Fiber Optic Cables Can Eavesdrop On Nearby Conversations

Source: Slashdot

Background

sciencehabit shares a report from Science Magazine: Cold War spies planted bugs in walls, lamps, and telephones. Now, scientists warn, the cables themselves could listen in. A fiber‑optic technique used to detect earthquakes can also pick up the faint vibrations of nearby speech — researchers reported this week at the General Assembly of the European Geosciences Union. Freely available artificial‑intelligence (AI) software turned the fiber‑optic data into intelligible, real‑time transcripts.

“Not many people realize that [fiber optic cables] can detect acoustic waves,” says Jack Lee Smith, a geophysicist at the University of Edinburgh who presented the result. “We show that in almost every case where you use these fibers, this could be a privacy concern.”

Distributed Acoustic Sensing (DAS)

Fiber optics can pick up sound through a technique called distributed acoustic sensing (DAS). An interrogator sends laser pulses down a cable and records the pattern of reflections from tiny glass defects along the fiber. When a seismic wave passes, it stretches and squeezes these defects, shifting the reflected light. Researchers use these shifts to build a picture of the earthquake.

DAS essentially turns a fiber cable into a long chain of seismometers that can detect:

• Earthquakes
• Volcano rumblings
• Cars and other traffic
• College marching bands

Although scientists sometimes install dedicated fiber lines for research, DAS can also be performed on dark fiber—unused strands in the global network that carry internet traffic.

Eavesdropping Experiment

Smith and his colleagues demonstrated that DAS can be used to eavesdrop:

1. Setup – They used an existing DAS system deployed to study coastal erosion.
2. Test source – A speaker was placed next to the cable and played pure tones, music, and speech.
3. Results –
   • Low‑frequency components of human speech (a few hundred hertz) were visible in the raw data without preprocessing.
   • Higher‑frequency speech required modest post‑processing but was still recoverable.
   • Feeding the data directly into Whisper, a free AI transcription tool, produced accurate real‑time transcripts.

Limitations

• The technique worked only for coiled cables exposed at the surface, and only up to about 5 m from the speaker.
• Burying the cable under ≈ 20 cm of dirt significantly degraded the speech signal.
• Straight cables, even when exposed and adjacent to the speaker, recorded speech poorly.

These findings highlight a potential privacy concern for fiber‑optic infrastructure, especially in urban environments where dark fiber is abundant.

Reference

Fiber‑optic cables can eavesdrop on nearby conversations (Science Magazine)