Discussion
The refractive index variation of a singal mode fiber is :
Answer: C
A single-mode fibre has a step change from core to cladding.
Standard single-mode fibre (such as ITU-T G.652) is a step-index fibre: a very thin core, about 8-10 micrometres across, with a uniform refractive index slightly higher than the uniform index of the cladding, so the index drops abruptly at the boundary. Because the core is so small, only one mode propagates, and intermodal dispersion does not arise, so there is no need for a graded profile.
A gradual, roughly parabolic decrease from the core axis is the graded-index profile used in multimode fibre to reduce modal dispersion. An index increasing from core to cladding would not guide light at all, since total internal reflection needs the core index to be higher.
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