Fiber Optics: How Light Carries the Internet

Total internal reflection

When light travels from a denser medium into a less dense one, Snell's law governs refraction: n₁·sin(θ₁) = n₂·sin(θ₂). As the incident angle θ₁ grows past the critical angle θ_c = arcsin(n₂/n₁), the refracted ray grazes the interface and beyond that angle no refracted ray exists at all — every photon bounces back. In a standard silica fiber, the core (n₁ = 1.4682) has a slightly higher refractive index than the cladding (n₂ = 1.4629), giving a critical angle of about 85.6°. Light entering the fiber beyond that angle from the axis reflects off the core-cladding boundary over and over without loss, guided down the fiber's length like a flexible pipe for light.

Numerical aperture and fiber structure

A standard fiber has three layers: a germanium-doped silica core (8-62.5 µm diameter), a pure-silica cladding (125 µm) with a lower refractive index that creates the total-internal-reflection boundary, and a UV-cured acrylate coating that protects against abrasion. The numerical aperture, NA = √(n₁² − n₂²), sets the acceptance cone of angles the fiber will actually guide — for standard SMF-28 fiber, NA ≈ 0.14 gives an acceptance half-angle of about 8°. Light entering outside that cone refracts into the cladding and is lost within centimetres, which is why fiber connectors must align cores to within about 1 µm for low insertion loss.

Single-mode versus multi-mode

Single-mode fiber has a core only ~8 µm wide — so narrow that only the fundamental mode can propagate, eliminating modal dispersion entirely and supporting bandwidth beyond 100 THz. It is the standard for long-haul telecom, submarine cables and FTTH networks. Multi-mode fiber has a larger 50-62.5 µm core, letting several modes travel at slightly different angles simultaneously; the resulting modal dispersion caps bandwidth-distance product at around 500 MHz·km, which limits it to short-range links under 550 m — typically data-centre and campus networking, where cost and ease of connection matter more than reach.

sin(θ_c) = n₂ / n₁
Silica fiber: n₁ = 1.4682 (core), n₂ = 1.4629 (cladding)
θ_c = arcsin(1.4629/1.4682) ≈ 85.6°

Amplifiers, WDM and undersea cables

Signal loss is unavoidable — silica's minimum attenuation window is about 0.2 dB/km at 1550 nm, which is why long-haul systems use the C-band. Erbium-doped fiber amplifiers (EDFAs), first demonstrated in 1987, amplify optical signals directly without ever converting them to electricity, boosting an entire 80+ channel DWDM system at once and making transoceanic links viable without electronic repeaters every few kilometres. Wavelength-division multiplexing then squeezes far more out of each strand of glass: packing roughly 80 channels at 400 Gbit/s onto the C-band yields about 32 Tbit/s from a single fiber pair. Over 99% of intercontinental internet traffic rides this exact stack through roughly 570 submarine cable systems totalling more than 1.4 million km, each cable powered by up to 15 kV of DC running alongside the fiber to feed repeaters every 40-100 km.

Frequently asked questions

Try it live

Everything above runs in your browser — open Optical Fiber & Total Internal Reflection, adjust core and cladding refractive indices, and watch rays bounce, escape, or stay trapped depending on the launch angle.