Optical communication

Optical communication converts information into controlled variations of light and reconstructs it at the receiver. A transmitter commonly uses a semiconductor laser, although light-emitting diodes remain useful for short, inexpensive links. Modulation may alter intensity, phase, frequency, or polarization; coherent systems measure phase and amplitude, while simpler direct-detection systems primarily measure received power. The channel is usually optical fiber, whose glass core guides light by total internal reflection.¹

A receiver uses a photodiode, transimpedance amplifier, and signal-processing circuitry to recover the data. Repeaters or optical amplifiers compensate for attenuation over long routes. Erbium-doped fiber amplifiers are especially important around the 1.55-micrometre window, where silica fiber has very low loss. Connector losses, chromatic dispersion, polarization effects, nonlinearities, and detector noise determine the practical reach and capacity of a link.

Fiber-optic networks achieve high capacity by combining low-loss glass with wavelength-division multiplexing. Several independently modulated wavelengths can share one fiber, and dense wavelength-division multiplexing carries many channels across the C and related bands. Digital coherent receivers, forward-error correction, and high-order modulation extract more information from each optical carrier while compensating for transmission impairments.²

The same principles operate at different scales: passive optical networks connect homes, terrestrial trunks interconnect cities, and submarine cables cross oceans. Standardized fiber categories distinguish properties such as core geometry, attenuation, and dispersion; ITU-T G.652 single-mode fiber is a widespread baseline for terrestrial and long-haul infrastructure.³ Optical links reduce the need for repeated electrical conversion, but network equipment still performs routing, switching, power management, and monitoring.

Free-space optical communication sends narrow laser beams through air or vacuum instead of a guided fiber. It can provide high data rates without a cable, including between buildings, aircraft, satellites, spacecraft, and ground stations. In space, the absence of atmospheric turbulence over most of the path is advantageous, while laser divergence permits small terminals and high directional security. NASA demonstrations have used laser communications to test much higher-rate spacecraft links than conventional radio systems can readily provide.

Atmosphere is the principal complication for terrestrial and Earth-space paths: clouds can block the beam, turbulence causes scintillation and beam wander, and pointing errors can interrupt reception. Systems therefore use acquisition and tracking, adaptive optics, site diversity, or a fallback radio link. Optical terminals also face eye-safety, alignment, weather, and regulatory constraints, so free-space systems complement rather than universally replace radio.

Optical communication is not limited to binary intensity modulation or ordinary internet cables. Plastic optical fiber is useful in vehicles and short indoor links because it is flexible and easy to terminate, though it generally has greater attenuation than silica fiber. Multimode fiber supports many propagation paths and is economical over short distances; single-mode fiber is preferred for long reach because it greatly reduces modal dispersion.

Research systems also use space-division multiplexing, in which multiple cores or modes share one fiber, potentially expanding capacity without laying a wholly new cable.⁴ At the other extreme, visible-light communication can encode data in LEDs while remaining apparently steady to human observers. Optical links can carry quantum states for quantum key distribution, but such systems are highly sensitive to loss and atmospheric conditions. The engineering boundary is therefore broad: from inexpensive local links to precision laser terminals and experimental quantum networks.

Photodiode

A semiconductor detector that converts incident optical power into an electrical current.

Wavelength-division multiplexing

A method of carrying multiple separately modulated optical wavelengths through one fiber.

Chromatic dispersion

Pulse spreading caused when different wavelengths propagate at different velocities in a fiber.

Coherent detection

Reception that uses a local optical oscillator to recover the signal's amplitude and phase.

Space-division multiplexing

A capacity-expansion technique that sends separate channels through distinct fiber cores or spatial modes.