude, phase, and polarization in free space. Multi-plane light conversion (MPLC) technology has been widely implemented for precise mode conversion control. Researchers at Cailabs in France achieved multiplexing/demultiplexing of multiple LP modes with high mode purity and low inter-modal crosstalk. Our research team has contributed significant advancements, developing a high-precision, non-destructive characterization technique for fiber microstructures, enabling 3D reconstruction of multi-core and few-mode fiber structures. We implemented mode multiplexing/demultiplexing technologies based on MPLC and fiber coupling, achieving high-efficiency mode conversion. Additionally, we proposed innovative few-mode fiber designs and pumping schemes to address gain imbalance in few-mode fiber amplifiers, supporting long-haul MDM transmission systems.Conclusions and ProspectsMode manipulation, a fundamental approach in MDM-based optical fiber communication systems, has demonstrated substantial research progress through the development of mode multiplexing/demultiplexing techniques and devices, offering innovative solutions for capacity expansion in fiber-optic communication systems. Our research team has contributed significantly in three key areas: 1) high-precision characterization of fiber microstructures, 2) advanced mode manipulation technologies, and 3) development of few-mode fiber amplifiers. As technology advances, mode manipulation applications are expanding beyond telecommunications into emerging fields such as optical imaging and photonic computing systems, while requiring more precise multidimensional parameter coordination and crosstalk suppression. Future research directions will emphasize exploring novel materials with exceptional optoelectronic properties, designing advanced photonic structures with subwavelength precision, and developing innovative methodologies for intelligent mode control. These developments are anticipated to advance MDM technology toward ultra-high-capacity optical networks while fostering interdisciplinary innovations in photonic information processing.

SignificanceAs a critical component in the Artificial Intelligence (AI) era, intelligent computing power is projected to reach 1037.3 EFLOPS by 2025, with a CAGR (Compound Annual Growth Rate) of 46.2% from 2023 to 2028. The optical network serves as the fundamental infrastructure for the digital economy. Optical interconnects facilitate long-distance, high-bitrate, and large-capacity transmission for inter-AI datacenter (AIDC) connections across national, regional, and edge AIDC networks. Various optical modules and connections are employed within different scales of intra-AI datacenters, utilizing either electronic packet switching-based architecture or optical switching-based architecture, to establish computing power pools that deliver abundant computing resources to end-users efficiently and flexibly.ProgressThis review comprehensively examines recent developments and key technologies in optical networks for AI datacenter interconnects. The discussion begins with the overall architecture of AI datacenter interconnects, encompassing inter-connections, intra-connections, and intelligent systems, while analyzing the general requirements of optical networks.For inter-AI datacenter connections, high-bitrate and large-capacity transmission is achieved through C+L bands 400 Gbit/s coherent techniques based on 128 Gbaud DSP and components. The review explores wavelength band extension, addressing challenges and recent developments in S band and other bands. The multi-core fibre (MCF) transmission advances with ITU standardization focusing on G.65x single mode fibre back-compatible MCF. Hollow core fibre (HCF) demonstrates significant advantages despite substantial challenges. The review compares various protection schemes to achieve high availability and reliability in optical networks. Lossless transmission emerges as a new challenge for optical networks supporting distributed datacenter training.For intra-AI data center connections, the primary objectives include reducing cost per bit and power consumption per bit while achieving lower power usage effectiveness (PUE). The review analyzes and compares different module types for electronic packet switching-based architecture, including retimed modules, linear-drive pluggable optics, co-packaged optics, and optical I/O. Optical switching is being evaluated and tested for new large-scale datacenter architectures, offering significant advantages. Given the presence of various massive modules in datacenters, intelligent operation and maintenance becomes essential.AI-native capabilities are crucial for optical networks, with particular attention given to digital twin and multi-agent applications in network operation and maintenance.The review summarizes the primary applications and associated requirements of AI datacenter interconnects, including computing power access, model training, deployment, and inference. Additionally, it presents current industrial practices for these applications.Conclusions and ProspectsThe rapid expansion of AI applications demands substantial computing power, creating significant challenges for AIDC optical interconnects. Optical networks provide the essential infrastructure for inter-AIDC and intra-AIDC connections, offering advantages in sustainability, ultra-broadband capability, intelligence, agility, low latency, high reliability, and cost-effectiveness. Despite these advances, optical networks face ongoing challenges in multi-bands transmission, new fibre-based transmission, convergent optical communication and sensing, and hybrid optical and electronic switching. Future research should focus on optimizing overall architecture, increasing transmission capacity, enhancing automation and intelligence, improving awareness and sensing capabilities, and achieving environmental sustainability in optical networks for AIDC, including both cluster and distributed datacenters, to deliver ubiquitous AI services.

SignificanceThe exponential growth of global data traffic has revealed fundamental limitations in traditional radio frequency (RF) communication, including spectrum scarcity, electromagnetic interference, and vulnerability to information leakage, which impede its capacity to meet 6G networks' demanding requirements for high speed, low latency, and reliable connectivity. Optical wireless communication (OWC), which utilizes light in visible, infrared, and ultraviolet bands as the transmission medium, emerges as a revolutionary solution, providing abundant spectrum resources, high data rates, and immunity to electromagnetic interference, among other benefits. However, despite its technological advantages, OWC encounters substantial security challenges including eavesdropping, signal interception, and channel disturbances at the physical layer that compromise its deployment in critical sectors such as military communications, satellite networks, and intelligent infrastructure. Addressing these security vulnerabilities is essential to realizing OWC's full potential and securing its position as a foundational technology for future intelligent communication systems.Progress OWC encompasses three core technical modalities designed for specific operational environmentsfree space optical (FSO) communication, visible light communication (VLC), and underwater wireless optical (UOWC) communication (Figs. 1?3). FSO systems employ near-infrared lasers to establish high throughput point-to-point links for long distance transmission, facilitating secure backhaul connectivity in urban networks, inter-satellite communications, and supporting communication requirements in emergency response scenarios (Fig. 1). Utilizing light emitting diodes operating in the 400?800 THz spectral band, VLC provides dual functionality of illumination and data transmission, enabling innovations in smart home automation, industrial internet of things ecosystems, and secure indoor positioning systems (Fig. 2). UOWC employs the blue-green light window where water absorption is minimal to overcome the bandwidth limitations of underwater acoustic communication, supporting applications in marine sensor networks, offshore exploration, and naval tactical operations (Fig. 3).The security challenges confronting OWC manifest across interconnected dimensionsspatial eavesdropping emerges from beam divergence in FSO and the broadcast nature of VLC, enabling passive interception through receiver deployment or beam splitting. Channel induced vulnerabilities, such as atmospheric turbulence in FSO and underwater multipath fading in UOWC, compromise signal integrity and expand attack surfaces. Protocol level risks, including RF side channel sniffing in VLC and beam splitting attacks in FSO, challenge traditional encryption methods.To address these, physical layer security technologies have been developed: chaotic encryption employs nonlinear systems like Lorenz and R?ssler models to generate unpredictable signals (Fig. 4), resisting eavesdropping in VLC and FSO despite synchronization challenges from atmospheric turbulence. Beam control techniques, such as orbital angular momentum (OAM) multiplexing, encode data into orthogonal spiral phase modes to complicate interception via mode specific decoding. In addition, optical fractal coding technology draws on the principles of fractal geometry and utilizes the self-similarity and recursiveness of fractal structures to efficiently compress and encrypt optical signals (Fig. 5). Quantum key distribution (QKD), based on the BB84 protocol, offers theoretically unbreakable key exchange via quantum mechanics but is limited by short transmission ranges and polarization decoherence (Fig. 6). Spread spectrum and optical code division multiple access enhance security through wideband spreading or orthogonal coding, though requiring high speed hardware and facing dynamic channel noise, while intelligent reflecting surfaces and artificial noise injection optimize signal phase and power in hybrid RF-FSO systems to boost legitimate links and degrade eavesdropper channels (Fig. 7). Adaptive enhancements, including dynamic power allocation and deep reinforcement learning, enable real time optimization of beam pointing and modulation, adapting to turbulent or underwater channel fluctuations, with anti-turbulence coding ensuring reliable transmission under varying conditions.Hybrid OWC architectures, such as RF-FSO links (Fig. 8) and satellite-air-ground integration, utilize complementary technologies to address single system limitations, enhancing reliability and security by dynamically switching between high speed FSO and robust RF links or forming resilient global networks (Fig. 9). Collaborative mechanisms, including multi-hop relay and hybrid non-orthogonal multiple access, optimize resource allocation and increase attack complexity through signal path diversification. In practical applications, unmanned aerial vehicle secure communication employs laser-VLC hybrid links with OAM multiplexing and quantum noise encryption to resist jamming during dynamic aerial ground data exchange (Fig. 10). Military tactical systems utilize narrow beam FSO with chaotic encryption to avoid electromagnetic interference, integrating high energy lasers for both communication and directed energy countermeasures (Fig. 11). Finally, civil applications include VLC for secure indoor networks in smart homes and healthcare, vehicle-to-everything networks based on VLC and hybrid links technologies, as well as UOWC for underwater sensor networks using blue-green light and adaptive optics to mitigate water scattering (Fig. 12).Conclusions and ProspectsOWC has progressed significantly through physical layer innovations, hybrid system designs, and application specific security solutions, establishing a foundation for secure, high reliability communication. However, key challenges persist: QKD faces obstacles in long distance transmission and cost-effective hardware, chaotic encryption requires robust control in turbulent channels, and hybrid networks demand improved cross layer security optimization, particularly in complex scenarios like air-water cross domain communication.Future research should prioritize three key directions to advance OWC technology. First, the integration of quantum security technologies, including advanced QKD protocols and quantum -resistant encryption into OWC systems, is essential for achieving absolute confidentiality. This necessitates the development of low-power, high -efficiency QKD modules compatible with both terrestrial and space -based OWC architectures. Second, the implementation of artificial intelligence and machine learning frameworks, particularly deep reinforcement learning for adaptive beamforming and neural network-based channel prediction, offers significant potential to enhance OWC resilience against dynamic eavesdropping and jamming threats. This approach enables real -time optimization of security parameters in response to environmental variations. Third, the expansion of OWC applications into emerging 6G and post 6G scenarios, such as ultra -reliable low -latency communication for autonomous vehicles, intelligent transportation systems, and distributed edge computing networks, requires interdisciplinary advancement across photonics, cryptography, and network engineering. Addressing these aspects will establish OWC as a foundation for next -generation networks, ensuring secure, efficient, and ubiquitous connectivity across terrestrial, aerial, and underwater domains, thus meeting the requirements of future intelligent systems.

SignificanceDigital twin technology has emerged as a transformative approach to bridging the gap between the physical and digital worlds, enabling unprecedented levels of intelligence, automation, and optimization in complex systems. In the context of optical communication networks, the rapid growth of data traffic driven by new applications such as Internet of Things (IoT), artificial intelligence (AI), cloud computing, and ultra-high-definition video imposes stringent requirements on network capacity, latency, reliability, and flexibility. Traditional optical networks, while benefiting from advances in coherent transmission, elastic optical networks, and software-defined networking, still face challenges such as conservative link configurations, static control mechanisms, and resource over-provisioning during fault recovery. These limitations hinder the efficient utilization of spectrum resources and the realization of intelligent, dynamic network management. Digital twin technology offers a solution by enabling real-time, high-fidelity mapping of the physical network into a digital word, supporting accurate perception, prediction, and intelligent decision-making throughout the entire network lifecycle. This paradigm shift is essential for evolving from passive maintenance to proactive, autonomous operation in next-generation optical networks.ProgressSignificant advances have been made in the development of digital twin technologies for intelligent optical networks, particularly in two core areas: high-precision physical-layer modeling and data-driven self-learning. For physical-layer modeling, a modular approach is widely adopted, where each network element—such as fibers, erbium-doped fiber amplifiers (EDFAs), Raman amplifiers (RAs), and wavelength selective switches (WSSs)—is independently modeled and then integrated to construct an end-to-end digital representation of the optical link (Fig. 1). Traditional white-box models are based on physical principles. For instance, the Manakov equation and the Gaussian noise (GN) model can be utilized to model fiber nonlinearity. Ordinary differential equations (ODEs) can be used to model EDFAs and RAs. Though these white-box models provide strong interpretability, they are often constrained by computational complexity and idealized assumptions. To overcome the limitations of white-box models, black-box models leveraging machine learning techniques such as neural networks have been introduced, achieving high prediction accuracy and computational efficiency while closely matching the results of conventional simulations (Figs. 3 and 7). Furthermore, grey-box models that combine physical knowledge with data-driven learning have demonstrated improved accuracy, robustness, and generalizability by integrating the strengths of both white-box and black-box approaches. This modeling paradigm has been successfully applied to the characterization of fibers, EDFAs, and RAs, where advanced machine learning methods such as active learning significantly reduce the required training data without sacrificing accuracy (Figs. 5, 8, and 9). Similar strategies have been extended to the modeling of filtering penalty, further enhancing the accuracy of optical network digital twins.After developing accurate physical models, data-driven self-learning becomes an essential technique for the deployment and continuous evolution of these models. This approach enables digital twins to autonomously adapt to dynamic network environments and maintain high modeling accuracy over time. The self-learning process primarily involves three key research aspects: telemetry, model inaccuracy correction, and input parameter uncertainty refinement. First, telemetry-driven data acquisition focuses on the real-time collection and integration of multi-source data from both the optical and digital domains. In addition to deploying devices such as optical power meters within optical links for monitoring purposes, acquiring real-time data from the digital signal processor (DSP) at the receiver has become a crucial approach. This method enables the real-time perception of multiple key metrics, including optical power evolution, nonlinear interference (Fig. 10), and filtering impairments (Fig. 11), thus providing a solid data foundation for the effective operation of subsequent self-learning algorithms. Second, in terms of model inaccuracy correction, techniques such as transfer learning and active learning have demonstrated the ability to significantly enhance the generalization and adaptability of models in real-world scenarios. Characterized by small sample sizes, the data dependency and training costs associated with large-scale deployment of digital twin systems can be reduced (Figs.12 and 13). Third, input parameter refinement techniques further formulate the uncertainty in input parameters as an optimization problem. By applying suitable optimization algorithms and various refining paradigms, uncertainties in the input parameters can be reduced, thereby reducing the impact of measurement noise and incomplete information on model performance (Figs. 14 and 15).Conclusions and ProspectsDigital twin technology is revolutionizing the management and optimization of optical communication networks by providing a dynamic, high-fidelity virtual representation of the physical infrastructure. The combination of physical-layer modeling and data-driven self-learning significantly enhances the accuracy, adaptability, and intelligence of network operations. Future research will focus on improving model scalability, data ecosystem integration, and trustworthiness. Challenges such as data sparsity, cross-domain generalization, and real-time deployment have yet to be solved. In the future, addressing these challenges through joint efforts among academia, industry, and research organizations will be essential for realizing the paradigm shift from passive maintenance to proactive prediction in optical network operations, thereby laying a solid foundation for the next generation of intelligent communication infrastructure.

SignificanceThe emergence of high-bandwidth technologies, including cloud computing, high-definition video streaming, and particularly the advancement of AI models, has intensified the requirements for optical networks regarding bandwidth and latency. Traditional single-mode fiber (SMF) primarily employs single-wave rate enhancement and system bandwidth extension to increase capacity. As single-wave 400 Gbit/s and extended C+L band systems become more prevalent, transmission systems based on SMF are anticipated to encounter challenges due to device bandwidth limitations and high nonlinearity in next-generation systems, potentially constraining practical capacity. Additionally, increasing low-latency demands and intensifying commercial competition necessitate optimization of fiber-induced latency. Space division multiplexing (SDM) fibers have been extensively researched to enhance per-fiber transmission capacity. However, SDM fiber-based systems exhibit either high nonlinearity comparable to SMF or signal crosstalk. Furthermore, their light guiding principle remains identical to traditional SMF, failing to address latency reduction.Anti-resonant hollow core fiber (AR-HCF), characterized by its distinctive fiber structure and guiding medium, offers advantages in loss, nonlinearity, backscattering, and latency, overcoming traditional transmission medium limitations and presenting novel solutions for large-capacity, low-latency networks. Recent years have witnessed the development of various fiber structures, including Kagome structure, single-ring negative curvature structure, conjoined-tube negative-curvature structure, and nested anti-resonant nodeless structure. Notably, nested anti-resonant nodeless fibers have achieved attenuation levels suitable for commercial applications. Substantial research has focused on supporting devices and transmission systems, encompassing splicing, AR-HCF to SMF adapters, optical time-domain reflection technology, and transmission performance demonstrations. AR-HCF deployment has occurred across multiple countries. Despite significant advances, industrial feasibility and performance enhancement face ongoing challenges, necessitating a comprehensive review of existing research to guide future technological development.ProgressThis review encompasses AR-HCF advancements, supporting devices, fiber deployment, and transmission system experiments. The analysis begins with detailed explanations of guiding principles, fabrication techniques, loss sources, loss reduction processes, and physical properties. Based on transmission system technical requirements, the review examines implementation schemes and performance of splicing, AR-HCF to SMF adapters, and optical time domain reflection technology for AR-HCF. Current splicing and adapter performance largely satisfies application requirements, while optical time domain reflection technology requires further development. Transmission experiments and deployment progress indicate achievement of 154.5 Tbit/s maximum unidirectional capacity and thousand-kilometer transmission distances in offline experiments. A 2 km AR-HCF transmission system demonstrates no transmission penalty with input power up to 39 dBm, benefiting long-distance transmission. Real-time transmission systems have achieved 270 Tbit/s bidirectional transmission, confirming potential for same-wavelength bidirectional transmission. Multiple fiber deployment cases and experimental demonstrations have been implemented across the United States, Europe, and China. The review concludes by addressing practical application challenges in fiber fabrication, testing methods, supporting devices, and standardization.Conclusions and ProspectsAR-HCF characteristics align effectively with optical network operator requirements, prompting operators worldwide to actively advance this technology. Further comprehensive research in fiber fabrication, supporting devices, and transmission system technologies remains essential to enhance AR-HCF development and transmission systems in both academic and engineering domains.

SignificanceInformation and communication serve as fundamental pillars of modern civilization and drivers of societal progress. Optical transport network (OTN) technology has achieved significant advancements as the core bearer in modern communication infrastructure, supporting ultra-high-speed backbone networks globally through functionalities like hard pipe isolation, transparent multi-service transmission, and multi-layer coordinated management. With per channel capacities exceeding 100 Gbit/s and end-to-end ms-level scheduling, OTN has become a cornerstone for large-capacity data transmission and cloud computing interconnections. However, terrestrial networks are inherently constrained by geographical environments and deployment costs, resulting in coverage gaps in remote areas such as oceans, deserts, and polar regions, and exhibiting insufficient resilience during natural disasters. To overcome these limitations, space networks, particularly low earth orbit (LEO) satellite constellations, are rapidly advancing, characterized by wide coverage, survivability, and flexible on-demand access, serving as a crucial complement to terrestrial infrastructure. Space optical communication is widely recognized as a promising solution for high-speed connectivity in space, offering distinct advantages such as ultra-high bandwidth, low transmission latency, and high security. Industry progress, including the deployment of optical inter-satellite links (OISL) on SpaceX’s second-generation Starlink satellites demonstrating single-link rate reaching 200 Gbit/s, demonstrate the engineering feasibility of large-capacity space optical networking based on inter-satellite laser links. The integration of advanced optical transport network technologies from the terrestrial domain with satellite laser communication capabilities gives rise to the space optical transport network (S-OTN). S-OTN is positioned as the optical layer transmission base for the future integrated space-air-ground-sea network, aligning with the ITU-T’s Network 2030 Vision and the objectives of the upcoming sixth-generation fixed network (F6G) to achieve ubiquitous coverage and intelligent connectivity by deeply integrating space network resources into its core architecture. Compared to traditional satellite microwave relay, S-OTN offers substantial improvements, including a capacity leap with single-satellite optical interface capacity potentially exceeding 400 Gbit/s, a reduction in cross-ocean end-to-end transmission delay to within hundreds of ms, and the ability to establish end-to-end trusted service pipelines in less than 1 s. By providing high-speed data transmission through large-capacity optical transport and ensuring low latency and high reliability through flexible optical networking techniques, S-OTN is poised to become a critical infrastructure supporting future F6G applications such as intelligent agent interconnectivity and ubiquitous sensing. However, deploying sophisticated optical transport technologies in the dynamic and challenging space environment presents a series of unique technical hurdles that necessitate dedicated research and development.ProgressThis review undertakes a systematic analysis of the pivotal technologies essential for the realization of S-OTN, building upon a proposed networking architecture suitable for large-scale deployments. We delve into critical technological domains, commencing with high-capacity inter-satellite optical transmission. This forms the high-speed backbone of the space layer but faces formidable challenges. These include long-distance signal degradation, precise pointing requirements, and dynamic disturbances such as satellite mobility and solar interference, all of which must be overcome to ensure reliable multi-Gbit/s to Tbit/s connectivity. Recent progress is evidenced by industry deployments demonstrating significant capacity, such as the optical inter-satellite links deployed by Starlink. Prior researches by Gao et al. and Li et al. have advanced adaptive optical interfaces and anti-disturbance control for inter-satellite links, specifically targeting stability and performance enhancement under dynamic conditions. Subsequently, we examine high-availability space-to-ground optical transmission, recognizing that the atmospheric channel presents a major impediment due to turbulence, scattering, and absorption. Progress in this area focuses on developing robust techniques to mitigate signal fading and interruptions. This includes advancements in adaptive optics and site diversity, as well as innovative adaptive reception schemes. For instance, Guo et al. have contributed research on utilizing mode mismatching multi-mode photonic lanterns to improve coupling efficiency despite atmospheric distortion. Furthermore, the integration of optical links with more weather-tolerant microwave systems through cooperative free space optical/radio frequency (FSO/RF) approaches with adaptive combining, analyzed by teams like Xu et al., shows promise for enhanced link reliability. The review further investigates multi-granularity onboard optical switching, a necessity for efficient traffic routing and resource management within dynamic satellite constellations. Research efforts, including those reviewed by Wu et al., address the complexities of implementing flexible switching fabrics capable of handling diverse data rates and protocols in a resource-constrained and radiation-hardened environment, often involving hybrid electro-optical approaches. Finally, we analyze large-scale elastic laser networking. As constellation sizes grow, the challenge of managing a highly dynamic topology with potentially thousands of nodes necessitates innovative networking paradigms beyond traditional terrestrial methods. Progress includes developing scalable control plane architectures that balance centralized coordination with distributed autonomy, as well as sophisticated techniques for dynamic routing, resource scheduling, and enhanced survivability. Notable work in this area includes the development of advanced survivability mechanisms like time window-based shared path protection designed for the specific dynamics of optical satellite networks by teams such as Yan et al. to maintain network resilience against frequent link state changes and failures. For each of these areas, we synthesize the state-of-the-art technological advancements and identify future evolution directions, analyzing the associated technical challenges from transmission, switching, and networking dimensions. The systematic analysis within this review is framed by a proposed multi-layered S-OTN architecture, which provides a functional basis for understanding the interactions and requirements across the orchestration, control, and transport planes essential for efficient management and coordination of the integrated space-ground network.Conclusions and ProspectsS-OTN is progressively transitioning from technological verification towards engineering deployment and large-scale application. This review highlights that while significant progress has been made, in-depth and detailed explorations are still required across various technical fronts to fully realize the potential of S-OTN. Future research should focus on developing intelligent perception-driven adaptive optical communication mechanisms, integrated communication and control collaborative processing architectures, enhanced robustness and redundancy optimization schemes for non-ideal environments, unified electro-optical switching structures with efficient encapsulation mechanisms, resource self-organization capabilities, multi-level protection strategies, and the evolution of space-ground integrated control architectures towards multi-level collaboration. Addressing these challenges is crucial for enhancing the transmission efficiency, anti-interference capability, and resource utilization of S-OTN, thereby supporting the future F6G infrastructure while guiding the practical deployment of S-OTN.

SignificanceAs the cornerstone of the modern information society, optical fiber communications serve as critical infrastructures supporting the development of next-generation information technologies such as the Internet, 5G communications, big data, cloud computing, and artificial intelligence. It has been designated as a strategically prioritized industry in China. However, with global data traffic growing exponentially, the transmission capacity of traditional single-mode fiber is rapidly approaching the Shannon limit, and the “capacity crisis” has become a major challenge restricting the development of future communications. Against this backdrop, the development of revolutionary new optical fiber communication technologies has become a strategic frontier fiercely contested by academia and industry worldwide. Few-mode fiber, with its unique spatial mode multiplexing characteristics, is regarded as the most promising technical pathway to break through current communication bottlenecks. Its core advantages are reflected in three key aspects: First, by utilizing multiple orthogonal spatial modes for mode-division multiplexing, it can significantly enhance the transmission capacity of a single fiber. Second, compared to alternative solutions like multi-core fiber, few-mode fiber achieves capacity expansion solely by increasing the number of transmission modes, giving it a distinct advantage in energy efficiency. Most importantly, it maintains strong compatibility with existing single-mode fiber infrastructure, ensuring relatively low deployment complexity. Due to the strengths, few-mode fiber technology is considered a key enabler for 6G communications, widely recognized as the most industrially viable solution for achieving ultra-high-capacity transmission at Pbit/s or even Ebit/s scales.ProgressThis paper reviews the research progress on mode multiplexers/demultiplexers, few-mode fibers, few-mode fiber amplifiers, and the unique equalization techniques specific to few-mode fiber communication systems. First, the advancements in mode division multiplexing/demultiplexing devices are introduced. The devices are primarily categorized into three types: free-space mode multiplexers/demultiplexers (Figure 4), all-fiber mode multiplexers/demultiplexers (Figure 8), and optical waveguide-based mode multiplexers/demultiplexers (Table 1). The working principles and performance parameters of each type are discussed in detail. Next, the types of few-mode fibers and their impairment measurement techniques are reviewed. The parameter characteristics and application scenarios of different few-mode fibers are presented, along with the measurement methods and performance evaluations of impairments unique to few-mode fibers (Table 2). Subsequently, the mechanisms and optimization approaches of few-mode erbium-doped fiber amplifiers and few-mode Raman fiber amplifiers are elaborated, focusing on aspects such as gain coefficients and differential mode gain. Finally, the multiple-input multiple-output (MIMO) equalization methods for few-mode fiber communication systems are summarized, including data-aided equalization methods and blind equalization methods without data assistance (Figures 34 and 36), along with some representative transmission experiments in few-mode fiber communication systems.Conclusions and ProspectsFew-mode fiber communication systems, as a crucial development direction for next-generation optical communication technologies, have attracted extensive attention from both academia and industry due to the significant potential in enhancing communication capacity. This review provides a detailed discussion on the research progress of key technologies in few-mode fiber communication systems. Although substantial advancements have been achieved in the key technologies, numerous challenges remain for their practical implementation. With continuous breakthroughs in core technologies and improvements in system integration capabilities, few-mode fiber communication systems are expected to achieve more efficient, stable, and cost-effective optical transmission in the future, which will provide strong support for meeting the ever-growing global demand for communication capacity.

SignificanceCoherent optical communication algorithms and chips have served as the primary catalyst for the advancement of longhaul optical transmission systems over the past two decades. This paper examines the developmental trajectory of coherent optical algorithms and chips for longhaul optical transmission, providing a comprehensive analysis of key algorithms essential for engineering applications, including analysis of parallelization processing and loop bandwidth, reduced sampling rate and clock recovery, non-integer-oversampling equalization, and forward error correction. Additionally, it explores how implementation constraints of low-complexity and low-power chips affect high-performance algorithm design.For future 1.6T and 3.2T longhaul optical transmission systems, maintaining QPSK mode as the single wavelength modulation format would require optical module baud rates exceeding 500 Gbaud/s, imposing substantial bandwidth and noise background requirements on optoelectronic components and oDSP chips for baseband signals. Furthermore, signal integrity in optical module routing connections under high baud rate conditions presents significant engineering challenges, with uncertain feasibility. Consequently, the aggregation of multiple optical carriers into super channel configurations emerges as a viable technical approach. However, multi-channel modes based on carrier aggregation limit spectral efficiency improvements. The wider channel spacing requirements reduce the channel count for 1.6T and 3.2T in single fibers, maintaining relatively constant total fiber capacity. Modern optical fiber cables contain hundreds or thousands of optical fibers. A more practical solution involves parallel aggregation of multiple optical fibers within multi-fiber cables to achieve next-generation ultra-large capacity longhaul transmission systems. This approach necessitates cost-effective multi-fiber shared optical amplification systems with fiber wavelength fusion capability and innovative coherent optical module architectures, signal processing algorithms, and efficient oDSP chips. These advancements facilitate reduced bit/km transmission costs, supporting the ongoing development of high-capacity longhaul optical transmission systems.ProgressWe propose a new spatial division multiplexing mode based on multi-fiber optical cables, as shown in Fig. 9. In a dual fiber parallel optical transmission system, the laser optical power of one wavelength is split and modulated by two 800G signals. The modulated data comes from two different 1.6T signals, and the remaining two 800G signals from the two 1.6T signals are modulated by the other wavelength split signal. This achieves laser sharing on both the transmitting and receiving sides, and the 1.6T signals are modulated on two adjacent wavelengths of the same fiber, ensuring controllable transmission delay. This architecture can evolve to a four-fiber mode, as shown in Fig. 10, where one laser is shared by four fibers while ensuring that the 3.2T signal is modulated on four adjacent wavelengths of the same fiber. For multiple wavelengths aggregated within the same fiber, considering non orthogonal multiplexing based on FTN shaping algorithm, as shown in Fig. 11, the channel spacing can be compressed to 800 GHz, achieving spectrum effectiveness (SE) of 4 bit/(s/Hz), corresponding to a 33% SE yield.Conclusions and ProspectsCoherent optical communication systems have experienced remarkable growth over the past two decades, advancing longhaul optical transmission rates from 40G QPSK to 800G QPSK and increasing single wave rates twentyfold. Coherent oDSP algorithms and ASIC chips represent the primary catalysts for this progress. The deceleration of Moore’s Law has imposed significant power constraints on chips, necessitating research focus on power-efficient algorithm design, including parallelism and loop bandwidth optimization, low sampling rate clock recovery and equalization, iterative error correction decoding, and Turbo equalization algorithms. The traditional approach of increasing baud rates under QPSK modulation for future 1.6T and 3.2T long-haul optical transmission systems has become unsustainable. This paper introduces a novel system solution, multi-fiber space division multiplexing, along with corresponding coherent optical modules, oDSP algorithms, and chip architectures, intended to support optical transmission advancement for the next two decades. This innovation faces considerable technical challenges, including multi-fiber shared optical amplifier design, new optical cross systems compatible with diverse fibers and wavelengths, cost-effective multi-fiber optical modules, and energy-efficient coherent oDSP-ASIC development. Successful longhaul optical transmission systems consistently achieve substantial reductions in bit/km transmission costs compared to previous generations while enabling continuous evolution of transmission rates and capacity upgrades.

SignificanceThe exponential growth in data transmission capacity during the information era has led to rapid developments in large models, high computing power, large clusters, and ultra-high rate, ultra-large capacity all-optical network communication, challenging traditional optical transmission windows. The transmission capacity of current optical fiber communication systems has encountered a “bottleneck”, making bandwidth expansion one of the most effective methods to increase communication capacity. The fiber amplifier serves a vital function in this context. Consequently, research on ultra-wideband fiber amplifiers enables broader communication bands for transmission applications.The amplification characteristics of fiber amplifiers are fundamentally linked to the performance of their doped optical fibers. Erbium-doped fibers (EDFs) maintain an essential position in wideband amplification within the C+L band, attributed to the luminescence characteristics of erbium ions. Amplification in other communication bands necessitates alternative doping elements. Bismuth-doped fibers (BDFs), a recent research focus, demonstrate versatility in forming various luminescent active centers through element combinations, exhibiting broadband luminescence characteristics within 1150?1700 nm, enabling effective amplification across O, E, S, L and U bands. Thulium-doped fibers (TDFs) utilize thulium ions’ wide gain spectrum characteristics in the 1450?2100 nm range, encompassing S, L and U bands.Recent years have witnessed substantial advancements in fiber amplifiers primarily based on EDFs, BDFs, or TDFs for achieving ultra-wideband amplification. Erbium-doped fiber amplifiers (EDFAs) continue to expand into the C+L band and L+ band regions. Bismuth-doped fiber amplifiers (BDFAs) have achieved notable progress in multiple-band amplification, leveraging bismuth ions’ broadband luminescence characteristics, particularly in O+E, E+S, and O+E+S bands. Thulium-doped fiber amplifiers (TDFAs) have emerged as crucial components for expanding amplification in S and U bands. Thus, analyzing these three primary types of doped fiber amplifiers holds substantial importance for achieving ultra-wideband amplification.ProgressResearchers have explored various EDFA configurations utilizing different optical components to enhance performance (Fig. 3). In 2019, Almukhtar et al. from the University of Malaya achieved approximately 14 dB flat gain between 1535 and 1605 nm through a two-stage dual-pass system. In 2023, Zhai et al. from the University of Southampton developed a dual-pump dual-pass amplifier system, achieving L-band gains exceeding 20 dB, representing a significant advancement in L-band fiber amplifiers (Fig. 7). In 2024, Liu et al. from Shanghai University fabricated PbS/Er co-doped fibers using atomic layer deposition (ALD) and modified chemical vapor deposition (MCVD), extending the bandwidth with gains above 20 dB to 1627 nm for the first time (Fig. 8).The BDFA employs bismuth active centers associated with phosphorus (BACs-P) in BDF for effective O-band amplification. In 2019, Thipparapu et al. from the University of Southampton constructed a dual-pump dual-pass optical fiber amplification system, achieving peak gains approaching 40 dB (Fig. 10). In 2023, Mikhailov et al. from the United States demonstrated gains exceeding 20 dB between 1255 and 1355 nm. In 2024, Chen et al. from Shanghai University implemented a 1240 nm dual-pump dual-pass amplification system, achieving maximum gains of 0.5 dB/m (Fig. 13). BDFAs achieve E-band amplification utilizing bismuth active centers associated with silicon (BACs-Si) in BDF. In 2024, Liu et al. from Huazhong University of Science and Technology achieved wideband amplification between 1390 and 1510 nm using only 16 m optical fiber length, with unit length gains reaching 4.06 dB/m in the dual-pass amplification system (Fig. 11).The BDFA can utilize both BACs-P and BACs-Si in BDF for high-performance O+E band amplification. In 2024, Zhai et al. from the University of Southampton demonstrated high-gain and wideband amplification across O, E, and S bands, achieving gains exceeding 20 dB between 1335 and 1473 nm at -10 dBm signal input power (Fig. 17). That same year, Yang et al. from Shanghai University fabricated BDF using ALD combined with MCVD technology, achieving gain intensities exceeding 15 dB between 1280 and 1495 nm (Fig. 18). Regarding L-band and L+U-band amplification, Liu et al. from Huazhong University of Science and Technology fabricated high-germanium-bismuth-doped fibers in 2024, achieving amplification between 1595 and 1670 nm using a 1550 nm pump, with gains reaching 26.3 dB at 1670 nm (Fig. 22).The S-band TDFA primarily utilizes fluoride and tellurium glass fibers with relatively low phonon energy as gain media. In 2006, Aozasa et al. conducted a systematic investigation of thulium ion doping concentration’s influence on S-band TDFA gain characteristics. With a thulium ion doping concentration of 6×10-3, they achieved a gain of 22 dB within the wavelength range of 1477?1507 nm (Fig. 23). In the U-band, Chen et al. (2014) employed thulium-germanium co-doped silica fibers as the gain medium, incorporating fiber gratings to optimize the amplifier system structure, and extended the gain bandwidth of the U-band fiber amplifier to 1628 nm (Fig. 24).Conclusions and ProspectsConsidering future market opportunities and development trajectories, new band optical fiber amplifiers demonstrate significant potential in data center optical interconnection applications, enabling the high-speed interconnection of AI computing power networks. EDFAs have achieved significant advances in the C+L band and L+ band, delivering gains exceeding 22 dB throughout the L band, and successfully extending the bandwidth with gains above 20 dB to 1627 nm. BDFAs have achieved amplification with gain intensities exceeding 15 dB within the wavelength range of 1280?1495 nm, while the gain per unit fiber length in the E+S band has reached 4.06 dB/m. TDFAs have demonstrated amplification with full-band gains greater than 25 dB in the S and U bands. Nevertheless, opportunities remain for optimizing doped ion concentrations in optical fibers, and enhancement of preparation processes is required to improve amplification system performance. Future developments integrating the mutual regulation mechanisms of erbium ions, bismuth ions, thulium ions and other ions, while focusing on optimization of preparation processes and amplification system parameters, will facilitate enhanced fiber amplifier performance during amplification using active doped fiber. The advancement of full-band ultra-wideband fiber amplifiers presents promising future prospects.

SignificanceAs the cornerstone of global information transmission, optical fiber communication networks carry over 90% of worldwide data traffic. ITU’s telecommunication standardization sector (ITU-T) has initiated research and standardization of the beyond 1 Tbit/s (B1T) standard, which defines 1.6 Tbit/s as the foundational rate for next-generation high-speed optical fiber communication. To achieve thousands of kilometers of electrical relay-free transmission over conventional fibers, low-order modulation formats have become essential, while spectral expansion serves as a fundamental technical enabler. By taking 1.6 Tbit/s as an example: If quadrature phase shift keying (QPSK) modulation is adopted, the symbol rate will increase from about 130 GBaud in the 400 Gbit/s era to about 500 GBaud, expanding the spectral width to 48 THz. This necessitates leveraging multi-band spectral resources across O/E/S/C/L/U bands. It is foreseeable that high-speed optical communication will break free from the decades-long C-band-centric evolution model, fully embracing a new paradigm based on multi-band and ultra-wide spectrum technologies. Four key challenges emerge. 1) Fiber spectral capability: G.652.D and G.654.E fibers, with cutoff wavelengths of 1260 nm and 1530 nm respectively, support long-haul C+L band transmission but fall short for ultra-wide spectrum demands exceeding 24 THz. 2) Photonic device innovation: Core components like modulators and lasers require material-level breakthroughs to develop ultra-broadband devices covering O to U bands. 3) Optical amplification advancement: Novel doped fiber amplification technologies must evolve to meet system requirements for full optical domain amplification. 4) Passive component evolution: The frequency response characteristics of wavelength selective switches and filters will critically impact overall spectral efficiency. Collectively, high-speed optical communication is transitioning to a paradigm featuring “spectrum-expanded fibers and capacity-expanded systems”, marking a transformative leap in technological development.ProgressG.652.D fiber, a critical derivative of standard single-mode fiber, has found extensive application in modern optical communication systems, particularly in metropolitan area networks and long-haul infrastructure. Compared to conventional G.652 fibers, G.652.D achieves reduced signal attenuation and distortion through refined manufacturing processes that minimize internal micro-defects. With its exceptional cost-performance ratio, G.652.D fiber is projected to remain the backbone for constructing high-capacity and long-distance optical communication networks for the foreseeable future. To address the demands of ultra-long-haul and high-capacity transmission, G.654 fiber was developed. This fiber features a pure silica core with a doped cladding, significantly reducing fiber loss by suppressing Rayleigh scattering. Introduced in the 2010s, G.654.E achieves a minimum loss of 0.14 dB/km while mitigating nonlinear effects by enlarging the core diameter and increasing the effective mode field area (Aeff). These enhancements drastically suppress stimulated Raman scattering (SRS) in C6T+L6T wide-spectrum systems, positioning G.654.E as a superior fiber choice for 400 Gbit/s and future Tbit/s-class ultra-long-haul backbone transmission. Currently, ultra-low-loss and large-effective-area fibers (e.g., G.654.E fiber) have only achieved a developed spectrum of 12 THz, which remains far from the ideal ultra-wide spectrum (24 THz). It is recommended that solid-core fibers adopt 24 THz spectrum expansion as a fundamental goal. Building on the basic fabrication processes of G.654.E fiber, research should focus on the relationship between cutoff wavelength, effective area, and attenuation to realize a new type of ultra-wide-spectrum fiber, addressing the near- to mid-term development requirements of high-speed optical communication. Given hollow-core fibers’ disruptive advantages over solid-core fibers across spectral bandwidth utilization, latency, attenuation, and nonlinearity suppression, high-speed optical communication systems leveraging hollow-core fiber architectures are poised to become the dominant paradigm in next-generation networks. Between 2018 and 2020, the Hollow-Core Fiber Group at the Optoelectronics Research Centre, University of Southampton, developed a 6-tube nested antinode-free hollow-core fiber (NANF-6) with an attenuation coefficient reduced to 0.28 dB/km. In 2022, the group further optimized the design, successfully fabricating a 5-tube double-layer nested antinode-free fiber (DNANF-5) with an attenuation of 0.174 dB/km, later pushing this value down to 0.138 dB/km. By 2024, in collaboration with Microsoft, the team achieved a groundbreaking attenuation coefficient of (0.08±0.03) dB/km, with their results published as a post deadline paper at OFC 2024. In August of the same year, researchers from Jinan University and China Mobile demonstrated another leap forward by fabricating an anti-resonant hollow-core fiber (AR-HCF) using a 4-tube truncated double-layer nested structure. This design achieved a record-low attenuation of 0.06 dB/km, surpassing the performance of conventional solid-core single-mode fibers and marking a historic milestone in ultra-low-loss optical fibers. In 2024, China Mobile leveraged the ultra-low backward Rayleigh scattering property of hollow-core fibers to propose a co-channel full duplex (CCFD) transmission concept. The team successfully demonstrated the world’s first 202.1 Tbit/s CCFD hollow-core fiber transmission, achieving identical performance to the unidirectional transmission while unlocking a fifth independent dimension, namely direction (following time slots, modulation, wavelength, and polarization) for optical multiplexing. In June 2024, China Mobile deployed the world’s first 800 Gbit/s hollow-core fiber transmission trial network in the Shenzhen?Dongguan metropolitan corridor, Guangdong Province. This pioneering field test rigorously validated the performance of AR-HCFs under real-world engineering stresses.Conclusions and ProspectsWith the advancement of ultra-wide spectrum Tbit/s optical fiber communication technologies, higher demands are being placed on fiber transmission performance. Novel solid-core fibers must meet two core requirements: first, achieving ultra-low loss across E+S+C+L bands to ensure efficient full-spectrum transmission; second, optimizing cross-sectional design to satisfy wide-spectrum communication requirements for parameters including cutoff wavelength and dispersion. To this end, it is recommended to establish an ultra-wide spectrum fiber technology advancement task force to systematically research technical solutions and promote G.65X standard formulation, thereby laying the foundation for next-generation fiber standards. AR-HCFs, utilizing air as the transmission medium, demonstrate revolutionary advantages including theoretically ultra-low latency, ultra-low loss, minimal nonlinear effects, and ultra-wide usable spectral bandwidth. Current progress shows transmission loss in hollow-core fibers has been reduced to <0.1 dB/km, but large-scale deployment requires coordinated industrial chain development. Critical challenges remain in mass production industrialization and efficient field splicing with existing networks.

SignificanceOptical Cross-Connects (OXCs) serve as fundamental components in backbone optical networks, facilitating optical signal switching, dynamic service provisioning, and rapid fault recovery. These systems constitute critical infrastructure supporting the advancement of the national digital economy. The continuous expansion of network capacity, coupled with emerging technologies such as multi-fiber links, multi-core fibers, and multi-band transmission, has substantially increased the switching degree of OXC nodes (Fig. 1), potentially extending to several hundreds of degrees. Traditional OXC architectures based on broadcast-and-select (B&S) or route-and-select (R&S) configurations face limitations due to the restricted port count of wavelength selective switches (WSSs), making them unsuitable for high-dimensional scenarios. Consequently, addressing existing technical constraints and developing innovative high-degree OXC architectures has become essential to meet future requirements for large-scale all-optical switching in backbone networks.ProgressTwo primary approaches exist for implementing high-degree OXCs. The first involves developing large-scale WSSs to extend existing OXC architectures, while the second utilizes small-scale WSS modules to construct high-degree OXCs. This paper examines three mainstream WSS fabrication technologies (Table 1), with liquid crystal on silicon (LCoS)-based WSSs emerging as the most viable solution for large-scale devices due to their inherent flexibility and scalability. Regarding high-degree OXCs constructed with small-scale WSSs, this paper presents the first systematic analysis of three representative solutions: the sparse fiber interconnection scheme, the modular OXC cascading scheme, and the multi-stage N×N WSS cascading scheme. The sparse fiber interconnection scheme, implemented on both line and add/drop sides, reduces WSS port requirements (Fig. 2 and Fig. 3). The add/drop side implementation achieves 40% cost reduction and 20% insertion loss reduction while maintaining comparable lightpath blocking performance. The modular cascading scheme enables interconnection of multiple small-scale modular OXC units through port bridging to form high-degree OXC nodes (Fig. 6). A hybrid architecture incorporating large-scale MEMS switches and WSSs, designated HMWC, enhances wavelength availability and approximates the blocking performance of traditional OXC architectures (Fig. 7). The multi-stage N×N WSS cascading scheme introduces a three-stage Clos-topology-based architecture, termed WSS-Clos OXC, implementing strictly non-blocking high-degree OXC nodes (Fig. 9). Supporting techniques including wavelength conversion (Fig. 10) and cost optimization (Fig. 11) are explored through modifications to intermediate-stage modules, addressing wavelength continuity and hardware complexity challenges.Conclusions and ProspectsThe sparse fiber interconnection scheme approximates traditional OXC performance under specific routing and wavelength assignment (RWA) algorithms while substantially increasing network operation and maintenance complexity. The modular OXC cascading scheme provides enhanced compatibility and scalability but encounters challenges regarding wavelength contention and increased insertion loss from multiple WSS stages. The multi-stage N×N WSS cascading scheme ensures strict non-blocking performance and achieves optimal cost-performance balance through architectural optimization, indicating significant potential for future implementation. Future research directions should encompass architecture design and performance evaluation of high-degree OXCs while addressing critical aspects such as system reliability, scalability, and integration of advanced photonic technologies.

SignificanceThe rapid advancement of technologies such as cloud computing, the internet of things (IoT), and artificial intelligence has driven exponential growth in global data traffic. By 2025, mobile phone users will constitute approximately 70.5% of the global population, with internet users reaching 5.56 billion, representing a 2.5% increase from 2024. Data traffic is projected to increase 2.5 times from 2024 to 2030. To address these demands, optical communication technologies continuously advance transmission capacity through innovations such as multi-dimensional multiplexing and advanced modulation formats. However, nonlinear effects in optical fibers, including self-phase modulation (SPM), cross-phase modulation (XPM), and four-wave mixing (FWM), combined with linear impairments like dispersion and polarization mode dispersion (PMD), present significant challenges to long-distance, high-capacity transmission. These effects distort signal waveforms, increase bit error rates, and fundamentally constrain transmission capacity and distance, creating critical challenges for next-generation ultra-high-speed systems. Traditional physical-layer compensation methods demonstrate limitations in dynamic adaptability and computational efficiency, necessitating the development of intelligent nonlinear equalization (NLE) techniques. Addressing these challenges remains essential for advancing next-generation ultra-high-speed optical communication systems, as they directly influence the reliability and scalability of global data infrastructure.ProgressTraditional methods for nonlinear compensation in fiber optic systems include digital backpropagation (DBP), perturbation theory-based nonlinear compensation (PNC), and Volterra series methods. DBP, proposed in 2008, compensates for transmission impairments by creating a virtual link in the digital signal processing (DSP) at the receiver end. Despite its high computational complexity, DBP has been optimized through approaches such as weighted DBP (WDBP), improved DBP (iDBP), and subband-processed enhanced split-step Fourier method (CB-ESSFM). These optimizations have significantly reduced computational overhead while maintaining or even improving performance, making DBP a benchmark for nonlinear compensation. PNC leverages perturbation theory to approximate nonlinear effects, offering low complexity and high performance. Recent advancements include triplet-correlative PNC (TC-PNC), which reduces computational complexity by sharing intermediate results in triplet calculations, and second-order PNC (SO-PNC), which extends perturbation theory to higher orders for better compensation accuracy. These methods demonstrate effectiveness in balancing computational efficiency with nonlinear distortion mitigation.Volterra series methods model nonlinearities using multi-order kernel functions, providing a flexible framework for analyzing and compensating nonlinear distortions. Improvements such as symmetric Volterra series nonlinear equalizers (symVSNE) and cascade structures have reduced complexity while maintaining performance, making Volterra methods suitable for high-order modulation formats and long-distance transmission systems.Artificial intelligence (AI) has emerged as a transformative approach, with deep neural networks (DNN) and hybrid architectures demonstrating significant potential. Learning-based digital backpropagation (LDBP) optimizes DBP steps using neural networks, enabling joint optimization of linear and nonlinear compensation parameters. Neural networks combined with perturbation methods (NN+PNC) enhance compensation performance by addressing traditional perturbation approximation limitations. Advanced architectures like long short-term memory (LSTM), convolutional neural networks (CNN), and Transformer-based models address temporal and spatial dependencies in nonlinear distortions. Notable advancements include physics-informed neural operators (PINO), which integrate physical laws with data-driven learning to ensure consistency with fiber optic transmission principles, and meta-learning approaches that enable rapid adaptation to new channel conditions with minimal training data. These innovations advance nonlinear equalization, offering robust solutions for next-generation optical communication systems.Conclusions and ProspectsThe research of nonlinear equalization techniques has evolved significantly, transitioning from traditional methods to data-driven and physics-informed machine learning approaches. While existing methods have enhanced compensation performance and reduced complexity, challenges persist in balancing computational efficiency, dynamic adaptability, and generalization across diverse channel conditions. Furthermore, numerous new nonlinear compensation methods remain in the research phase, necessitating the development of practical nonlinear compensation methods for next-generation optical transmission systems.Future research should prioritize several key directions to advance NLE toward practical implementation and broader applicability. First, the development of efficient architectures remains essential to address the computational limitations of current algorithms. Second, physics-embedded learning paradigms warrant emphasis to bridge the gap between data-driven and model-driven approaches. Specifically, incorporating the nonlinear Schr?dinger equation (NLSE) or Manakov equations directly into neural operators—through physics-informed neural networks (PINN) or differentiable solvers—will enhance model interpretability, reduce training data requirements, and improve generalization across diverse fiber parameters. Third, achieving cross-scenario robustness requires innovative adaptation mechanisms, particularly for dynamically reconfigurable optical networks. Transfer learning techniques could adapt pre-trained NLE models for novel fiber types (e.g., hollow-core or multi-core fibers) with minimal retraining, while federated learning architectures may enable collaborative model optimization across geographically distributed network nodes while maintaining data privacy. These advancements will accelerate the transition from laboratory prototypes to field-deployable solutions, facilitating the development of terabit-per-second fiber-optic networks.

SignificanceAs the largest ecosystem covering 71% of Earth’s surface, the ocean harbors abundant biological resources, mineral deposits, and energy reserves, holding irreplaceable strategic value for sustainable development. With rapid advancements in marine resource exploitation, environmental monitoring, and military applications, traditional acoustic communication technologies can no longer meet the growing demand for high-speed data transmission in modern ocean exploration. Underwater wireless optical communication (UWOC) technology has emerged as a pivotal development direction in underwater communications due to its remarkable advantages of high bandwidth (up to Gbit/s level), low latency (ms level), and low power consumption. This technology demonstrates vast application prospects in submarine observation networks, underwater internet of things (IoT), marine resource exploration, and military security. However, the complexity and uniqueness of marine environments, including high water attenuation, dynamic turbulence effects, platform position uncertainty, and intense background light interference, pose significant challenges for practical UWOC implementation. Therefore, systematically reviewing UWOC research progress and analyzing its development trends carry substantial theoretical and practical significance for advancing this technology.Progress Recent years have witnessed breakthrough developments in three key aspects of UWOC technology transmission, propagation, and reception.In transmission technology, significant efforts have focused on developing high-power blue-green lasers and micro-LED arrays. Notably, Tsinghua University-Berkeley Shenzhen Institute developed a blue single-layer quantum dot micro-LED achieving 2 Gbit/s transmission over 3 m underwater with a bit error rate below the forward error correction threshold (Fig. 1). National Chiao Tung University implemented a semipolar InGaN/GaN green micro-LED array achieving 3.129 Gbit/s communication rates (Fig. 1). Advanced modulation formats like 16-QAM OFDM have substantially improved spectral efficiency, with experiments demonstrating 12.5 Gbit/s transmission over 1.7 m underwater (Fig. 2). University of Science and Technology of China realized 1 Gbit/s transmission over 130 m using PAM4 modulation with avalanche photodiode detectors.In propagation technology, beam shaping techniques have achieved important breakthroughs in complex underwater environments. Bessel beams and Airy beams exhibit superior anti-interference capabilities under turbulent and bubbly conditions due to their unique non-diffracting and self-healing properties (Fig. 3). Beijing University of Posts and Telecommunications developed Bessel beams using fiber micro-axicon technology that effectively mitigate signal attenuation caused by absorption and Mie scattering. Fudan University’s 2024 proposal of self-focusing Airy beams combined with wavelength division multiplexing increased system data rates by 91%. Multi-dimensional multiplexing techniques including wavelength division, polarization, and orbital angular momentum multiplexing have dramatically enhanced system capacity. Fudan University achieved 20.09 Gbit/s over short distances using wavelength division multiplexing (Fig. 4), while Ocean University of China verified the feasibility of orbital angular momentum multiplexing at 20 Mbit/s rates (Fig. 4).Reception technology has seen remarkable progress in high-sensitivity detectors and advanced signal processing algorithms. The application of single-photon avalanche detectors (SPAD) and silicon photomultipliers (SiPM) significantly improved communication performance under low-light conditions. Xi’an Institute of Optics and Precision Mechanics achieved 0.34 bit-1 receiver sensitivity using 32-PPM modulation with high-performance photon-counting detectors (Table 2). In signal processing, deep learning-assisted equalization methods demonstrate outstanding advantages. Fudan University first introduced Gaussian kernel-aided deep neural network equalizers into underwater visible light communication systems, achieving 1.5 Gbit/s transmission over 1.2 m underwater. In 2025, Beijing University of Posts and Telecommunications proposed a machine vision-based intelligent turbulence perception mechanism that effectively addresses turbulence effects caused by thermohaline gradients and bubbles.Engineering prototype development and sea trials have achieved notable accomplishments worldwide. The Xi’an Institute of Optics and Precision Mechanics developed a series of prototypes demonstrating outstanding performance in deep-sea applications, with their LED array-based system achieving 20 Mbit/s communication rates at 3509 m depth in the South China Sea aboard the “Deep Sea Warrior” manned submersible (Fig. 10). In practical applications like submarine pipeline laying monitoring, their prototypes successfully transmitted HD video in real-time over 18 m@c=0.5 m-1 (equivalent communication distance under Class I water quality conditions: 300 m) under sea state 4?5 conditions (Fig. 11). Internationally, Sonardyne’s BlueComm system has been practically deployed for unmanned underwater vehicle swarm communications, supporting rates up to 500 Mbit/s (Table 4).Conclusions and ProspectsUWOC technology has made significant progress in both theoretical research and engineering applications after years of development, yet still faces challenges in communication distance and environmental adaptability. Based on current research and technological trends, future UWOC development will focus on the following aspects.1) Extended range and higher rates. Developing higher-power blue-green lasers, optimizing beam shaping techniques, and adopting more efficient modulation schemes may break through current distance limitations. The U.S. Navy has initiated research on high-energy high-repetition-rate lasers targeting systems exceeding 1 Gbit/s over 100 m.2) Multi-modal communication integration. Hybrid systems combining optical, acoustic, and RF communications will become a major trend. The marine space-time reference network (Fig. 12) demonstrates the concept of an integrated air-space-ground-sea network that leverages the strengths of various communication methods.3) Integrated communication and sensing. Future UWOC systems will not only transmit data but also enable environmental sensing and target detection through optical signal analysis, providing richer data support for marine research.4) Intelligent and autonomous systems. Deep integration of artificial intelligence will enhance system adaptability. Deep learning-based channel estimation, signal processing, and link optimization algorithms will enable intelligent responses to complex marine environmental changes.5) Standardization and practical implementation. As the technology matures, establishing unified communication protocols and performance evaluation standards will be crucial for transitioning UWOC from laboratory research to large-scale practical applications.

SignificanceThe rapid advancement of global information infrastructure, particularly in the context of global satellite internet and integrated space-air-ground networks, presents unprecedented challenges for modern communication systems regarding capacity and performance. While radio frequency (RF) communication has traditionally been the cornerstone of communication systems, it increasingly exhibits fundamental limitations at the physical layer: restricted spectrum availability, limited data transmission rates, and vulnerability to interference. These constraints have become particularly evident in B5G and subsequent-generation communication systems. Free space optical communication (FSOC) technology, characterized by its ultra-wide bandwidth, high-speed transmission, robust anti-interference capabilities, and enhanced security, has emerged as a promising solution for next-generation high-performance communication systems. It is widely acknowledged as a crucial enabling technology for establishing integrated space-air-ground information networks.FSOC technology demonstrates a clear trajectory toward enhanced capacity, expanded coverage, and functional integration. A notable research direction involves the convergence of laser communication and ranging technologies. This integrated approach combines high-speed laser communication with high-precision laser ranging, enabling both large-capacity information transmission and precise navigation capabilities, spatial positioning, and multi-mission functionality. The increasing complexity of space missions necessitates enhanced satellite payload capabilities in terms of integration, environmental adaptability, and functional versatility. Integrated systems exhibit superior mission adaptability and system scalability, establishing themselves as the predominant direction for future satellite communication payloads.Although there have been many advancements in establishing satellite-ground links, interstellar laser communication, network architecture design, modulation mechanisms, and system integration, and there has also been a comprehensive review of key FSOC technologies, a unified analytical framework that covers various FSOC system architectures and their critical technical components is still needed. Future FSOC systems must address complex scenarios involving multiple modulation formats and communication-perception coordination, placing increased demands on communication terminals regarding modulation adaptability and environmental awareness. This paper provides a systematic review of recent FSOC research achievements by major international space agencies and Chinese research institutions, analyzes the development status of various FSOC systems, and incorporates recent advances in integrated communication-ranging studies. Based on this analysis, the paper outlines the evolution toward multi-modal architectures supporting multiple modulation formats and integrated communication-ranging functionalities in future communication terminals. Additionally, this work presents design specifications and field experimental results from a self-developed multi-modal communication terminal board, offering theoretical guidance and research references for understanding FSOC system evolution, identifying technical challenges, and developing future-oriented integrated space-air-ground optical communication networks.ProgressTable 1 summarizes representative FSOC achievements and specific parameters from various countries. The EDRS-A satellite, launched in 2016, successfully achieved its designated orbit, featuring a laser communication terminal with a 45000 km communication range and 1.8 Gb/s transmission rate. SA plans to launch DRS-D in 2025, targeting data communication capabilities of 80000 km range and 3.6 Gb/s transmission rate. In 2021, the Japanese JDRS satellite established successful two-way FSOC links with ground stations, achieving downlink data rates of 2.5 Gb/s from geosynchronous orbits. China initiated related projects in the 1990s. In 2024, the Changguang Satellite completed interstellar laser communication experiments at 10 Gb/s and 100 Gb/s, demonstrating technical capability for direct, high-speed transmission of high-resolution remote sensing images. Recent research has emphasized integrated laser communication and ranging technology, as detailed in Table 2. The U.S. lunar laser communication program in 2013 implemented an innovative combination of 1550 nm wavelength and pulse position modulation (PPM) technology, achieving sub-centimeter ranging accuracy (<1 cm) while enabling uplink speeds of 20 Mb/s and downlink speeds of 622 Mb/s. In 2024, the Beijing Institute of Telemetry Technology team achieved 14.1 mm ranging error using code element phase difference measurement with 625 Mb/s BPSK modulation.While these studies have advanced multi-modulation compatibility mechanisms, hardware schemes, and sensitivity indices, most research remains limited to offline simulations or laboratory verifications. A comprehensive multimode modulation and demodulation link has not been established, and system functions remain predominantly separated. Furthermore, integrated communication and ranging design requirements have not been fully addressed.Results and DiscussionsThis paper proposes and develops a multimodal communication ranging integrated system for space laser communication, building upon previous research findings. To evaluate the performance of the multimodal communication terminal under authentic long-distance spatial channel conditions, an FSO link spanning approximately 510 meters was established. The experiments were conducted using BPSK format at 625 Mb/s rate, with a symbol period of 1.6 ns. The achieved ranging root mean square error of 47 ps, approximately 2.9% of the codeword period, as presented in Table 4, demonstrates that the proposed system achieves high-accuracy ranging capability at the sub-codeword level while maintaining stable operation under real-time communication conditions. Fig. 8 illustrates the sensitivity of the multimodal receiver at various communication rates (0.625 Gb/s, 1.25 Gb/s, 2.5 Gb/s, 5 Gb/s, 10 Gb/s). The BER threshold of the FEC was established at 3.8×10-3. The sensitivity measurement for the 10 Gb/s DP-QPSK signal reached -46 dBm.Conclusions and ProspectsThis paper presents a systematic review of research advancements and engineering implementations in the field of air-heaven-heaven integrated FSOC network, both domestically and internationally. The FSOC system is evolving from single-link operations to a sophisticated system characterized by multi-layer architecture, broad coverage, and enhanced coordination capabilities. It is emerging as a crucial supporting technology for establishing high-bandwidth, low-latency, ubiquitous interconnected communication networks. This paper thoroughly examines the key technological challenges confronting FSOC systems, particularly regarding diversification of modulation formats, heterogeneity of link types, and integration of communication and ranging capabilities.Based on the above discussion, this paper presents and develops a multimodal communication and ranging integrated terminal system designed for air-to-air fusion network applications. The system features multimodal modulation format compatibility, adaptive rate control, and sub-code level ranging fusion capabilities. The system employs a unified hardware and software architecture, supporting mainstream modulation modes including OOK, BPSK, QPSK, and DP-QPSK. It implements signal modulation/demodulation and integrated ranging functionality through an FPGA integration platform.The system design methodology and experimental verification results outlined in this paper provide viable engineering approaches and technical support for developing next-generation FSOC communication terminals characterized by high adaptability, flexibility, and scalability. The research findings are anticipated to contribute significantly to the development of future air-to-air integrated network systems.

SignificanceWith the growing demand of ocean exploration, underwater wireless communication has drawn great interest in recent years. Underwater acoustic communication suffers low bandwidth and high latency, while radio frequency signal attenuates rapidly in the water due to the skin effect. In contrast, underwater optical wireless communication (UOWC) exhibits distinct advantages such as high data rate and low latency, making it a promising solution for underwater wireless communication. However, the dynamic underwater environments and non-ideal device characteristics bring challenges for UOWC systems. Considering these challenges, the technological advancements and application prospects of UOWC are presented to provide reference for its future development.ProgressExisting UOWC technologies primarily focus on channel equalization, modulation and coding, signal detection, tracking and multi-user access. Channel equalization for UOWC systems includes pre-equalization and post-equalization, where hardware-based pre-equalization is an effective approach to compensate for the nonlinear device characteristics at the transmitter side, and post-equalization typically employs signal processing techniques to eliminate the inter-symbol interference at the receiver side. Commonly used modulation schemes in UOWC include on-off keying, pulse amplitude modulation, pulse position modulation, and pulse width modulation. Channel coding schemes in UOWC are mainly categorized into block codes (e.g., Reed-Solomon code and Bose-Chaudhuri-Hocquenghem code) and convolutional codes (e.g., low-density parity-check code and Turbo code). Both model-driven and data-driven detection schemes have been proposed for UOWC systems based on various detectors. Tracking is an effective method to guarantee link stability and signal quality of UOWC, which can be classified into imaging and non-imaging categories. Multi-user access technologies in UOWC primarily include time division multiple access, code division multiple access, non-orthogonal multiple access, and hybrid schemes. Off-line or real-time UOWC systems have been developed incorporating these transmission technologies. Nevertheless, existing UOWC systems still face significant challenges such as dynamic underwater environments and non-ideal device characteristics, which require further development of UOWC technologies.Conclusions and ProspectsUOWC is considered as a promising underwater wireless communication technology, due to its distinct advantages including high data rate, low latency, and easy deployment. First, the UOWC channel characteristics are elaborated in the aspect of link and device, presenting the main challenges of UOWC in practical scenarios. Then, the advancements of UOWC technologies in channel compensation, modulation and coding, signal detection, tracking, and multi-user access are introduced subsequently. Next, the development progress of existing UOWC systems is reviewed. Finally, prospects and discussions are presented on the applications of UOWC. Future efforts involve adaptive transmission and integration of different communication technologies to construct intelligent ocean networks.

SignificanceQuantum key distribution (QKD) represents a paradigm shift in secure communication, as it enables the establishment of cryptographic keys with information-theoretic security based on fundamental principles of quantum mechanics. As such, QKD is of great significance for ensuring information security, especially in the current era of increasing data exchange volume and frequency. Despite its promise, the widespread practical deployment of QKD still faces several persistent challenges. These include limited noise resistance, limited communication bandwidth, and difficulties in network compatibility.To overcome these limitations, high-dimensional quantum key distribution (HD-QKD) has emerged as a powerful solution, offering enhanced performance in several key dimensions. First, the HD states have higher information capacity, so each photon can encode more information, thereby improving the transmission efficiency of information. Second, early research has shown that high-dimensional systems exhibit excellent noise tolerance, whether it is environmental or resulting from eavesdropping attacks. Third, high-dimensional quantum states exhibit richer variations in higher-dimensional Hilbert spaces, which can greatly expand potential application boundaries. Recently, this feature has been used to explore the implementation of multi-mode compatible networks. In this review, we present an overview of the state-of-the-art developments in HD-QKD technologies. We discuss the current state of development and the challenges associated with various technological approaches.ProgressHD-QKD based on temporal, spatial, and hybrid degrees of freedom (DoFs) has advanced significantly, yet faces technical and security challenges. In time-domain encoding, early two-dimensional phase-coding systems utilized Mach-Zehnder interferometers (MZIs) for time-bin states, while energy-time entanglement-based HD-QKD, pioneered by Ali-Khan et al. (2007), demonstrated multi-bit transmission potential. In 2013, Mower et al. integrated security analysis tools for continuous-variable QKD (CV-QKD) and provided an upper bound on information leakage under collective attacks. Subsequent works by Zhang et al. integrated conjugate Franson interferometry, enabling experimentally observable security monitoring. In addition, Islam et al. proposed a comprehensive theoretical framework for HD discrete-variable QKD (DV-QKD) systems, taking into account security aspects including finite-size effects. They also implemented four-dimensional time-bin QKD experiments using cleverly designed cascaded asymmetric MZIs. Unfortunately, due to the difficulty of operating the degree of freedom in the time domain, this encoding method often suffers from low decoding efficiency.Spatial-mode encoding via orbital angular momentum (OAM) offers high-dimensional Hilbert spaces but struggles with mode stability and fiber compatibility. Breakthroughs in specialty fibers (e.g., ring-core and hollow-core fibers) enabled OAM transmission over 25 km, while free-space experiments validated urban feasibility. However, slow spatial light modulators (SLMs) hinder real-time OAM manipulation. Path encoding via multi-core fibers (MCFs) achieved 4D entanglement distribution over 11 km and hybrid time-path systems over 52 km, highlighting scalability for telecom networks. However, spatial-mode-based encoding still faces significant challenges in extending transmission distances, and phase disturbances in long-distance channels remain difficult.HD-QKD systems based on single degrees of freedom face practical constraints. Combining multiple degrees, such as polarization and OAM, expands the Hilbert space and eases technical challenges. In 2019, Han’s team utilized vector vortex photon states to map OAM manipulations onto polarization, enabling high-fidelity, low-error HD-QKD (0.60% error rate). The subsequent works extended communication distances to 25 km, enhancing system stability and reducing real-time errors. The scheme is downward-compatible with lower-dimensional QKD, supporting flexible quantum networking and advancing scalable quantum communication infrastructure.Experimental realization of entangled HD-QKD systems faces significant complexity beyond 2D implementations, currently remaining in early exploration stages. Spontaneous parametric down-conversion (SPDC) serves as the primary method for generating high-dimensional entanglement, utilizing DOFs like OAM, path, or energy-time, or by creating hyperentanglement across multiple DOFs simultaneously. Key challenges include ensuring high-fidelity entanglement distribution and mutually unbiased basis measurements, with each DOF presenting inherent limitations: OAM suffers from spatial mode transmission issues, path requires complex phase stabilization, and time encoding involves difficult measurements.OAM entanglement states, while theoretically unlimited in dimension (demonstrated up to 10010 dimensions), struggle with distribution (limited to 3 km in free space or 1 km in few-mode fiber) and require complex measurement techniques like phase plates or multi-plane light conversion. Path encoding allows convenient high-dimensional state preparation and measurement via optical elements but relies on multi-core fiber for transmission, demanding active phase compensation over distance. Time encoding performs well in robustness and single-mode fiber compatibility, achieving distribution over hundreds of kilometers. However, measurement typically necessitates complex interferometers, though QKD requirements simplify this somewhat.Hybrid encoding via hyperentanglement leverages SPDC’s natural ability to entangle multiple DOFs, mitigating individual DOF limitations. Systems based on polarization and time-bin hyperentanglement are fiber-compatible and simplify measurements, enabling 50 km HD-QKD demonstrations. Beyond QKD, hyperentanglement enables advanced protocols like efficient single-copy entanglement purification. This technique uses hyperentangled states to perform deterministic CNOT operations within a single photon pair, thereby purifying one DOF against depolarization noise. It vastly outperforms traditional two-copy purification in efficiency. Experimental implementations by Hu et al. and Ecker et al. in 2021 successfully demonstrated fidelity boosts and restored QKD performance over noisy links, proving its vital role in enabling practical long-distance quantum communication.Conclusions and ProspectsIn summary, HD-QKD using the weak coherent sources shows increasing dimensions and rates, surpassing 2D-QKD performance in some aspects. However, practical adoption faces significant hurdles. Time-phase encoding offers compatibility but suffers from efficiency loss and extended time slots at higher dimensions. Spatial mode encodings avoid time issues but lack standard fiber compatibility and face atmospheric distortion in free space, requiring advanced wavefront correction. Multicore fiber is promising for path encoding but needs improved phase stabilization. Fortunately, with advances in experimental techniques, the limitations in each DOF are gradually being mitigated, though considerable progress is still required. HD entanglement offers enhanced capabilities and noise resilience for quantum communications. In particular, recent subspace encoding protocols have significantly improved the noise resistance and environmental adaptability of HD entangled QKD, offering a promising technological pathway for deploying QKD in complex environments. In addition, HD systems based on hyperentanglement also offer advantages in protocols such as entanglement purification and entanglement swapping, which can improve the operational efficiency of these processes. Experimentally, research on HD entangled QKD is gradually transitioning from short-range proof-of-principle demonstrations to systems capable of supporting long-distance transmission. However, high-dimensional quantum states are highly susceptible to channel noise, and achieving high-fidelity transmission and measurement over long distances remains one of the key technical challenges in this field. Fortunately, with the continuous development of theory and experiments, we believe that in the near future, HD-QKD will soon demonstrate its advantages in practical applications and play a key supporting role in the future development and upgrading of quantum networks.

SignificanceOptical chaos secure communication, a physical-layer encryption technology, represents an innovative solution addressing vulnerabilities in traditional cryptographic algorithms during the quantum computing and artificial intelligence era. This approach utilizes chaotic lasers as hardware-based “root keys”, combining high-speed capabilities, compatibility with existing fiber-optic infrastructure, and resistance to algorithmic decryption. In contrast to quantum key distribution, which encounters practical deployment challenges, optical chaos communication utilizes chaotic dynamics’ inherent unpredictability to facilitate secure data transmission and key exchange. This technology demonstrates strategic significance for national cybersecurity protection and the advancement of next-generation secure communication networks. This paper presents a comprehensive review of technological developments, challenges, and future directions in optical chaos secure communication, emphasizing chaotic laser generation, synchronization, communication frameworks, key distribution, and integrated chaotic light sources.ProgressThis review is structured into four main sections, each addressing a critical aspect of optical chaos secure communication.The first section examines the fundamental principles of chaotic laser generation and synchronization. Various methods for generating chaotic signals are explored, including optical feedback, optical injection, and optoelectronic feedback. These techniques utilize the nonlinear dynamics of semiconductor lasers and optoelectronic oscillators to generate chaotic outputs with high complexity and unpredictability. The analysis includes distinctions between master-slave and common-signal-driven synchronization schemes, which are fundamental for ensuring transmitter and receiver synchronization in secure communication.The second section focuses on the development of optical chaos communication systems, particularly those based on semiconductor lasers and optoelectronic oscillators. We review the progress in transmission rates, communication distances, and system integration, emphasizing the role of artificial intelligence (AI)-assisted digital signal processing in optimizing synchronization performance. For instance, AI-based models have been employed to compensate for signal distortions caused by transmission noise and to improve the accuracy of chaos synchronization. We also discuss the challenges associated with achieving high-speed communication over long distances, such as the impact of fiber dispersion and nonlinear effects on synchronization quality. Recent breakthroughs, including the use of broadband chaotic sources and advanced modulation techniques, have enabled data transmission rates of up to 100 Gbit/s over distances exceeding 100 km, demonstrating the potential of optical chaos communication for practical applications.The third section examines the progress in chaos-based key distribution, where chaotic signals are used as physical entropy sources for secure key generation. We discuss various techniques for extracting random keys from synchronized chaotic signals, including chaos masking and random key distribution systems. These methods leverage the high correlation between chaotic signals generated by synchronized lasers to ensure that only the legitimate users can generate identical keys. However, challenges such as synchronization recovery time, key rate, and distribution distance remains a significant obstacle for high-speed and long-distance key distribution. Recent research has focused on reducing synchronization recovery time through the use of open-loop synchronization structures and optimizing key generation algorithms to improve key rates. Recently, key distribution rates of up to 0.75 Gbit/s over 160 km of optical fiber have been achieved, demonstrating the potential of chaotic key distribution in practical applications.The fourth section highlights the advancements in chaotic light sources, which are critical for enhancing the performance of optical chaos communication systems. Significant progress has been made in broadening the bandwidth of chaotic signals through techniques such as external optical injection, mutual injection, and the use of high-nonlinearity fibers. Integrated chaotic lasers, including monolithic and hybrid integrated designs, have been developed to achieve compact and efficient chaotic sources. Novel laser structures, such as short-cavity DFB lasers, microcavity lasers, and long-cavity Fabry-Perot lasers, have demonstrated the ability to generate broadband chaotic signals with suppressed time-delay signatures. These advancements not only improve the complexity and unpredictability of chaotic signals but also pave the way for the miniaturization and practical deployment of chaos-based communication systems.Conclusions and ProspectsDespite substantial advances in enhancing optical chaotic systems’ performance and capabilities, several key challenges remain in fully realizing optical chaos secure communication’s potential for real-world applications.The achievement of ultra-high-speed transmission with stable synchronization over long distances remains a significant challenge in optical chaotic communication. While transmission rates have improved, chaotic systems continue to face limitations from synchronization errors and environmental disturbances. AI-assisted signal processing, synchronization, and error correction demonstrate considerable potential for addressing these constraints, enhancing system efficiency and enabling faster, more precise synchronization. Furthermore, expanding key distribution capabilities remains essential for optical chaotic communication advancement. Although chaotic synchronization provides a robust foundation for secure key exchange, efficiency constraints arise from synchronization recovery time and distribution distance. Future research should prioritize reducing recovery time and improving key distribution system scalability for enhanced key exchange efficiency.Looking ahead, optical chaotic communication presents transformative potential for secure communication systems, offering an alternative to conventional cryptographic methods. The ongoing integration of AI, improvements in chaotic laser technologies, and advances in key distribution techniques will expand the possibilities in secure optical communication. As communication networks become faster and more interconnected, optical chaos secure communication provides a promising solution to meet the increasing demand for robust, high-performance, and physically secure communication systems.

SignificanceOptical fiber communication has revolutionized global information technology by providing a high-capacity, low-loss transmission medium. Traditional silica-based solid-core fibers, however, have reached their theoretical performance limits due to intrinsic material absorption, nonlinearities, and Rayleigh scattering losses. Hollow-core fibers (HCFs), which guide light primarily within air rather than glass, substantially reduce these intrinsic limitations. This review examines near-infrared (NIR, 0.8?2.5 μm) HCFs, which are increasingly essential for ultra-high-capacity optical communication networks due to their superior characteristics including ultra-low attenuation, ultra-low latency, minimal nonlinearities, and broad bandwidth transmission.ProgressThis review systematically analyzes recent developments and key technologies of near-infrared HCFs. It initially presents four fundamental light-guiding mechanisms: photonic bandgap guidance using periodic dielectric structure; anti-resonant guidance based on thin-walled capillaries inducing Fabry-Perot-like interference; Bragg reflection guidance enabled by multilayered radial index profiles; and metallic or dielectric-coated mechanisms that utilize mirror-like or interference-based reflection from inner cladding surfaces. Among these, anti-resonant hollow-core fibers (AR-HCFs) have garnered the most attention in recent years, particularly in the forms of nested anti-resonant nodeless fibers (NANFs) and double-nested variant (DNANFs), which have achieved record-low attenuation level around 0.28 dB/km. These advances stem from minimizing surface scattering, leakage loss, and mode coupling by enhancing structural symmetry and eliminating node points.The review further examines key performance parameters, including attenuation, nonlinearity, chromatic dispersion, and polarization-mode dispersion (PMD). Transmission losses in AR-HCFs primarily arise from structural leakage losses, surface scattering induced by interface roughness, and mode coupling due to microstructural asymmetry. Recent innovations in fabrication processes, including refined structural designs, precise control of capillary wall thickness, and nodeless structure, have substantially reduced these losses. Nonlinearity in HCFs is several orders lower compared to traditional solid-core fibers, enabling higher power thresholds for optical transmission, which proves especially beneficial for next-generation ultra-high-speed data networks.Building on these performance advantages, the review explores the expanding application landscape of HCFs, encompassing high-capacity data transmission, mid-infrared laser systems, quantum communication, and precision sensing. The ultra-low latency and minimal nonlinear impairments of AR-HCFs render them particularly suitable for quantum key distribution, enhancing the fidelity and security of quantum state transmission. Additionally, the broad transmission window and low-loss characteristics of HCFs have enabled promising results in mid-infrared laser delivery and ultra-sensitive gas detection, further demonstrating their versatility in emerging photonic technologies.Conclusions and ProspectsDespite substantial advancements, HCFs still face challenges such as manufacturing uniformity, long-length fabrication, and efficient integration with existing optical networks. Future research aims to further reduce attenuation, improve bending insensitivity, and achieve large-scale, cost-effective production. Moreover, integrating advanced materials and innovative structural designs will enable additional application fields, particularly in quantum communication, high-power laser delivery, and environmental sensing. Continued interdisciplinary research will advance HCFs technology towards broad practical deployment in global optical communication infrastructures.

SignificanceDigital fiber optic link monitoring has emerged as a pivotal technique for ensuring the high reliability, stability, and performance optimization of modern optical communication systems. With the continuous evolution of network architectures, driven by the exponential growth of data traffic, cloud computing, and emerging latency-sensitive applications, the demand for real-time, fine-grained monitoring of optical link conditions has become increasingly critical. Traditional performance monitoring schemes, which primarily focus on macroscopic indicators such as bit error rate and optical signal-to-noise ratio, offering only limited insight into the internal state of transmission links, are insufficient for meeting the requirements of intelligent and adaptive optical networks.By leveraging advanced digital signal processing techniques at the receiver, digital fiber optic link monitoring enables the non-intrusive reconstruction and continuous observation of essential physical-layer parameters, including optical power distribution, chromatic dispersion, polarization-dependent loss, and nonlinear impairments. This capability provides network operators with real-time visibility into the link’s physical behavior, facilitating proactive fault diagnosis, predictive maintenance, and dynamic performance optimization. As optical networks evolve toward software-defined and self-optimizing architectures, digital link monitoring is expected to play a central role in supporting autonomous control, resource allocation, and closed-loop network management.ProgressSubstantial progress has been made in the development of digital fiber link monitoring techniques. Early studies primarily focused on correlation-matching-based power profile estimation (PPE), which utilized perturbation models combined with mirror-path construction to infer the spatial distribution of optical power along the fiber link. Although correlation method (CM)-PPE demonstrates the feasibility of in-band power profile estimation, its spatial resolution is limited, and the obtained results are often relative rather than absolute, constraining its practical applicability for fault localization and link performance analysis.To address these shortcomings, the minimum mean square error (MMSE)-based power profile estimation method is subsequently proposed. This approach employs a least squares fitting framework to construct an unbiased estimator by minimizing the mean square error between the observed nonlinear signal distortions and those predicted by perturbation theory. Compared to earlier methods, MMSE-PPE significantly improves the accuracy of power profile reconstruction and can be readily extended to multi-parameter monitoring, including polarization-dependent loss and chromatic dispersion variations.With increasing demand for robustness and adaptability in practical deployments, algorithmic enhancements have been introduced to further improve the performance of these estimation techniques. Regularization methods, spatial response function correction, and deconvolution strategies have been explored to mitigate estimation bias, suppress noise, and enhance the model’s resilience to complex and dynamic network conditions, such as amplifier gain fluctuations and polarization mode dispersion.Digital backpropagation (DBP) reconstructs the signal propagation process by numerically solving the nonlinear Schr?dinger equation in the reverse propagation direction, thereby enabling the estimation of various transmission-induced impairments with high accuracy. The incorporation of deep neural networks into the DBP framework further enhances its adaptability, allowing it to dynamically optimize weight parameters and accommodate a wide range of link configurations and nonlinear distortion regimes. In addition to power profile estimation, DBP-based schemes have shown potential in amplifier gain spectrum analysis, and device behavior modeling, including wavelength-selective switch (WSS) effects, while simultaneously compensating for nonlinear impairments.Furthermore, research efforts have extended digital monitoring frameworks toward multi-parameter estimation, targeting quantities such as chromatic dispersion, polarization mode dispersion, and differential group delay. These methods often employ template-based models in combination with advanced correlation matching techniques to facilitate comprehensive link state identification and classification under realistic network conditions.Conclusions and ProspectsDespite the promising achievements in digital fiber link monitoring, several technical challenges remain unresolved. Most notably, the computational complexity associated with high-accuracy inverse problem-solving remains a significant bottleneck, particularly for long-haul or high-capacity transmission systems where real-time estimation is essential. Moreover, many existing algorithms are inherently dependent on idealized link assumptions and may exhibit reduced robustness when confronted with complex, time-varying network environments.Addressing these challenges will require future research efforts to prioritize algorithmic simplification and computational efficiency. The integration of deep learning models is expected to facilitate the development of adaptive, data-driven inversion frameworks capable of handling multi-parameter coupling effects and generalizing across diverse link conditions.In summary, digital fiber optic link monitoring is transitioning from theoretical research to practical engineering applications and is expected to become an indispensable component of future intelligent optical networks. This technology will not only enhance the efficiency and reliability of optical transmission systems but will also enable advanced functionalities, including real-time fault detection, resource optimization, predictive maintenance, and autonomous self-healing. As the convergence of digital monitoring, machine learning, and sensing technologies continues to evolve, it is anticipated that digital fiber link monitoring will play a pivotal role in the development of next-generation self-optimizing and self-aware optical communication infrastructures.

ObjectiveSpace laser communication is a communication technology that uses laser as carrier to realize high-speed and reliable data transmission in space. Its fundamental concept is to support efficient data transmission between platforms in space, on earth, and at sea through high transmission rates, strong anti-interference abilities, and excellent confidentiality. Currently, the cutting edge of space laser communication technology lies in the development of networking capabilities. This transition moves from the traditional point-to-point communication model to a multi-node collaborative network architecture, aiming to establish an intelligent, full-coverage communication network. However, in the current space laser communication networking scenario, the classic point-to-point laser communication terminal remains the core configuration. Multi-node chain networking is achieved by increasing the number of laser terminals. To realize a set of laser communication terminals capable of adapting to different types and numbers of laser links, the system should undergo software and hardware reconstruction. This will allow the establishment of laser links with multiple communication nodes to enable information transmission. We propose an optical theory and network architecture based on rotating parabolic multi-mirror splicing and discuss the challenges and breakthroughs in key technologies such as multi-beam isolation and multi-node acquisition and tracking. We provide a reference for technological progress in space optical communication networking.MethodsTo use a set of laser terminals and establish laser links with multiple communication nodes simultaneously, information transmission is realized. The optical theory and networking configuration of multi-mirror splicing based on the rotating paraboloid are proposed. The core theory is the optical principle of the rotating paraboloid. The paraboloid is rotated 180° around its axis of symmetry, and the resulting curved surface is the rotating paraboloid. The increase in the number of mirror splices, or “planes”, brings the system closer to the ideal rotating paraboloid. This ensures effective beam energy utilization and communication link margin while allowing more laser communication nodes to be dynamically reconstructed. The ultimate goal is to create a hardware system that can support optical communication networking with any number of nodes through software configuration. The theory and configuration of the one-to-many laser communication networking architecture based on the rotating paraboloid provides a 360° omnidirectional field of view and a theoretical pitch field of view above 100°. This setup can enable multi-point duplex simultaneous laser communication over a larger range and at longer distances.Results and DiscussionsThe scientific research team successfully carries out a high-altitude multi-node laser communication networking test in Lulang, Xizang, and verified the key technology for dynamic networking of space platforms. The test adopts the self-developed optical transceiver system: the master optical transceiver provides 360° omnidirectional coverage and ±30° pitch scanning of a 200 μrad narrow beam through a rotating parabolic array antenna, while the slave optical transceiver uses a 90 mm aperture antenna with a 100 μrad divergence angle design. In the experiment, a three-node network is established, consisting of the master optical transceiver on the ground, the airship-based slave optical transceiver at 3300 m, and the mountaintop slave optical transceiver. A 2.5 Gbit/s high-speed transmission has been achieved over a link distance of 2 km, and synchronized returns of the gondola, unmanned aerial vehicle (UAV) aerial photography, and ground monitoring video has been successfully completed. The measured data shows that the coarse-fine composite tracking system provides a dynamic tracking accuracy better than 6 μrad (Fig. 8), and the fine tracking system significantly improves communication quality. Compared to the non-fine tracking system, the bit error rate increases from 10-5 to better than 10-9 (Fig. 9), which verifies the capability for rapid multi-node chain-building and stable transmission in harsh environments. This achievement provides important technical support for building a space-based high-speed laser communication network, marking a breakthrough in the field of space laser networking.ConclusionsCompared to traditional point-to-point laser communication terminals, the rotating paraboloid configuration offers a similar 360° azimuth and 100° elevation beam control range, with millisecond-level mechanical servo speed. Its effective transmitting and receiving aperture exceeds 50 mm, and its multi-mirror splicing control and single sub-optical path integrated transmitting and receiving capabilities are unprecedented. As the number of nodes increases, the volume and weight advantages of this configuration will become more apparent. In comparison with non-mechanical solid-state scanning laser communication networking configuration, the rotating paraboloid multi-mirror splicing optical antenna configuration provides a wider beam control range than the liquid crystal phased array (±12° azimuth and elevation) and a larger effective transmitting and receiving aperture than the optical waveguide phased array (8 mm). This is achieved while maintaining the ability to transmit and receive multiple beams simultaneously. Additionally, unlike polarization/energy diversity networking terminals based on classical optics, which require an additional set of polarization/energy diversity devices for each node, the rotating paraboloid formed by multiple mirrors offers excellent scalability. It allows for the addition of more nodes to the laser communication networking without changing the hardware structure. In conclusion, the multi-reflector antenna configuration based on the rotating paraboloid and the associated laser communication networking device introduced in this paper offers high antenna gain and link margin, scalable software reconfiguration for large-scale networking, and a relatively mature technical system, which can provide a reference for the technological progress of space optical communication networking.

ObjectiveConventional multi-core fiber design methodologies face significant challenges, including inefficient processes and high computational requirements, particularly when addressing complex multi-objective optimization problems that depend on parameter sweeps and trial-and-error iterations. The non-uniqueness problem arising from multi-parameter coupling further compounds these limitations. This research presents a machine learning (ML)-based collaborative design framework to address these challenges for randomly coupled multi-core optical fiber (RC-MCF) and few-mode MCF (FM-MCF), aiming to enhance design efficiency, minimize computational resource utilization, and resolve the complexities associated with multi-parameter interactions in fiber design.MethodsFor RC-MCFs, a forward prediction model is constructed by integrating neural networks (NNs) and optimization algorithms, comprising three sequential stages: first, a classification model (NN 1) is trained on structural parameters including core radius (r), refractive index difference (Δ), and twisting peak rate (Tp) to predict the optimal core spacing (Λ) with high accuracy; second, a sparrow search algorithm-optimized random forest model (SSA-RF) is employed to classify spatial mode dispersion (SMD) into ordered labels, thereby enhancing feature representation for subsequent analysis; third, a particle swarm optimization-optimized random forest model (PSO-RF) utilizes the enhanced features—including the predicted Λ and SMD labels—to regress SMD values, capturing the complex nonlinear relationships between structural and optical properties. For FM-MCFs, two inverse design strategies are proposed: the first is an independent neural network model (NN 2), a five-layer NN with L2 regularization that directly maps target optical performance parameters—such as inter-core crosstalk(IC-XT), differential mode group delay (DMGD), effective area (Aeff), and bending loss (BL)—to structural parameters including r1, r2, w1, Δ1, α, Λ, and dOCT; the second is a NN-PSO joint model (NN 3+PSO), where a forward NN (NN 3) first learns the mapping from structural parameters to performance metrics, and this model is then integrated with a particle swarm optimization (PSO) algorithm to serve as a fitness function for searching optimal structural parameters that meet predefined target performance criteria.Results and DiscussionsThe implementation of these frameworks demonstrates significant effectiveness: for RC-MCFs, the NN 1 model achieves a core spacing prediction accuracy of approximately 0.989, as validated by a confusion matrix, while the SMD prediction exhibits a Pearson correlation coefficient (PCC) of approximately 0.9561 [Fig. 5(b)], indicating strong alignment between predicted and actual values; notably, the ML model substantially improves computational efficiency, reducing prediction time from hours for traditional numerical simulations to milliseconds after training (Table 1), enabling rapid design iterations. For FM-MCFs, the NN 2 model achieves PCC values of 0.9811 for κ, 0.9974 for DMGD, and 0.9870 for Aeff, with a mean square error (MSE) of approximately 5×10-4, while the NN 3+PSO model further enhances accuracy, achieving PCC values of 0.9988 for κ, 0.9996 for DMGD, and 0.9914 for Aeff, with an MSE of approximately 3×10-5 [Fig. 10(a)]; significantly, all predicted designs satisfy the stringent BL requirement of less than 0.001 dB/km under standard bending conditions (R=140 mm), as verified by COMSOL simulations [Fig. 10(b)]. The ML frameworks collectively reduce computational resource consumption by over 90% compared to traditional methods, effectively resolving the non-uniqueness problem in multi-parameter optimization and demonstrating the effectiveness of feature enhancement and deep learning in modeling complex nonlinear relationships between fiber structures and optical performances.ConclusionsThis research demonstrates the successful implementation of ML-assisted design frameworks for RC-MCFs and FM-MCFs, achieving significant improvements in both design efficiency and prediction accuracy. The forward prediction model for RC-MCFs and the inverse design strategies for FM-MCFs illustrate the transformative potential of machine learning in addressing the limitations of conventional design approaches, offering a reliable and scalable solution for developing next-generation large-capacity optical transmission systems. Although the current models demonstrate robust performance across typical parameter ranges, future research will focus on enhancing prediction accuracy in extreme parameter regions through data augmentation techniques and model optimization—such as implementing weighted loss functions—and expanding the applicability of these frameworks to other complex fiber designs, thereby advancing the field of optical fiber engineering.

ObjectiveThe seamless integration of communication and sensing has become a crucial factor in redesigning optical network architectures. This integration enables multi-dimensional sensing capabilities while supporting the intelligent evolution of information infrastructure, addressing contemporary network demands. The demand for ultra-low-frequency (ULF) distributed acoustic sensing (DAS) continues to grow in essential applications such as geophysical exploration, structural health monitoring, and marine seismic monitoring. However, current fiber optics integrated sensing and communication (ISAC) systems encounter substantial challenges, including limited sensitivity to low-frequency signals and suboptimal spectral resource utilization due to the separation of communication and sensing functions in conventional systems. To address these challenges, this paper introduces a novel co-wavelength-channel ISAC system architecture based on frequency division multiplexing (FDM). The system achieves concurrent high-capacity communication and ULF sensing by integrating a digital subcarrier multiplexing (DSM) signal for communication and a linear frequency-modulated pulse signal for sensing within the same wavelength channel.MethodsWe investigate the performance of proposed ISAC system for ULF DAS and high-capacity communication using standard single-mode fiber (SSMF), which is widely deployed in existing optical networks. At the transmitter side, DSM and sensing signals are generated independently but share a common laser source. The signals are then combined in the optical domain through FDM, leveraging the flexible spectrum allocation capability of the DSM signal. The FDM scheme used in the proposed system is classified into two configurations, depending on the placement of the sensing signal relative to the DSM signal. As for in-band and out-of-band FDM, the sensing signal is placed within and outside the frequency band of DSM signal, respectively. Meanwhile, a careful guard-band allocation is required to minimize the interference between communication and sensing signals. In this study, the sensing and communication performance is investigated between in-band and out-of-band FDM schemes. After transmission over the fiber, the forward-propagated ISAC signal is received using a coherent receiver. The received signal undergoes digital bandpass filtering to extract the communication signal, which is then equalized to recover the transmitted data. Simultaneously, the backscattered ISAC signal, which carries the sensing information, is processed. An optical filter is first applied to remove the DSM signal, followed by either direct detection or coherent detection to reconstruct the vibration information imposed on the fiber. The sensing performance comparison between direct detection and coherent detection is also studied under the condition of ULF sensing. Finally, we examine the bit error ratio (BER) performance of DSM signals after transmission over a 38 km SSMF link, and the sensing sensitivity utilizing a piezoelectric transducer (PZT) at the end of SSMF to generate an ULF acoustic wave.Results and DiscussionsThe proposed ISAC system effectively demonstrates both high-capacity optical transmission and ULF DAS capabilities over a 38 km SSMF link. Experimental results indicate that while the out-of-band FDM scheme delivers a higher distributed sensing signal-to-noise ratio (SNR) by minimizing interactions between communication and sensing signals (Fig. 9), its increased device bandwidth requirements result in a lower communication Q-factor (Fig. 10). The in-band FDM scheme achieves a more optimal balance between spectral utilization and communication performance. Experimental findings confirm that the direct detection scheme surpasses coherent detection in low-frequency vibration sensing (Fig. 11). This advantage is particularly significant in applications requiring high sensitivity to ULF signals, such as seismic monitoring and structural health diagnostics. Through optimization of spectral positioning and guard-band allocation between communication and sensing signals, and implementation of direct detection with autocorrelation demodulation, the proposed ISAC system achieves ULF sensing while maintaining high communication capacity. Based on dense wavelength division multiplexing (DWDM) technique, the transmission of 96-channnel dual polarization 16-state quadrature amplitude modulation (DP-16QAM) DSM signals reaches the BER threshold of 20% soft-decision forward error correction coding (SD-FEC). The system maximizes communication capacity by inserting guard-band only within one wavelength channel while maintaining full utilization of remaining channels for high-speed data transmission, achieving an aggregated communication capacity of 34.44 Tbit/s (Fig. 12). The ULF sensing capability is validated through vibration experiments using a PZT. The sensing system demonstrates a sensitivity of 5.49 nε /Hz@0.1 Hz with a spatial resolution of 20 m (Fig. 13).ConclusionsWe demonstrate the feasibility of integrating high-capacity optical communication with ULF DAS using SSMF. By leveraging the flexible spectrum allocation of DSM signals, the high-speed DSM signal and the linear frequency-modulated pulse sensing signal are frequency-division multiplexed within the same wavelength channel. After optimizing the spectral positioning and guard-band allocation between communication and sensing signals, together with utilizing the direct detection and autocorrelation demodulation for sensing signals, we successfully achieve a sensitivity of 5.49 nε /Hz @0.1 Hz with a spatial resolution of 20 m, together with a transmission capacity record of 34.44 Tbit/s over 38 km SSMF. These results validate the feasibility of enhancing existing SSMF communication networks with sensing capabilities for ULF monitoring, paving the way for the intelligent evolution of optical networks.

ObjectiveSpace laser communication has emerged as a transformative technology addressing the increasing demands of next-generation space exploration and science missions. Conventional microwave communication systems face fundamental constraints: limited capacity, intense competition for spectrum resources, and exponential increases in size, weight, and power consumption with increasing transmission distance and data rates. These limitations significantly impede deep-space exploration, Earth observation constellations, and satellite networks that require high-data-rate links across thousands to tens of thousands of kilometers without repeaters.Coherent optical communication systems, particularly those utilizing homodyne detection, present a compelling solution in this context. Leveraging the ultra-short wavelength and high frequency of laser light, these systems deliver substantially higher bandwidth potential and superior receiver sensitivity compared to direct detection or conventional microwave-based systems. This sensitivity is essential for overcoming the substantial link losses inherent in space-based free-space optical links. Additionally, coherent detection provides exceptional background light rejection, a crucial capability in the space environment saturated with solar radiation. However, achieving the necessary phase and frequency synchronization between the signal light and local oscillator light for homodyne detection presents significant technical challenges, particularly under dynamic space conditions (e.g., platform vibrations, large Doppler shifts). The development of robust, high-sensitivity, and power-efficient homodyne receivers therefore represents a cornerstone for realizing the full potential of high-capacity space laser communications.MethodsThis research successfully developed and validated a high-performance 10 Gbit/s binary phase-shift keying (BPSK) optical homodyne receiver prototype based on the Costas loop architecture. The prototype receiver includes several key technical achievements. 1) Delayed XOR phase-frequency detector (PFD). A PFD architecture (Fig. 2) is implemented using a time-delayed XOR gate preceded by limiting amplifiers. Theoretical analysis [Eq. (6)] and numerical simulations (Fig. 3 and Fig. 4) optimized the delay, balancing a -6.25 to 6.25 GHz frequency capture range with high phase discrimination sensitivity while eliminating BPSK modulation artifacts. 2) Auxiliary-controlled composite optical phase-locked loop (OPLL). A dual-loop architecture (Fig. 5) including a fast loop and a slow loop has been proposed and realized. The fast loop with MHz bandwidth uses an acousto-optic frequency shifter (AOFS) -50 to 50 MHz range controlled by an analog active loop filter (LF) (Fig. 6) with lead-lag compensation for loop stability. The slow loop with kHz bandwidth uses the piezoelectric transducer (PZT) -30 to 30 GHz range of a narrow-linewidth fiber laser controlled via digital LF implemented with digital signal processor (DSP) and digital to analog converter (DAC). In addition, an auxiliary controller monitors LF output and filtered PFD error. These enabled an autonomous frequency scanning, lock detection, and proactive Doppler compensation by coordinating the fast/slow loops. 3) Prototype implementation & testing. A 10 Gbit/s BPSK receiver (Fig. 7) is built using a 90° optical hybrid (within the error range of -5° to 5), balanced photodetectors (10 GHz bandwidth), home-made PFD/LF boards, AOFS, and fiber laser based local oscillator laser. The performance verification is completed through the following four tests. 1) Autonomous locking tests (Fig. 8). Measuring acquisition time under ±12 GHz initial frequency offset conditions of -12 to 12 GHz. 2) Doppler tolerance tests (Fig. 10). Apply a sine frequency scan of -8 to 8 GHz at a rate of 600 MHz/s. 3) Sensitivity tests (Fig. 12). Measuring BER vs. received power using a PRBS 223-1 pattern. 4) Phase noise analysis. Integrating residual phase error from PFD output spectra (Fig. 13).Results and DiscussionsThe prototype demonstrated exceptional performance exceeding typical space communication requirements. 1) Autonomous acquisition. Lock acquisition completed in ≤7.3 s under extreme initial frequency offset conditions of -12 to 12 GHz (Fig. 9), demonstrating robust link initialization essential for operational systems. The auxiliary controller’s scanning and detection capability proved instrumental in this achievement. 2) Doppler tolerance. The receiver compensated simulated Doppler shifts of -8 to 8 GHz magnitude at 600 MHz/s slew rates [Fig. 11(b)], substantially exceeding typical maximum requirements. Notably, disabling the auxiliary control resulted in immediate loss of lock [Fig. 11(c)], confirming its essential role in extending the effective tracking range beyond the AOFS’s inherent ±50 MHz limit. 3) Phase-locking fidelity. Measured loop bandwidth reached ~500 kHz (Fig. 13), enabling effective suppression of broadband phase noise. Integrated residual phase error is 3.6°, substantially below the ~10° threshold for low-penalty BPSK demodulation, confirming loop stability under closed-loop operation. 4) Sensitivity with discrete implementation. Achieved a sensitivity of -41.3 dBm at BER is 1×10-7 (Fig. 14), exceeding the -40 dBm benchmark for 10 Gbit/s space transmission links. The sensitivity at BER (1×10-9) is -39.5 dBm, with clear eye opening observed (Fig. 14, inset). This approaches theoretical homodyne limits and surpasses reported integrated alternatives at similar speeds. 5) System robustness. The composite loop design effectively integrated the wide tuning range of the PZT-based slow loop (±30 GHz) with the rapid response of the AOFS-based loop (10 MHz bandwidth), resolving the inherent trade-off in single-loop designs. The auxiliary control’s sophisticated management of frequency drift prevention ensured sustained lock under dynamic conditions.ConclusionsThis research successfully designed, implemented, and rigorously validated a high-performance 10 Gbit/s BPSK optical homodyne receiver prototype based on the Costas loop architecture. The developed prototype represents a significant milestone in space coherent receiver technology utilizing discrete components. The results demonstrate the technical maturity and space-readiness of the Costas loop homodyne architecture, fulfilling the critical requirements for high-sensitivity, high-data-rate space laser communications. The prototype’s performance confirms its advantages in laser phase and frequency noise tolerance, acquisition robustness, and Doppler resilience compared to alternative coherent receiver configurations. Future improvements can focus on size reduction, power consumption optimization, support for higher-order modulation formats, and system integration. These technical achievements establish a fundamental platform for developing next-generation integrated coherent terminals essential for high-capacity inter-satellite links and provide significant reference value for the engineering implementation of coherent receivers in space optical communications.

ObjectiveAs satellite networks continue to expand in scale and service demands grow rapidly, satellite networks face significant challenges due to limited spatial resources and insufficient transmission efficiency. Traditional switching mechanisms struggle to meet the demands of high-capacity, low-latency, and high-reliability inter-satellite communications. This challenge is especially pronounced as satellite constellations scale from hundreds to tens of thousands of satellites. Emerging applications such as ultra-high-definition video streaming, on-board artificial intelligence (AI) real-time inference, and edge computing on low Earth orbit satellites impose new and stringent performance requirements on on-board switching systems. These emerging applications demand on-board switching systems to support throughput at the Tbit/s level, end-to-end latency in the millisecond range, and energy efficiency on the order of μJ/bit, in order to meet the stringent power constraints of satellite platforms. However, traditional electrical switching systems are limited by interface bandwidth and power density, causing their switching capacity to stagnate at several hundred gigabits per second, making it difficult to support Tbit/s level traffic loads. Optical switching technology, with its ultra-wide bandwidth and low transmission loss, has increasingly become a focus of research and industry. However, on-board optical switching lacks flexible multi-granularity transmission capabilities, resulting in reduced switching efficiency when handling diverse types of traffics. In this context, we propose a high-capacity on-board optical/electric hybrid switching technology aimed at overcoming the physical limitations of traditional switching systems. This novel switching architecture integrates the flexible control of electrical switching with the high bandwidth advantages of optical switching, achieving both high throughput and low latency.MethodsWe propose a resource-constrained, high-capacity multi-directional optical/electric hybrid switching architecture that integrates multi-granularity electrical switching based on spatial frames with reconfigurable optical switching technology. The design aims to address the differentiated requirements of single-direction fine-grained multi-granularity traffic and multi-direction coarse-grained traffic switching within satellite payloads under limited spatial resource conditions. Specifically, the architecture consists of three main components: the master control module, the all-optical switching module, and the spatial frame switching module. The spatial frame switching module handles the switching of small-granularity and medium-granularity traffic within the satellite network by performing fine-grained switching inside the module. For large-granularity traffic switching across different directions within the satellite, data is modulated onto optical carriers and switched between directions via the all-optical switching module. The all-optical switching module adjusts its internal microlens array according to control information, directing input optical signals to the target spatial frame switching modules. This enables large-granularity data switch across multiple spatial directions between spatial frame switching modules. Additionally, this paper proposes a traffic rate adaptive matching technique based on the generic mapping procedure (GMP) and a low-insertion-loss, high-isolation, non-blocking all-optical switching technology. The former addresses the rate mismatch problem when multiplexing multiple signals into optical channel payload unit (OPU) frames and enables adaptive rate matching for signals from different clock domains. The latter overcomes the excessive insertion loss commonly encountered in traditional optical switching.Results and DiscussionsExperimental results demonstrate that the developed high-capacity on-board optical/electric hybrid switching platform successfully achieves full-granularity optical channel data unit (ODU) 0/ODU 2 switching and all-optical switching under a power consumption of no more than 90.72 W. By applying the service rate adaptive matching technique based on GMP, our payload successfully achieves adaptive rate matching from ODU 2 services to OPU 4. This resolves the clock domain mismatch issue commonly encountered in the asynchronous mapping of signals to higher-order OPU k frames in traditional electrical switching. Meanwhile, by employing the low-insertion-loss, high-isolation, non-blocking all-optical switching technology, the proposed system achieves all-optical switching with an insertion loss of no more than 2 dB (Table 1), significantly outperforming the loss levels of traditional all-optical switching modules. In addition, this paper presents an adaptability analysis of the potential effects of space environmental factors—such as radiation and temperature fluctuations—on the payload. By employing radiation-hardened field-programmable gate arrays (FPGAs), space-grade temperature-tolerant firmware, high-process materials, and real-time monitoring and adjustment through control software, the system’s reliability and stability under harsh space environmental conditions have been significantly enhanced.ConclusionsTo address the core challenges of high capacity, low latency, and energy efficiency in satellite networks, we introduce an innovative resource-constrained on-board optical/electric hybrid switching architecture, along with two key technologies: a service rate adaptive matching technique based on the GMP and a low-insertion-loss, high-isolation, non-blocking all-optical switching technology. Through the deep integration of spatial frame-based electrical switching and reconfigurable optical switching, this study achieves coordinated control of full-granularity ODU 0/ODU 2 electrical switching and all-optical switching under conditions of low power consumption and minimal resource usage. This work provides a highly reliable switching solution for mega satellite constellations. Its innovative hybrid control architecture and spatial frame scheduling mechanism significantly improve resource utilization in satellite networks. Moreover, it offers Tbit/s-level switching infrastructure for 6G space-air-ground integrated networks and lays a solid technical foundation for the development of optical/electric collaborative technologies in space-air-ground integrated communication systems.

ObjectiveThe rapid advancement of data-intensive technologies, including the Internet of Things (IoT) and cloud computing, presents unprecedented capacity challenges for current optical networks. Traditional optical communication networks, primarily utilizing silica single-mode fiber (SMF), have historically achieved exponential growth in information transmission capacity. However, the Kerr nonlinearity inherent in single-mode fibers imposes limitations, with maximum throughput approaching the theoretical limit of approximately 100 Tbit/s, insufficient for future data traffic demands. In response, space-division multiplexing (SDM) technology has emerged as a crucial solution to overcome the capacity constraints of single-mode fibers by enabling multiple independent data channels within a single wavelength through parallel spatial channels, offering a revolutionary approach to expanding optical network capacity.MethodsThis research presents a high-capacity, long-haul multidimensional multiplexing transmission system utilizing weakly coupled multi-core few-mode fiber (MC-FMF), achieving efficient transmission of 48 Gbaud broadband signals through a weakly coupled 19-core 6-mode fiber. The system combines SDM technology with wavelength-division multiplexing (WDM) and polarization-division multiplexing (PDM) technologies. This implementation facilitates independent transmission of four spatial modes (LP11a, LP11b, LP21a, and LP21b) and their corresponding polarization across 80 wavelength channels.Results and DiscussionsThe fabrication process optimization for the multi-core few-mode fiber employed in this petabit-scale multidimensional multiplexing system is illustrated in this paper, requiring comprehensive optimizations across three key stages: core deposition, multi-core preform fabrication, and fiber drawing (Fig. 1). In the core rod deposition stage, we employ our proprietary plasma chemical vapor deposition (PCVD) technology, combined with high-purity raw materials specifically developed for MC-FMF. Through systematic optimization of deposition parameters and material ratios, we implement advanced PCVD techniques including multi-time deposition, precision doping, and interface stress control to ensure accurate realization of the computer-simulated core-cladding structures. The process optimization particularly focuses on enhancing the structural fidelity of multiple cores while maintaining low attenuation characteristics through precise stress management. The 19 cores are arranged in concentric rings to minimize inter-core crosstalk, with a uniform core spacing of 44 μm specifically designed to suppress internal crosstalk. At 1550 nm, this 19-core fiber exhibits a typical inter-core crosstalk below -40 dB/30 km, demonstrating negligible coupling effects between cores that can be effectively disregarded in system operation. Considering the geometric dimensions and refractive index profile of each core, every individual core can support six spatial modes: LP01, LP11a, LP11b, LP21a, LP21b, and LP02 (Fig. 3). Building upon these outstanding characteristics, the proposed high-performance weakly coupled multi-core few-mode fiber establishes a solid foundation for future deployment of high-capacity, long-haul multidimensional multiplexing optical communication systems.Multiple-input multiple-output (MIMO) equalization, a fundamental algorithm in multidimensional multiplexing transmission systems, is widely employed to compensate for inter-channel crosstalk. Traditional multiplexing transmission systems, multiple-input multiple-output time-domain equalizers (MIMO-TDEs) exhibit high computational complexity that increases linearly with tap length. This paper addresses these limitations by proposing an MIMO frequency-domain equalizer (FDE) incorporating a decision-based carrier phase recovery (DBCPR) module (Figs. 5 and 6). The integrated decision-based carrier phase recovery module enhances phase noise resistance while improving channel estimation accuracy. This approach substantially reduces computational complexity while improving phase noise tolerance.The bit error rate (BER) performance analysis of eight sub-channels after long-haul transmission through multi-core few-mode fiber reveals comparable BER characteristics, attributed to mode-dependent loss (MDL) suppression through optimized loop configuration. Signal degradation occurs with increasing transmission distance due to progressive pulse broadening and accumulated mode coupling. Advanced digital signal processing (DSP) algorithms at the receiver enable few-mode signals after 1000-km transmission to maintain BER threshold with 20% overhead. Uniform performance across wavelength channels is achieved through independent channel attenuation optimization via wavelength selective switch (WSS). Constellation diagrams demonstrate effective impairment compensation through the proposed MIMO-FDE-DBCPR. The SDM-MDL-PDM multidimensional multiplexing system achieves a total line rate of 1.945 Pbit/s.ConclusionsThis study provides essential technical references for advancing next-generation high-capacity optical transmission systems, demonstrating the significant potential of MC-FMF in multidimensional multiplexed optical communications. The research successfully implemented a petabit-scale multidimensional multiplexing transmission system utilizing weakly coupled MC-FMF. The system incorporates an innovative MIMO-FDE-DBCPR for multidimensional signal processing, which substantially reduces computational complexity while enhancing phase noise tolerance. Experimental results demonstrate that following 1000-km transmission, all wavelength channels meet the BER threshold, validating the system’s efficiency and reliability. This investigation presents valuable technical guidelines for the design and optimization of next-generation high-capacity optical transmission systems, while reinforcing the substantial application potential of MC-FMF in multidimensional multiplexed optical communications.

ObjectiveThe 6th generation (6G) mobile networks envision a fully connected world, indicating that next -generation information systems will feature enhanced bandwidth, increased access points, improved energy efficiency, and sophisticated functionalities. The evolution of 6G technologies presents unprecedented challenges to modern electronic warfare. Advanced anti-jamming radar detection techniques, utilizing higher frequency bands and sophisticated modulation methods, substantially enhance information rates and capacities. However, these advancements render traditional jamming techniques based on electronic hardware ineffective due to bandwidth limitations. For instance, digital radio frequency (RF) memory (DRFM) technologies are constrained by digital-analog conversion /digital-analog conversion (DAC/ADC) sampling rates, with instantaneous bandwidths rarely exceeding 2 GHz. Microwave photonics has emerged as a solution to overcome these electronic limitations, garnering significant attention from academia and industry. Unlike conventional microwave systems focused on advancing singular communication or sensing technologies, microwave photonics, characterized by high frequency, broad bandwidth, low loss, and tunability, enables multifunctionality and compact structure. To address the convergence of large-capacity communication, radar detection, and electronic jamming in the 6G context, we present a communication-jamming functionally integrated microwave photonic RF front-end based on a single optical modulator.MethodsIn this paper, we proposed a novel communication-jamming functionally integrated microwave photonic RF front-end. The suggested scheme utilizes a dual-polarization optical in-phase and quadrature (IQ) modulator, where the intercepted enemy radar signal or communication signal is modulated on the X-polarization of the carrier via IQM-X, biased at the linear operation point to achieve carrier suppression single-sideband (CS-SSB) modulation. Simultaneously, an RF signal for carrier reconstruction is modulated on the Y-polarization of the carrier via IQM-Y to generate velocity deception information. The IQM-Y bias is adaptively controlled using a self-developed modulator bias control module, enabling transitions between single-false-target, multi-false-target, and blinking-false-target configurations. The dual-polarization optical IQ modulator output signal undergoes polarization alignment before entering a photonic RF memory (PRFM) structure based on an active fiber loop for cyclic storage, generating range deception information and expanding transmission capacity. The stored signal is subsequently detected by a photodetector (PD) to obtain the reconstructed communication signal or range-velocity compound jamming signal.Results and DiscussionsIn experiments, the high-fidelity storage capacity for a 12 GHz RF signal has reached 600 μs in a 300 m fiber optic loop, corresponding to 400 false targets in the range dimension (Table 1 and Fig. 2). Further, the optical carrier is reconfigured by the self-developed bias control module to achieve the integration and switching of single-false-target, multi-false-target, and blinking-false-target jamming functions. (Fig. 3). In multi-false-target generation, the number of false targets within the 10 dB effective bandwidth is kept at 9, ensuring that the total number of false targets during storage reached 3600 (400×9), each carrying different range-velocity deception information. Additionally, by simply adjusting the frequency of the reconstruction signal, fast tuning of the velocity deception can be achieved. On the other hand, the communication transmission for a 16QAM signal with 0.8 Gbaud bandwidth at 12 GHz is also verified. The transmission distance has increased to 90 km, and more than 300 information copies have been generated within a 6.17% error vector magnitude (EVM) penalty (Figs. 4 and 5).ConclusionsThe research findings demonstrate the exceptional storage, jamming, and transmission capabilities of the proposed scheme. The X/Ku band storage capacity achieved 600 μs with 3600 range-velocity compound false targets, while the communication transmission distance extended to 90 km. The solution requires only a dual-polarization optical IQ modulator, utilizing IQM-X for radar or communication signal loading and IQM-Y for carrier reconstruction to achieve velocity deception. The integration of PRFM enables range deception and enhanced transmission capacity. This scheme presents a viable alternative to traditional DRFM, offering a promising solution for future electronic warfare applications.

ObjectiveThe exponential growth of data center interconnection (DCI) traffic, driven by cloud computing, AI-driven analytics, and hyperscale applications, has exposed critical limitations in conventional intensity modulation/direct detection (IM/DD) systems. While IM/DD systems remain widely adopted for their low power consumption and simplicity, their capacity and reach are fundamentally constrained by fiber chromatic dispersion (CD) and bandwidth limitation, particularly in single-wavelength 400 Gbit/s-and-beyond scenarios. Coherent optical interconnections, with their superior spectral efficiency (SE) and digital CD compensation capability, have emerged as a promising solution for next-generation 800 Gbit/s‒1.6 Tbit/s DCI system. However, the high cost and power overhead of high-speed analog-to-digital converters (ADCs) and digital signal processing (DSP) chips—key enablers of coherent detection—remain significant adoption barriers. Furthermore, achieving ultra-high SE through increasing symbol rates exacerbates inter-symbol interference (ISI) in bandwidth-constrained channels, demanding advanced receiver-side equalization that further escalates system complexity. Existing methodes to mitigate ISI, such as feed-forward equalizers (FFEs), maximum likelihood sequence estimation (MLSE), or transmitter-side pre-filtering, often trade off noise resilience for bandwidth efficiency, limiting their practicality in cost-sensitive DCI environments. We address these intertwined challenges by proposing a novel symbol-domain multiplexing (SDM) framework that simultaneously reduces bandwidth demand, enhances noise tolerance, and simplifies transceiver complexity. By rethinking conventional modulation paradigms, we aim to break the “bandwidth vs. complexity” deadlock in high-capacity DCI systems, providing a scalable pathway to 800 Gbit/s‒1.6 Tbit/s deployments while alleviating ADC/DSP-related cost and power constraints.MethodsTo address these challenges, we propose a novel symbol multiplexing method based on joint coded modulation, specifically designed to enhance spectral efficiency and bandwidth/baud rate utilization in coherent optical interconnection systems. The proposed method builds a linear coded relationship between quadrature amplitude modulation (QAM) symbols using forward error correction (FEC) encoding. By applying a “many-to-one” symbol mapping strategy, every two 5 bit 32 QAM symbols are combined and mapped onto a single constellation point, effectively generating a symbol-multiplexed 16 QAM signal. This method increases the symbol capacity from the conventional 4 bit per 16 QAM symbol to 5 bit, resulting in a 25% improvement in spectral efficiency and bandwidth utilization per baud.The symbol multiplexing process in this work goes beyond conventional modulation formats by integrating the benefits of joint coding and modulation. Traditional modulation methods treat coding and modulation as separate layers, but by tightly coupling them, symbol multiplexing allows for a more efficient representation of information. This strategy is particularly advantageous in short-reach coherent systems where channel bandwidth is constrained because of hardware and design limitations. As data rates increase, scaling up symbol rates leads to more severe ISI and higher demands on DSP and hardware. The proposed method mitigates these issues by reducing the required system bandwidth.Results and DiscussionsTo support the proposed transmitter-side symbol multiplexing, a corresponding joint demodulation and decoding method is introduced at the receiver. This receiver architecture performs parallel iterative decoding, leveraging soft information exchange between the FEC decoder and the QAM demodulator. The iterative process refines the symbol likelihoods, effectively improving the extrinsic information transfer (EXIT) curves and boosting the overall decoding performance. This technique enhances the reliability of symbol decisions in the presence of channel noise and ISI, leading to better bit error rate (BER) performance at lower optical signal-to-noise ratios (OSNRs).The theoretical performance of the proposed symbol multiplexing method is analyzed under bandwidth-constrained conditions. Simulation and analysis demonstrate that this method provides notable gains in noise tolerance compared to conventional modulation formats. Specifically, by enabling more bits to be transmitted per symbol while maintaining manageable constellation sizes, the method improves trade-offs between complexity, power, and performance.To validate the feasibility and performance of the proposed method, a 400 Gbit/s short-reach coherent optical interconnection system is implemented and experimentally evaluated. In this setup, the symbol-multiplexed 16 QAM signal is transmitted over a limited-bandwidth link, and the joint demodulation-decoding algorithm is applied at the receiver. Experimental results show that, compared to a conventional 16 QAM signal with the same effective data rate, the symbol-multiplexed 16 QAM requires only 0.8× system bandwidth to achieve effective data transmission. Moreover, it delivers a 3.97 dB OSNR gain, confirming the method’s robustness and spectral efficiency advantages.ConclusionsThe outcomes of this work indicate that symbol multiplexing based on joint coded modulation holds strong potential as an enabling technology for next-generation high-speed optical interconnections, especially in environments such as cloud data centers where efficient bandwidth usage and low power consumption are critical. By enhancing bandwidth utilization and providing better resilience to noise and ISI, this method can significantly contribute to the scalability and energy efficiency of future optical interconnection systems.

ObjectiveLaser communication has attracted significant attention in the communications field due to its advantages, such as high bandwidth, strong anti-interference capability, and low probability of interception. With the growing demands for telemetry and data transmission of unmanned aerial platforms, the development of laser communication links for non-orbital platforms has become an urgent and critical need. Unlike satellite-based laser communications, non-orbital platforms face unique challenges, including trajectory uncertainty, rapid and unpredictable attitude changes, and highly complex electromagnetic environments. These factors impose more stringent requirements on the rapid establishment of laser communication links. In response to these challenges, we propose a novel dynamic aiming correction method that integrates directional microwave measurement with particle filtering. By utilizing directional microwave beams to measure relative positional changes across platforms, the method effectively overcomes the limitations of absolute spatial positioning systems. It significantly reduces the field of uncertainty (FoU), thus shortening the scanning coverage time. Moreover, it demonstrates strong adaptability in conditions characterized by electromagnetic interference and navigation denial, which are frequently encountered in practical scenarios.MethodsTo rapidly establish laser communication links between unmanned aerial platforms, we propose a dynamic correction method that fuses directional microwave pointing measurements with particle filtering. First, we analyze the influence of inertial navigation position errors on the acquisition scanning coverage time to determine the FoU requirements for non-orbital laser link establishment. Then, we construct a flight dynamics model and a directional microwave pointing measurement model. Based on a particle filter iterative algorithm with a particle weight resampling mechanism, the method effectively reduces the acquisition scanning coverage area under non-orbital and complex electromagnetic conditions. The effectiveness of the proposed method is validated through simulation-based equivalent experiments, showing significantly reduced scanning coverage times in both formation and cross-maneuvering flight scenarios. Furthermore, a maritime field experiment, in which one terminal is deployed on a sea-based test vessel and the other mounted on a six-degree-of-freedom turntable at the ground station, confirms the effectiveness and robustness of the method, providing a reliable foundation for the engineering application of laser communication between non-orbital platforms.Results and DiscussionsSimulation-based equivalent experiments are conducted to compare the performance of three approaches: relying solely on inertial navigation, direct directional microwave measurement, and the proposed particle-filter-based dynamic aiming correction. The results show that the acquisition scanning coverage time is 86 s when relying solely on inertial navigation (Fig. 1), and 197 s when using direct directional microwave measurement without filtering. In contrast, the proposed method reduces the acquisition scanning coverage time to 10 s in the formation flight scenario (Fig. 4) and 16 s in the cross-maneuvering flight scenario (Fig. 5), and demonstrates insensitivity to link distance variations (Fig. 5), indicating superior efficiency and robustness. A field experiment conducted over a 25 km sea path measured an average acquisition time of 12 s with a standard deviation of 1.5 s (Fig. 6). The maritime terminal utilized the vessel’s motion and engine-induced vibrations to emulate the complex dynamics of aerial platforms in flight, while the ground-based terminal emulated corresponding motion states via the turntable. This setup realistically simulated the non-orbital dynamic conditions and generated critical data supporting the validation of the proposed dynamic aiming correction method. This study aims to address the challenges of establishing laser communication links for non-orbital platforms in complex environments. Analysis results indicate that, compared with all-optical acquisition methods, the proposed approach supports communication requirements over a broader airspace. Furthermore, in contrast to traditional methods that rely on high-precision global positioning system / inertial navigation system (GPS/INS), the proposed method achieves shorter acquisition times and greater robustness without the need for satellite navigation assistance.ConclusionsWe demonstrate that the proposed particle filter-based dynamic aiming correction method significantly improves laser link establishment efficiency in both formation and cross-maneuvering flight scenarios. Compared with methods relying solely on inertial navigation, the acquisition scanning coverage efficiencies are improved by factors of 8.6 and 5.4, respectively; compared with direct directional microwave measurement, the improvments are by factors of 19.7 and 12.3. In the 25 km maritime laser link establishment experiment, the method achieved an average acquisition time of 12 s, consistent with simulation analysis results. Moreover, the acquisition time is insensitive to variations in link distance, demonstrating excellent performance and robustness. The proposed method exhibits low dependency on inertial navigation accuracy, microwave communication stability, and satellite navigation. It is even capable of independently completing the acquisition task under complex electromagnetic conditions, thus meeting the engineering requirements for rapid laser link establishment between non-orbital platforms.

ObjectiveAddressing optical physical layer security for metropolitan-optimized optical transport network (M-OTN) presents a critical challenge for telecom operators. This paper introduces and experimentally validates a methodology for real-time optical service unit (OSU) optical signal time-domain scrambling integrated with decoy-state quantum key distribution (DS-QKD). The system processes OSU optical signals in real-time utilizing tunable Fabry-Perot cavity (FPC) with dynamically updated and synchronized keys. The DS-QKD system implements the decoy-state BB84 protocol and polarization coding for seed key distribution. The research demonstrates effective end-to-end optical physical layer security for M-OTN (OTU2, 10.709 Gbit/s) data transmission under real-time key update conditions.MethodsFigure 2 illustrates the operational principle of real-time OSU optical signal time-domain random scrambling integrated with the DS-QKD system. The system employs a symmetric encryption architecture, incorporating a DS-QKD transmitter and receiver, with key transmission via DS-QKD. Through the quantum channel, the transmitter communicates a random seed key to the receiver without service data transmission. The DS-QKD system initially transfers the seed key to the local field-programmable gate array (FPGA), which maintains the seed key and establishes a running key pool. The transmitter's FPGA then utilizes a running key from the pool to scramble the input OSU optical signal. Concurrently, it transmits the synchronization marker to the receiver's FPGA through the synchronization channel. Upon receiving the synchronization marker, the receiver’s FPGA employs the corresponding running key from its pool to descramble the received OSU optical signal. FPC facilitates the time-domain scrambling of the OSU optical signal (Fig. 3). Each FPC incorporates an independent temperature control module (TCM), and the scrambling/descrambling controller modifies the FPCs’ parameters using the running key after transmitter-receiver synchronization, specifically adjusting the cavity’s optical thickness for time-domain scrambling/descrambling.Results and DiscussionsThe eye diagrams of the experimental results for OSU optical signal scrambling and descrambling (Fig. 6), with Fig. 6(a) and Fig. 6(b) showing the original and scrambled signals, respectively. The scrambled signal differs substantially from the original 10.709 Gbit/s non-return-to-zero (NRZ) signal due to the FPC array's bit overlapping scrambling. This confirms the scrambler’s effectiveness in disrupting the temporal position relationship between bits, rendering the OSU optical signal undigitizable. The unperturbed eye diagrams are shown in Fig. 6(c) and Fig. 6(d), respectively. Figure 7 illustrates the system’s running key performance, while Fig. 8 shows the bit error rate (BER) performance of the OSU signal after backhaul (B2B). These results confirm the effective enhancement of optical physical layer security.ConclusionsThis research presents and experimentally validates an OSU optical physical layer security protection method utilizing real-time optical signal time-domain scrambling. DS-QKD provides the seed key, enabling running key generation between the transmitter and receiver. System performance testing confirms that only authorized users employing the synchronous scrambler/de-scrambler and correct running key can successfully recover OSU data. Without the synchronization running key, eavesdroppers cannot extract the OSU optical signal’s digital features. The proposed method enhances M-OTN security by implementing protection in the optical domain, supplementing traditional electrical domain encryption algorithms.

ObjectiveDeveloped distributed acoustic sensing (DAS) makes it possible to simultaneously sense and communicate via a single fiber link. In a long-haul optical fiber link, relay amplifiers are required to simultaneously amplify forward communication signals and Rayleigh backscattering (RBS) signals. However, cascading relay amplifiers progressively accumulate and amplify amplified spontaneous emission (ASE) noise, elevating the DAS system’s intensity noise and thereby limiting the sensing and communication distance. To this end, we conduct research on the design of multi-span relay amplification scheme in an optical fiber link. A noise model of cascaded relay amplification is established, and the influence of the relay amplification scheme on the signal-to-noise ratio (SNR) of RBS signals is theoretically analyzed. Experiments are performed, and these results confirm the validity of the theoretical analysis. We focus on analyzing the impact of key parameters of relay amplification on the noise performance of the DAS system, excluding hardware design of the relay amplifier.MethodsBy establishing a noise model for cascaded bidirectional erbium-doped fiber amplifiers (EDFAs), we investigate factors influencing the SNR (RSN) of RBS signals in DAS systems, including EDFA gain (G), nonlinear threshold (Pth), single span length (L), and the number of cascaded spans (M). The critical threshold for determining whether a DAS system can acquire sensing signals is when the RSN drops to 1. Based on this condition, it is essential to comprehensively optimize system parameters such as pump power (Pp), pulse width (Tp), the correlation coefficient of ASE noise (2nsphνΔν), gain coefficients (Ga, Gb), single span length (L), and the number of cascaded spans (M) to achieve the best sensing and communication performance. Finally, a DAS system with a cascaded amplified link is constructed to experimentally validate the effectiveness of the noise model.Results and DiscussionsFor optical fiber links, each additional relay amplifier contributes to a cumulative increase in the DAS system’s noise. To maintain the SNR, it is necessary to shorten the single span length (L) and reduce the EDFA gain (G). Specifically, it can be implemented by referring to Eq. (7). Additionally, optimizing the DAS system’s probe pulse can enhance the RBS signals strength while mitigating excess noise introduced by nonlinear effects such as modulation instability (MI) and stimulated Brillouin scattering (SBS). This approach involves precisely controlling the probe pulse’s peak power and pulse width, and extending the linear frequency sweep range to exceed the fiber’s Brillouin gain bandwidth.ConclusionsWe conduct a comprehensive study on the design of multi-span relay amplification for integrated sensing-communication fiber links. By establishing a noise model for cascaded bidirectional EDFAs, the effects of EDFA gain (G), ASE noise (PASE), single span length (L), and the number of cascaded spans (M) on the SNR of RBS signals are investigated. Theoretical and experimental results show that this relay scheme supports a maximum single span length of 108 km. Accounting for optical nonlinear noise, we further elucidated the relationship between EDFA gain constraints and probe pulse characteristics (peak power, pulse width, and linear-frequency sweep range). This study provides a solid theoretical foundation for optimizing multi-span relay amplification in integrated sensing-communication fiber links, with significant practical relevance for fiber infrastructure monitoring and security applications.

ObjectiveThis research investigates the water immersion phenomenon in hollow-core anti-resonant fibers (HC-ARF) following structural damage under real-world conditions, such as outdoor deployment, conduit breach, or accidental damage, and its subsequent effects on transmission performance. The study aims to elucidate the water immersion mechanism and establish the temporal relationship between water penetration depth and duration through experimental measurements. Additionally, this research quantifies the impact of water immersion on signal attenuation. Understanding these dynamics and their implications is crucial for evaluating HC-ARF’s long-term reliability, failure modes, and maintenance requirements in practical applications, while also informing improvements in environmental resilience design. These insights are essential for ensuring operational stability and predicting the longevity of optical communication and sensing systems.MethodsThis investigation comprises three interconnected phases—theoretical simulation, experimental measurement, and performance testing—to analyze water immersion dynamics in fractured HC-ARF and associated transmission degradation mechanisms. The theoretical simulation utilizes the Washburn capillary imbibition equation Lt=γ?r?cosθ2η?t to model water immersion dynamics in HC-ARF microtubes. The depth-time relationships are derived by varying surface tension γ, viscosity η, equivalent tube diameter r, and contact angle θ. The experimental phase involves immersing HC-ARF sample end-faces of various lengths in water under ambient conditions, with depth-time data recorded throughout immersion. Comparative analysis of experimental and theoretical results across different fiber lengths and immersion conditions reveals key factors influencing water immersion behavior. The performance evaluation examines attenuation effects using long-length fiber samples subjected to fracture-immersion treatment (192 h). The methodology involves sequential excision of the water-contaminated segment while recording output spectra, followed by removal of multiple redundant segments with spectral documentation after each cut. All spectra are captured using a broadband source and optical spectrum analyzer, with attenuation spectra measured via the cut-back method.Results and DiscussionsTheoretical simulations based on the Washburn equation, adapted to the specific fiber structure, produce water immersion depth-time curves and predict final immersion depths under various initial gas pressures. Experimental data indicate that internal gas pressure is the primary determinant of immersion kinetics. During 192 h of immersion testing, all 10 m samples demonstrate final immersion depths exceeding 9.5 m, with immersion velocity decreasing under end-face sealing conditions due to pressure accumulation from gas compression. Longer fibers (0.5 km and 1.0 km samples) exhibit similar depth-time patterns, reaching approximately 36 m immersion depth after 192 h, though variations from simulations suggest structural non-uniformities. Attenuation analysis reveals that complete performance restoration requires removing both the water-contaminated segment and an additional 2 m redundancy section. Given the consistent cross-sectional geometry, internal pressure conditions, and non-optimal baseline attenuation across tested samples, further validation with diverse fiber types remains necessary to establish comprehensive dynamic models and degradation principles.ConclusionsThis research combines theoretical modeling with experimental validation to examine water immersion dynamics in HC-ARF and resultant attenuation effects. The Washburn equation-based theoretical depth-time correlations are confirmed through systematic immersion experiments across various fiber lengths, demonstrating that immersion velocity depends primarily on internal gas pressure and fiber geometry, reaching maximum rates during single-end sealed immersion. Extended fibers of 0.5 km and 1.0 km demonstrate uniform 36 m immersion depths after 192 h, while removing 2 m beyond maximum immersion depth effectively restores baseline attenuation. Variations between experimental and theoretical results suggest structural non-uniformities, leading to a practical recommendation of removing contaminated segments plus 2 m redundancy sections to remediate post-fracture water damage.

ObjectiveWith the advancement of ultra-wideband amplifiers, multi-band transmission has emerged as one of the most effective strategies to enhance the capacity of standard single-mode fiber systems. Currently, optical transmission across the C+L band has already been commercially implemented. Among potential candidates for further spectral extension, the S-band appears to be the most promising. However, the incorporation of the S-band into the transmission system introduces pronounced power tilt from higher to lower frequencies due to the influence of stimulated Raman scattering (SRS), which results in significantly degraded performance in the S-band compared to the C-band. Consequently, the expected linear capacity growth with increased bandwidth cannot be fully realized. To address this challenge, techniques such as power pre-emphasis, advanced digital signal processing (DSP) algorithms, and the deployment of Raman amplifiers have been introduced into S+C+L band transmission systems, achieving substantial performance improvements. Nevertheless, the introduction of Raman amplification also brings increased system complexity and power consumption. In this study, we propose a joint optimization approach combining power pre-emphasis and entropy minimization, aiming to explore the performance enhancement potential of these two techniques within a lumped amplification transmission link.MethodsIn this study, a joint optimization scheme combining power pre-emphasis and group-wise entropy minimization is proposed. Initially, system performance is enhanced through optimized power pre-emphasis. Based on the resulting performance distribution, transmission channels are grouped, and for each group, the worst-performing channel is selected as the representative target for entropy optimization, ensuring error-free transmission within the entire group. During the power pre-emphasis stage, we investigate both uniform and non-uniform power allocation strategies. The non-uniform optimization is guided by a 3 dB criterion accounting for both linear and nonlinear noise, while the uniform and non-uniform schemes targeting maximum capacity are also explored. All optimization processes are conducted using a particle swarm optimization algorithm. In the entropy optimization stage, a look-up table is first established using generalized mutual information, signal-to-noise ratio, and entropy values under probabilistic shaping 256 quadrature amplitude modulation (PS-256-QAM) to determine initial values. These are then fine-tuned within the transmission system to minimize entropy. The proposed optimization framework is validated through numerical simulations, demonstrating clear improvements in overall system performance.Results and DiscussionsIn the power optimization phase, the average power levels and spectral tilts across different frequency bands are first optimized using three distinct strategies: non-uniform channel power optimization, uniform channel power optimization, and a criterion-guided power control strategy referred to as ASENLI. The comparative performance of these approaches is systematically illustrated in Fig. 5. Specifically, the corresponding signal-to-noise ratio (SNR), amplified spontaneous emission (ASE), and nonlinear interference (NLI) ratios achieved under the non-uniform and ASENLI strategies are shown in Figs. 5(a)?(c), respectively. For the uniform strategy, the optimal launch power is determined to be 0.967 dBm, and the associated SNR values are also presented in Fig. 5(a).Fig. 5(b) shows the optimized launch power profiles obtained using both the non-uniform and ASENLI methods, revealing that these two strategies yield closely matched power distributions. As demonstrated in Fig. 5(d), the system capacity achieved by the ASENLI strategy is only 0.06% lower than that obtained by maximizing SNR, indicating that the criterion-based strategy is well-suited for power optimization in multi-band transmission systems. Furthermore, comparison with the flat-optimal launch power scheme reveals that the non-uniform strategy delivers a 3.4% enhancement in total system capacity, confirming its effectiveness in improving the performance of S+C+L band optical fiber transmission. Following power optimization, a rapid multi-channel grouping mechanism is established to enable the implementation of entropy loading, thus validating the proposed joint active-passive optimization framework. To achieve this, edge and center channels within each band are evaluated via a sliding-window simulation method, and these results are compared with full-band predictions obtained through a closed-form analytical model, as shown in Fig. 6. Although a degree of discrepancy in SNR values exists between the simulation and theoretical model, the overall trend alignment validates the feasibility of rapid grouping based on the modeled full-spectrum SNR distribution. The wavelength division multiplexing (WDM) system channels are subsequently grouped in sets of 30. According to the look-up table provided in Fig. 3, the channel with the lowest SNR in each group—specifically, channels indexed at 30, 60, 90, 120, 150, and 180—is selected for initial entropy assignment, with corresponding initial entropy values of 7.6, 7.4, 7.3, 7.2, 7.0, and 6.8 bit/symbol. These values are further refined via the proposed entropy-loading method, which performs fine adjustments based on the weakest channel within each group to ensure the normalized generalized mutual information (NGMI) meets the 0.833 threshold. Resulting adjusted entropy values are 7.4657, 7.4657, 7.3056, 7.0703, 6.7134, and 6.5854 bit/symbol, respectively. To evaluate the capacity enhancement offered by the joint optimization strategy, three transmission scenarios are simulated and compared in terms of system generalized mutual information (GMI): 1) uniform launch power with 128-QAM, 2) ASENLI-based power control with 128-QAM, and 3) ASENLI-based power control with PS-256-QAM. These simulation results are shown in Fig. 7. Across the L, C, and S bands, launch power optimization contributes GMI improvements of 1.53%, 7.12%, and 15.22%, respectively. On top of this, entropy loading further enhances GMI by 11.71%, 7.03%, and 7.66% in each corresponding band. These findings indicate that while benefits of power optimization are most prominent in the S band, entropy loading delivers consistent and substantial capacity gains across all spectral bands.ConclusionsIn this study, a joint active-passive optimization strategy, integrating criterion-guided power control and grouped entropy loading, is proposed to enhance the transmission capacity of S+C+L band optical systems. This strategy is designed to simultaneously mitigate the impact of stimulated Raman scattering (SRS) and maximize spectral channel utilization. The proposed framework begins with the application of particle swarm optimization to jointly optimize the average launch power and spectral tilt across the S, C, and L bands. Building upon these optimized power profiles, entropy is then loaded group-wise based on system performance, aiming to further elevate the total system capacity. A simulation platform incorporating 180 channels across the S+C+L band, based on the mean-field theory, is constructed to evaluate and compare the system capacities under different power allocation strategies, including non-uniform channel power optimization, uniform power optimization, and the proposed criterion-guided power control approach. Results demonstrate that non-uniform power optimization yields the highest system capacity. However, the proposed criterion-guided strategy, despite employing a sub-optimal power allocation from an engineering perspective, achieves 99.4% of the optimal capacity while exhibiting significantly faster convergence, making it more practical for real-world deployment. Following entropy loading, the GMI sees further improvement when compared to the uniform power optimization baseline, with GMI increases of 1.53%, 7.12%, and 15.22% observed in the L, C, and S bands, respectively. When combined with group-wise entropy refinement, additional GMI enhancements of 11.71%, 7.03%, and 7.66% are attained. These findings confirm the effectiveness of the proposed joint optimization strategy in delivering comprehensive capacity gains across all spectral bands.

ObjectiveThe downlink laser communication link from satellites to the ground, a critical component of the Integrated Space-Ground-Air Information Network, demonstrates an increasing demand for high-capacity optical communication with minimal interruption probability. However, laser transmission remains highly susceptible to atmospheric turbulence, which substantially impairs the communication quality of satellite-to-ground laser links. While adaptive optics (AO) technology, the predominant turbulence compensation approach, effectively enhances coupling efficiency into single-mode fibers, its compensation performance proves insufficient under strong turbulence conditions.MethodsThis paper presents an atmospheric turbulence compensation method that integrates AO and mode diversity reception (MDR). The system processes the distorted spatial light beam through initial wavefront correction via an AO system, followed by mode-diverse reception using a multi-plane light converter (MPLC) based on multimode fiber. This turbulence compensation method was implemented in a high-orbit satellite-to-ground laser communication experiment at the Lijiang station. The experiment utilized a three-channel digital coherent reception system to achieve stable transmission of a 1 Gbit/s binary phase shift keying (BPSK) signal.Results and DiscussionsIn this high-orbit satellite-to-ground laser communication experiment, bidirectional satellite-to-ground acquisition was achieved within 30 seconds. As shown in Fig. 3, the tracking system error under AO closed-loop conditions maintained below 1 μrad for both X-axis and Y-axis tracking. After AO wavefront correction, the root mean square (RMS) wavefront error reduced from 0.82λ to 0.10λ, as illustrated in Fig. 4, confirming the adaptive optics system’s wavefront compensation capability. At the receiver end, the distorted light beam underwent AO correction before reception through a MPLC based on multimode fiber. As shown in Fig. 5, under AO closed-loop conditions, the output power at the single-mode end of the MPLC exhibited significant improvements. Notably, the SC-based MDR method enhanced the power at a complementary cumulative probability (CCDF) of 0.9 by 3.94 dB compared to the single AO compensation scheme, as illustrated in Fig. 6. Additionally, real-time digital signal processing results based on a field-programmable gate array (FPGA) demonstrate that under AO closed-loop conditions, the selection combining-based MDR method improved the probability of the bit error rate (BER) being less than 1×10-3 from 72.0% to 91.1%, as illustrated in Fig. 7. These findings demonstrate the substantial advantages of the combined AO and MDR method over individual AO or MDR compensation, markedly enhancing satellite-to-ground laser communication system performance in complex atmospheric environments.ConclusionsThis paper introduces an AO combined with MDR turbulence compensation strategy designed to effectively mitigate turbulence interference in high-orbit satellite-to-ground laser communication links. An on-orbit demonstration and verification experiment, conducted at the Lijiang ground station with a high-orbit satellite, achieved stable transmission of a 1 Gbit/s BPSK signal. The experimental results confirm the advantages of combining AO and MDR compared to single AO implementation. Furthermore, the AO-MDR mechanism demonstrates synergistic effects between the two components. This advancement holds significant practical implications for future high-orbit satellite-to-ground laser communication experiments and may facilitate further development of satellite-to-ground laser communication technology.