Fiber optic communication

Communication with light

Fiber optic communication sends information through glass or plastic fibers using light. Instead of electrical signals in copper wires, information is encoded onto optical signals from lasers or LEDs.

Fibers are central to the internet, long-distance telephone systems, data centers, medical devices, and sensors.

Guiding light

A fiber guides light through total internal reflection or, more generally, waveguiding. The core has refractive index slightly larger than the cladding:

$n_{core}>n_{clad}$

For a simple step-index fiber, the numerical aperture in air is approximately

$NA=sqrt{n_{core}^2-n_{clad}^2}$

This describes the range of input angles that can be guided.

Modulation

Information is transmitted by modulating light. The simplest digital method turns light intensity on and off to represent bits. More advanced systems modulate phase, frequency, amplitude, or polarization.

A basic optical carrier can be represented as

$E(t)=E_0cos(omega_c t)$

Modulation changes one or more properties of this carrier to encode information.

Attenuation

As light travels through fiber, its power decreases due to absorption, scattering, bending losses, and connector losses. Attenuation is often measured in decibels per kilometer.

If optical power changes from $P_0$ to $P$, the decibel change is

ight)$$ Low attenuation allows signals to travel long distances before amplification or regeneration is needed. ## Dispersion Dispersion spreads optical pulses. If a pulse broadens too much, neighboring bits overlap and errors occur. Major types include modal dispersion, chromatic dispersion, and polarization mode dispersion. Modal dispersion occurs when different paths or modes take different times. Single-mode fibers reduce this by allowing only one spatial mode. Chromatic dispersion occurs because different wavelengths travel at different speeds. ## Single-mode and multimode fiber Multimode fiber has a larger core and supports many propagation modes. It is easier to couple light into but suffers more modal dispersion. Single-mode fiber has a small core and supports one main mode, enabling long-distance high-bandwidth transmission. Long-haul communication usually uses single-mode fiber. ## Wavelength-division multiplexing Wavelength-division multiplexing sends many channels through the same fiber using different wavelengths. Each wavelength carries a separate data stream. At the receiver, optical filters separate the channels. This greatly increases fiber capacity. ## Optical amplifiers Optical amplifiers boost light signals without converting them to electrical signals. Erbium-doped fiber amplifiers are widely used near telecommunications wavelengths. They use stimulated emission, connecting fiber communication directly to laser physics. ## The big idea Fiber optic communication uses guided light to carry data with high bandwidth and low loss. Total internal reflection, modulation, attenuation, dispersion, multiplexing, and optical amplification all matter. The technology is a major practical triumph of wave optics, lasers, and materials science.