Short note: Multi mode optical fiber

Multimode optical fibre has a core wide enough (50 or 62.5 µm, with cladding of 125 µm) to allow many light rays (modes) to travel along different paths at the same time. A cheap LED source is typically used, with wavelengths of 850 nm and 1300 nm.

Step-index:   cladding ____________
              core    \/\/\/\/  rays zig-zag by
              cladding ‾‾‾‾‾‾‾‾‾‾‾‾ total reflection
Graded-index: refractive index falls smoothly
              from centre to edge, so rays bend
              gently and arrive nearly together.

Types

• Step-index multimode: the core has uniform refractive index, with an abrupt drop at the cladding. Rays travelling at steep angles take longer paths, so the pulse spreads (modal dispersion). Bandwidth is lowest.
• Graded-index multimode: the refractive index decreases gradually from the centre outwards. Rays at the edge move faster, which compensates for their longer path, so dispersion is much smaller.

Features

• Cheaper than single-mode (LED sources, easy connectors, easy alignment).
• Higher modal dispersion, so limited distance (typically 300 m to 2 km) and bandwidth (up to 10 Gbps over short links).
• Used in LANs, campus backbones and data centres.