Structure of fiber-optic cable

A fiber-optic cable consists of a glass or plastic core, surrounded by cladding with a lower refractive index. Light injected into the core at an angle greater than the critical angle undergoes total internal reflection, repeatedly bouncing off the core-cladding boundary and traveling down the fiber with minimal loss. An outer jacket provides physical protection.

  +---------------------------------------+
  |              Jacket                    |
  |  +-----------------------------------+  |
  |  |            Cladding               |  |
  |  |   +---------------------------+   |  |
  |  |   |          Core             |   |  |  <- light travels here
  |  |   +---------------------------+   |  |
  |  |                                    |  |
  |  +-----------------------------------+  |
  +---------------------------------------+

Propagation modes

1. Multimode, step-index — core has uniform refractive index; light rays take multiple different paths ("modes") bouncing at different angles, causing modal dispersion (rays arrive at slightly different times), limiting distance/bandwidth. Larger core diameter (~50–62.5 µm), cheaper LEDs as light source.

2. Multimode, graded-index — core's refractive index decreases gradually from center to edge, causing light to travel in curved paths. Rays taking longer geometric paths travel faster (lower refractive index near the edge), which reduces modal dispersion compared to step-index multimode.

3. Single-mode — very narrow core (~8–10 µm), allowing only one straight-line mode of light to propagate. Eliminates modal dispersion almost entirely, supporting the highest bandwidth and longest distances, but requires a precise, more expensive laser light source.

Single-mode is used for long-haul/backbone links; multimode is used for shorter, cheaper LAN/building-scale runs.