Sparsely repeated 21.7 Tb/s Net-Rate Transoceanic Transmission with 266 km Ultra-Long Spans Enabled by Low IMI and Low loss Hollow Core Fiber

This paper demonstrates a record-breaking 21.7 Tb/s net-rate transoceanic transmission over 6,660 km using 266-km ultra-long spans of low-interference hollow-core fiber, achieving the feat with fewer than 30 repeaters through a combination of a newly designed fiber, high-power amplification, and adaptive channel rates.

The Gist

Imagine the internet as a massive, global highway system where data travels as streams of light. For decades, we've been driving these light streams through standard glass fibers (Single-Mode Fiber). It works well, but it's like driving on a road that has speed bumps (signal distortion) and requires a gas station (a repeater) every 60 to 80 miles to keep the car running. To cross an ocean, you need over 100 of these gas stations, which makes the cable expensive, heavy, and hard to maintain.

This paper describes a breakthrough that is like inventing a super-highway made of empty air instead of glass, allowing cars to drive for hundreds of miles without stopping.

Here is the breakdown of their achievement in simple terms:

1. The New Road: "Hollow Core" Fiber

Instead of sending light through a solid glass thread, the researchers used a Hollow Core Fiber (HCF).

• The Analogy: Think of standard fiber as a long, solid glass rod. Light has to push through the glass, which slows it down and causes it to bounce around (creating noise).
• The Innovation: The new fiber is like a straw. The light travels through the empty air inside the straw. Because it's not touching glass, it travels faster, loses less energy, and doesn't get "jittery" (non-linear effects).
• The Specific Design: They used a special type called GTA-ST-HCF. Imagine the straw has a special internal structure (a "support tube" and "gap tubes") that acts like a guardrail. It keeps the light perfectly centered in the middle of the air, preventing it from hitting the walls and getting messy. This solves a major problem where light usually bounces into different paths and interferes with itself.

2. The Challenge: The "Gas Pockets"

Even though the road is amazing, there was a problem. The air inside the fiber isn't a perfect vacuum; it contains a tiny bit of gas.

• The Analogy: Imagine driving on a perfect highway, but every few miles, there are invisible "gas pockets" that try to suck the energy out of your car's engine at specific frequencies. In the fiber, these are called Gas Line Absorption (GLA) peaks. If your data signal hits these pockets, it gets eaten alive.
• The Solution: The researchers didn't try to remove the gas (which is hard). Instead, they became smart drivers. They used Adaptive Channel Rates.
  • When the road is clear, they drive fast (high speed, 135 GBaud).
  • When they approach a "gas pocket," they slow down and take a narrower lane (lower speed, 30–52 GBaud) so they don't hit the bad spot.
  • This is like a GPS that automatically reroutes traffic to avoid construction zones, keeping the flow moving smoothly even with obstacles.

3. The Engine: A Super-Charged Booster

To keep the light traveling such long distances, they needed a powerful amplifier.

• The Analogy: Standard fiber systems use a small booster every 60 miles. This team used a high-power amplifier (like a massive turbocharger) that could push the signal much further.
• The Result: Instead of needing a gas station every 60 miles, they could go 266 kilometers (about 165 miles) between boosters.

4. The Big Win: Crossing the Ocean with Fewer Stops

The team tested this system by simulating a transoceanic journey.

• The Distance: They sent data across 6,660 kilometers (roughly the distance from New York to London).
• The Capacity: They moved 21.7 Terabits per second. To visualize this: that's enough data to stream thousands of 4K movies simultaneously, or download the entire Library of Congress in a few seconds.
• The Efficiency: Because the spans are so long, they only needed 25 repeaters (gas stations) for the whole trip.
  • Old Way: You would need over 100 repeaters for the same distance.
  • New Way: You need less than 30.

Why Does This Matter?

This is a "paradigm shift" for the future of the internet.

1. Cheaper Cables: Fewer repeaters mean the cable is lighter, cheaper to build, and easier to lay on the ocean floor.
2. Faster Deployment: Building a cable with 25 stops is much faster than building one with 100.
3. Lower Maintenance: Fewer devices on the ocean floor means fewer things can break.
4. Future Proof: As we need more internet speed, this technology can handle it without needing to lay entirely new cables.

In summary: The researchers built a "straw" that lets light fly through air, taught the data how to dodge invisible gas pockets by changing its speed on the fly, and used a super-charged engine to cross the ocean with a fraction of the usual stops. It's a massive leap forward for global communication.

Technical Summary

1. Problem Statement

Current long-distance optical communication systems rely on standard single-mode fibers (SMF), which face fundamental limitations regarding nonlinearity, latency, and cost-per-bit. While Anti-Resonant Hollow Core Fibers (AR-HCF) offer a promising alternative with ultra-low loss and reduced nonlinearity, their deployment in transoceanic systems has been hindered by two primary physical impairments:

• Inter-Modal Interference (IMI): Coupling between the fundamental mode and higher-order modes degrades signal quality over long distances.
• Gas Line Absorption (GLA): Specific absorption peaks within the fiber (caused by gas molecules trapped in the core) create spectral notches that disrupt transmission, particularly in the C-band.

Existing HCF demonstrations have typically required shorter spans (e.g., ~127 km) to mitigate these issues, resulting in a high number of repeaters (100+) for transoceanic distances, which increases deployment costs and maintenance complexity.

2. Methodology

The authors employed a recirculating loop experimental setup to simulate transoceanic transmission over ultra-long distances.

• Fiber Technology: The core of the experiment utilized a newly designed Gap Tube Assisted Support Tube Hollow Core Fiber (GTA-ST-HCF).
  • Architecture: This design combines a support tube for ultra-low confinement loss with a gap tube mechanism for modal filtering.
  • Performance: It achieves both low attenuation and high modal purity, significantly suppressing higher-order mode coupling.
  • Span Configuration: The loop consisted of a single span of 266 km, composed of 23 spliced spools of GTA-ST-HCF.
• Amplification: A high-power Erbium-Ytterbium doped fiber amplifier (EYDFA) operating in the C-band was used, providing a launch power of 34 dBm with a maximum noise figure of 4.68 dB.
• Transmission Setup:
  • Signal: A fully loaded Wavelength Division Multiplexed (WDM) comb covering 4.8 THz, plus a Channel Under Test (CUT).
  • Modulation: Dual-polarization 16-QAM with symbol rates up to 135 GBaud.
  • DSP: Standard coherent detection with offline processing including matched filtering, CD compensation, MIMO equalization, and SC-LDPC coding (rates 0.4–0.9) for error-free decoding.
• Adaptive Spectral Management: To overcome GLA peaks, the team implemented adaptive channel rates. Instead of a fixed baud rate, the symbol rate was dynamically adjusted per channel to ensure the signal spectrum avoided the specific wavelengths of gas absorption lines.

3. Key Contributions

• New Fiber Design: Demonstration of the GTA-ST-HCF, which achieves a record-low IMI. The standard deviation of power fluctuations was reduced by 6.4-fold compared to previous HCF designs (reaching an IMI of approximately -58 dB/km).
• Ultra-Long Span Record: Successful transmission over a 266 km single span, more than double the length of previous HCF transoceanic demonstrations (typically ~127 km).
• Spectral Optimization Strategy: A novel approach to WDM transmission that varies baud rates (from 135 GBaud down to 30 GBaud) to navigate around GLA spectral notches while maintaining high spectral efficiency.
• System Integration: Integration of high-power amplification (34 dBm) with low-loss HCF to maximize the reach per amplifier.

4. Experimental Results

The system was tested over varying distances (simulated by round trips in the loop):

• Attenuation: The 266 km span exhibited a loss coefficient of 0.098 dB/km, resulting in a total span loss of approximately 27 dB (including splicing and adaptor losses).
• Capacity vs. Distance:
  • 5,320 km: Achieved a net throughput of 23.5 Tb/s (Spectral Efficiency: 4.82 b/s/Hz).
  • 6,660 km: Achieved a net throughput of 21.7 Tb/s (Spectral Efficiency: 4.48 b/s/Hz).
  • 7,990 km: Achieved a net throughput of 15.9 Tb/s (Spectral Efficiency: 3.3 b/s/Hz).
• Repeater Count: The 6,660 km transmission was achieved with only 25 repeaters (spaced by 266 km). In contrast, standard SMF systems typically require over 100 repeaters for similar distances.
• Performance Metrics:
  • Achievable Information Rate (AIR) reached 590 Gbps per lambda at 10,000 km.
  • Despite GLA penalties in the second absorption band (2–3 dB SNR penalty), the adaptive baud rate strategy maintained uniform performance across the C-band.
• Capacity-Distance Product: The results represent a significant leap in capacity-distance products for single-core fibers, effectively doubling the net throughput of previous HCF records while using spans twice as long.

5. Significance and Impact

This paper represents a paradigm shift in submarine optical networking:

1. Cost Reduction: By increasing span lengths from ~55–100 km to 266 km, the number of repeaters required for transoceanic cables can be reduced by up to 5-fold. This drastically lowers cable manufacturing costs, deployment time, and long-term maintenance expenses.
2. Feasibility of HCF: It proves that AR-HCF is viable for real-world transoceanic applications, overcoming the historical barriers of IMI and GLA through advanced fiber design and adaptive signal processing.
3. Future Scalability: The successful demonstration of 21.7 Tb/s net rates over ultra-long spans lays the groundwork for next-generation undersea cables capable of handling exponentially growing global data traffic with lower latency and energy consumption.

In conclusion, the authors have successfully demonstrated the first transoceanic WDM transmission using 266 km spans, validating the combination of low-loss/low-IMI hollow core fiber, high-power amplification, and adaptive spectral management as a viable path toward the future of high-capacity submarine communications.