Which of the following is not a usual classification of optical fibre ?
Single mode graded index
The two independent labels. A fibre is described by how many modes it supports and by how the refractive index varies across the core. In principle that gives four combinations, but only three are found in practice:
| Type | Core diameter | Used for |
|---|---|---|
| Single mode step index | 8–10 μm | long-haul, highest bandwidth |
| Multimode step index | 50–200 μm | short links, cheap sources |
| Multimode graded index | 50–62.5 μm | LAN links, reduced modal dispersion |
| Single mode graded index | — | not a usual class |
Why the missing combination makes no sense. A graded index profile exists for one purpose: to equalise the transit times of the many modes in a multimode fibre. Rays that stray towards the cladding travel further, but the index there is lower so they travel faster, and all modes arrive together — which cuts modal dispersion dramatically.
A single mode fibre, by definition, carries only one mode. With just one path there are no differing transit times to equalise, so grading the index would serve no purpose. Single mode operation is obtained instead by shrinking the core until the normalised frequency falls below cut-off:
\(V=\dfrac{2\pi a}{\lambda}\,NA \lt 2.405\)
and a step profile does that perfectly well.
Note on the boundary of the rule. Specialised dispersion-shifted and dispersion-flattened single mode fibres do use tailored index profiles — but their purpose is to move the zero-dispersion wavelength by controlling material and waveguide dispersion, not to equalise modal delays, so they are not called "graded index" in this classification.
Hence, the classification that does not usually exist is single mode graded index.
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