How Fibre Optic Cables Work — Principles and Technology

Optical fibre is an ultra-thin glass strand capable of transmitting data at the speed of light. But how does it actually work? This article explains the physical principles, the structure of the cable, and why this technology has replaced copper for modern telecommunications.

The principle: transmitting data with light

A fibre optic transmission system relies on three components:

• An optical transmitter (laser or LED): converts the electrical signal (digital data) into light pulses
• The fibre optic cable: guides these light pulses over distances from a few metres to 80+ kilometres
• An optical receiver (photodiode): converts the light back into an electrical signal usable by the equipment

Data is encoded as binary signals: light on = 1, light off = 0. Billions of these pulses per second enable data rates of 10 Gbps to 100+ Gbps.

Light travels through glass at about 200,000 km/s (2/3 of the speed of light in a vacuum). It is this fundamental speed that makes optical fibre unbeatable for telecommunications.

Structure of a fibre optic cable

A fibre optic cable is made up of several concentric layers, from the inside out:

1. The core

This is the transmission channel. A cylinder of ultra-pure glass (doped silica) through which the light propagates. Diameter: 9 µm for single-mode (thinner than a human hair), 50 µm for multimode.

2. The cladding

A layer of glass surrounding the core, with a slightly lower refractive index. It is this index difference that confines the light within the core through total internal reflection. Standard diameter: 125 µm.

3. The primary coating

A polymer layer (acrylate) that protects the bare fibre against micro-bending and moisture. Diameter: 250 µm.

4. The outer jacket

The final mechanical protection: PVC for indoor use, LSZH (halogen-free) for sensitive environments, or armoured steel for outdoor use. It is this layer that gives the cable its colour (yellow = single-mode, orange/aqua = multimode).

Total internal reflection — the heart of how it works

The physical principle that allows light to travel through the fibre is total internal reflection. When light passes from a medium with a high refractive index (the core) to a medium with a lower index (the cladding), it is totally reflected if the angle of incidence exceeds a critical threshold.

Result: the light bounces in a zigzag pattern inside the core, guided along the entire length of the fibre without escaping. This is why we speak of an optical waveguide.

Single-mode vs multimode — two ways to guide light

The diameter of the core determines how many modes (paths) of light can propagate:

• Single-mode (9 µm): a single mode of light. No modal dispersion → long distance (10–80+ km). Standard for FTTH and telecommunications.
• Multimode (50 µm): several modes of light. Modal dispersion → short distance (< 550 m). Used in data centres for the lower cost of transmitters.

To learn more about the standards (OS2, OM1–OM5), see our article Classification of optical fibres.

Why optical fibre is superior to copper

For a complete comparison, see our article Advantages and disadvantages of optical fibre.

FAQ — How optical fibre works