High-Resolution Coherent DFS Over 20km Ultra-Low-Loss Anti-Resonant Hollow-Core Fiber with Live Traffic

This paper demonstrates sub-meter resolution coherent distributed fiber sensing over 20 km of ultra-low-loss anti-resonant hollow-core fiber while simultaneously maintaining live 1.2 Tbps traffic on an adjacent channel.

The Gist

Imagine you are trying to listen to a whisper in a massive, empty cathedral. Now, imagine that whisper is coming from a tiny vibration in the walls, but the cathedral is so quiet and the walls are so smooth that the sound bounces back incredibly weakly. That is essentially the challenge scientists faced with a new type of "super-fiber" used for the internet.

Here is the story of how they solved it, explained simply.

The Problem: The "Ghost" Fiber

For decades, internet data has traveled through glass fibers (like long, thin strands of glass). Recently, engineers invented a new kind of fiber called Hollow-Core Fiber (HCF).

• The Analogy: Think of a traditional fiber as a solid glass rod. Light travels through the glass. The new HCF is like a straw. The light travels through the air inside the straw, not the glass walls.
• The Benefit: Because light isn't touching the glass, it travels faster (lower latency) and loses less energy (lower loss). It's like a car driving on a perfectly smooth, empty highway instead of a bumpy road.
• The Catch: Because the light is mostly in the air, it doesn't bounce off the "walls" very much. If you want to use this fiber to "listen" to vibrations (like a security system that detects footsteps or earthquakes), it's incredibly hard because the signal is so faint. It's like trying to hear a pin drop in a library using a microphone that only picks up loud noises.

The Challenge: The "Blind Spot"

The researchers wanted to use this 20-kilometer (12-mile) long "straw" fiber to do two things at once:

1. Carry live internet traffic (1.2 Terabits per second—that's enough to stream thousands of 4K movies simultaneously).
2. Act as a giant microphone to detect tiny vibrations (acoustic sensing) along the whole length of the cable.

The problem was that the connections where the "straw" meets the old "glass" cables created huge echoes (reflections). These echoes were so loud they drowned out the tiny whispers (vibrations) they were trying to hear. It was like trying to hear a baby crying while someone was blasting a fire alarm right next to your ear.

The Solution: The "Stabilized Flashlight" and "Smart Code"

The team from Nokia Bell Labs and their partners came up with a clever two-part solution:

1. The Stabilized Laser (The Steady Hand)
Usually, lasers wiggle a little bit, which creates noise. The team used a special, ultra-stable laser.

• The Analogy: Imagine trying to take a photo of a fast-moving car in the dark with a shaky hand. The photo comes out blurry. They used a laser that is as steady as a rock, allowing them to take a "crystal clear photo" of the fiber's interior without the image shaking.

2. The Smart Code (The Secret Handshake)
Instead of just sending a simple pulse of light, they sent a complex, repeating code (like a secret handshake).

• The Analogy: Imagine shouting "Hello" in a crowded room. No one hears you. But if you shout a specific, complex song that only your friend knows, they can pick it out of the noise. They used a code that allowed them to filter out the "noise" of the fiber and the "blasting fire alarm" of the connection points, isolating the tiny vibrations they wanted to hear.

The Results: Hearing the Whisper

With this new system, they achieved three amazing things:

1. Super Sharp Vision: They could pinpoint exactly where a vibration happened with sub-meter resolution.
   • Analogy: If the fiber was a 20km long hallway, they could tell you exactly which tile on the floor someone stepped on, down to less than the length of a ruler (30cm).
2. No Traffic Jam: They did this while the fiber was carrying 1.2 Terabits of live internet traffic.
   • Analogy: They managed to listen to a whisper in the room without the people talking on the phone (the internet traffic) getting any static or dropping their calls.
3. Finding the "Splices": They could find the exact spots where two pieces of fiber were joined together (splices) and measure how much light was lost there. This helps engineers fix weak spots in the network.

The Big Picture

This paper proves that we can finally use these amazing, super-fast "air-filled" fibers for real-world security and monitoring. Before this, the fibers were too "quiet" to listen to. Now, we have a way to turn these high-speed internet cables into giant, sensitive microphones that can detect earthquakes, construction work, or even intruders, all without slowing down the internet for anyone.

In short: They turned a "ghostly" fiber that was too quiet to listen to, into a super-sensitive ear that can hear a pin drop while the whole world is talking on the phone.

Technical Summary

1. Problem Statement

The rapid development of Anti-Resonant Hollow-Core Fibers (AR-HCF) offers significant advantages for optical telecommunications, including ultra-low attenuation (<0.11 dB/km), reduced latency, and high-power handling capabilities. However, implementing Distributed Acoustic Sensing (DAS) or Distributed Fiber Sensing (DFS) on these fibers presents unique challenges:

• Weak Backscattering: Unlike solid-core fibers, AR-HCFs guide light through an air core, drastically reducing light-matter interaction. The Rayleigh Backscattering Coefficient (RBC) from the air core is extremely low (approx. -90 to -100 dB/m), and backscattering from the glass microstructure is even lower (-115 to -150 dB/m). This makes detecting acoustic vibrations highly difficult.
• Parasitic Reflections: Coupling standard Single-Mode Fibers (SMF) to HCF requires adapters (e.g., Mode Field Adapters), which introduce strong Fresnel reflections. These reflections create "dead zones" and limit the dynamic range of traditional sensing systems.
• Resolution vs. Range Trade-off: Conventional Optical Time Domain Reflectometers (OTDR) struggle with HCFs. Short pulses are needed for high resolution but lack the energy to overcome the weak backscatter over long distances. Previous attempts achieved only ~1.5 m resolution over long distances using impractically long acquisition times or pulse amplification.
• Coexistence with Live Traffic: Most sensing systems cannot operate simultaneously with high-capacity data traffic without causing interference or signal degradation.

2. Methodology

The authors developed a Coherent Distributed Fiber Sensing (DFS) system utilizing a stabilized laser source to overcome the limitations of HCF backscattering.

• Fiber Under Test: A 20.2 km span of novel Support Tube Hollow-Core Fiber (ST-HCF) manufactured by YOFC, featuring a loss of <0.1 dB/km at 1550 nm. The fiber consists of two spools connected by a splice, with SMF patches at both ends coupled via custom Mode Field Adapters (MFA) with angle-cutting and anti-reflection coatings.
• Sensing Architecture:
  • Stabilized Laser: A custom Optical Frequency Discriminator (OFD) was used to minimize laser frequency noise, addressing coherence loss issues.
  • Advanced Probing: The system employs long Golay codes (221 symbols) at modulation rates up to 500 Mbaud. This allows for high energy injection per probing interval without sacrificing resolution.
  • Signal Processing: A matched filter at the receiver extracts the Channel State (CS) rapidly, while a high-pass filter removes ultra-low frequency mechanical noise (<20 Hz).
• Live Traffic Integration: A second wavelength carrying 1.2 Tbps of live traffic (using a Nokia PSI-M transponder) was multiplexed onto the same fiber adjacent to the sensing channel. The system monitored the Uncorrected Code Blocks (UCB) to ensure the sensing process did not degrade the communication signal.
• Acoustic Excitation: A 3-axis translation stage was mounted on the fiber splice at the 8.1 km mark to induce controlled mechanical oscillations for testing.

3. Key Contributions

1. Sub-Meter Resolution on HCF: The team achieved a spatial resolution of 0.3 meters over a 20 km HCF span, a significant improvement over previous OTDR limitations (typically >1.5 m or requiring hours of acquisition).
2. Ultra-Low-Loss Characterization: They successfully characterized the propagation loss of the ST-HCF as 0.086 ± 0.005 dB/km using bidirectional DFS measurements, confirming the fiber's ultra-low loss capabilities.
3. Precise Splice Localization: The system precisely identified splicing losses and adapter losses (estimated <0.3 dB per adapter), demonstrating the ability to pinpoint faults in long HCF spans despite strong parasitic reflections from the SMF-HCF interfaces.
4. Non-Disruptive Sensing: The study demonstrated, for the first time, acoustic sensing on HCF while maintaining uninterrupted 1.2 Tbps live traffic on an adjacent channel. The system operated successfully with launch powers up to 23 dBm without increasing the UCB rate.

4. Experimental Results

• Backscattering Analysis:
  • The RBC of the SMF patch was measured at -76 dB/m, while the HCF air core RBC was estimated at -93.7 dB/m (consistent with theory).
  • The residual backscattering from the HCF glass structure was estimated at -125.1 dB/m, confirming the extreme difficulty of sensing in these fibers.
  • The RBC difference between SMF and HCF was approximately 49.1 dB.
• Resolution and Loss:
  • The Full Width at Half Maximum (FWHM) of peaks around splice regions was measured at 60 cm, validating the 0.3 m native gauge length.
  • The total end-to-end loss was 2.3 dB, with the two adapters and splices contributing approximately 0.56 dB combined.
• Acoustic Detection:
  • The system successfully detected a 40 Hz mechanical oscillation induced at the 8.1 km splice point.
  • The peak was clearly visible in the Power Spectral Density (PSD) despite the RBC from the air being around -93 dB/m and the event occurring in a region with RBC powers below -100 dB/m.
• Traffic Impact:
  • At launch powers below 13 dBm, traffic loss occurred due to added attenuation.
  • However, at launch powers up to 23 dBm, the system showed zero Uncorrected Code Blocks (UCB), proving that the high-power sensing signal did not interfere with the 1.2 Tbps data stream.

5. Significance

This work represents a major milestone in the practical deployment of Hollow-Core Fibers. By proving that high-resolution (sub-meter) sensing is possible on ultra-low-loss HCF without compromising live high-capacity traffic, the authors have removed a critical barrier to the adoption of HCF in real-world optical transport networks.

The results suggest that HCFs can now serve a dual purpose: acting as the backbone for next-generation high-speed data transmission while simultaneously functioning as a sensitive, distributed sensor network for infrastructure monitoring (e.g., detecting vibrations, leaks, or intrusions) over tens of kilometers. This paves the way for "smart" optical networks that do not require separate sensing fibers or downtime for maintenance.