Optical Fibre Communication Basics A Complete Simple Guide

Optical fibre communication sends information as pulses of light through a thin glass or plastic fibre, about the thickness of a human hair. Light stays inside by total internal reflection and travels kilometres with very low loss. At the receiving end, light pulses are converted back into electrical signals.

This technology matters because it carries huge amounts of data at light speed with tiny interference. Modern internet backbones, long distance phone calls, cable TV and data centres all depend on optical fibres. It replaced copper for high speed long links.

Key Concepts

Core: Central glass region with higher refractive index where light travels. Cladding: Outer glass layer with lower refractive index surrounding core. Total internal reflection: Light hitting boundary above critical angle reflects fully inside. Numerical aperture: Measure of light gathering ability, no unit. Attenuation: Loss in fibre, as low as 0.2 dB per km in good fibres. Multiplexing: Sending many wavelengths together in one fibre.

Formulas

Critical angle: sin of critical angle equals refractive index of cladding divided by refractive index of core. Angles in degrees, indices have no unit. Assumes light going from denser to rarer medium.

Numerical aperture: NA equals square root of (core index squared minus cladding index squared). Also equals sin of maximum acceptance angle in air.

Acceptance angle: Maximum angle from fibre axis at which light entering will still reflect totally inside.

Worked Example

Core index is 1.50 and cladding index is 1.48. Find critical angle and NA. Step 1: sin critical equals 1.48 divided by 1.50 equals 0.9867. Critical angle equals arcsin 0.9867 approx 80.6 degrees from normal, meaning light almost parallel to boundary stays inside. Step 2: NA equals sqrt(1.50 squared minus 1.48 squared) equals sqrt(2.25 minus 2.1904) equals sqrt(0.0596) approx 0.244. Step 3: Acceptance angle equals arcsin 0.244 approx 14.1 degrees. Light entering within this cone will propagate through the fibre.

Common Misconceptions

• Light travels in straight empty tube: Wrong. Fibre is solid glass, light reflects zigzag inside core.
• Optical fibre carries electric current: Wrong. It carries light, immune to electromagnetic interference and does not conduct electricity.
• Thicker fibre always better: Wrong. Single mode thin core fibres give highest bandwidth over long distance, thick multimode fibres have more dispersion.

Real-World Applications

Under sea cables connect continents for internet. FTTH brings broadband directly to homes. Hospitals use fibre bundles for endoscopy imaging inside body. Industries use fibre sensors for temperature and strain. Data centres use short fibre links for superfast server connections.

Optical fibre guides light by total internal reflection between core and cladding, enabling low loss, high bandwidth communication. Its calculation of critical angle and numerical aperture decides how light enters and stays inside for kilometres.