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Short note: Multimode graded index fibre

Short answer

A multimode graded-index fibre has a core index that decreases parabolically from the axis to the cladding, so rays follow curved paths and arrive at nearly the same time, giving low intermodal dispersion. It has a large core (50–62.5 µm), works with cheap LEDs, and suits LANs and data centres up to a few km.

Write a short note on multimode graded index fiber.

A multimode graded-index (GI) fibre is a fibre whose core refractive index is highest at the axis and decreases gradually (usually parabolically) to the cladding value at the core edge, and which supports many modes.

Index profile

n(r) = n1 √(1 − 2Δ (r/a)^α),   r < a
n(r) = n2,                     r ≥ a

α = 2 (parabolic) is the usual profile. Core diameter is 50 or 62.5 µm, cladding 125 µm.

Ray propagation

 cladding ______________________________
           .-.      .-.      .-.
 core    /    \   /    \   /    \
 ----*----------------------------->
          \  /     \  /     \  /
 cladding ______________________________

Rays bend gradually towards the axis by continuous refraction and follow sinusoidal or helical paths. Rays far from the axis travel longer paths but through lower-index (faster) glass, so they arrive at almost the same time as the axial ray.

Features

  • Intermodal dispersion much lower than step-index multimode (bandwidth ~0.5–2 GHz·km versus ~20–50 MHz·km).
  • Larger core than single mode, so cheap LED/VCSEL sources and connectors can be used.
  • Loss about 2.5 dB/km at 850 nm, 0.6–1 dB/km at 1300 nm.
  • Applications: LANs, data centres (OM3/OM4 fibre), campus and building backbones up to a few km.

✓ Answer checked by Claude Opus 5.5 (high effort)

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