Fiber Optics: How Light Carries the Global Internet

01 The Copper Ceiling

Through the 1970s, long-distance communication depended on coaxial cable and microwave relay, both constrained by physics that no amount of engineering could fully finesse. Electrical signals lose energy in proportion to frequency and distance, so every kilometre of copper demands compensation, and every amplifier adds noise.

The measurement that mattered was attenuation in decibels per kilometre. Coaxial cable at gigahertz frequencies loses on the order of 5 to 20 dB per km; a modest 30 km run can attenuate a signal by a factor of a billion. Telephone network economics were built around this ceiling: repeaters every couple of kilometres, tens of thousands of copper pairs in trunk bundles, and a hard cap on how much data a single physical channel could carry.

Fiber optics inverted the constraint. Guided light in ultrapure glass suffers attenuation measured in fractions of a decibel per kilometre at the right wavelength, and because light is not an electrical signal, the channel is immune to the electromagnetic interference that plagues dense copper plant.

02 Total Internal Reflection

An optical fiber is a dielectric waveguide: a core of doped silica with a refractive index slightly higher than the surrounding cladding. Light entering the core within the acceptance cone strikes the core-cladding boundary at grazing angles beyond the critical angle and reflects back with near-perfect efficiency, bouncing along the axis for kilometres.

The geometry looks unforgiving and works generously. Single-mode fiber has a core only about 8 to 9 micrometres across, roughly a tenth the width of a human hair, yet guides light with such fidelity that a signal can travel 80 to 100 km between amplifiers. Modal dispersion, the smearing of pulses in thick multimode fiber, is eliminated by making the core so narrow that only one propagation path survives.

That single-mode design choice, standardized in the 1980s, is why the same physical glass now serves both short building links and transoceanic routes.

03 Glass So Pure a Kilometre of It Is Mostly Window

The limiting loss is not the reflection but the glass itself. Fused silica scatters light, with Rayleigh scattering falling as the fourth power of wavelength, and absorbs at impurity resonances, above all traces of water in the form of hydroxyl ions that create the famous attenuation peak near 1,383 nm.

Manufacturing suppresses both. Preforms are built by depositing ultra-pure vapour-phase chemicals and collapsing them, then drawn into fiber in towers hundreds of metres tall. Modern single-mode fiber measures roughly 0.15 to 0.20 dB per km at 1,550 nm, a window of transparency where photons ride for tens of kilometres between amplifiers.

The amplifier was as decisive as the fiber itself. Before the erbium-doped fiber amplifier matured around 1990, regeneration meant converting every channel to electronics and back; afterwards, a single pump laser could amplify all wavelengths in a fiber simultaneously, without any optical-to-electrical conversion.

04 Wavelength Division Multiplexing

Once light itself carries the signal, the usable bandwidth of a fiber is bounded chiefly by how many distinct wavelengths transceivers can combine and separate. Dense wavelength division multiplexing stacks dozens to more than a hundred carriers, each modulated at 100 to 800 Gbps, onto a single 9-micrometre core.

Coherent detection, borrowed from radio engineering, let operators encode more bits per symbol and compensate dispersion in silicon rather than in the glass. Per-channel rates climbed from 2.5 Gbps in the early 1990s to multi-terabit laboratory demonstrations on a single fiber.

The result is an inversion of scarcity. The marginal cost of bandwidth along a lit route collapsed, reshaping both the economics of the internet backbone and the design of data centers, which now run the same coherent optics over distances measured in racks rather than oceans.

05 The Submarine Backbone

Roughly 99 percent of intercontinental data traffic travels on submarine cables: several hundred systems linking continents along the seabed, many following routes charted a century ago for telegraph wire. A modern transatlantic cable carries hundreds of terabits per second on a handful of fiber pairs inside a tube about as wide as a garden hose.

The investment is cyclical and enormous, with individual builds costing hundreds of millions of dollars, and it concentrates risk. Cable faults from anchors and trawlers interrupt traffic routinely; regional outages reroute petabits across alternate paths until a repair ship arrives.

Capacity growth on these routes tracks the internet's own expansion. Lit capacity estimates on major routes grew from tens of terabits in 2010 to well over a petabit per route by the mid-2020s, with hyperscale cloud and AI operators now the majority investors in new systems.

06 Limits and Tradeoffs

The physics remains bounded. Rayleigh scattering sets a floor near 0.14 dB per km that no purification can beat, so capacity growth now comes from spectral efficiency, wider amplification bands, and spatial multiplexing in multi-core and few-mode fibers rather than from purer glass.

Nonlinear effects grow with optical power and channel count, which means more channels eventually interfere with one another, and dispersion must be managed digitally at the receiver. Fiber is also fragile in tension: minimum bend radius complicates dense routing, though bend-insensitive designs have largely retired the problem indoors.

The hardest constraint is economic rather than physical. Trenching the last mile to a home can cost more than everything optical behind it, which is why fiber deployment depth still varies enormously between and within countries.

07 Legacy and Outlook

In four decades, fiber went from laboratory curiosity to the substrate of the world's data layer, carrying traffic that grows every couple of years over glass that has changed remarkably little since the 1980s standards were written.

Current work points along two axes: hollow-core fibers that guide light through air rather than solid glass, promising lower latency and gains approaching the speed of light in vacuum, and co-packaged optics that dissolve the copper links between chips and transceivers. Both respond to the same customer: the data center, now the largest single consumer of new optical capacity as AI training clusters sprawl across buildings.

The throughline of the technology is a single idea executed relentlessly: keep the signal as light for as long as possible. Every generation of fiber engineering has been a fight to delay, by one more hundred kilometres, the moment photons must become electrons again.