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.

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.

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.