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Construction and types of optical fibre

Short answer

An optical fibre consists of a doped-silica core (n1), a pure-silica cladding (n2, n1>n2), an acrylate buffer and an outer jacket. Three principal types exist: step-index multimode (large core, many modes, short links), step-index single-mode (small core, single mode, long haul) and graded-index multimode (parabolic index profile, low modal dispersion, LANs).

Explain the construction and types of optical fiber with necessary diagrams.

An optical fibre is a thin, flexible glass (or plastic) waveguide that carries light by total internal reflection.

Construction

  cross-section             side view
   .-------------.      jacket  ================
  /   jacket      \     buffer  ----------------
 |  .-----------.  |    clad n2 ________________
 | /  cladding   \ |    core n1  ---> light --->
 | |   .-----.   | |    clad n2 ________________
 | |   | core|   | |    buffer  ----------------
 | \   '-----'   / |    jacket  ================
  \ '-----------' /
   '-------------'
  • Core: central glass region of higher index n1 (silica doped with germanium) that carries the light.
  • Cladding: surrounding glass of slightly lower index n2 (n1 > n2) that makes TIR possible; standard diameter 125 µm.
  • Buffer coating: acrylate layer(s) (to 250 µm) that protect against moisture, scratches and microbending.
  • Jacket and strength members: outer PVC/PE sheath and Kevlar yarn that give mechanical strength in a cable.

Types of optical fibre

TypeCore sizeIndex profileModesUse
Step-index multimode50–200 µmAbrupt stepMany, zig-zagShort links
Step-index single mode8–10 µmAbrupt stepOneLong haul
Graded-index multimode50/62.5 µmParabolicMany, curvedLAN
   SI multimode:  /\/\/\/\/\  large modal dispersion
   Single mode:   ---------->  no modal dispersion
   GI multimode:  ~~~~~~~~~~>  low modal dispersion

Fibres may also be classified by material: all-glass (silica), plastic-clad silica, and all-plastic (POF) fibres.

✓ Answer checked by Claude Opus 5.5 (high effort)

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