- The paper demonstrates a 21.7 Tb/s transoceanic transmission over 6660 km using 266 km ultra-long spans with fewer than 30 repeaters.
- The paper employs a recirculating loop and adaptive modulation, achieving ultra-low loss (0.098 dB/km) and minimal IMI (-68.8 dB/km) with advanced DSP techniques.
- The paper indicates significant CAPEX/OPEX improvements for future submarine and metro networks by enabling sparse repeater deployment with innovative HCF design.
Ultra-Long Span Transoceanic Transmission with Low IMI Hollow Core Fiber
Introduction
This work demonstrates a significant advancement in long-haul optical transmission, specifically in the context of sparsely repeated transoceanic data transfer using new hollow core fiber (HCF) technology. The study presents a wavelength-division multiplexed (WDM) system achieving a 21.7 Tb/s net data rate over 6660 km fiber spans, where each span measures 266 km and fewer than 30 repeaters are required. Central to this result is the deployment of a newly designed Gap Tube Assisted Support Tube Hollow Core Fiber (GTA-ST-HCF) exhibiting ultra-low loss and minimal inter-modal interference (IMI).
Experimental Architecture and Methodology
The experimental setup is based on a recirculating loop incorporating a 266-km GTA-ST-HCF span, high-power Erbium-Ytterbium co-doped fiber amplifiers, and adaptive channel symbol rates. The transmitted signal is a fully loaded C-band WDM grid, with each channel modulated as dual polarization 16-QAM. Signal integrity is preserved using a combination of Nyquist shaping, polarization scrambling, and dynamic gain equalization. High-resolution optical time-domain reflectometry confirms the linear (low) loss profile at 0.098 dB/km and excellent splicing/insertion performance.
A critical innovation is the tailored design of the GTA-ST-HCF. Its support tube architecture minimizes confinement loss, and the gap tube assists in modal filtering, resulting in a measured IMI of -68.8 dB/km—over a sixfold improvement in power fluctuation stability compared to typical HCFs. This architectural refinement is indispensable for maintaining high AIRs over ultra-long spans and for suppressing modal noise that would otherwise degrade SNR and throughput.
Sophisticated receiver-side DSP (chromatic dispersion compensation, 2x2 MIMO equalization, etc.) supports high-fidelity signal analysis. Performance metrics include SNR, NGMI, and net rate (using multi-rate SC-LDPC code rates between 0.4 and 0.9).
The study provides extensive data on per-channel and aggregate system performance. For a representative channel at 192.220 THz, an AIR up to 590 Gbps/lambda at 10,000 km is documented, reflecting the efficacy of ultra-long span/repeater spacing. Importantly, channel baud rates are adaptively assigned to avoid gas line absorption (GLA) peaks—symbol rates are maximized where GLA is minimal (e.g., 135 GBaud), and reduced near absorption lines (as low as 30 GBaud), optimizing spectral efficiency and minimizing net throughput penalty from absorption-induced notches.
Measured after 6660 km, the aggregate AIR reaches 23.4 Tb/s, with a net throughput of 21.7 Tb/s (SE = 4.48 b/s/Hz), accommodated by 25 repeaters—a scenario directly applicable to transatlantic deployments. At greater distances (7990 km), the system maintains a net throughput of 15.9 Tb/s, showcasing robust resilience even under increased spectral impairment. Notably, penalties from residual GLA induce only a 2-3 dB SNR reduction within affected sub-bands, confirming the effectiveness of the variable-baud WDM design.
Compared to state-of-the-art systems (e.g., 11185 km at 50.9 Tb/s using ~55 km spans in SMF, or 127 km spans in previous HCF work), this result achieves a twofold increase in throughput and more than doubles the feasible span length per amplifier. Moreover, if deployed in metro or submarine cable scenarios, a fivefold reduction in the number of repeaters is projected, with direct benefits for both CAPEX and OPEX.
Implications and Future Directions
The findings signify a critical step in leveraging HCF for ultra-long-haul optical networking, addressing two chronic barriers—attenuation and modal interference. The substantial IMI reduction, achieved without a tradeoff in loss, validates advanced HCF geometries as a foundation for the next generation of metro and submarine links. Lower repeater density translates into simpler, more reliable infrastructure, potentially revolutionizing system architecture for oceanic and continental backbone networks.
Practically, this work suggests that future deployment models will prioritize ultra-long-span HCF with adaptive transceiver-side signal management. Entropy loading and advanced water-filling algorithms could further increase system capacity, although these come with increased transceiver complexity and DSP demands. Additional research is warranted to examine lower-order modulation formats (such as QPSK) for even longer reach, and to generalize these results to optical bands beyond the C-band.
Theoretically, the reduction of inline amplification nodes will have profound effects on the fundamental cost and reliability models of global optical communications. The improved understanding of IMI and GLA management in HCF fiber opens new directions for fiber engineering and transmission strategy co-design.
Conclusion
This paper offers the first demonstration of >21 Tb/s net WDM transmission across 6660 km of 266 km span HCF, enabled by a novel GTA-ST-HCF with record low IMI and loss characteristics. The adaptive channel rate strategy allows for effective spectral efficiency despite GLA limitations, enabling sparse repeater placement and laying groundwork for a paradigm shift in high-capacity, low-density transoceanic and metro optical systems. The results strongly support further exploration of ultra-long span HCF systems as viable successors to SMF-based transoceanic cables.
