Optical fiber and its advantages

An optical fiber is a thin, flexible strand of very pure glass or plastic that guides light along its length. It is used to carry information as light pulses.

Structure

  +-----------------------------------+
  |  Jacket (protective coating)      |
  |  +-----------------------------+  |
  |  |  Cladding (n2)              |  |
  |  |  +-----------------------+  |  |
  |  |  |  Core (n1 > n2)       |  |  |
  |  |  +-----------------------+  |  |
  |  +-----------------------------+  |
  +-----------------------------------+

Working principle: total internal reflection

The core has a refractive index n1 higher than the cladding index n2. Light that strikes the core-cladding boundary at an angle (measured from the normal) greater than the critical angle θc = sin⁻¹(n2/n1) is totally reflected back into the core. It zigzags along the core without leaving it, even when the fiber is bent.

  ray  \  /\  /\  /\  /
 ------>\/  \/  \/  \/ ----> out

In a system, an LED or laser converts the electrical signal to light, the fiber carries it, and a photodiode at the far end converts it back.

Advantages over copper or coaxial cable

• Very large bandwidth and high data rate (Gbps to Tbps).
• Very low attenuation, so fewer repeaters are needed over long distances.
• Immune to electromagnetic interference and crosstalk, because it carries light.
• Better security: it is difficult to tap without detection.
• Small size and light weight; thin cables carry many channels.
• No spark or fire risk (no electrical current), so it is safe in hazardous areas.
• The raw material (silica) is abundant, so it is cheap per unit bandwidth.