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PointStep-index fibreGraded-index fibre
Index profileCore index \(n_1\) is constant and drops abruptly (in a step) to \(n_2\) at the claddingCore index is highest on the axis and falls gradually (roughly parabolically) to \(n_2\) at the cladding
Ray pathZig-zag: straight lines, total internal reflection at the core–cladding boundaryHelical or sinusoidal curves, by continuous refraction
ModesSingle-mode or multimodeUsually multimode
Intermodal dispersionLarge in multimode fibreVery small, because rays travelling farther move through lower-index (faster) regions
Bandwidth / data rateLower (multimode)Higher
NA\(NA=\sqrt{n_1^2-n_2^2}\), same everywhereVaries with radial distance; maximum on the axis
Number of modes\(N\approx V^2/2\)\(N\approx V^2/4\)
Core diameterSM: 5–10 µm; MM: 50–200 µm50–100 µm
Cost / fabricationSimple, cheaperComplex, costlier
UsesShort links (MM), long-haul (SM)Medium-distance LANs
 Step index: n |  ____            Graded index: n |   /\
               | |    |                           |  /  \
               |_|    |_                          |_/    \_
                 r -->                               r -->
Doubtly AI · 25 Oct 2026 AI-verified · Oct 2026 Report
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