Light Trapped in Glass
Optical fibers guide light through a simple but elegant mechanism: total internal reflection. When a light ray inside the glass core strikes the core-cladding boundary at an angle steeper than the critical angle, it bounces back completely rather than passing through. This critical angle depends on the ratio of refractive indices — and it's the reason light can travel hundreds of kilometers through a hair-thin glass strand with remarkably low loss.
Numerical Aperture and Acceptance Cone
The numerical aperture of a fiber determines which rays can successfully enter and propagate. NA equals the square root of (n₁² − n₂²), and it defines the maximum acceptance half-angle via θ = arcsin(NA). Rays entering at steeper angles refract out through the cladding and are lost. This simple geometry governs how efficiently light couples from a source into the fiber.
Single-Mode vs Multi-Mode
When the core is large enough (typically > 50 µm), many different ray paths — modes — can propagate simultaneously. Each mode travels a slightly different effective distance, causing pulse spreading called modal dispersion. Shrinking the core to about 8 µm forces only the fundamental mode to propagate, eliminating modal dispersion and enabling data rates of terabits per second over hundreds of kilometers.
Real-World Fiber Networks
The global internet backbone runs on single-mode silica fibers operating at 1550 nm, where attenuation drops to just 0.2 dB/km. Wavelength-division multiplexing packs hundreds of independent channels into a single fiber, each carrying 100+ Gbps. Submarine cables spanning ocean floors carry over 99% of intercontinental data traffic — a testament to the power of total internal reflection at scale.