Phase-coherent optical frequency transfer is essential for optical clock networking, relativistic geodesy, and distributed precision metrology. However, realizing coherent optical networks spanning thousands of kilometers in standard single-mode fiber (SMF) generally requires densely distributed amplifiers or repeater stations together with complex operational control, while long-term instability remains limited by thermally driven residual phase fluctuations. Here we show that hollow-core fiber (HCF) can simultaneously improve transfer instability and relax the reach limitation of long-span optical frequency transfer. Compared with SMF, HCF exhibits lower fiber-induced phase noise and shorter propagation delay, supporting improved short-term instability, while its much lower thermal sensitivity supports nearly one-order-of-magnitude better long-term instability. In addition, for long-haul HCF links, no observable stimulated Brillouin scattering induced saturation is found up to the maximum available injected power of 34 dBm, whereas the threshold of an equal-length SMF link remains only a few dBm. Together with the lower attenuation achievable in modern HCF, this enables ultra-long single-span optical frequency transfer. Using a 152 km HCF link with an average attenuation of 0.18 dB/km, we demonstrate single-span optical frequency transfer, achieving a fractional frequency instability of 7.3 x 10^-21 at 10,000 s and a fractional uncertainty of 1.8 x 10^-20. These results establish HCF as a transmission medium that simultaneously improves instability and extends single-span reach, opening a practical route toward future intercontinental optical frequency networks with ultrahigh precision.

In this Letter, we experimentally demonstrate a 16.7 Tb/s co-frequency co-time full-duplex 10 km mobile fronthaul on C band by harnessing the ultralow loss, nonlinearity, dispersion, and Rayleigh backscattering of anti-resonant hollow-core fiber. 112 GBaud four-level pulse amplitude modulation signals are adopted with direct detection. Bit error rates of all the channels are below 6.7% hard-decision forward error correction threshold of 3.8 × 10-3. Few sensitivity penalties of inter-channel nonlinear interference and backward crosstalk are observed as 0.2 dB and 0.5 dB, respectively. This work shows that anti-resonant hollow-core fiber is a promising medium for implementing a high-capacity 6 G fronthaul architecture.

The co-propagation of quantum and classical signals through shared optical fibers is crucial for scalable quantum networks. However, this coexistence is fundamentally limited by spontaneous Raman scattering (SpRS) from the bright classical light, which generates overwhelming noise that disrupts the single-photon-level quantum signals. Here, we overcome this long-standing challenge by leveraging the inherently ultralow nonlinearity of hollow-core fiber (HCF) to suppress SpRS noise. By operating both the quantum time synchronization (QTS) and classical optical frequency transfer (OFT) signals within the telecom C-band, separated by only ~10 nm, we successfully demonstrate their simultaneous transmission over a 122-km HCF link. With a classical OFT power of 1 mW, the QTS performance shows negligible degradation, maintaining sub-picosecond time stability at 2000 s, while the OFT achieves a fractional frequency instability of 10^-20. Near-sub-picosecond QTS stability is preserved even when the classical power is increased to 3 mW. Furthermore, simulations based on our experimental data indicate that with next-generation low-loss HCF, the platform can tolerate classical powers beyond 10 mW and extend the QTS range to over 500 km. By realizing a unified quantum-classical time-frequency distribution framework, this work establishes HCF as a highly capable and practical platform for future scalable quantum networks.

Optical networks aim to achieve a higher rate per channel and larger capacity per fiber while lowering the cost per bit. Ultra-wide band (UWB) wavelength-division multiplexing (WDM) over single-mode fibers has been widely studied, particularly in the S+C+L-band. However, inter-channel stimulated Raman scattering (ISRS) imposes scalability challenges and limits system capacity. Anti-resonant hollow-core fiber (AR-HCF) emerges as a promising transmission medium for UWB systems due to its ultra-low nonlinearity. The air-guiding mechanism not only brings ultra-low loss but also results in ultra-low Rayleigh backscattering, allowing for direction to be treated as an independent dimension. In this paper, we leverage these advantages to enhance the fiber capacity, demonstrating co-frequency co-time full-duplex (CCFD) S+C+L-band WDM transmission over 100 km AR-HCF to highlight its potential. Since the capacity is primarily constrained by optical/electrical components rather than AR-HCF itself, we employed multiple strategies, including a homemade high-power S-band integrable tunable laser assembly (ITLA) and optical domain equalization (OEQ) to improve the performance of coherent transceivers. It is important to note that gas absorption, as an extrinsic factor, has caused significant impairments in certain channels, which can be partially mitigated by conventional digital signal processing (DSP) techniques. With all these efforts, the capacity of one direction was experimentally measured to be 188.8 Tb/s under a bidirectional transmission configuration, corresponding to an equivalent aggregate bidirectional capacity of 377.6 Tb/s over 100-km AR-HCF. The result underscores the advantages of AR-HCF for ultra-wideband bidirectional transmission.

Coherent 400G technology can enhance the capacity and reduce the power consumption per bit in long-haul optical networks with transmission distances of over 1,500 km. Advances in optical components, chips and optical-layer infrastructure will enable 1,500 km transmission with about 0.08 nJ b−1 efficiency per optical module, allowing optical fibre networks to become a backbone of everyday communications. In this Review, we describe the key technologies necessary for long-haul large-capacity 400G optical transmission. First, we determine that the quadrature phase-shift keying format is appropriate to implement such an optical network. Accordingly, optical components, chips and optical-layer infrastructure are required to support 130 GBd symbol rate and 12-THz-wide optical bandwidth. Next, we summarize the benchmarking experimental demonstrations of long-haul large-capacity optical transmission with coherent 400G signals. Finally, we outline the remaining challenges such as unified C + L-band optics and provide guidance for future researchers to bring higher-bandwidth and longer-range communications beyond 400G to telecom networks. Long-haul large-capacity 400G optical transmission over 1,500 km is possible through advanced fibre-optic systems. This Review provides a holistic view of the signal modulation, spectrum plans, optical components and optical-layer infrastructure necessary to achieve those networks.

Conventional C+L-band transmission systems are typically implemented using two parallel and independent optical layer architectures, leading to increased system complexity and reduced robustness under strong inter-band coupling. To address this issue, this work investigates unified C+L-band optical amplification techniques and compares discrete integration schemes with erbium–bismuth co-doped and cascaded configurations in terms of gain flatness, noise performance, and implementation complexity. For high-speed DCI (Data Center Interconnect) ZR applications, a single-stage dual-pumped erbium–bismuth-doped fiber amplifier (EBDFA) is proposed, enabling 12-THz ultra-wideband amplification across the C+L band with a gain exceeding 21 dB and a noise figure below 5.6 dB. Based on this design, a unified C+L-band transmission system employing pure doped-fiber amplification is constructed. Without distributed Raman amplification (DRA), seamless C6T+L6T transmission is experimentally demonstrated. Over 80 km and 160 km of standard G.652.D fiber, aggregate capacities of 80.8 Tb/s and 78.5 Tb/s are achieved, respectively, with a maximum per channel rate of 1 Tb/s. These results demonstrate broadband, high-capacity C+L-band transmission with a significantly simplified optical-layer architecture, highlighting the practicality of the proposed scheme for equipment-dense DCI scenarios.

In recent years, anti-resonant hollow-core fibers (AR-HCFs) have made remarkable progress, surpassing the loss limit of conventional silica solid-core fibers, and achieving attenuation below 0.1 dB/km. Due to their ultra-low latency, negligible nonlinearity, and broad transmission bandwidth, AR-HCFs have attracted significant attention from both academia and industry, positioning them as a foundational technology for optical communications over the next half-century. However, large-scale industrial production of AR-HCFs remains underdeveloped, and prior research has been largely confined to laboratory-scale experiments using short fiber segments, failing to fully explore their performance potential or identify deployment-related challenges. To address this gap, we present the field deployments of four structurally distinct AR-HCF cables spanning 10 similar to 42.7 km. Post-deployment measurements demonstrate an average link loss reduction from 0.599 dB/km to 0.113 dB/km, with the lowest recorded span loss reaching 0.065 dB/km, scoring the world's first fiber cable deployment to break the 0.1 dB/km. Leveraging these deployed links, we conduct real-time coherent transmission experiments at 800 Gb/s to 1.2 Tb/s across S-, C-, and L-bands, validating the feasibility of co-time co-frequency full-duplex (CCFD) transmission in hollow-core fibers and characterizing the impact of gas absorption (e.g., CO2 id="240"> and H2 id="241">O) on system performance. Despite these impairments, we achieve the first field demonstration of real-time terabit-scale coherent transmission over the S+C+L band, delivering a record aggregate capacity of 114.9 Tb/s over 137.36 km using 105 channels.

Antiresonant hollow-core fibres (AR-HCFs) have recently reached attenuation far below the Rayleigh-scattering limit of silica, but their inherently multimode nature remains a major challenge for practical systems requiring high modal purity. In particular, suppressing higher-order modes (HOMs) at the 1 dB/m level while maintaining sub-0.1 dB/km fundamental-mode (FM) loss is difficult because conventional filtering strategies rely on tuning nested-tube dimensions, a design freedom that becomes increasingly restricted in the ultralow-loss regime. Here, we propose a new HOM-control mechanism in an interstitial-tube-assisted double nested anti-resonant nodeless fiber (IT-DNANF) by introducing angular offset of the interstitial tubes. Instead of using nested cavities as the primary tuning element, the proposed approach exploits the gap region between adjacent cladding tubes as a leakage-adjacent modal-control interface. Numerical simulations show that the offset increases both FM and HOM losses, but with a substantially stronger sensitivity for HOMs, leading to rapid enhancement of differential modal loss. Furthermore, when the gap-region FM is tuned into phase matching with the core HOM, strong coupling to a high-leakage state is induced, resulting in a pronounced HOM-loss peak. Using the practical criterion of HOM losslarger than 1 dB/m, we identify optimized IT-DNANF designs that achieve rapid HOM stripping while maintaining FM loss below 0.05 dB/km at 1550 nm. This work establishes angular offset as a physically distinct and manufacturability-friendly degree of freedom for mode purification in ultralow-loss hollow-core fibres.

This paper explores HCF communication systems, focusing on deployment, architecture evolution for its properties, novel channel impairments, waveband selection and system challenges, outlining key application directions. © 2026 The Author(s)

The coexistence of quantum information and classical signals in a single fiber is essential for future quantum networks that leverage the well-established optical fiber infrastructure. Although multiplexing technologies can separate quantum and classical signals, pure silica core fibers (PSCFs) remain fundamentally limited by the high nonlinearity, which generates substantial Raman scattering and four-wave mixing noise. Hollow-core fibers (HCFs), guiding light predominantly in air, offer an attractive solution with intrinsically ultra-low nonlinearity and strongly suppressed nonlinear noise. In this work, we demonstrate the entanglement-based key coexisting with data over an 18-km HCF link. We achieve time-encoded high-dimensional quantum key distribution (HD-QKD) carrying 0 dBm of bidirectional received power, corresponding to a theoretical data capacity of up to 2.3 Tbps. During 24 hours of continuous operation, an average secret key rate (SKR) of 10.56 kbps is obtained. Theoretical analysis further predicts SKRs above 135 kbps over transmission distances exceeding 200 km using state-of-the-art low-loss HCFs. These results show significantly improved performance compared with PSCF-based systems and highlight the potential of HCFs for scalable quantum-classical coexistence compatible with the architectures of established fiber-optic networks.

We demonstrate real-time transmission of 134-Tb/s capacity with all channels operating at 1.276-Tb/s, supporting bidirectional transmission across 15.75-THz triple-band over 75-km G.654.E fiber for data centre interconnection.

We demonstrate real-time 82-channel transmission over 80 km using an E2E-unified C+L-band system across a seamless 102 nm spectrum. The unified system employing a single-stage EBDFA achieves 80 Tb/s total capacity with most channels at 1 Tb/s.

We demonstrate a 214-Tb/s S+C+L band optical signal over 2×75-km G.654 transmission in an 18.7-THz bandwidth using only DFAs. The net bit rate per wavelength exceeds 1 Tb/s for each of the 204 channels.

We report the first real-time 128Tb/s co-frequency co-time full-duplex transmission over the first deployed 20km AR-HCFs in complex urban duct network in China by leveraging extremely low distributed Rayleigh backscattering of AR-HCF.

We report the first metro-scale real-time 32 lambda x1.2-Tb/s DP-64QAM-PCS transmission over field-deployed 85.4-km DNANF-5 fibers with low-loss of 0.21dB/km.

We present the first single-channel 1.001-Tb/s DP-36QAM-PCS recirculating transmission over 73 loops of 146.77-km ultra-low-loss low-IMI DNANF-5 fiber, achieving a record transmission distance of 10,714.28 km.

Significance As 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. Progress G. 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 Prospects With 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.

The first real-time 62.1 Tb/s transmission over a seamless 87 nm spectrum across 80 km G.652.D fiber is demonstrated using a unified C+L-band system. The prototype employs broadband EBDFA, simplifying dual-band optics for high-speed data center interconnect.

A record unrepeatered 10.7-Tb/s (>1.2-Tb/s/λ) 8λ-WDM transmission with EDFAs only over 217.1-km anti-resonant hollow-core fiber (AR-HCF) was experimentally demonstrated by leveraging AR-HCF’s ultralow nonlinearity, in which high-spectral-efficiency single-carrier 148-GBd DP-144QAM-PCS signals were used.

Single-channel DP-144QAM-PCS 1.09Tbps transmission over 100km AR-HCF with 0.164 dB/km at 1550nm for decentralized intelligent training was experimentally verified for the first time. Nonlinearity-free input power is improved from 7dBm for G.652 to above 15dBm.