The Decay of Light

In a standard single-mode fiber (G.652), attenuation at $1550\,\text{nm}$ is approximately $0.2\,\text{dB/km}$. After $100\,\text{km}$, the signal power drops by $20\,\text{dB}$ (99%). Without amplification, a signal sent from New York would be mathematically non-existent long before it reached the Mid-Atlantic Ridge.

EDFA: The Heart of the Repeater

The Erbium-Doped Fiber Amplifier (EDFA) revolutionized subsea comms by allowing all WDM channels to be amplified simultaneously in the optical domain.

A short segment of fiber is doped with Erbium ions ($\text{Er}^{3+}$). When 'pumped' with a high-power laser at $980\,\text{nm}$ or $1480\,\text{nm}$, the erbium ions are excited to a higher energy state. When a signal photon passes through, it triggers Stimulated Emission, causing the ions to drop back to a ground state while releasing a new photon identical to the original.

Raman Amplification

While EDFAs use a dedicated doped fiber, Raman Amplification uses the transmission fiber itself as the gain medium. This relies on Stimulated Raman Scattering (SRS).

A high-power pump signal is sent into the fiber (often in the reverse direction). When pump photons collide with silica molecules, they lose energy to molecular vibrations (phonons) and are re-emitted as lower-frequency photons that match the data signal's frequency, effectively boosting it.

The Raman frequency shift in silica fiber.

Spectral Tilt and OSNR

In a cable with 100+ repeaters, even a $0.1\,\text{dB}$ gain imbalance per repeater accumulates into a $10\,\text{dB}$ 'tilt' across the spectrum. High-frequency channels might see massive gain while lower frequencies vanish into the noise floor.

Conclusion

The physics of subsea repeaters is what transforms a simple strand of glass into a global neural network. By manipulating erbium ions and Raman shifts miles below the surface, we effectively negate the laws of attenuation and keep the world connected.