Fiber Optics: How Light Carries the Internet

The internet's backbone is a global network of glass fibers thinner than a human hair, each carrying dozens of simultaneous data streams encoded on different wavelengths of infrared light. These cables span oceans and transit billions of data packets every second — powered by nothing but photons and total internal reflection.

1. Total Internal Reflection

When light travels from a denser medium (high refractive index n₁) to a less dense medium (n₂ < n₁), Snell's law governs refraction:

n₁ \cdot sin(θ₁) = n₂ \cdot sin(θ₂)

As the incident angle θ₁ increases past the critical angle θ_c, the refracted ray grazes parallel to the interface (θ₂ = 90°), and beyond θ_c, no refracted ray exists — all light bounces back. This is total internal reflection:

sin(θ_c) = n₂ / n₁ For silica fiber: n₁ = 1.4682 (core), n₂ = 1.4629 (cladding) θ_c = arcsin(1.4629/1.4682) \approx 85.6°

Light entering the fiber at angles beyond θ_c from the fiber axis bounces off the core-cladding boundary repeatedly without loss — guided down the fiber's length. The glass carries the signal like a flexible pipe for light.

2. Fiber Structure & Numerical Aperture

A standard optical fiber consists of three cylindrical layers:

Core: Germanium-doped silica (GeO₂–SiO₂). Diameter 8–62.5 µm. Slightly higher refractive index than cladding.
Cladding: Pure silica, diameter 125 µm. Lower refractive index — creates the boundary for TIR.
Primary coating: UV-cured acrylate, outer diameter 245–900 µm. Protects against abrasion and moisture. Color-coded for identification.

The Numerical Aperture (NA) specifies the acceptance cone of light angles the fiber will guide:

NA = sin(θ_acceptance) = \sqrt(n₁² - n₂²) Typical SMF-28: NA \approx 0.14 \to acceptance half-angle \approx 8°

Light entering outside this cone refracts into the cladding and is lost within centimeters. Fiber connectors must align cores to ≤1 µm accuracy for low insertion loss.

3. Single-Mode vs Multi-Mode

Single-Mode Fiber (SMF)

Core diameter: ~8 µm

Only the fundamental LP₀₁ mode propagates. Eliminates modal dispersion — all photons travel the same path. Bandwidth exceeds 100 THz.

Use: Long-haul telecom, submarine cables, FTTH PON networks. Most internet backbone fiber is SMF-28 compliant.

Multi-Mode Fiber (MMF)

Core diameter: 50–62.5 µm

Multiple modes propagate simultaneously at different angles. Modal dispersion spreads pulses — limits bandwidth × distance to ~500 MHz·km.

Use: Short-range data center connections (<550 m), campus networks, test equipment. Graded-index MMF minimizes dispersion somewhat.

The single-mode core of 8 µm is so small because the single-mode condition requires core diameter < ~2.4 · λ / (π · NA) — at 1310 nm wavelength this yields ~8.6 µm for NA = 0.14.

4. Attenuation & Dispersion

Signal degrades over distance due to two main effects:

Attenuation (signal power loss):

Rayleigh scattering: Submicron density fluctuations frozen into the glass scatter light. Decreases as 1/λ⁴ — shorter wavelengths scatter more. Fundamental limit of silica fiber.
IR absorption: Silica absorbs strongly beyond 1700 nm. OH⁻ ion impurities add absorption peaks at 1380 nm ("water peak").
Minimum attenuation window: ~0.2 dB/km at 1550 nm. This is why most long-haul systems use the C-band (1530–1565 nm).

After 100 km at 0.2 dB/km: loss = 20 dB \to power reduced by factor 100

Dispersion (pulse spreading):

Chromatic dispersion: Different wavelengths travel at different speeds. In SMF-28, zero-dispersion wavelength λ₀ ≈ 1310 nm. At 1550 nm, D ≈ 17 ps/(nm·km).
Polarization mode dispersion (PMD): Two polarization modes propagate at slightly different speeds due to core ellipticity.

5. EDFA Optical Amplifiers

Erbium-Doped Fiber Amplifiers (EDFAs) amplify optical signals without converting to electrical. A short section (~10 m) of silica fiber is doped with Er³⁺ ions. When pumped by a 980 nm or 1480 nm laser diode, the erbium ions reach an excited state. Signal photons at 1530–1565 nm (C-band) stimulate emission — the same mechanism as laser gain — amplifying the signal photons directly.

Key advantage: EDFAs amplify all wavelengths in the C-band simultaneously. A single EDFA amplifies 80+ DWDM channels at once. Gain: 20–40 dB. Noise figure: 3–6 dB (quantum noise limit). First demonstrated in 1987 by Desurvire, Payne, and Townsend — the invention that made transoceanic fiber links viable without electronic repeaters every few kilometres.

6. Wavelength-Division Multiplexing

Rather than increasing bit rate on a single channel, Wavelength-Division Multiplexing (WDM) transmits many independent data streams on different wavelengths simultaneously through a single fiber. Dense WDM (DWDM) uses <100 GHz channel spacing (ITU grid, ~0.8 nm).

C-band capacity: ~80 channels × 400 Gbit/s each = 32 Tbit/s per fiber pair (Alcatel-Lucent, 2024)
Multiplexer: Thin-film interference filters or arrayed waveguide gratings (AWGs) combine wavelengths onto one fiber
Demultiplexer: Same devices in reverse separate wavelengths at the receiver
Advanced modulation: DP-QPSK, DP-16QAM encode 2–4 bits per symbol per polarization

Record capacity: NTT demonstrated 22.9 Pbit/s over 13 km of 4-core fiber in 2022. Commercial transoceanic systems typically achieve 100–200 Tbit/s total capacity per cable.

7. Undersea Cables and the Global Network

Over 99% of intercontinental internet traffic travels through submarine fiber cables — there are ~570 systems totaling over 1.4 million km of cable as of 2024. These cables withstand enormous challenges:

Water pressure: At 8 km depth, 80 MPa. Cable protected by polyethylene, steel armor wires, and copper power conductor.
Power feed: High-voltage DC power (up to 15 kV, 1 A) runs along the cable to power EDFA repeaters every 40–100 km.
Lifetime: Designed for 25 years. Major breaks (~100/year) caused by anchors, trawling, submarine landslides.
Latency: Speed of light in fiber ≈ 0.67c. New York to London (6,000 km): ~30 ms. Shortest route cables are designed for financial HFT (Hibernia Express: 59.6 ms RTT).