Optical fiber
A thin glass fibre of core + cladding structure that guides light by total internal reflection along its length. Loss < 0.2 dB/km at 1.55 μm in silica single-mode fibre; the backbone of global telecommunications.
Definition
An optical fibre is a thin, flexible glass or plastic rod consisting of a high-index core surrounded by a lower-index cladding; light launched into the core is guided along the fibre's length by repeated total internal reflection at the core–cladding interface. The numerical aperture NA = √(n_core² − n_cladding²) specifies the cone of acceptance angles that will couple into guided modes. Single-mode fibres have core diameters of about 9 μm for 1.55 μm wavelength — small enough that only one transverse mode propagates — and form the backbone of long-distance telecommunications. Multimode fibres have 50–62.5 μm cores, support dozens of modes, and are used for short-haul datacentre interconnects.
The modern era began in 1970 when Corning developed low-loss silica fibre below 20 dB/km; by the 2000s, optical-fibre losses at 1.55 μm had been pushed below 0.2 dB/km, allowing signals to travel tens of kilometres between amplifiers. The three telecom wavelength windows are 850 nm (multimode datacentre), 1.31 μm (low-dispersion single-mode metro), and 1.55 μm (low-loss single-mode long-haul). Wavelength-division multiplexing (WDM) pumps dozens to hundreds of independent wavelength channels through a single fibre, each carrying 10–400 Gbit/s, giving aggregate capacities of 50+ Tbit/s per fibre pair. Submarine fibre cables carry more than 99% of all transoceanic data traffic; Internet, telephone, and video all depend on fibre-optic transmission at the physical layer.