How fiber-optic cables work

01Light becomes signal

A fiber-optic cable converts electrical data into light, sends that light through a glass strand, and converts it back. A laser or LED at one end flashes on and off billions of times per second. Each pulse is a binary one or zero. At the far end a photodetector catches the light and recreates the electrical signal.

The cable itself is passive. It does not amplify or interpret. It simply guides light from one end to the other with very little loss, which is why a strand of glass thinner than a human hair can carry millions of phone calls simultaneously.

02Total internal reflection

The guiding principle is total internal reflection. When light traveling through a dense medium strikes the boundary with a less dense medium at a shallow enough angle, it reflects entirely back instead of refracting out. The fiber exploits this by using two kinds of glass: a dense core surrounded by a less dense cladding.

Light launched into the core at a sufficiently shallow angle bounces along the core-cladding boundary without escaping. The angle at which light can enter and still be guided is called the acceptance cone, and it is set by the refractive index difference between core and cladding.

03The glass is engineered

The glass in a fiber is not ordinary window glass. It is ultra-pure silica doped with carefully chosen elements to control its refractive index. Impurities that would scatter or absorb light are reduced to parts per billion, because even tiny contamination adds up over kilometers.

Single-mode fibers have cores so narrow, around 9 micrometers, that light travels in essentially one path. Multimode fibers have wider cores, 50 or 62.5 micrometers, allowing multiple paths. Single-mode fibers carry signals farther because the single path avoids inter-symbol interference, but they require more precise and expensive transceivers.

04Pulses survive long distances

Even the purest glass attenuates light. Over tens of kilometers the signal weakens and spreads. Optical amplifiers called EDFAs, erbium-doped fiber amplifiers, boost the signal without converting it back to electricity, which lets a single fiber link span thousands of kilometers.

Dispersion is the other enemy. Different wavelengths travel at slightly different speeds, spreading pulses apart. Engineers compensate by choosing wavelengths where dispersion is minimal, around 1310 and 1550 nanometers, and by designing dispersion-compensating fibers that undo the spreading.

05Many signals share one fiber

A single fiber does not carry one signal. Wavelength division multiplexing assigns different colors of light to different data streams, then combines them into one fiber. A modern long-haul cable might carry 80 or more wavelengths, each at 100 gigabits per second or more.

The fiber is a shared medium, but the sharing happens in the optical domain. Each wavelength exits through its own filter at the far end. This is how a cable the diameter of a garden hose can carry terabits of data across an ocean.

06The cable protects the fiber

The glass strand is fragile. A practical cable wraps it in layers: a primary coating, a buffer tube, strength members of steel or aramid yarn, a waterproof gel or water-blocking powder, and an outer sheath. Submarine cables add copper tubes for power and heavier armor against anchors and currents.

The electronics at each end, lasers, amplifiers, receivers, are where most of the cost and complexity lives. The fiber is cheap; the systems around it are not.

07Reliability comes from redundancy

A single broken fiber is a problem. A cable cut can sever many fibers at once. Networks are designed with redundant routes so traffic can reroute. Submarine cables are laid in pairs or rings, and landing stations connect to inland networks through multiple paths.

The technology works because the physics is dependable and the engineering accounts for what the physics cannot guarantee. Light will travel through glass; the rest is about making sure the glass survives and the signal arrives.